east african rift is what type of plate boundary
16734089
The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the south-west Indian Ocean and south-east Atlantic Ocean. A divergent tectonic plate boundary separating the Somali Plate to the north from the Antarctic Plate to the south, the SWIR is characterised by ultra-slow spreading rates (only exceeding those of the Gakkel Ridge in the Arctic) combined with a fast lengthening of its axis between the two flanking triple junctions, Rodrigues () in the Indian Ocean and Bouvet () in the Atlantic Ocean. Geological setting Spreading rates The spreading rate along the SWIR varies: the transition between slow (30 mm/yr) and ultra-slow (15 mm/yr) spreading occur at magnetic anomaly C6C (ca. 24 Ma). This occurs between 54°–67°E, the deepest, and perhaps coldest and most melt-poor, part of Earth's mid-ocean ridge system. Crustal thickness decreases quickly as spreading rates drop below c. 20 mm/yr and in the SWIR there is an absence of volcanic activity along stretches of ridge axis. Along large sections, the SWIR runs obliquely relative to the spreading direction, typically about 60°. Because obliquity increases ridge length while decreasing mantle upwelling rates, the SWIR is transitional between slow and ultra-slow ridges. The slow-spreading sections of the SWIR have magmatic segments linked by transform faults, while the ultra-slow sections lack such transforms and have magmatic segments linked by amagmatic troughs. Diffuse plate boundaries Spreading in the SWIR is slow, but the plate boundary is intersected by the much slower but more diffuse Nubian–Somalian boundary. The variation in spreading rates indicate the SWIR is not a spreading centre between two rigid plates, but that the previously assumed single African Plate north of the SWIR is in fact divided into three plates: the Nubian, Lwandle, and Somalian plates. The location on the SWIR of this "diffuse" triple junction between the Nubian, Somali, and Antarctic plates has been estimated to between 26°E and 32°E or just west of the Andrew Bain transform fault. This diffuse triple junction forms the southern end of the East African Rift system. In situ Jurassic rocks 180 Ma-old rocks, dated from zircons in diorite and gabbro, were dredged from a location south of the SWIR in 2010. This age is comparable to that of the break-up of Gondwana, the opening of the Indian Ocean, and emplacement of the Karoo Large Igneous Province (179-183 Ma) — in sharp contrast the Neogene age of the ocean floor near the SWIR. It can be assumed the rocks were deposited near the SWIR by an external force, such as an ice-rafting or a tsunami, but the SWIR is located far away from any continental margin and rocks of similar age have been reported from the Mid-Atlantic Ridge. If the rocks came directly out of the mantle it would have lost most of its isotopic lead. Ice-rafted dropstones commonly show sign of rounding. Hydrothermal circulation at mid-ocean ridges can, however, bring intrusive rocks into the shallow mantle, and it is possibly a good candidate in this case. Most rocks
Southwest Indian Ridge
9549806
The Explorer Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located about west of Vancouver Island, British Columbia, Canada. It lies at the northern extremity of the Pacific spreading axis. To its east is the Explorer Plate, which together with the Juan de Fuca Plate and the Gorda Plate to its south, is what remains of the once-vast Farallon Plate which has been largely subducted under the North American Plate. The Explorer Ridge consists of one major segment, the Southern Explorer Ridge, and several smaller segments. It runs northward from the Sovanco Fracture Zone to the Queen Charlotte Triple Junction, a point where it meets the Queen Charlotte Fault and the northern Cascadia subduction zone. Geology This divergent boundary first formed about 5-7 million years ago when the northern end of the Juan de Fuca Plate broke off along the Nootka Fault to form the Explorer Plate. This apparently had some important ramifications for regional geologic evolution. When this change was completed, Cascade Arc volcanism from Northern California to southwestern British Columbia returned and the present-day Cascade Range and Olympic Mountains started to form. The oceanic crust is moving away from the Explorer Ridge to either side. On the eastern side the eastward moving Explorer Plate is being subducted under the North American Plate. The belt of volcanoes along the Pacific Ranges are the direct results of this collision. The western side of the Explorer Ridge is associated with the northwest trending Pacific Plate which has formed the Queen Charlotte Fault, an active transform fault along the coast of British Columbia and southeast Alaska. The Explorer Ridge is also seismically active. Most seismicity recorded in this area occurred on or near the Explorer Transform Fault Zone. The relatively shallow depth of the Southern Explorer Ridge (up to 1800 m) in comparison with most other segments of the northeast Pacific spreading centers suggests that there has been considerable volcanic activity along this segment in the past 100,000 years. Notable features along the ridge The Explorer Ridge includes a deep rift valley which runs along the axis of the ridge along nearly its entire length. This rift marks the actual boundary between adjacent tectonic plates, where magma from the mantle reaches the seafloor, erupting as lava and producing new crustal material for the plates. Before 2002 Explorer Ridge was the least explored of the northeast Pacific spreading centers, even though it was known to have robust hydrothermal activity and is seismically active. Along the Southern Explorer Ridge lies a large hydrothermal vent area called Magic Mountain. It is an unusual hydrothermal site, with its off-axis location and relatively long-lived activity. The source of the hydrothermal fluid that fuels Magic Mountain probably rises along fault systems associated with a recent episode of rifting that, in turn, followed a massive outpouring of lava. These vents are forming seafloor massive sulfide deposits on the ocean floor. Many strange deep-water creatures have been found here. See also Geology of the Pacific Northwest Gorda Ridge
Explorer Ridge
1268367
The East Pacific Rise (EPR) is a mid-ocean rise (usually termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), at a divergent tectonic plate boundary, located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from (north to south) the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate. It runs south from the Gulf of California in the Salton Sea basin in Southern California to a point near , where it joins the Pacific-Antarctic Ridge (PAR) trending west-south-west towards Antarctica, near New Zealand (though in some uses the PAR is regarded as the southern section of the EPR). Much of the rise lies about off the South American coast and reaches a height about above the surrounding seafloor. Overview The oceanic crust is moving away from the East Pacific Rise to either side. Near Easter Island the rate is over per year which is the fastest in the world. However, on the northern end, it is much slower at only roughly per year. On the eastern side of the rise, the eastward-moving Cocos and Nazca plates meet the westward moving South American Plate and the North American Plate and are being subducted under them. The belt of volcanos along the Andes and the arc of volcanoes through Central America and Mexico are the direct results of this collision. Due east of the Baja California Peninsula, the Rise is sometimes referred to as the Gulf of California Rift Zone. In this area, newly formed oceanic crust is intermingled with rifted continental crust originating from the North American Plate. Near Easter Island, the East Pacific Rise meets the Chile Rise at the Easter Island and Juan Fernandez microplates, trending off to the east where it subducts under the South American Plate at the Peru–Chile Trench along the coast of southern Chile. This portion of the Rise has been referred to as the Cape Adare-Easter Island Ridge, Albatross Cordillera, Easter Island Cordillera, Easter Island Rise, and Easter Island Swell. Parts of the East Pacific Rise have oblique spreading, such as the Nazca–Pacific plate boundary between 29°S and 32°S. This is seafloor spreading that is not orthogonal to the nearest ridge segment. The southern extension of the East Pacific Rise (the PAR) merges with the Southeast Indian Ridge at the Macquarie Triple Junction south of New Zealand. The southern stretch of the East Pacific Rise is also one of the fastest-spreading divergent boundaries on Earth, peaking at /year. Along the East Pacific Rise the hydrothermal vents called black smokers were first discovered by the RISE project in 1979, and have since been extensively studied. These vents are forming volcanogenic massive sulfide ore deposits on the ocean floor. Many unique deep-water creatures have been found with vents, that subsist in a chemosynthetic ecosystem rather than one using photosynthesis. See also Pacific-Antarctic Ridge Lamont seamount
East Pacific Rise
37577368
The tectonic evolution of the Transantarctic Mountains appears to have begun when Antarctica broke away from Australia during the late Cretaceous and is ongoing, creating along the way some of the longest mountain ranges (at 3500 kilometers) formed by rift flank uplift and associated continental rifting. The Transantarctic Mountains (TAM) separate East and West Antarctica. The rift system that formed them is caused by a reactivation of crust along the East Antarctic Craton. This rifting or seafloor spreading causes plate movement that results in a nearby convergent boundary which then forms the mountain range. The exact processes responsible for making the Transantarctic Mountains are still debated today. This results in a large variety of proposed theories that attempt to decipher the tectonic history of these mountains. Three main causes for uplift Thermal mechanisms, such as mantle plumes and magma injections into the crust, cause rift zones and thermal expansion of the mountain belt. Convergence of crustal material occurs when the rift zone pushes the West Antarctic plate into the much stronger East Antarctic plate. Flexural uplift is a mechanical process where a tectonic plate is broken by a deep normal fault resulting in the footwall uplifting rock. This uplift is associated with isostatic rebound. West Antarctic Rift System influence The West Antarctic Rift System is located just west of the Transantarctic Mountains, and is considered to be the largest influence in the history of the Transantarctic Mountains. The rift is a broad region where slow extension has occurred for approximately 100 My. Until recently the influence of this rift was debated because little was known about the region due to a lack of geological and geophysical data. Uplift history Breakup of the super-continent Gondwana started in the early Jurassic around 184 million years ago (Ma), but Antarctica did not break up from Australia until the late Cretaceous (80 Ma). Just before the breakaway in the Late Jurassic and Early Cretaceous, rifting began to occur near the soon-to-be Transantarctic Mountains. It is uncertain whether this first episode of rifting caused any uplift of the mountain range. Some researchers say this episode is a reactivation of rift that was formed during the Gondwanide Orogeny, but it has never been proven. This event was followed by low-angle extensional faulting along the East Antarctic plate. No uplift occurred during this initial phase of faulting due to counteracting erosional events. Shortly after, during the Mid Cretaceous (100 Ma), the West Antarctic Rift System began to form. Certain models show that the rapid rifting and intense thermal forces are due to a shallow (50 km depth) lithosphere-asthenosphere boundary under the Transantarctic Mountains. Rifting in this area is still going on today. Such a long period of rifting leads to uplift due to prolonged lateral heat conduction in the mountain belt. This is backed by seismic evidence. Initial surface uplift began approximately 55 Ma during the Early Cenozoic era. Rift flank uplift occurs when the crust on the side of a rift zone converges with a stronger
Tectonic evolution of the Transantarctic Mountains
16592799
The Azores Triple Junction (ATJ) is a geologic triple junction where the boundaries of three tectonic plates intersect: the North American Plate, the Eurasian Plate and the African Plate. This triple junction is located along the Mid-Atlantic Ridge (MAR) amidst the Azores islands, nearly due west of the Strait of Gibraltar. It is classed as an R-R-R triple junction of the T type (for its shape), as it is an intersection of the Mid-Atlantic Ridge running north–south and the Terceira Rift which runs east-southeast. The spreading rate along the MAR does not change abruptly at the ATJ, instead decreasing from 22.9±0.1 mm/yr at 40°N to 19.8±0.2 mm/yr at 38°N. This means the ATJ is not a simple triple junction where three tectonic plates meet at a point. The transitional range of spreading rates instead indicates the presence of a microplate, commonly referred to as the Azores Microplate, although the observed behaviour can also be explained in terms of a diffuse boundary. Its northern boundary intersects the MAR between 39.4°N and 40.0°N and its southern between 38.2°N and 38.5°N. The microplate moves about 2 mm/yr east-northeast along its Nubian boundary. References Notes Sources Tectonics of the Azores External links Main tectonic structures that can be defined in the area of the Azores Triple Junction Triple junctions Mid-Atlantic Ridge
Azores Triple Junction
74779525
On at 23:11 DST (22:11 UTC), an earthquake with a moment magnitude of 6.9 and maximum Mercalli intensity of IX (Violent) struck Morocco's Al Haouz Province. The earthquake's epicenter was southwest of Marrakesh, near the town of Ighil and the Oukaïmeden ski resort in the Atlas Mountains. It occurred as a result of shallow oblique-thrust faulting beneath the mountain range. At least 2,960 deaths were reported, with most occurring outside Marrakesh. Damage was widespread, and historic landmarks in Marrakesh were destroyed. The earthquake was also felt in Spain, Portugal, and Algeria. It is the strongest instrumentally recorded earthquake in Morocco, the deadliest in the country since the 1960 Agadir earthquake, and the second-deadliest earthquake of 2023 after the Turkey–Syria earthquakes. Its magnitude also makes it the largest earthquake on the African continent since the 2006 7.0 Mozambique earthquake and the largest in North Africa since the 1980 7.1 El Asnam earthquake. Over 2.8 million people from Marrakesh and areas surrounding the Atlas Mountains were affected, including 100,000 children. Following the earthquake, many countries offered humanitarian assistance, and Morocco announced a three-day period of national mourning. Tectonic setting Morocco lies close to the Azores–Gibraltar Transform Fault, which is the boundary between the African Plate and the Eurasian Plate. This zone of right-lateral strike-slip becomes transpressional at its eastern end, with the development of large thrust faults. To the east of the Strait of Gibraltar, in the Alboran Sea, the boundary becomes collisional in type. Most of the seismicity in Morocco is related to movement on that plate boundary, with the greatest seismic hazard in the north of the country close to the boundary. In 2004, Al Hoceima was struck by a magnitude 6.3 earthquake that killed 628 people and left 926 injured. A magnitude 7.3 earthquake affecting neighboring Algeria in 1980 killed 2,500 people. The Atlas Mountains are an intracontinental mountain belt that extends from Morocco to Tunisia. These mountains formed from a collision during the Cenozoic. The mountain range reaches its highest elevation to the west, in Morocco. The High Atlas formed during the reactivation of an ancient rift from the Triassic. However rather than extensional forces, the reactivation compressed the rift feature due to the collisional feature in the north. Seismicity in Morocco is concentrated in the country's northern region and the Alboran Sea. South of the Rif, seismic activity is sparse but spread across the Middle Atlas, High Atlas, and Anti-Atlas. Seismicity in the Saharan Atlas is limited, and absent in the Saharan region south of the belt; it is also less active eastwards in Algeria and Tunisia. Previously, the largest earthquake recorded in the Atlas Mountains was the 5.9 earthquake that struck Agadir in 1960. Earthquakes in the Atlas Mountains display focal mechanisms of strike-slip, thrust or a combination of both (oblique-slip). Earthquake The Marrakesh–Safi earthquake is the strongest instrumentally recorded in Morocco's history, surpassed only by upper estimates of the 1755 Meknes earthquake, at 6.5–7.0. It occurred at depth, and had a magnitude of 6.8, according
2023 Al Haouz earthquake
493465
The North American Plate is a tectonic plate containing most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific Plate (which borders the plate to the west). It extends eastward to the seismically active Mid-Atlantic Ridge at the Azores Triple Junction plate boundary where it meets the Eurasian Plate and Nubian Plate. and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust. The interior of the main continental landmass includes an extensive granitic core called a craton. Along most of the edges of this craton are fragments of crustal material called terranes, which are accreted to the craton by tectonic actions over a long span of time. Much of North America west of the Rocky Mountains is composed of such terranes. Boundaries The southern boundary with the Cocos Plate to the west and the Caribbean Plate to the east is a transform fault, represented by the Swan Islands Transform Fault under the Caribbean Sea and the Motagua Fault through Guatemala. The parallel Septentrional and Enriquillo–Plantain Garden faults, which run through Hispaniola and bound the Gonâve Microplate, are also a part of the boundary. The rest of the southerly margin which extends east to the Mid-Atlantic Ridge and marks the boundary between the North American Plate and the South American Plate is vague but located near the Fifteen-Twenty Fracture Zone around 16°N. On the northerly boundary is a continuation of the Mid-Atlantic Ridge called the Gakkel Ridge. The rest of the boundary in the far northwestern part of the plate extends into Siberia. This boundary continues from the end of the Gakkel Ridge as the Laptev Sea Rift, on to a transitional deformation zone in the Chersky Range, then the Ulakhan Fault between it and the Okhotsk Plate, and finally the Aleutian Trench to the end of the Queen Charlotte Fault system (see also: Aleutian Arc). The westerly boundary is the Queen Charlotte Fault running offshore along the coast of Alaska and the Cascadia subduction zone to the north, the San Andreas Fault through California, the East Pacific Rise in the Gulf of California, and the Middle America Trench to the south. On its western edge, the Farallon Plate has been subducting under the North American Plate since the Jurassic period. The Farallon Plate has almost completely subducted beneath the western portion of the North American Plate, leaving that part of the North American Plate in contact with the Pacific Plate as the San Andreas Fault. The Juan de Fuca, Explorer, Gorda, Rivera, Cocos and Nazca plates are remnants of the Farallon Plate. The boundary along the Gulf of California is complex. The gulf is underlain by the Gulf of California Rift Zone, a series of rift basins and transform fault segments from the northern end of the East Pacific Rise in the mouth of the gulf to the San Andreas
North American Plate
526164
The Arabian Plate is a minor tectonic plate in the Northern and Eastern Hemispheres. It is one of the three continental plates (along with the African and Indian plates) that have been moving northward in geological history and colliding with the Eurasian Plate. This collision is resulting in a mingling of plate pieces and mountain ranges extending in the west from the Pyrenees, crossing Southern Europe to Iranian plateau, forming the Alborz and the Zagros Mountains, to the Himalayas and ranges of Southeast Asia. Lexicology The Arabian Plate is a designation of the region, and it is also sometimes referred to as the Arab Plate. Borders The Arabian Plate consists mostly of the Arabian Peninsula; it extends westward to the Sinai Peninsula and the Red Sea and northward to the Levant. The plate borders are: East, with the Indo-Australian plate, at the Owen Fracture Zone South, with the African Plate to the west and the Somali Plate and the Indo-Australian Plate to the east West, a left lateral fault boundary with the African Plate called the Dead Sea Transform (DST), and a divergent boundary with the African Plate called the Red Sea Rift which runs the length of the Red Sea; North, convergent boundary with the Anatolian Plate and Eurasian Plate, including the East Anatolian Fault, Zagros fold and thrust belt, and Makran Trench. History The Arabian Plate was part of the African Plate during most of the Phanerozoic Eon (Paleozoic–Cenozoic), until the Oligocene Epoch of the Cenozoic Era. The Red Sea rifting began in the Eocene, and the separation of Africa and Arabia occurred approximately in the Oligocene, and since then the Arabian Plate has been moving toward the Eurasian Plate. The opening of the Red Sea rift led to volcanic activity. There are volcanic fields called the Older Harrats, such as Harrat Khaybar and Harrat Rahat, cover parts of the western Arabian Plate. Some activity still continues especially around Medina, and there are regular eruptions within the Red Sea. The collision between the Arabian Plate and Eurasia is pushing up the Zagros Mountains of Iran. Because the Arabian Plate and Eurasian Plate collide, some cities such as those in southeastern Turkey (which is on the Arabian Plate) may undergo earthquakes, tsunamis, and volcanoes. Countries and regions Countries within the plate include Bahrain, Djibouti, Iraq, Jordan, Kuwait, Oman, Qatar, Saudi Arabia, Syria, United Arab Emirates and Yemen. Regions include the Anti-Lebanon Mountains (Lebanon), parts of Awdal (Somalia/Somaliland), the Khuzestan Province (Iran), the Southeastern Anatolia Region (Turkey), and the Southern Denkalya Subregion (Eritrea). References Arabian Peninsula Cenozoic geology of Africa Cenozoic geology of Asia Geology of Africa Geology of Asia Geology of the Indian Ocean Natural history of West Asia Tectonic plates
Arabian Plate
1609792
The Ruzizi (also sometimes spelled Rusizi, French: Rivière Ruzizi) is a river, long, that flows from Lake Kivu to Lake Tanganyika in Central Africa, descending from about to about above sea level over its length. The steepest gradients occur over the first , where hydroelectric dams have been built. Further downstream, the Ruzizi Plain, the floor of the Western Rift Valley, has gentle hills, and the river flows into Lake Tanganyika through a delta, with one or two small channels splitting off from the main channel. The Ruzizi is a young river, formed about 10,000 years ago when volcanism associated with continental rifting created the Virunga Mountains. The mountains blocked Lake Kivu's former outlet to the drainage basin of the Nile and instead forced the lake overflow south down the Ruzizi and the drainage basin of the Congo. Course Along its upstream reaches, the river forms part of the border between Rwanda on the east with the Democratic Republic of the Congo (DRC) on the west. Further downstream, it forms part of the border between the DRC and Burundi, and its lowermost reach lies entirely within Burundi. To the west, the Fizi Baraka mountains tower over the river. The Bridge of Concord, Burundi's longest bridge, crosses the river near its mouth. Tributaries of the Ruzizi River include the Nyamagana, Muhira, Kaburantwa, Kagunuzi, Rubyiro and Ruhwa, among others. The Ruzizi River, flowing south into Lake Tanganyika, is part of the upper watershed of the Congo River. Nineteenth-century British explorers such as Richard Francis Burton and John Hanning Speke, uncertain of the direction of flow of the Ruzizi, thought that it might flow north out of the lake toward the White Nile. Their research and follow-up explorations by David Livingstone and Henry Morton Stanley established that this was not the case. The Ruzizi flows into Lake Tanganyika, which overflows into the Lukuga River about south of Ujiji. The Lukuga flows west into the Lualaba River, a major tributary of the Congo. Geology Rifting, the slow pulling apart of a tectonic plate, has produced the East African Rift system and its many basins and lakes. The system, on the boundary between the African Plate (Nubian Plate) and the Somali Plate, has two branches, both oriented north–south. Rifting in the western branch, called the Albertine Rift, began between 25 and 10 million years ago. The Ruzizi River lies along the western rift, which includes, from north to south, lakes Albert, George, Edward, Kivu, Tanganyika, Rukwa, Malawi, and others. Uplift associated with the rifting altered the connections among the region's water bodies. About 13,000 to 9,000 years ago, volcanic activity blocked Lake Kivu's former outlet to the watershed of the Nile. The volcanism produced mountains, including the Virungas, which rose between Lake Kivu and Lake Edward, to the north. Water from Lake Kivu was then forced south down the Ruzizi. This, in turn, raised the level of Lake Tanganyika, which overflowed down the Lukuga River. Variations in uplift and climate have caused the Ruzizi and
Ruzizi River
69072081
The 1969 Sardo earthquakes, which occurred from March 29, 1969 to April 05, 1969 was an earthquake swarm of three events with magnitudes of 6.2 and 6.1 in the Afar Region, Ethiopia. All three tremors had estimated intensities of between IX (Violent) and VIII (Severe) respectively. It killed a total of about 20–40 people and further injuring 160 as well. Tectonic setting The Afar Region is set in a tectonic boundary with three plates in a Y-shaped configuration. This makes Ethiopia home to one of the most seismic regions in all of Africa. It is also located in the Main Ethiopian Rift, part of the greater East African Rift which have been known to have been where frequent geological activity, one of the most common is the possible separation of East Africa from the mainland. The Main Ethiopian rift is a central valley some 84km wide and is extending ESE-WNW at a rate of about 2.5mm/yr. Ethiopia has experienced numerous earthquake similar to this such as in 1961 and 1847. Earthquakes The first of the earthquake sequence had a magnitude of 6.2 and was the most destructive of the three causing the 20-40 deaths and 160 injuries as well as most of the damage. The second and third earthquakes both with magnitudes of 6.1 were fortunately less worse however still caused some damage and recorded with high intensities. Numerous surface faultings and displacement were also caused, the main event caused left-lateral displacements and throws northeast of 75 cm near Sardo. The April event also caused visible surface faultings near Sardo with 70 cm vertically and 50 cm horizontally. Damage The village of Sardo (Serdo) suffered most of the impact due to it being the nearest to the epicenter. Poor infrastructure and framing was said to be blamed for the widespread severity of the damage. Some of the local huts in the area were made mostly of mud, which also resulted in the many deaths. The sequence triggered numerous rockslides blocking roads and transportation between the other parts of Ethiopia, as well as the outside in nearby Djibouti where moderate jolts were felt. Many of the residents who survived the event fled to the nearby village of Loggia. See also List of earthquakes in Ethiopia List of earthquakes in 1969 East African Rift Great Rift Valley, Ethiopia References External links Summary of the Sardo earthquakes Earthquakes in Africa Natural disasters in Ethiopia Sardo Sardo earthquake Earthquake clusters, swarms, and sequences 1969 disasters in Ethiopia
1969 Sardo earthquakes
26613027
Fracture zones are common features in the geology of oceanic basins. Globally most fault zones are located on divergent plate boundaries on oceanic crust. This means that they are located around mid-ocean ridges and trend perpendicular to them. The term fracture zone is used almost exclusively for features on oceanic crust; similar structures on continental crust are instead termed transform or strike slip faults. The term fracture zone has a distinct geological meaning, but it is also used more loosely in the naming of some oceanic features. Fracture zones are much longer than wide, but may have feature complexity within their width. Not all named fracture zones are active, indeed only the central portion of those still forming usually is, in an area of active transform faulting associated with a mid-ocean ridge. Classic fracture zones remain significant ocean floor features with usually different aged rocks on either side of the fracture zone due to past tectonic processes. Some fracture zones have been created by mid-ocean ridge segments that have been subducted and that part may no longer exist. Pacific Ocean Most fracture zones in the Pacific Ocean originate from large mid-ocean ridges (also called "rises") such as the East Pacific Rise, Chile Rise and Juan de Fuca Ridge. The plates that host the fractures are Nazca, Pacific, Antarctic, Juan de Fuca and Cocos among others. Fracture zones being subducted under Southern and Central America are generally southwest-northeast oriented reflecting the relative motion of Cocos, Nazca and the Antarctic Plates. Chile Rise The fracture zones of the Chile Rise trend in a west to east fashion with the most southern ones taking a slightly more southwest to northeast orientation. This non-perpendicular relation to Chile's coast reflects the oblique subduction of Nazca Plate under southern Chile. West of Chile rise the fracture zones are hosted in the Antarctic Plate. Some fracture zones such as Chile and Valdivia make up large sections of the Nazca-Antarctic Plate boundary. East Pacific Rise The East Pacific Rise includes the Pacific-Antarctic Rise (Pacific Plate and Antarctic Plate boundary) in some usages and in others relates only to the boundaries between the Pacific Plate and the Nazca Plates which includes the Juan Fernández Plate and Easter Microplate. Nazca Plate boundary Pacific-Antarctic Rise Galapagos Rise Panama Fracture Zone Western Pacific Some of the fracture zones in the western Pacific Ocean are associated with the smaller plate boundaries of the active back-arc basin spreading center of the North Fiji Basin being the Hunter Fracture Zone and North Fiji Fracture Zone. The Parece Vela Rift (Parece Vela Fracture Zone Province) is also associated with the back-arc basin of the Parece Vela Basin (West Mariana Basin) at the intersection of the Philippine Sea Plate and Mariana Plate. South of the Equator Vening Meinesz Fracture Zone Cook Fracture Zone Hunter Fracture Zone North Fiji Fracture Zone North of the Equator Victoria Fracture Zone Central Fracture Zone Parece Vela Rift (Parece Vela Fracture Zone Province) Kokugan Fracture Zone Owashi Fracture Zone Ojirowashi Fracture Zone
List of fracture zones
1658029
The Amurian microplate (or Amur microplate; also occasionally referred to as the China Plate, not to be confused with the South China Subplate) is a minor tectonic plate in the northern and eastern hemispheres. The Amurian Plate is named after the Amur River, which forms the border between the Russian Far East and Northeast China. It is bounded on the north, west, and southwest by the Eurasian Plate, on the east by the Okhotsk Plate, to the southeast by the Philippine Sea Plate along the Suruga Trough and the Nankai Trough, and the Okinawa Plate, and the Yangtze Plate. The Amurian Plate may have been involved in the 1975 Haicheng earthquake and the 1976 Tangshan earthquake in China. Boundaries The Amurian microplate is a division within the Eurasian plate, with an unknown western boundary, defined on the south by the Qinling suture zone in central China and the Baikal Rift Zone and Stanovoy Mountains on the north. The Baikal Rift Zone is considered a boundary between the Amurian Plate and the Eurasian Plate. GPS measurements indicate that the plate is slowly rotating counterclockwise. The boundary between the Okhotsk Plate is the eastern margin of the Sea of Japan. Geography It covers northeastern China, the Korean Peninsula, the Sea of Japan, Shikoku, Kyushu, southwest Honshu (Kansai, Chūgoku), eastern Mongolia and the south of Russian Far East. See also North China Craton References Further reading Dongping Wei and Tetsuzo Seno. 1998. Determination of the Amurian Plate Motion. Mantle Dynamics and Plate Interactions in East Asia, Geodynamics Series. v.27, edited by M. F. J. Flower et al., 419p, AGU, Washington D.C. (abstract ) Tectonic plates Geology of Japan Geology of China Geology of Korea Geology of North Korea Geology of South Korea Geology of the Russian Far East Geology of the Pacific Ocean
Amurian microplate
60104192
The geology of Africa is varied and complex, and gives rise to the wide variety of landscapes found across the continent. The African continent rests over two main plates. The African plate, accounting for the whole of north Africa, and the Somali plate, which accounts for the eastern side of mid and southern Africa. The Somali plate is moving away from the African plate in a split from Djibouti in the north, to Eswatini in the south. The parting of these two plates formed the southern part of what used to be known as The Great Rift Valley. In geological terms, the African and Somali plate separation has formed the East African Rift System (EARS), comprising two separate rifts systems - the Eastern Rift Valley, and a western branch known as the Albertine Rift. Two massive domes were formed, the Kenyan dome and the Ethiopian dome (known as the Ethiopian Highlands). The Albertine Rift follows the western edge of the Kenyan dome. This runs from Lake Malawi in the south, up into Lake Rukwa, Lake Tanganyika and Lake Albert in the north, where it ends. The Kenyan dome has the eastern branch of the EARS (known as The Gregory Rift) running through its middle, and contains most of what we historically saw as the Great Rift valley. Lake Victoria lies in the middle of the dome, with the Gregory Rift to the east of it. The Gregory rift has Lake Eyasi & Lake Manyara at its southern end, running north up to the west of Nairobi and continuing on through Kenya to Lake Turkana - which lies between the northern edge of the Kenyan dome and the southern edge of the Ethiopian dome. The Ethiopian dome is split down the middle by the Eastern Rift, formed by the developing plate boundary, and has formed a valley running from Lake Chamo and Lake Abaya in the south, widening through Lake Shala, Lake Ziwa and Lake Koku, until it reaches its widest point some from Djibouti. Major geological events Vredefort impact structure Geological features Central African Shear Zone Geologists of Africa Maria Wilman Hugo Dummett Geological maps See also :Category:Geologic groups of Africa :Category:Stratigraphy of Africa References Works cited Further reading Africa's Top Geological Sites (35th International Geological Congress Commemorative Volume), 2016 External links United States Geological Survey maps of the geology of Africa (PDF) Interactive map of the topography of Africa - five selectable sources, including; OpenTopoMap, ESRI Topo & CyclOSM
Geology of Africa
146184
Tectonic plate interactions are classified into three basic types: Convergent boundaries are areas where plates move toward each other and collide. These are also known as compressional or destructive boundaries. Obduction zones occurs when the continental plate is pushed under the oceanic plate, but this is unusual as the relative densities of the tectonic plates favours subduction of the oceanic plate. This causes the oceanic plate to buckle and usually results in a new mid-ocean ridge forming and turning the obduction into subduction. Orogenic belts occur where two continental plates collide and push upwards to form large mountain ranges. These are also known as collision boundaries. Subduction zones occur where an oceanic plate meets a continental plate and is pushed underneath it. Subduction zones are marked by oceanic trenches. The descending end of the oceanic plate melts and creates pressure in the mantle, causing volcanoes to form. Back-arc basins can form from extension in the overriding plate, in response to the displacement of the subducting slab at some oceanic trenches. This paradoxically results in divergence which was only incorporated in the theory of plate tectonics in 1970, but still results in net destruction when summed over major plate boundaries. Divergent boundaries are areas where plates move away from each other, forming either mid-ocean ridges or rift valleys. These are also known as constructive boundaries. Transform boundaries occur when two plates grind past each other with only limited convergent or divergent activity. Convergent boundaries (subduction zones) The oceanic Nazca Plate subducts beneath the continental South American Plate at the Peru–Chile Trench. Just north of the Nazca Plate, the oceanic Cocos Plate subducts under the Caribbean Plate and forms the Middle America Trench. Oceanic crust of the South American Plate subducts under the Caribbean Plate in the Lesser Antilles subduction zone. The Cascadia subduction zone is where the oceanic Juan de Fuca, Gorda and Explorer Plates subduct under the continental North American plate. The oceanic Pacific Plate subducts under the North American Plate (composed of both continental and oceanic sections) forming the Aleutian Trench. The oceanic Pacific Plate subducts beneath the continental Okhotsk Plate at the Japan Trench. The oceanic Philippine Sea Plate subducts beneath the Eurasian Plate at the Ryukyu Trench. The oceanic Pacific Plate subducts under the oceanic Philippine Sea Plate forming the Mariana Trench. The oceanic Philippine Sea Plate is subducting under the Philippine Mobile Belt forming the Philippine Trench and the East Luzon Trench. The Eurasian Plate is subducting under the Philippine Mobile Belt at the Manila Trench. The Sunda Plate is subducting under the Philippine Mobile Belt at the Negros Trench and the Cotobato Trench. The oceanic Australian Plate is subducted beneath the continental Sunda Plate along the Sunda Trench. The oceanic Solomon Sea Plate is subducting beneath the South Bismarck Plate and the New Hebrides Plate driven by the mutual movements of the Australian and Pacific plates and local spreading centres. The oceanic Pacific Plate is subducting under the Tonga Plate at the Tonga Trench driven
List of tectonic plate interactions
20711683
The Mariana Plate is a micro tectonic plate located west of the Mariana Trench which forms the basement of the Mariana Islands which form part of the Izu–Bonin–Mariana Arc. It is separated from the Philippine Sea Plate to the west by a divergent boundary with numerous transform fault offsets. The boundary between the Mariana and the Pacific Plate to the east is a subduction zone with the Pacific Plate subducting beneath the Mariana. This eastern subduction is divided into the Mariana Trench, which forms the southeastern boundary, and the Izu–Ogasawara Trench the northeastern boundary. The subduction plate motion is responsible for the shape of the Mariana plate and back arc. Geological history Subduction at the Mariana plate has been going on for over 50 million years. Some theories of the origin of this microplate is that when the Pacific plate began to subduct beneath the Philippine plate the volcanism and spreading ridge started to make an arc. This geological activity caused the section of the Philippine plate to break off and become the Mariana microplate. The Mariana Islands consist of volcanoes that are active and dormant and are made up of volcanic and sedimentary rocks from the Pleistocene. Defining Features As the Pacific Plate subducts beneath the Mariana Plate, it creates a trench. This is the Mariana Trench, and it is the deepest trench in the world. Another result of this subduction is the Mariana Islands. These are formed from dehydration of the subducting, old oceanic crust creates melt, and the melt rises to the surface through a volcano. This volcanism has been occurring for almost 50 million years. The rock type in the area is volcaniclastic sediments on top of igneous rocks. The source of these rocks are from crustal spreading. Just off the eastern coast of Mariana there are big seamounts made of serpentinized periodic. They are formed from mud volcanism. The composition for the seamounts differs in the Izu–Ogasawara Trench and Mariana systems which indicate regional changes in geology. Tectonic Behavior Eastern Convergent Boundary The tectonic plate is approximately 100 km thick and converging to the east at a rate of 50–80 mm/yr with the Pacific Plate subducting at 60–100 mm/yr This eastern subduction is divided into the Mariana Trench, which forms the southeastern boundary, and the Izu–Ogasawara Trench the northeastern boundary. The Izu–Ogasawara Trench and Mariana subduction zones are traveling at different rates. While the northern section of the Izu–Ogasawara Trench plate is subducting at 44 mm/yr, the southern section subducts at 14 mm/yr. The subducting Pacific Plate dips at about 10 degrees and directed 83 degrees west of north. The northern subduction zone is expanding by rifting while the southern contains a strike slip fault. Seismologists have been studying how the subducting slabs underneath the Mariana island arc are entering the lower mantle and being redirected horizontally, deflecting off the upper to lower mantle transition zone. Western Divergent Boundary The Mariana Plate is also separating at a rate of 30 mm/yr from the Philippine Plate
Mariana Plate
57697542
The geology of Socotra is part of the national geology of Yemen. Ancient Precambrian metamorphic rocks are intruded by younger igneous rocks, overlain by limestones and other marine sediments that deposited during marine transgression periods in the Cretaceous and the past 66 million years of the Cenozoic. The island is on the Somali Plate, which rifted away from the Arabian mainland within the past 60 million years. Tectonics The geological history of what is now the island of Socotra began with the formation of the Arabian-Nubian Shield between 780 and 600 million years (Ma) ago through the accretion of terranes and sections of continental crust. The origins of the terranes composing the part of the shield that now borders the Gulf of Aden, including Socotra, remains unclear, as does the time frame of their accretion. It is possible that this area formed from fragments of an even older paleocontinent. This assembly formed part of the East African Orogeny, after which it experienced a long period of relatively tectonic stability. The breakup of the Arabian-Nubian Shield began about 35 Ma with the development of the Afar plume. Activity at the plume created the Ethiopia-Yemen Continental Flood Basalts, and exposed the shield to hot mantle material from below, leading to the onset of rifting some time before 28 Ma. A divergent plate boundary developed, dividing the shield into the Arabian Plate and the Somali Plate. As the plates separated, water from the Indian Ocean filled the newly created basin, resulting in the Gulf of Aden. The spreading center has formed a mid-ocean ridge called the Aden Ridge. This rift divided Socotra and the Arabian Peninsula; prior to rifting, Socotra was contiguous with what is now the Dhofar Governorate in southern Oman. Extension along the eastern segments of the Aden Ridge increases the separation of the Arabian Peninsula from Somalia (and Socotra) at the rate of about . Additionally, for approximately the last 10 Ma, the associated geologic forces have subjected Socotra to slow tectonic uplift. Stratigraphy Precambrian The oldest rocks on Socotra date from the Precambrian, with a minimum age of approximately 800 Ma. These comprise metasedimentary rocks, primarily schist and gneiss, formed under the conditions of amphibolite facies metamorphism. Also part of the island's basement are several types of igneous rock. Plutonic granites were formed from intrusions of magma from beneath the surface, while andesite, dacite and rhyolite were deposited by lava flows thick. Breccia and tuff were the result of more recent, and more explosive, volcanism. These periods of volcanic activity have not been precisely dated, but the resulting rocks are cut by the granite and gabbro of the Hagghier Mountains, whose formation has been established as Precambrian. The Hagghier Mountains represent the largest exposure of basement rocks on the island. Other Precambrian outcrops exist at Ras Momi and Ras Shu'ub, at the extreme east and west of the island, respectively, and in the Qalansiyah valley in the northwest. The neighboring islands of Abd al Kuri and Samhah also have
Geology of Socotra
24816163
The Iberian Plate is a microplate typically grouped with the Eurasian Plate that includes the microcontinent Iberia, Corsica, Sardinia, the Balearic Islands, the Briançonnais zone of the Penninic nappes of the Alps, and the portion of Morocco north of the High Atlas Mountains. The Iberian plate is a part of the Eurasian plate. Neoproterozoic The Iberian plate came into existence during the Cadomian Orogeny of the late Neoproterozoic, about 650–550 Ma, on the margin of the Gondwana continent, involving the collisions and accretion of the island arcs of the Central Iberian Plate, Ossa-Morena Plate, South Portuguese Plate. The three plates have never separated substantially from each other since that time. Mesozoic In the Mesozoic, Late Jurassic Africa started moving east, and the Alpine Tethys opened. Subsidence related to this caused deep deposits of sediments on the east and some sediment remnants in pop downs in central parts of Spain. Two stages of rifting occurred in the east, one from Later Permian to Triassic, and the second from Late Jurassic to early Cretaceous. On the south side deposits of carbonates and clastic sediments formed a shelf in shallow water during late Triassic and Liassic times. This was rifted in Toarcian times (Early Jurassic 190 Ma). Active rifting was complete by 160 Ma. After this thermal subsidence occurred till the end of Cretaceous. During this time rifting separated North America from Africa forming a transform zone. In the late Triassic and early Jurassic there were two stages of rifting involving extension and subsidence on the western margin of Iberia. It also extended the western margin. The Iberian Abyssal Plain, off the west coast of Portugal and Spain, formed 126 Ma. This separated Newfoundland's Grand Banks, with Galicia Bank and Flemish Cap being split at 118 Ma. By Early Cretaceous, 110 Ma rifting occurs on west and north west edges. During the time of the supercontinent Pangea, the Iberian plate was joined to Armorica (Northern France). During the break-up of Pangea, in the early Cretaceous, the Bay of Biscay started opening around 126 Ma and completed by 85 Ma. This created the Biscay Abyssal Plain, and parted the Iberian plate from the Trevelyan Escarpment. During this time Iberia rotated anticlockwise relative to Eurasia. This caused the subduction of the Ligurian Basin onto the eastern side. This formed the Betic nappe stack. After 85 Ma the Atlantic Ocean opening started between Ireland and Greenland. This left the Bay of Biscay as a failed rift. Cenozoic The rotation of Iberia and its relation to the formation of the Pyrenees has been difficult to decipher with certainty. Detailed aeromagnetic measurements from the sea floor offshore of the Grand Banks of Newfoundland show that Iberia moved as part of the African plate from late Cretaceous to mid-Eocene time, with a plate boundary extending westward from the Bay of Biscay. When motion along this boundary ceased, a boundary linking extension in the Kings Trough to compression along the Pyrenees came into existence. Since the late Oligocene, the Iberian
Iberian Plate
1985950
The Jordan Rift Valley, also Jordan Valley also called the Syro-African Depression, is an elongated depression located in modern-day Israel, Jordan and the West Bank. This geographic region includes the entire length of the Jordan River – from its sources, through the Hula Valley, the Korazim block, the Sea of Galilee, the (Lower) Jordan Valley, all the way to the Dead Sea, the lowest land elevation on Earth – and then continues through the Arabah depression, the Gulf of Aqaba whose shorelines it incorporates, until finally reaching the Red Sea proper at the Straits of Tiran. History and physical features The Jordan Rift Valley was formed many millions of years ago in the Miocene epoch (23.8 – 5.3 Myr ago) when the Arabian Plate moved northward and then eastward away from Africa. One million years later, the land between the Mediterranean and the Jordan Rift Valley rose so that the sea water stopped flooding the area. Alternatively, it was a fall in the sea level that caused the disconnection, or a mix between the two phenomena - researchers have not yet reached a consensus. The geological and environmental evolution of the valley since its inception in the Oligocene can be seen in a variety of sedimentary and magmatic rock units, preserved as continuous sequences in the deeper basins. The outcropping formations around the basins represent alternating deposition and erosion phases. The lowest point in the Jordan Rift Valley is in the Dead Sea, the lowest spot of which is below sea level. The shore of the Dead Sea is the lowest dry land spot on Earth, at below sea level. With its flanks rising sharply to almost above sea level in the west, and similarly in the east, the rift is a significant topographic feature over which a few narrow paved roads and difficult mountain tracks lead. The valley north of the Dead Sea has long been a site of agriculture because of water available from the Jordan River and numerous springs located on the valley's flanks. Dead Sea Transform The plate boundary that extends through the valley is variously called the Dead Sea Transform (DST) or Dead Sea Rift. The boundary separates the Arabian Plate from the African Plate, connecting the divergent plate boundary in the Red Sea (the Red Sea Rift) to the East Anatolian Fault in Turkey. The DST fault system is generally considered to be a transform fault that has accommodated a northwards displacement of the Arabian Plate. This interpretation is based on observation of offset markers, such as river terraces, gullies and archaeological features, giving horizontal slip rates of several mm per year over the last few million years. GPS data give similar rates of present-day movement of the Arabian Plate relative to the Africa Plate. It has also been proposed that the fault zone is a rift system that is an incipient oceanic spreading center, the northern extension of the Red Sea Rift. In 1033, the rift valley was struck by a magnitude 7.3
Jordan Rift Valley
16701679
The Red Sea Rift is a mid-ocean ridge between two tectonic plates, the African Plate and the Arabian Plate. It extends from the Dead Sea Transform fault system, and ends at an intersection with the Aden Ridge and the East African Rift, forming the Afar Triple Junction in the Afar Depression of the Horn of Africa. The Red Sea Rift was formed by the divergence between the African and Arabian Plates. The rift transitioned from a continental rift to an oceanic rift. Magnetic anomalies suggest that the spreading rate on either side of the Red Sea is about 1 cm/year. The African plate has a rotation rate of 0.9270 degrees/Ma, while the Arabian plate has a rotation rate of 1.1616 degrees/Ma. Spreading model A two-stage spreading model explains the tectonic evolution in this region. The first major rift motion was seen in the lower/middle Eocene, followed by major seafloor spreading in the late Eocene and early Oligocene. This was followed by a period of 30 Ma of no motion, during which a large amount of evaporites were deposited. After this quiet period of deposition, a new period of activity started about five million years ago. This new phase of spreading caused considerable disturbance to the sediments that were deposited, which created an unstable situation as the crust and sediments parted and the axial trough evolved. Normal faulting along the rift valley during earthquakes shows that the extensional motion is continuing. Mechanism of rifting A three-step process has been proposed for the mechanism of rifting. First, a thermal anomaly developed in the mantle in the earliest stages of rifting, causing the rise of the asthenosphere and the thinning of the subcrustal continental lithosphere. There have been several mechanisms proposed to achieve this, such as convective thermal thinning. This was followed by decompressions, which occurred due to uplift related to the gradual stretching and thinning of the crust as rifting continued. Stretching and thinning can take place either according to a symmetrical, pure shear, extension model, or due to an asymmetric, detachment-delamination model. Basaltic dykes are also injected during the stretching and thinning. As the basaltic injections become restricted to a narrow axial zone, true seafloor spreading initiates with the Vine-Matthews-type magnetic anomaly stripes. Axial propagation of the oceanic rift occurs, resulting in a continuous axis of spreading. The rift may be intersected by a shear or fracture zone, which act as locked zones and prevent further propagation. Zones of compression may develop. Composition While there is a general agreement that the axial trough of the Red Sea originated by seafloor spreading, and therefore is underlain by oceanic crust, the nature of the crust beneath the main trough and coastal plains of the Red Sea is still controversial, leading to the development of a few theories. One theory suggests that the entire Red Sea basin is underlain by oceanic crust, while another theory claims that the main trough is underlain only in part by oceanic crust. A third theory suggests that outside
Red Sea Rift
74061512
The Lau-Colville Ridge is an extinct oceanic ridge located on the oceanic Australian Plate in the south-west Pacific Ocean extending about from the south east of Fiji to the continental shelf margin of the North Island of New Zealand. It was an historic subduction boundary between the Australian Plate and the Pacific Plate and has important tectonic relationships to its east where very active spreading and subduction processes exist today. It is now the inactive part of an eastward-migrating, 100 million year old Lau-Tonga-Havre-Kermadec arc/back-arc system or complex and is important in understanding submarine arc volcanism because of these relationships. To its west is the South Fiji Basin whose northern bedrock is Oligocene in origin. Geography It is divided into two segments as the ridge is flanked on its eastern side by two southward propagating back-arc basins, the northern Lau Ridge (called historically the South Fiji Ridge and also known as the Fiji Ridge, Neckes Ridge and Southern Fiji Ridge) that forms the western boundary of the Lau Basin and has contributed to many islands south east of Fiji. The Lau Basin to its east currently contains 6 million year old active spreading centers between the Australian and Niuafo'ou or Tonga microplates. The southern Colville Ridge (also known as Khrebet Kolvil-Lau from Russian Хребет Колвил-Лау)forms the western boundary of the Havre Trough. The Havre Trough has had rift extension for the last 2 million years between the Australian plate and the Kermadec microplate and this extends into Zealandia's continental Taupō Rift and New Zealand's Taupō Volcanic Zone. Geology The remnant andesitic volcanic arc Lau Ridge was mainly active between 14 and 6 million years ago although late volcanism persisted until 2.5 million years ago. The Colville Ridge has been found to have volcanics aged between 7.5 to 2.6 million years ago. The composition range is from mafic picro-basalt to dacite with arc-type element patterns intermediate between Pacific mid ocean ridge basalt and subducted lithosphere. The South Fiji Basin to the west of the Lau-Colville Ridge contains back‐arc basin basalts up to 33 million years old that were erupted during spreading and some ocean island like basalts erupted several million years after spreading ceased in the basin which was before 22 million years ago Directly to the east of the ridge in its central portion the Lau basin contains an area of sediment that has been called the Western Sedimentary Basin. The sediments in the Havre Trough can be up to thick. Tectonics The late Miocene rifting of about 6 million years ago initiated in what was an old forearc (called the Vitiaz Arc) and was in Cretaceous-Eocene arc crust that became part of the northern Lau Ridge. Currently back-arc spreading to the east of the Lau Ridge in the adjacent Lau Basin occurs along a number of spreading centers and the Valu Fa Ridge (VFR). As we come south down the Lau Basin spreading rates decrease being for the Central Lau Spreading Centre (CLSC) /year, just above Eastern Lau Spreading
Lau-Colville Ridge
1561831
The geology of the Himalayas is a record of the most dramatic and visible creations of the immense mountain range formed by plate tectonic forces and sculpted by weathering and erosion. The Himalayas, which stretch over 2400 km between the Namcha Barwa syntaxis at the eastern end of the mountain range and the Nanga Parbat syntaxis at the western end, are the result of an ongoing orogeny — the collision of the continental crust of two tectonic plates, namely, the Indian Plate thrusting into the Eurasian Plate. The Himalaya-Tibet region supplies fresh water for more than one-fifth of the world population, and accounts for a quarter of the global sedimentary budget. Topographically, the belt has many superlatives: the highest rate of uplift (nearly 10 mm/year at Nanga Parbat), the highest relief (8848 m at Mt. Everest Chomolangma), among the highest erosion rates at 2–12 mm/yr, the source of some of the greatest rivers and the highest concentration of glaciers outside of the polar regions. This last feature earned the Himalaya its name, originating from the Sanskrit for "the abode of the snow". From south to north the Himalaya (Himalaya orogen) is divided into 4 parallel tectonostratigraphic zones and 5 thrust faults which extend across the length of Himalaya orogen. Each zone, flanked by the thrust faults on its north and south, has stratigraphy (type of rocks and their layering) different from the adjacent zones. From south to north, the zones and the major faults separating them are the Main Frontal Thrust (MFT), Subhimalaya Zone (also called Sivalik), Main Boundary Thrust (MBT), Lesser Himalaya (further subdivided into the "Lesser Himalayan Sedimentary Zone (LHSZ) and the Lesser Himalayan Crystalline Nappes (LHCN)), Main Central thrust (MCT), Higher (or Greater) Himalayan crystallines (HHC), South Tibetan detachment system (STD), Tethys Himalaya (TH), and the Indus‐Tsangpo Suture Zone (ISZ). North of this lies the transhimalaya in Tibet which is outside the Himalayas. Himalaya has Indo-Gangetic Plain in south, Pamir Mountains in west in Central Asia, and Hengduan Mountains in east on China–Myanmar border. From east to west the Himalayas are divided into 3 regions, Eastern Himalaya, Central Himalaya, and Western Himalaya, which collectively house several nations and states. Making of the Himalayas During Late Precambrian and the Palaeozoic, the Indian subcontinent, bounded to the north by the Cimmerian Superterranes, was part of Gondwana and was separated from Eurasia by the Paleo-Tethys Ocean (Fig. 1). During that period, the northern part of India was affected by a late phase of the Pan-African orogeny which is marked by an unconformity between Ordovician continental conglomerates and the underlying Cambrian marine sediments. Numerous granitic intrusions dated at around 500 Ma are also attributed to this event. In the Early Carboniferous, an early stage of rifting developed between the Indian subcontinent and the Cimmerian Superterranes. During the Early Permian, this rift developed into the Neotethys ocean (Fig. 2). From that time on, the Cimmerian Superterranes drifted away from Gondwana towards the north. Nowadays, Iran, Afghanistan and Tibet are partly made up
Geology of the Himalayas
12800
The Great Rift Valley () is a series of contiguous geographic trenches, approximately in total length, that runs from Lebanon in Asia to Mozambique in Southeast Africa. While the name continues in some usages, it is rarely used in geology as it is considered an imprecise merging of separate though related rift and fault systems. This valley extends northward for 5,950 km through the eastern part of Africa, through the Red Sea, and into Western Asia. Several deep, elongated lakes, called ribbon lakes, exist on the floor of this rift valley: Lake Malawi and Lake Tanganyika are examples of such lakes. The region has a unique ecosystem and contains a number of Africa's wildlife parks. The term Great Rift Valley is most often used to refer to the valley of the East African Rift, the divergent plate boundary which extends from the Afar Triple Junction southward through eastern Africa, and is in the process of splitting the African Plate into two new and separate plates. Geologists generally refer to these evolving plates as the Nubian Plate and the Somali Plate. Theoretical extent Today these rifts and faults are seen as distinct, although connected, but originally, the Great Rift Valley was thought to be a single feature that extended from Lebanon in the north to Mozambique in the south, where it constitutes one of two distinct physiographic provinces of the East African mountains. It included what today is called the Lebanese section of the Dead Sea Transform, the Jordan Rift Valley, Red Sea Rift and the East African Rift. These rifts and faults were formed 35 million years ago. Asia The northernmost part of the Rift corresponds to the central section of what is called today the Dead Sea Transform (DST) or Rift. This midsection of the DST forms the Beqaa Valley in Lebanon, separating the Mount Lebanon range from the Anti-Lebanon Mountains. Further south it is known as the Hula Valley separating the Galilee mountains and the Golan Heights. The Jordan River begins here and flows southward through Lake Hula into the Sea of Galilee in Israel. The Rift then continues south through the Jordan Rift Valley into the Dead Sea on the Israeli-Jordanian border. From the Dead Sea southwards, the Rift is occupied by the Wadi Arabah, then the Gulf of Aqaba, and then the Red Sea. Off the southern tip of Sinai in the Red Sea, the Dead Sea Transform meets the Red Sea Rift which runs the length of the Red Sea. The Red Sea Rift comes ashore to meet the East African Rift and the Aden Ridge in the Afar Depression of East Africa. The junction of these three rifts is called the Afar Triple Junction. Africa The East African Rift follows the Red Sea to the end before turning inland into the Ethiopian highlands, dividing the country into two large and adjacent but separate mountainous regions. In Kenya, Uganda, and the fringes of South Sudan, the Great Rift runs along two separate branches that are
Great Rift Valley
25025301
Slab pull is a geophysical mechanism whereby the cooling and subsequent densifying of a subducting tectonic plate produces a downward force along the rest of the plate. In 1975 Forsyth and Uyeda used the inverse theory method to show that, of the many forces likely to be driving plate motion, slab pull was the strongest. Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at oceanic trenches. This force and slab suction account for almost all of the force driving plate tectonics. The ridge push at rifts contributes only 5 to 10%. Carlson et al. (1983) in Lallemand et al. (2005) defined the slab pull force as: Where: K is (gravitational acceleration = 9.81 m/s2) according to McNutt (1984); Δρ = 80 kg/m3 is the mean density difference between the slab and the surrounding asthenosphere; L is the slab length calculated only for the part above 670 km (the upper/lower mantle boundary); A is the slab age in Ma at the trench. The slab pull force manifests itself between two extreme forms: The aseismic back-arc extension as in the Izu–Bonin–Mariana Arc. And as the Aleutian and Chile tectonics with strong earthquakes and back-arc thrusting. Between these two examples there is the evolution of the Farallon Plate: from the huge slab width with the Nevada, the Sevier and Laramide orogenies; the Mid-Tertiary ignimbrite flare-up and later left as Juan de Fuca and Cocos plates, the Basin and Range Province under extension, with slab break off, smaller slab width, more edges and mantle return flow. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere does not directly cause motion by the friction of such basal forces. The North American Plate is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. Ridge push is thought responsible for the motion of these plates. The subducting slabs around the Pacific Ring of Fire cool down the Earth and its core-mantle boundary. Around the African Plate upwelling mantle plumes from the core-mantle boundary produce rifting including the African and Ethiopian rift valleys. See also Mid-ocean ridge Seafloor spreading Ridge push References Further reading Geodynamics Geophysics Plate tectonics Subduction Geology theories
Slab pull
17236819
Mount Bisoke (also Visoke) is an active volcano in the Virunga Mountains of the Albertine Rift, the western branch of the East African Rift. It straddles the border of Rwanda and the Democratic Republic of the Congo, but the summit is located in Rwanda. It is located approximately 35 km northeast of the town of Goma and the adjacent Lake Kivu. Geology Bisoke, like all the peaks in the Virunga Mountain Range, is a volcano created by rift action on the forming divergent boundary of the East African Rift which is slowly bisecting the African plate. Bisoke has two recorded eruptions which happened in 1891 and 1957. The most recent eruption happened 11 km north of the summit, and it formed two small cones on the volcano's north flank. There is evidence that the area where this eruption occurred is still geologically active, suggesting the likelihood of future activity at Bisoke. The volcano has two crater lakes, one being the largest of the range. Geography The mountain is within the Rwandan Volcanoes National Park and the Congolese Virunga National Park. The steep slopes of the peak are densely covered with equatorial rainforest and alpine meadows. The summit does not gather snow, but is often shrouded in fog. Bisoke is one of the mountains that provide habitat for the endangered mountain gorilla, and the Karisoke Research Center founded by Dian Fossey is in the valley to the west. Industry/Tourism Being within two national parks, it is by law off-limits to most standard wilderness industries such as logging, farming, or mining. Aside from visitors to the parks searching for gorillas or other wildlife, the peak is popular with mountaineers. It can be climbed in a day from the Rwandan side, and the Rwanda Development Board (RDB) leads two-day excursions to it and nearby Mount Karisimbi, most often out of the nearby city of Ruhengeri. The climb is considered steep but walkable. The local tourism industry was devastated during the 1990s by several bloody uprisings and wars in central Africa including the Rwandan Civil War (1990–1994), Rwandan genocide (1994), First Congo War (1996–1997) and Second Congo War (1998–2003), and Burundian Civil War (1993–2005). During this time, military action and refugee relocation wreaked ecological havoc (including deforestation and poaching of endangered species) upon the Volcanoes and Virunga National Parks and the surrounding area. In recent years, both parks have been somewhat secured, and the tourism industry of the area has been recovering, despite ongoing conflicts and, from 2020-2022, the damage caused by COVID-19 lockdowns. References External links Volcanoes of the Great Rift Valley Mountains of the Democratic Republic of the Congo Virunga Mountains Stratovolcanoes of the Democratic Republic of the Congo Active volcanoes Democratic Republic of the Congo–Rwanda border International mountains of Africa Stratovolcanoes of Rwanda Holocene stratovolcanoes
Mount Bisoke
45252083
The formation of the Gulf of Mexico, an oceanic rift basin located between North America and the Yucatan Block, was preceded by the breakup of the Supercontinent Pangaea in the Late-Triassic, weakening the lithosphere. Rifting between the North and South American plates continued in the Early-Jurassic, approximately 160 million years ago, and formation of the Gulf of Mexico, including subsidence due to crustal thinning, was complete by 140 Ma. Stratigraphy of the basin, which can be split into several regions, includes sediments deposited from the Jurassic through the Holocene, currently totaling a thickness between 15 and 20 kilometers. Basin countries Tectonics Pangaea Beneath the sediments of the Gulf of Mexico basin, most of the pre-Triassic basement rocks are believed to be allochthonous thrust sheets sutured during the formation of Pangaea. However, it was during the break-up of the supercontinent that the foundation for the Gulf of Mexico sediments would be laid. Prior to the rifting which formed the Gulf of Mexico basin, extensional deformation in the Late-Triassic caused by the breakup up of Pangaea, and more specifically the rifting of the Atlantic Ocean, created basement graben formations which filled with terrestrial red bed sediments, and volcanic sediments from the eruption of the Central Atlantic Magmatic Plume. The plume erupted 60,000 cubic kilometers of flood basalts over ancestral North and South America, Africa and parts of Europe. In addition to the basalt flows, other important igneous rock types include diabases associated with the grabens and red beds, and overlapping, north-northwest trending dike swarms related to the tectonic volcanism of the breakup of Pangaea. Early to mid-Jurassic Separation of the North and South American Plates in the Early-Mid Jurassic beginning with the rotation of the Yucatan Block, along with changes in sea level and thermal activity from the active rift, created a shallow marine basin wherein thick Jurassic salts and evaporites could be deposited. These evaporites overlay a thick transitional crust, the local basement rock prior to rifting, and deposition of salts continued over the forming oceanic crust as rifting spread the sea floor throughout the Jurassic. It was during the Jurassic, approximately 140–160 million years ago, that the shape of the Gulf of Mexico as we know it was formed. The unique shape of the Gulf of Mexico, surrounded on all sides by continental crust, is the result of two different tectonic boundaries: an ocean-continent transform boundary, and a magmatic plume fueled seafloor spreading center active contemporaneously in regards to geologic time. The transform boundary caused two approximately 22° counterclockwise rotations of the Yucatan Block away from the North American plate. One rotation happened prior to seafloor spreading, and the second rotation happened while the basin spread, creating the current geographical shape of the Gulf of Mexico and the current placement of the Yucatan Peninsula. The active rifting of the Late-Triassic to Mid-Jurassic, and the stratigraphy which depended strongly on that tectonic development, is followed by a relatively calm tectonic period in the Late-Jurassic. The Late-Jurassic Gulf of Mexico is characterized
Gulf of Mexico basin
16759956
The Karlıova Triple Junction is a geologic triple junction of three tectonic plates: the Anatolian Plate, the Eurasian Plate and the Arabian Plate. Location The Karlıova Triple Junction is found where the east–west trending North Anatolian Fault intersects the East Anatolian Fault coming up from the southwest, and is ~700 km distant from the Maras Triple Junction. Because each arm of the junction is a transform fault (F), the Karlıova Triple Junction is an F-F-F type junction. Geometry The Karliova triple junction is a geological intersection point located in eastern Turkey, where the boundaries of the Arabian Plate, the Eurasian Plate, and the African Plate meet. It is a complex structure with multiple faults and fractures, resulting from the collision and convergence of these tectonic plates. The geometry of the Karliova triple junction is characterized by three intersecting arms, each representing the boundary of one of the plates. The arms meet at an angle of approximately 120 degrees, forming a Y-shaped configuration. The Arabian Plate boundary extends to the southeast, the African Plate boundary extends to the southwest, and the Eurasian Plate boundary extends to the north. References "New constraints on the Karliova Triple Junction between Arabia, Eurasia and Anatolia", A. Hubert-Ferrari, Geophysical Research Abstracts, Vol. 9, 06822, 2007. MAY 1, 2003 BİNGÖL (TURKEY) EARTHQUAKE, Preliminary Report (Updated on May 13, 2003) Triple junctions Plate tectonics Geology of Turkey
Karlıova Triple Junction
8226212
The Mornos () is a river in Phocis and Aetolia-Acarnania in Greece. It is long. Its source is in the southwestern part of the Oiti mountains, near the village Mavrolithari, Phocis. It flows towards the south, and enters the Mornos Reservoir near the village Lefkaditi. The dam was completed in 1979. It leaves the reservoir towards the west, near Perivoli. The river continues through a deep, sparsely populated valley, and turns south near Trikorfo. The lower course of the Mornos forms the boundary between Phocis and Aetolia-Acarnania. The Mornos empties into the Gulf of Corinth about 3 km southeast of Nafpaktos. Geology of the Mornos rift valley The Mornos river erodes the Mornos rift valley, which crosses the trough between Parnassos and Pindus geotectonic zones. Those zones are simply mountain chains trending generally NW to SE. They were formed during the Hellenic orogeny, when the mountain zones of Greece were thrust upward in folds due to compression caused by the subduction of the African Plate under the Eurasian Plate. The old subduction zone remains as the Hellenic trench. The zones closest to the subduction are the outer Hellenides, where Hellenides is the geologic term for the mountains of Greece. The inner Hellenides are further east. Some compression continues today. However, in the Miocene (23-5 Ma) another geologic regime began, the extensional, as part of a process called back-arc extension. In this regime, not yet totally understood, the overriding plate behind the arc raised by the subduction, termed the "back arc," began to extend itself, pushing the arc, here the Hellenic arc, back over the subduction, causing what is known as "slab-rollback," in which the actual line of subduction moves in a direction opposite to the subduction even while the latter is still subducting and compressing. This extension in the back-arc region, causing normal faults, even while reverse faults are still collecting in the compression zone, opened the Aegean Sea starting in the Miocene, and the Corinth rift along the Corinth fault, a normal one, starting less than 2 Ma in the Pleistocene (5-3 Ma). As a result of this "pull-apart" stress across the outer Hellenides, the Gulf of Corinth and its extension on the west, the Gulf of Patras, divided the Peloponnesus from the mainland and moved it south. The seismic zone created by all this deformation is one of the most intense on Earth. In addition to causing the main fault, the stress of extension relieved itself over a number of smaller approximately parallel subfaults within the Corinth Basin and to the north of it. One of these is the Amphissa-Arachova fault system, containing the Delphi fault, and creating the Pleistos rift valley. Further west is the Mornos fault, creating the Mornos rift valley, still separated from the valley to the east by mountains of the outer Hellenides. Mornos artificial lake Mornos Lake was created as a reservoir for the city of Athens, which is populated by about 3.1 million people, representing about 40% of the population of Greece.
Mornos
16701604
The Afar Triple Junction (also called the Afro-Arabian Rift System) is located along a divergent plate boundary dividing the Nubian, Somali, and Arabian plates. This area is considered a present-day example of continental rifting leading to seafloor spreading and producing an oceanic basin. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift. The latter extends a total of from the Afar Triangle to Mozambique. The connecting three arms form a triple junction. The northernmost branching arm extends north through the Red Sea and into the Dead Sea, while the eastern arm extends through the Gulf of Aden and connects to the Mid-Indian Ocean ridge further to the east. Both of these rifting arms are below sea level and are similar to a mid-ocean ridge. The third rifting arm runs south extending around through the countries of Kenya, Uganda, the Democratic Republic of Congo, Rwanda, Burundi, Tanzania, Zambia, Malawi and, finally, Mozambique. This southern rifting arm is better known as the East African Rift or the East African Rift System (EARS), when it includes the Afar Triangle. Doming and rifting A rift is the result of pulling apart or extension of lithosphere, including the crust, caused by mantle upwelling where hotter asthenosphere magma rises up into the colder lithosphere to stretch and thin it. The triple rift is thought to have begun in the Late Cretaceous epoch to the Paleogene period. At that time the African plate was experiencing far-field stresses caused by portions of the northern boundary of the African plate subducting under the Eurasian plate. Today, the Arabian plate is experiencing a crustal down pull, or slab pull, that has separated from the African plate. At the same time as the subduction in the north, there was mantle upwelling causing the crust to down warp and swell into domes throughout the East African Rift System. The Kenyan dome has been studied extensively. The plume is thought to have begun under Lake Tana in Ethiopia. Based on the environmental correlations and current topographic locations of the Jurassic Upper Limestone and Cretaceous Upper Sandstone, the net rock uplift of the Ethiopian Plateau would be since . The thinned Ethiopian lithosphere could have resulted in ponding from mantle plume and subsequent uplift. Gani et al. (2007) propose that episodic increase of incision of the Ethiopian Plateau suggests episodic growth rates within the plateau, since the incision rates have no correlation to the past climate events. As an effect of Archimedes' principle of isostatic rebound, 2.05 km uplift has occurred within the last 30 million years. Baker et al. (1972) also suggest that the uplift of this area is sporadic and divided by long periods of stability and erosion. Some periods of uplift are recorded at the end of the Cretaceous that resulted in of uplift and the end of the Neogene with a staggering in magnitude. The Ethiopian dome experienced its largest uplift coinciding with the end of the Neogene uplift associated with the Kenyan dome. It
Afar Triple Junction
61943
Lake Malawi, also known as Lake Nyasa in Tanzania and Lago Niassa in Mozambique, () is an African Great Lake and the southernmost lake in the East African Rift system, located between Malawi, Mozambique and Tanzania. It is the fourth largest freshwater lake in the world by volume, the ninth largest lake in the world by area and the third largest and second deepest lake in Africa. Lake Malawi is home to more species of fish than any other lake in the world, including at least 700 species of cichlids. The Mozambique portion of the lake was officially declared a reserve by the Government of Mozambique on June 10, 2011, while in Malawi a portion of the lake is included in Lake Malawi National Park. Lake Malawi is a meromictic lake, meaning that its water layers do not mix. The permanent stratification of Lake Malawi's water and the oxic-anoxic boundary are maintained by moderately small chemical and thermal gradients. Geography Lake Malawi is between and long, and about wide at its widest point. The lake has a total surface area of about . The lake is at its deepest point, located in a major depression in the north-central part. Another smaller depression in the far north reaches a depth of . The southern half of the lake is shallower; less than in the south-central part and less than in the far south. The lake has shorelines on western Mozambique, eastern Malawi, and southern Tanzania. The largest river flowing into it is the Ruhuhu River, and there is an outlet at its southern end, the Shire River, a tributary that flows into the very large Zambezi River in Mozambique. Evaporation accounts for more than 80% of the water loss from the lake, considerably more than the outflowing Shire River. The outflows from Lake Malawi into the Shire River are vital for the economy as the water resources support hydropower, irrigation and downstream biodiversity. Concerns have been raised over the future climate change impacts of Lake Malawi due to the recent decline in lake levels and the overall drying trend. The climate in the lake region is already experiencing changes, with the temperatures predicted to increase throughout the country. The lake is about southeast of Lake Tanganyika, another of the great lakes of the East African Rift. The Lake Malawi National Park is located at the southern end of the lake. Geological history Malawi is one of the major Rift Valley lakes and an ancient lake. The lake lies in a valley formed by the opening of the East African Rift, where the African tectonic plate is being split into two pieces. This is called a divergent plate tectonics boundary. Malawi has typically been estimated to be 1–2 million years old (mya), but more recent evidence points to a considerably older lake with a basin that started to form about 8.6 mya and deep-water condition first appeared 4.5 mya. The water levels have varied dramatically over time, ranging from almost below current
Lake Malawi
521081
The African Plate, also known as the Nubian Plate, is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plate to the west (separated by the Mid-Atlantic Ridge); the Arabian Plate and Somali Plate to the east; the Eurasian Plate, Aegean Sea Plate and Anatolian Plate to the north; and the Antarctic Plate to the south. Between and , the Somali Plate began rifting from the African Plate along the East African Rift. Since the continent of Africa consists of crust from both the African and the Somali plates, some literature refers to the African Plate as the Nubian Plate to distinguish it from the continent as a whole. Boundaries The western edge of the African Plate is a divergent boundary with the North American Plate to the north and the South American Plate to the south which forms the central and southern part of the Mid-Atlantic Ridge. The African Plate is bounded on the northeast by the Arabian Plate, the southeast by the Somali Plate, the north by the Eurasian Plate, the Aegean Sea Plate, and the Anatolian Plate, and on the south by the Antarctic Plate at the Southwest Indian Ridge. All of these are divergent or spreading boundaries with the exception of the northern boundary and a short segment near the Azores known as the Terceira Rift. Components The African Plate includes several cratons, stable blocks of old crust with deep roots in the subcontinental lithospheric mantle, and less stable terranes, which came together to form the African continent during the assembly of the supercontinent Pangea around 250 million years ago. The cratons are from south to north, the Kalahari Craton, Congo Craton, Tanzania Craton and West African Craton. The cratons were widely separated in the past, but came together during the Pan-African orogeny and stayed together when Gondwana split up. The cratons are connected by orogenic belts, regions of highly deformed rock where the tectonic plates have engaged. The Saharan Metacraton has been tentatively identified as the remains of a craton that has become detached from the subcontinental lithospheric mantle, but alternatively may consist of a collection of unrelated crustal fragments swept together during the Pan-African orogeny. In some areas, the cratons are covered by sedimentary basins, such as the Tindouf Basin, Taoudeni Basin and Congo Basin, where the underlying archaic crust is overlaid by more recent Neoproterozoic sediments. The plate includes shear zones such as the Central African Shear Zone (CASZ) where, in the past, two sections of the crust were moving in opposite directions, and rifts such as the Anza Trough where the crust was pulled apart, and the resulting depression filled with more modern sediment. Modern movements The African Plate is rifting in the eastern interior of the African continent along the East African Rift. This rift zone separates the African Plate to the
African Plate
16703290
The Aden Ridge is a part of an active oblique rift system located in the Gulf of Aden, between Somalia and the Arabian Peninsula to the north. The rift system marks the divergent boundary between the Somali and Arabian tectonic plates, extending from the Owen Transform Fault in the Arabian Sea to the Afar Triple Junction or Afar Plume beneath the Gulf of Tadjoura in Djibouti. The Gulf of Aden is divided east to west into three distinct regions by large-scale discontinuities, the Socotra, Alula Fartak, and Shukra-El Shiek transform faults. Located in the central region, bounded by the Alula Fartak fault and Shukra-El Shiek fault, is the Aden spreading ridge. The Aden Ridge connects to the Sheba Ridge in the eastern region and to the Tadjoura Ridge in the western region. Due to oblique nature of the Aden Ridge, it is highly segmented. Along the ridge there are seven transform faults that offset it to the north. Initiation of rifting Extension of the Gulf of Aden rift system began in the late Eocene - early Oligocene (~35 Ma ago), caused by the northeast escape of the Arabian plate from the African plate at a rate of ~2 cm/yr, and the development of the Afar plume. Extension eventually gave way to seafloor spreading, first initiated near the Owen transform fault ~18 Ma ago. Seafloor spreading then propagated as far west as the Shukra-El Shiek fault at a rate of ~14 cm/yr ~6 Ma ago rifting propagated west of the Shukra-El Shiek fault until terminating at the Afar plume. The Afar plume is believed to have contributed to the initiation of the Aden ridge, due to the flow of hot mantle material being channeled along the thin lithosphere beneath the Gulf of Aden. Currently, the Aden Ridge is undergoing extension at a rate of ~15 mm/yr. Segmentation of the Aden Ridge Compared to its neighboring ridges, the Aden ridge is much more segmented. The Aden Ridge is broken up by seven transform faults with ridge segments of 10 – 40 km. In contrast, the Sheba Ridge is broken by only three transform faults and the Tadjoura Ridge continues essentially uninterrupted to the Afar Plume. Sauter et al. (2001) proposed that variations in the spacing of spreading cells along ridges is a result of spreading rate; i.e., larger spacing results from slower spreading rates. However, the variation in spreading rates across the Gulf of Aden, 18 mm/yr in the east and 13 mm/yr in the west, is not great enough to explain the significant variation in spreading cell length between the Aden ridge and its neighboring ridges. One likely cause for the segmentation of the Aden ridge is its distance from the Afar plume. The westernmost region of the Gulf, where the Tadjoura Ridge is located, has an anomalously high mantle temperature due to its proximity to the Afar plume. The result of this is higher degrees of melting and magmatism below the ridge, which allows for longer spreading segments without transform
Aden Ridge
891796
The Australian Plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when India broke away and began moving north. Australia and Antarctica had begun rifting by and completely separated a while after this, some believing as recently as , but most accepting presently that this had occurred by . The Australian plate later fused with the adjacent Indian Plate beneath the Indian Ocean to form a single Indo-Australian Plate. However, recent studies suggest that the two plates have once again split apart and have been separate plates for at least 3 million years and likely longer. The Australian Plate includes the continent of Australia, including Tasmania, as well as portions of New Guinea, New Zealand and the Indian Ocean basin. Scope The continental crust of this plate covers the whole of Australia, the Gulf of Carpentaria, southern New Guinea, the Arafura Sea, the Coral Sea. The continental crust also includes northwestern New Zealand, New Caledonia and Fiji. The oceanic crust includes the southeast Indian Ocean, the Tasman Sea, and the Timor Sea. The Australian Plate is bordered (clockwise) by the Eurasian Plate, the Philippine Plate, the Pacific Plate, the Antarctic Plate, the African Plate and the Indian Plate. It is however known from movement studies that this definition of the Australian Plate is 20% less accurate than one that assumes independently moving Capricorn, and Macquarie microplates. Geography The northeasterly side is a complex but generally convergent boundary with the Pacific Plate. The Pacific Plate is subducting under the Australian Plate, which forms the Tonga and Kermadec Trenches, and the parallel Tonga and Kermadec island arcs. It has also uplifted the eastern parts of New Zealand's North Island. The continent of Zealandia, which separated from Australia and stretches from New Caledonia in the north to New Zealand's subantarctic islands in the south, is now being torn apart along the transform boundary marked by the Alpine Fault. South of New Zealand the boundary becomes a transitional transform-convergent boundary, the Macquarie Fault Zone, where the Australian Plate is beginning to subduct under the Pacific Plate along the Puysegur Trench. Extending southwest of this trench is the Macquarie Ridge. The southerly side is a divergent boundary with the Antarctic Plate called the Southeast Indian Ridge (SEIR). The subducting boundary through Indonesia is not parallel to the biogeographical Wallace line that separates the indigenous fauna of Asia from that of Australasia. The eastern islands of Indonesia lie mainly on the Eurasian Plate, but have Australasian-related fauna and flora. Southeasterly lies the Sunda Shelf. To the east of Indonesia there appears to be under the Indian Ocean a deformation zone between the Indian and Australian Plates with both earthquake and global satellite navigation system data indicating that India and Australia are not moving on the same vectors northward and have started a process of again separating. This zone is along the northern NinetyEast Ridge
Australian Plate
20697684
The Aegean Sea Plate (also called the Hellenic Plate or Aegean Plate) is a small tectonic plate located in the eastern Mediterranean Sea under southern Greece and western Turkey. Its southern edge is the Hellenic subduction zone south of Crete, where the African Plate is being swept under the Aegean Sea Plate. Its northern margin is a divergent boundary with the Eurasian Plate. The seafloor in this region is about 350 m below sea level, while the adjacent Black Sea and Mediterranean Sea are 1300–1500 m deep. For this reason it is considered a high plateau between the seas. Evidence suggests the Aegean Plate contains thinned continental crust, rather than oceanic crust. Since its creation the crust has been thinned through various processes, including post-orogenic collapse and crustal extension. This extension is responsible for the formation of the Gulf of Corinth. Previous observations of the region's motion described the crust under the Aegean Sea as a part of the Anatolian Plate, and the different directions of motion were explained as the plate rotating counterclockwise. Further measurements found that motion of the Aegean region differed from the previous model, so the two plates are now considered distinct from each other. Development The Aegean Sea area is thought to be an actively extending back-arc region due to slab rollback on the Hellenic subduction zone. This has resulted in extensive normal faulting and rifting, as well as the formation of a back-arc basin. This environment has created a number of horst and graben features on the seafloor, similar to basin and range topography. Many of the Aegean Islands are peaks from these features reaching above sea level. The southern part of the plate is 20–22 km thick, while the northern part of the plate is 32–40 km thick, which suggests that the extensional environment has only recently begun affecting the northern region. Prior to the extensional environment, the region underwent the Aegean Orogeny (), followed by crustal thinning due to post-orogenic collapse. This period enabled metamorphism and then exhumation of many types of metamorphic rock found on the Aegean islands. Seismic activity The Aegean Sea and surrounding area is seismically active because of the Hellenic subduction zone, as well as the extension of the Aegean Plate. The African plate is subducting under the Aegean Plate at a rate of about 40 mm/year, causing shallow earthquakes near the fault and deeper earthquakes near the Greek volcanic arc. Some seismic activity is a result of the extension of the plate, which creates east–west trending faults that can slip and cause earthquakes. References Further reading External links Tectonic plates Aegean Sea Geology of Greece Geology of Turkey
Aegean Sea Plate
497007
A rift valley is a linear shaped lowland between several highlands or mountain ranges produced by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear depression may subsequently be further deepened by the forces of erosion. More generally the valley is likely to be filled with sedimentary deposits derived from the rift flanks and the surrounding areas. In many cases rift lakes are formed. One of the best known examples of this process is the East African Rift. On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges in continental crust or in oceanic crust. They are often associated with a number of adjoining subsidiary or co-extensive valleys, which are typically considered part of the principal rift valley geologically. Earth's rift valleys The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of sea floor spreading. Examples of this type of rift include the Mid-Atlantic Ridge and the East Pacific Rise. Many existing continental rift valleys are the result of a failed arm (aulacogen) of a triple junction, although there are two, the East African Rift and the Baikal Rift Zone, which are currently active, as well as a third which may be, the West Antarctic Rift System. In these instances, not only the crust but entire tectonic plates are in the process of breaking apart forming new plates. If they continue, continental rifts will eventually become oceanic rifts. Other rift valleys are the result of bends or discontinuities in horizontally-moving (strike-slip) faults. When these bends or discontinuities are in the same direction as the relative motions along the fault, extension occurs. For example, for a right lateral-moving fault, a bend to the right will result in stretching and consequent subsidence in the area of the irregularity. In the view of many geologists today, the Dead Sea lies in a rift which results from a leftward discontinuity in the left lateral-moving Dead Sea Transform fault. Where a fault breaks into two strands, or two faults run close to each other, crustal extension may also occur between them, as a result of differences in their motions. Both types of fault-caused extension commonly occur on a small scale, producing such features as sag ponds or landslides. Rift valley lakes Many of the world's largest lakes are located in rift valleys. Lake Baikal in Siberia, a World Heritage Site, lies in an active rift valley. Baikal is both the deepest lake in the world and, with 20% of all of the liquid freshwater on earth, has the greatest volume. Lake Tanganyika, second by both measures, is in the Albertine Rift, the westernmost arm of the active East African Rift. Lake Superior in North America, the largest freshwater lake by area, lies in the ancient and dormant Midcontinent Rift. The largest subglacial lake, Lake Vostok, may also lie in an ancient rift
Rift valley
16733975
The Rodrigues Triple Junction (RTJ), also known as the Central Indian [Ocean] Triple Junction (CITJ) is a geologic triple junction in the southern Indian Ocean where three tectonic plates meet: the African Plate, the Indo-Australian Plate, and the Antarctic Plate. The triple junction is named for the island of Rodrigues which lies north-west of it. The RTJ was first recognized in 1971, then described as a stable R-R-R (ridge-ridge-ridge) triple junction based on coarse ship data. Boundaries The boundaries of the three plates which meet at the Rodrigues Triple Junction are all oceanic spreading centers, making it an R-R-R type triple junction. They are: the Central Indian Ridge (CIR, between the African and Indo-Australian plates) with a spreading rate of 50 mm/yr; the Southwest Indian Ridge (SWIR, between the African and Antarctic plates) 16 mm/yr; and the Southeast Indian Ridge (SEIR, between the Indo-Australian and Antarctic plates) 60 mm/yr. The SEIR has the highest spreading rates at the RTJ, and, while now considered an intermediate spreading centre, it was a fast spreading ridge between anomalies 31 and 22, with a rate of 110 km/myr at anomaly 28. The spreading rate is similar in the CIR but slower and the ridge has a more complex geometry. The SWIR has ultra-slow spreading rates, a rough topography, and great number of large offset fracture zones. All three boundaries are themselves intersected by diffuse boundaries: the CIR is intersected by the Indian–Capricorn boundary; the SEIR by the Capricorn–Australian boundary; and the SWIR by the Nubian–Somalian boundary. For example, the East African Rift divides Africa into the Nubian and Somalian plates. These plates converge in the southern part of the rift valley (2 mm/yr) but diverge in the northern part (6 mm/yr) and a very slight difference in spreading rates across the central part of the ultra-slow SWIR indicates there is a vague triple junction somewhere south of Madagascar. Tectonic evolution The RTJ was born when the Seychelles microcontinent drifted off the Indian Plate at 64 Ma and the Carlsberg Ridge opened. Since then the RTJ has moved eastward from south of Madagascar (modern coordinates) to its current location. Since 65 Ma the RTJ has been migrating north-east at a decreasing rate: originally the velocity was 10 cm/yr, at 43 Ma 2.6 cm/yr, and since 41 Ma around 3.6–3.8 cm/yr. The stability in migration rate around 41 Ma coincides with the bend in the Hawaiian–Emperor seamount chain — hinting at a global reorganisation of tectonic plates at this time. Originally considered a stable RRR (ridge-ridge-ridge) triple junction, the RTJ is now believed to be an unstable RRF (ridge-ridge-fault) triple junction in which the axis of the CIR is offset eastward by 14 km/myr because of differences in spreading rates between the SEIR and CIR. This is a configuration similar to that of the Galapagos Triple Junction in the east Pacific. Each time the RTJ offset eastward a new segment is added to CIR. resulting in a constant length for the SEIR while CIR constantly lengthens.
Rodrigues Triple Junction
145705
The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North American from the Eurasian Plate and the African Plate, north and south of the Azores Triple Junction. In the South Atlantic, it separates the African and South American plates. The ridge extends from a junction with the Gakkel Ridge (Mid-Arctic Ridge) northeast of Greenland southward to the Bouvet Triple Junction in the South Atlantic. Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level, for example in Iceland. The ridge has an average spreading rate of about per year. Discovery A ridge under the northern Atlantic Ocean was first inferred by Matthew Fontaine Maury in 1853, based on soundings by the USS Dolphin. The existence of the ridge and its extension into the South Atlantic was confirmed during the expedition of HMS Challenger in 1872. A team of scientists on board, led by Charles Wyville Thomson, discovered a large rise in the middle of the Atlantic while investigating the future location for a transatlantic telegraph cable. The existence of such a ridge was confirmed by sonar in 1925 and was found to extend around Cape Agulhas into the Indian Ocean by the German Meteor expedition. In the 1950s, mapping of the Earth's ocean floors by Marie Tharp, Bruce Heezen, Maurice Ewing, and others revealed that the Mid-Atlantic Ridge had a strange bathymetry of valleys and ridges, with its central valley being seismologically active and the epicenter of many earthquakes. Ewing, Heezen and Tharp discovered that the ridge is part of a 40,000-km (25,000 mile) long essentially continuous system of mid-ocean ridges on the floors of all the Earth's oceans. The discovery of this worldwide ridge system led to the theory of seafloor spreading and general acceptance of Alfred Wegener's theory of continental drift and expansion in the modified form of plate tectonics. The ridge is central to the breakup of the hypothetical supercontinent of Pangaea that began some 180 million years ago. Notable features The Mid-Atlantic Ridge includes a deep rift valley that runs along the axis of the ridge for nearly its entire length. This rift marks the actual boundary between adjacent tectonic plates, where magma from the mantle reaches the seafloor, erupting as lava and producing new crustal material for the plates. Near the equator, the Mid-Atlantic Ridge is divided into the North Atlantic Ridge and the South Atlantic Ridge by the Romanche Trench, a narrow submarine trench with a maximum depth of , one of the deepest locations of the Atlantic Ocean. This trench, however, is not regarded as the boundary between the North and South American Plates, nor the Eurasian and African Plates. Islands The islands on or near the Mid-Atlantic Ridge, from north to south, with their respective highest peaks and location, are: Northern Hemisphere
Mid-Atlantic Ridge
43819057
The Lwandle Plate is one of three tectonic microplates, along with the Rovuma Plate and Victoria Plate, that make up the African Plate with the Somali Plate and the Nubian Plate. Its discovery is very recent, so the velocity of the plate is neither well known nor well understood. Many experiments are ongoing to quantify this. The Lwandle Plate lies between 30°E and 50°E, sharing a boundary with the Nubian, Somali, and Antarctic Plates. The Lwandle Plate is largely oceanic, lying off the southeast coast of Africa. It is currently believed that the southern part of Madagascar forms part of the Lwandlean Plate, with one of the plate boundaries cutting through the island. Discovery of the Lwandle Plate For many years it was widely accepted that rifting in the East African Rift system, 22–25 million years ago, resulted in the splitting of the African Plate into 2 smaller plates – the Somali Plate and the Nubian Plate. However, most recently, through the application of GPS technology and integration of earthquake data, it was discovered that the rift created three additional “microplates” – the Lwandle Plate, Victoria Plate, and Rovuma Plate. Earthquakes occur most often at plate boundaries and have been used as a guide to predict the locations of multiple plate boundaries. “Lwandle’s” existence was postulated after studying earthquake data in areas that were once assumed to be the interiors of the Nubian and Somali plates. By including the Lwandlean Plate in their calculation, researchers were able to more accurately solve for the intersection between the East African Rift and the Southwest Indian Ridge. GPS technology and data was also introduced in an effort to show with certainty the difference between the Somali Plate and the Nubian Plate. Using the GPS data, researchers could measure the velocity of tectonic plates from their interiors. The existence of the Lwandle Plate was first quantified using the Lwandle-Antarctica-Nubia plate circuit closure constrained by spreading rates and transform azimuths along the Southwest Indian Ridge. Mathematical plate circuit closure is attained while including the existence of Lwandle; using plate circuit closure, plate velocities can be calculated from the velocities of other plates. In 2008, the entire East African Rift System was successfully "quantified" through the integration of the GPS and earthquake data with the spreading rate and transform azimuth constraints. There have been very few studies that directly study the Lwandlean Plate. Previous studies about Lwandle have primarily been focused on attempting to quantify the mechanics of the African Rift System. For this reason, the presumed evolution and formation of the microplates are not well known. Evolution Lwandle was assumed to be part of the Somali plate, at first. Here are two models that show how thinking about Lwandle has evolved. Boundary types The majority of Lwandle's plate boundaries are not well understood. What is known is: Southern boundary – The Southwest Indian Ridge, an ultra-slow spreading ridge with a slow spreading rate of about 12–18 mm/yr, acts as the southern boundary. This spreading ridge
Lwandle Plate
65584572
The Victoria Microplate or Victoria Plate is a small tectonic plate in East Africa. It is bounded on all sides by parts of the active East African Rift System. It is currently rotating anticlockwise. Its boundaries are close to those of the mainly Archaean Tanzania Craton, with the two arms of the rift system having propagated along the surrounding Proterozoic shear belts. To the northwest, west and southwest it has a boundary with the Nubian Plate, to the northeast and east with the Somali Plate and to the southeast with the Rovuma Plate. References Tectonic plates Geology of Africa
Victoria Microplate
12990367
Easter Plate is a tectonic microplate located to the west of Easter Island off the west coast of South America in the middle of the Pacific Ocean, bordering the Nazca Plate to the east and the Pacific Plate to the west. It was discovered from looking at earthquake distributions that were offset from the previously perceived Nazca-Pacific Divergent boundary. This young plate is 5.25 million years old and is considered a microplate because it is small with an area of approximately . Seafloor spreading along the Easter microplate's borders have some of the highest global rates, ranging from /yr. Structure and tectonics (present) From the 1970s to 1990s, multiple efforts were made to collect data on the area, including several magnetic and gravitational anomaly surveys. These surveys show that Easter plate is uniquely shallow, bordered by spreading centers and transform boundaries, with a triple junction located at the southern and northern tip. Along the eastern border, there are several spreading centers south of 27° S and 3 northward propagating rift to the north of 27° S. The axis further north is a graben reaching a depth of approximately 6000 m. Northward propagation of the eastern rifts is continuous at a speed of /yr. The spreading ridge between 26° S and 27° S has a spreading rate of /yr, but is asymmetrical on Nazca Plate side. Bathymetry data shows the depth is near 26°30' S and progressively gets deeper to the north, reaching depths of in an axial valley. There is approximately a gap at the northern end of the east rift with no rift connecting the northern boundary to the eastern boundary. The northern border has wide ridges, greater than 1 km tall, linked side-by-side with the steeper slopes to the south. The southern trough area sits deeper than the areas to the north. The very eastern end of the northern border has pure strike-slip motion, while the western end is marked by the Northern Pacific-Nazca-Easter triple junction. This triple junction is a stable rift-fracture-fracture zone with anomalous earthquakes occurring to the northeast portion, indicating a possible second spreading axis. The rest of the northern boundary to the east and west of the triple junction are colinear transform boundaries. A trough, approximately deep, borders the north along this transform boundary to the east connecting to a deep hole, called the "Pito Deep" because of its close proximity to the Pito Seamount, at the northeastern limit. The western border is divided into two parts. The west section has 2 spreading segments running north to south with spreading rates that approximately range from /yr. These segments are connected by sinistrally slipping transform faults around 14°15' S. A relay basin runs north to south along the southernmost segment as a result of past counter-clockwise rotation. The southwest consists of one slower spreading center (/yr) that runs northwest to southeast until joining the southern transform boundary. Like the western end of the northerner border, the southern end also has an inferred rift-rift-fracture triple junction,
Easter Microplate
44041683
The South China Sea Basin is one of the largest marginal basins in Asia. South China Sea is located to the east of Vietnam, west of Philippines and the Luzon Strait, and north of Borneo. Tectonically, it is surrounded by the Indochina Block on the west, Philippine Sea Plate on the east, Yangtze Block to the north. A subduction boundary exists between the Philippine Sea Plate and the Asian Plate. The formation of the South China Sea Basin was closely related with the collision between the Indian Plate and Eurasian Plates. The collision thickened the continental crust and changed the elevation of the topography from the Himalayan orogenic zone to the South China Sea, especially around the Tibetan Plateau. The location of the South China Sea makes it a product of several tectonic events. All the plates around the South China Sea Basin underwent clockwise rotation, subduction and experienced an extrusion process from the early Cenozoic to the Late Miocene. The geological history can be classified into five tectonic evolutionary stages. (1) rift system development (2) sea floor spreading, (3) subsidence of the South China Sea, (4) closure of the South China Sea Basin and (5) uplift of Taiwan. Rift system development In the initial stage of the development of South China Sea, a basin was developed by extension to form two passive margins. The consensus is that the extension propagated from the northeast to the southwest, although some experts argue that the southwest basin is in fact older. The rifting and multiple grabens initiated around 55 Ma, based on seismic profiles across the southern China Shelf. The rifting intensified around 50 Ma due to the collision of the Indian and Eurasian plates. Two different models on how the extension initiated have been proposed, by Wang (2009) and by Cullen (2010). Wang's model for South China Sea rifting proposes a different area of rift development. The north and northeastern parts of the South China Sea formed their rifts earlier in the Paleocene. The south and southwestern parts of the South China Sea showed a later rifting around the Eocene or later. The difference in rifting and time gap between the northeastern and southwestern regions indicate the South China Sea is not a geologically homogeneous area, and its lithosphere could be divided into two areas, southwest and northeast according to its tectonic evolution. The reasons behind these differences in its rifting stage could be various, such as impact from different plates and different distribution of plumes under the crust. The Red River Fault along the western boundary of the South China Sea was believed to influence the rifting in the south and southwestern regions. Strike-slip faults. Cullen indicated that the South China Sea Basin's rifting could be traced back to the late Cretaceous and the extension concluded in two episodes during the Cenozoic. The first episode of extension occurred in the Early Paleocene and was widely distributed. The first rift system was located mainly in the Dangerous Ground (SE of South
Tectonics of the South China Sea
24573187
The Madagascar Plate or Madagascar block is a tectonic plate holding the island of Madagascar. It was once attached to the Gondwana supercontinent and later the Indo-Australian Plate. Rifting in the Somali Basin began at the end of the Carboniferous 300 million years ago, as a part of the Karoo rift system. The initiation of Gondwana breakup, and transform faulting along the Davie Fracture Zone, occurred in the Toarcian (about 182 million years ago) following the eruption of the Bouvet (Karoo) mantle plume. At this time East Gondwana, comprising the Antarctic, Madagascar, Indian, and Australian plates, began to separate from the African Plate. East Gondwana then began to break apart about 115–120 million years ago when India began to move northward. Between 84–95 million years ago rifting separated Seychelles and India from Madagascar. Since its formation the Madagascar block has moved roughly in conjunction with Africa, and thus there are questions as to whether the Madagascar Plate should be still considered a separate plate. Assembly of Gondwana Madagascar was formerly located in the central part of the supercontinent Gondwana. It contains part of the East African Orogen, which formed in the Neoproterozoic to Cambrian assembly of the Gondwana. This heavily influenced the geology of central and northern Madagascar. The entire island can be divided into four tectonic and geologic units: the Antongil block, the Antananarivo block, the Bekily Belt in the south, and the Bemarivo Belt in the far north. The Antongil Block is characterized by a 3.2 Ga gneiss intruded by granite that has undergone greenschist facies metamorphism. The Antananarivo Block contains 2.5 Ga gneiss layered with younger granitoids and gabbros. It has metamorphosed to granulite facies conditions. The Bemarivo Belt contains two regions, metasedimentary gneisses in the southern part and granitic domes in the north. The Bekily Belt is made up of mostly sedimentary protoliths and granulite and upper amphibolite grade gneisses. The blocks in the northern part of the island are made up of Archean cratonic material. The Antongil block has been linked with the Dharwar Formation of India, however the Antananarivo block to the west has been too heavily altered to link easily to another continent. The central part of the island contains metasediments from African and Indian continental shelves. This is the Itremo Group, which also contains intrusions of material from the Antongil block. The Itremo sheet was folded in the amalgamation of Madagascar ~700 Mya, and now contains upright folds, divergent reverse faults, and strike-slip faults. Rifting The Madagascar plate experienced two major rifting events during the break-up of Gondwana. First, it separated from Africa about 160 Mya (million years ago), then from the Seychelles and India 66–90 Mya. The first rifting event, separation from Somalia and the rest of Africa, caused displacement along the Davie Ridge, in the Mozambique Channel to the west of the islands, a now extinct transform. The rifting is also associated with extensive deformation as well as volcanism in the late Cretaceous and Cenozoic (Eocene to Miocene). The second
Madagascar Plate
1791751
The Somali Plate is a minor tectonic plate which straddles the Equator in the Eastern Hemisphere. It is currently in the process of separating from the African Plate along the East African Rift Valley. It is approximately centered on the island of Madagascar and includes about half of the east coast of Africa, from the Gulf of Aden in the north through the East African Rift Valley. The southern boundary with the Nubian-African Plate is a diffuse plate boundary consisting of the Lwandle Plate. Geology The Arabian Plate diverges to the north forming the Gulf of Aden. The Indian Plate, Australian Plate, and Antarctic Plate all diverge from the Somali Plate forming the eastern Indian Ocean. The Somali-Indian boundary spreading ridge is known as the Carlsberg Ridge. The Somali-Australian boundary spreading ridge is known as the Central Indian Ridge. The Somali-Antarctic boundary spreading ridge is known as the Southwest Indian Ridge. The western boundary with the African Plate is diverging to form the East African Rift, which stretches south from the triple junction in the Afar depression. The southern boundary with the Nubian-African Plate is a diffuse plate boundary with the Lwandle Plate. The Seychelles and the Mascarene Plateau are located northeast of the Madagascar. Tectonic history From the Kibaran orogeny fused the Tanzanian and Congo cratons. From 1000 to 600 Ma the super-continent Gondwana was formed and the Pan-African orogeny sutured the Tanzanian and Kalahari cratons. The rifting of Gondwana occurred from 190 Ma to 47 Ma separating Madagascar from the eastern coast of Africa and placing the Seychelles/Mascarene Plateau northeast of Madagascar. The rifting of the Red Sea started around and the first rifting occurred in the northern West African Rift System around . See also Victoria Microplate References Further reading Emerick, C. M., 1985, Age progressive volcanism in the Comores Archipelago and Northern Madagascar, Oregon State Univ., Corvallis, PhD Thesis, Abstract Tectonic plates Geology of Africa Geology of the Indian Ocean Geology of Somalia
Somali Plate
64734427
The volcanic system of Krýsuvík (or Krísuvík, both pronounced in Icelandic), also Krýsuvík-Trölladyngja volcanic system, is situated in the south–west of Iceland on the Reykjanes peninsula. It is located in the middle of Reykjanes and on the divergent plate boundary of the Mid-Atlantic Ridge which traverses Iceland. It was named after the Krýsuvík area which is part of it and consists of a fissure system without a central volcano. However, there are some indications—namely, the discovery by geophysical methods of what scientists interpret as a buried caldera, combined with the well-known, vigorous hydrothermal system above it—that an embryonic central magma chamber may already exist or be actively developing. The volcanic system has a length of , a width of around , covers an area of , and its highest elevation is . It is one of 4 (or up to 7, depending on the source) volcanic systems situated within the Reykjanes Volcanic Belt. The volcanic systems are arranged en echelon and at an angle that varies from 20–45° to the direction of the rift zone on the divergent plate boundary traversing Reykjanes. General characteristics The volcanic system of Krýsuvík has no central volcanic edifice, but rather a fissure swarm that is long, is composed of a mixture of volcanic and tectonic fissures and faults, of which are volcanic fissures. Recent geophysical work on the system, employing magnetotelluric resistivity sampling and modeling, indicates the presence of a buried caldera with possibly an embryonic central magma chamber beneath the system. This suggests that it could eventually develop into a central volcano like the similar, but more mature Hengill volcanic system farther to the east on the peninsula. There are no known submarine fissures of the system which nevertheless reaches from the south coast in direction south–west to north–east over the Reykjanes Peninsula. The northernmost fissures are thought to reach Lake Rauðavatn on the outskirts of Reykjavík. There are no ice-covered volcanoes connected to the Krýsuvík system, but Lake Kleifarvatn lies within the system and geothermal activity is found at the lake bottom. The Krýsuvík volcanic system has a tendency to effusive basaltic fissure eruptions; the last eruption took place in the 14th century. The Fagradalsfjall fissure swarm that erupted in 2021 was initially considered potentially a branch or a secondary part of the Krýsuvík volcanic system, but it is now usually considered a separate volcanic system. The eruption products of the Krýsuvík system consist exclusively of basalt. Eruptions The volcanic system is centered on the divergent plate boundary on Reykjanes peninsula. It it is easier there for magma to reach the surface, because of the multitude of tectonic and volcanic faults and fissures in such regions. There have been at least 10 volcanic episodes within the volcanic system in the last 8,000 years. These episodes each comprised many single eruptions and were most probably connected to rifting. Some Holocene eruptions have been dated specifically, especially the eruption that produced Búrfellshraun (ca. 5290 BP). Since the time of settlement in Iceland, which is
Krýsuvík (volcanic system)
60481245
Project FAMOUS (French-American Mid-Ocean Undersea Study) was the first-ever marine scientific exploration by manned submersibles of a diverging tectonic plate boundary on a mid-ocean ridge. It took place between 1971 and 1974, with a multi-national team of scientists concentrating numerous underwater surveys on an area of the Mid-Atlantic Ridge about west of the Azores. By deploying new methods and specialized equipment, scientists were able to look at the sea floor in far greater detail than ever before. The project succeeded in defining the main mechanisms of creation of the median rift valley on the Mid-Atlantic Ridge, and in locating and mapping the zone of oceanic crustal accretion. Study area The Project FAMOUS study area was located on a section of the Mid-Atlantic Ridge about west of the Azores (Sao Miguel) at 36° 50’ north latitude. It includes a -wide median valley or rift valley on the crest of the Mid-Atlantic Ridge that trends slightly east of north. Within the median valley lies the present boundary between the North American and African tectonic plates. The floor of the rift valley is deep and wide and the bounding rift mountains are at a depth of about , or about above the floor. The rift valley is long and it is offset to the eastward in the north at Fracture Zone A; in the south, it is offset westward at Fracture Zone B. Methodology A significant obstacle in marine surveys was the use of echo sounders with a wide transmit beam, which smeared-out details of the sea floor features. The crustal accretion or creation process was thought to take place over a few kilometers width of sea floor, which was below the resolution of ship echo sounders. Thus, near-bottom and on-bottom approaches were employed along with new sonar mapping tools. Investigations included airborne magnetics, advanced surface ship sonar, and geophysical measurements, seismology, deep-towed instruments, large format bottom photography, fixed on-bottom instruments, and on-bottom dives with research manned submersibles in the rift valley of the Mid-Atlantic Ridge. The Woods Hole Oceanographic Institution (WHOI) in Massachusetts provided surface ships and the submersible ALVIN; the French provided surface ships and the bathyscaph Archimède and submersible CYANA. The British conducted side scan sonar surveys and on-bottom seismic experiments. Lead institutions were WHOI and the French Centre Oceanologique de Bretagne, Brest, France. Project leaders were James Heirtzler, Claude Riffaud, and Xavier Le Pichon. Operational challenges In the 1960s, Canadian scientists had begun a detailed study of the Mid-Atlantic Ridge at a latitude of 45° N that included multiple expeditions by surface ships. With Project FAMOUS located on the ridge in more clement latitudes around 37° N, a coordinated multi-national, multi-ship series of more than twenty expeditions took place over four years, between 1971 and 1974. Bilateral briefing meetings were held as new expeditions were completed. The unique operational features of Project FAMOUS included the use of newly developed narrow-beam and multibeam echo sounders along with deeply-towed instruments and manned submersibles to achieve a new, higher level of resolution
Project FAMOUS
71621276
In 1990, present day South Sudan was rocked by a series of violent earthquakes. It started with the largest event, a 7.2, and continued with multiple very large aftershocks for the next couple of months. It contains some of the largest recorded earthquakes anywhere in Africa. Tectonic setting The East African Rift System (EARS) is a system of rifts and associated rift lakes within the eastern portion of the African continent. It accommodates the internal breakup of Africa. It is a boundary between the Somali and African Plate. The Victoria and Rovuma microplates help accommodate additional more local stresses. The eastern portion of the rift runs from the Afar rift to Tanzania, while the western portion runs from Lake Albert (Africa) in Uganda all the way down to Mozambique. The Aswa Rift Zone (ARZ) is an important geologic structure in the area near the earthquakes. The ARZ may act as a broad zone of faulting that links together the eastern and western portions of the EARS. The mainshock occurred along a fault where the ARZ and EARS meet. Earthquake sequence May 20 mainshock The first and largest earthquake in the sequence struck near Juba at 02:22:01 on May 20, 1990. The event had a of 6.6, with a and of 7.2, and struck at a depth of . Its focal mechanism shows left-lateral (sinistral) strike-slip faulting, which is consistent with the shear forces in the area of the ARZ. The event killed 31 people and damaged some buildings in Juba, and some in Moyo, Uganda. This event is believed to be the largest earthquake ever recorded in South Sudan, and is one of the largest events known in all of Africa. May 24 foreshock At 19:34:44 on May 24, 1990, another large earthquake struck southern South Sudan. It occurred at a depth of with a of 6.1, of 6.5-6.8 and () of 6.5-6.6. The focal mechanism solution varies, with some showing the earthquake being the result of normal dip-slip faulting, while others suggesting it was the result of reverse dip-slip faulting instead. May 24 mainshock At 20:00:08, only a little over 25 minutes after the foreshock, an even stronger earthquake struck the region: the second strongest of the whole sequence. This event was a 6.6, 7.0, 7.1 quake that occurred at a depth of . The focal mechanisms by various sources again disagree, with some showing normal faulting, with others showing strike slip faulting. It caused additional structural damage to buildings previously affected by the first and strongest shock, however, no casualties occurred. The worst damage was observed in uninhabited areas. July 9 mainshock After the sequence had largely finished, another large shock struck the area for a final time. The last big earthquake in the sequence was a 6.0, 6.4-6.5 and 6.6 event. It occurred at a depth of . Focal mechanism solutions are split, with a roughly equal proportion of agencies and studies preferring a strike-slip event, with another equal proportion believing it to be a normal faulting
1990 South Sudan earthquakes
39341090
A transfer zone in geology is an area where deformational strain is transferred from one structural element to another typically from fault to fault in rift systems. Therefore, listric faults and monoclinal folds in the hanging wall are typical structures linked by transfer zones; however, complexities do exist. The terms interbasin and intrabasin transfer zones have been proposed to delineate the magnitude of the transfer zone. Transfer zones can be described according to the fault dip directions; synthetic or conjugate and according to their deformation style; convergent or divergent. Transfer zones can be farther identified by its maturity or (fault propagation evolution); whether the major fault relationship is approaching, overlapping, collateral or collinear. Since transfer zones are normally found in extensional settings many studies have been done within the East African rift system and the Gulf of Suez rift system. Transfer zones have also played a role in hydrocarbon exploration and extraction within the Albertine graben. Interbasin transfer zones Interbasin transfer zones often deal with major boundary rifts. Interbasin transfer zones could be interbasin ridges, broad faulted highs, or major relay ramps, all which have a large influence on the rift system. Intrabasin transfer zones Intrabasin transfer zones are within the constraints of the interbasin transfer zones and are normally an order of magnitude smaller. Intrabasin transfer zones could be relay ramps within en echelon normal faults, or minor fault jogs. The southern margin of the Gulf of Evvia near Atalanti shows the magnitude of difference between interbasin transfer zones with accommodating distances up to 10 km as well as intrabasin transfer zones that accommodate approximately 1 km or less. Synthetic transfer zones Synthetic transfer zones must have major normal faults dipping in the same direction. This includes relay ramps. Relay ramps have been studied within the East African rift system (Lake Malawi) and the Gulf of Suez rift system. Transfer zone drainage basins consist of any sediments that will eventually travel through the transfer zone and in the half graben. The Gulf of Evvia is an example where a relay ramp plays a major role in drainage. Footwall drainage is limited in the Gulf of Evvia while transfer zone drainage is more pronounced. Since size of the drainage basin is one of the major controls on sediment flux, both interbasin and intrabasin synthetic relay ramp type transfer zones must have a major influence on sedimentation into the overall system. While intrabasin transfer zones are smaller they still act as a conduit for sedimentation. Conjugate transfer zones Conjugate transfer zones must have major normal faults dipping in the opposite direction. This classification can be farther divided into convergent and divergent transfer zones. Convergent transfer zones Convergent transfer zones dip toward each other and in result more complex faulting and folding can occur in the zones between them. Divergent transfer zones Divergent transfer zones dip in opposite direction and often result in topographical highs. Transfer zone fault propagation evolution Major fault boundaries within rift systems tend to expand with time based
Transfer zone
31343672
The 2002 Kalehe earthquake occurred on October 24 at 06:08 UTC. This earthquake had a magnitude 6.2, and the epicenter was located in Democratic Republic of the Congo, near Lake Kivu. Two people were reported dead. Building damage was reported in Goma, Lwiro, Kalehe, and Mugeri. The seismicity, volcanism, and uplift in the basin of Lake Kivu delimits the rift of a tectonic plate. Lake Kivu belongs to the western branch of the East African Rift System (EARS). The western branch is usually divided into several segments, and Lake Kivu belongs to the northern segment. Tectonic setting Lake Kivu is formed in an active rift basin that is part of the western branch of the East African Rift System. The basin is a half-graben, with the main controlling fault on the western side. The other margin is mainly unfaulted and flexural in type. The active extension is WNW–ESE trending, perpendicular to the main border fault. The displacement rate across this part of the EARS rift system is in the range 3–5 mm per year. Most earthquakes in the rift system are normal in type. Earthquake The earthquake had a focal mechanism consistent with normal faulting and a hypocentral depth of 11.0 km. These observations have been combined with the analysis of InSAR data and the known geology of the area to conclude that the earthquake was a result of movement on one of the east-dipping rift border faults. The earthquake was preceded by an eruption in the Nyiragongo volcanic area to the north of Lake Kivu, in January 2002. This is interpreted to be a diking event. Analysis of the effect of the diking on the stress field to the south suggests that faulting like that seen in the earthquake may have been promoted by this event. See also List of earthquakes in 2002 List of earthquakes in the Democratic Republic of the Congo References External links 2002 disasters in the Democratic Republic of the Congo Earthquakes in the Democratic Republic of the Congo Kalehe 2002 in the Democratic Republic of the Congo
2002 Kalehe earthquake
47202223
The Agulhas Basin is an oceanic basin located south of South Africa where the South Atlantic Ocean and south-western Indian Ocean meet. Part of the African Plate, it is bounded by the Agulhas Ridge (part of the Agulhas-Falkland Fracture Zone) to the north and the Southwest Indian Ridge to the south; by the Meteor Rise to the west and the Agulhas Plateau to the east. Numerous bathymetric anomalies hint at the basin's dynamic tectonic history. Geology In a Late Paleocene (59-56 Ma) reconstruction of the opening of the South Atlantic Ocean (i.e. the separation of South America and Africa during the Gondwana break-up) the Meteor Rise lies conjugate to the Islas Orcadas Rise (east of the Falkland Plateau). The separation of the Meteor Rise and the Islas Orcadas Rise marks the beginning of the formation of the Agulhas Basin. The Agulhas Ridge extends from the northern tip of the Meteor Rise towards the Agulhas Bank south of South Africa. The ridge, however, ends abruptly in a small plateau at where it intersects a northeastward-trending spreading centre (the Agulhas Rift) that was abandoned during the Early Paleocene (61 Ma). The presence of a short-lived tectonic plate between these structures was first proposed by . They named it the Malvinas Plate and proposed that it was active from 90 Ma until the spreading ceased in the Agulhas Basin at 65 Ma. The plate is located at a proto-Bouvet Triple Junction. found that Late Cretaceous (100-66 Ma) fracture zones generated on the Agulhas Rift do not align with those north of the Agulhas Fracture Zone and cannot therefore have been formed by the spreading of South America and Africa. Furthermore, magnetic anomalies on the Malvinas Plate do not align with their conjugates on the African Plate if the spreading rates and directions of South America and Africa are used as a guide. They also noted that the Agulhas Fracture Zone do not lay perpendicular to traces of the South America-Africa spreading north of it and cannot, therefore, have been generate by this spreading. The Agulhas Rift is the abandoned Malvinas-Africa ridge crest. 97 Ma the plate boundary in the Agulhas Basin was reorganised when the Mid-Atlantic Ridge made an eastward jump. This brought the boundary towards the Agulhas Plateau where excessive volcanism was building a large igneous province. The inception of the Malvinas Plate accompanied this shortening of the Agulas Fracture Zone. 61 Ma, the Malvinas Plate was finally incorporated into the African Plate when the Malvinas-Africa ridge was abandoned as the result of a westward ridge jump along the Agulhas-Falkland Fracture Zone. This second ridge jump reduced one of the most spectacular fracture zones in Earth's history — in length — to . Oceanography The Agulhas Current flows south along the African east coast. When it reaches the southern tip of Africa, it retroflects back into the Indian Ocean. At this retroflection it leaks warm core eddies known as Agulhas rings into the South Atlantic. This mesoscale anti-cyclonic rings feed the Atlantic
Agulhas Basin
57138990
The Wessex Basin is a petroleum-bearing geological area located along the southern coast of England and extending into the English Channel. The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the English Channel is of similar size. The basin is a rift basin that was created during the Permian to early Cretaceous in response to movement of the African plate relative to the Eurasian plate. In the late Cretaceous, and again in the Cenozoic, the basin was inverted as a distant effect of the Alpine orogeny. The basin is usually divided into 3 main sub-basins including the Winterborne-Kingston Trough, Channel Basin, and Vale of Pewsey Basin. The area is also rich in hydrocarbons with several offshore wells in the area. With the large interest in the hydrocarbon exploration of the area, data became more readily available, which improved the understanding of the type of inversion tectonics that characterize this basin. Tectonic mechanisms Lithospheric thinning and crustal extension led to the subsidence and consequently the creation of asymmetric graben. These events happened in pulses starting in the Permian and concluding in the early Cretaceous period. By mid Cretaceous the active crustal extension ceased and the region underwent a period of unfaulted subsidence in part due to the thermal relaxation effects from the earlier lithospheric action. Inversion began starting in the late Cretaceous and carried on through Cenozoic times coinciding with the Alpine orogeny happening at the now convergent boundary of the African and Eurasian plates. Basement structure and stratigraphy Starting at the basement of the structure, the area of interest overlies Variscan externides consisting of Devonian and Carboniferous sediments in imbricated thrust sheets. The Hercynian basement was rather thin in origin with the thrust emplacement in a north-northwest trend developed most likely with a strike-slip fault. After the conclusion of the initial Hercynian deformation, normal reactivation of these same faults occurred along with differential subsidence beginning in the late Carboniferous as a direct result from the northwest/southeast trending wrench movements. The sands created within this period lie in an unconformable manner above the Devonian and Carboniferous basement. The asymmetrical grabens are found within these sands and through the use of dating techniques mark the point in history of basin initiation during the late Carboniferous. Following this stage the basin underwent a period of erosion removing approximately 10 km of sediment from the area with the following Permian sediment deposition being dictated by the former Hercynian structure. This semi-arid, desert sedimentation occurred on the western portion of the basin with the clasts eventually thinning as they progress towards the east side of the basin. Triassic sediments sit unconformably above the Permian layer with the makeup being mainly sands and silts. Like the former stage, the Triassic sediments were first concentrated on the west side of the basin but also appeared in the southwest as a direct result from marine transgression. The Jurassic period followed with the same marine sedimentation, but by the end of the period,
Wessex Basin
6842670
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on the amount of stretching involved. Stretching is generally measured using the parameter β, known as the beta factor, where t0 is the initial crustal thickness and t1 is the final crustal thickness. It is also the equivalent of the strain parameter stretch. Low beta factor In areas of relatively low crustal stretching, the dominant structures are high to moderate angle normal faults, with associated half grabens and tilted fault blocks. High beta factor In areas of high crustal stretching, individual extensional faults may become rotated to too low a dip to remain active and a new set of faults may be generated. Large displacements may juxtapose syntectonic sediments against metamorphic rocks of the mid to lower crust and such structures are called detachment faults. In some cases the detachments are folded such that the metamorphic rocks are exposed within antiformal closures and these are known as metamorphic core complexes. Passive margins Passive margins above a weak layer develop a specific set of extensional structures. Large listric regional faults dipping towards the ocean develop with rollover anticlines and related crestal collapse grabens. On some margins, such as the Niger Delta, large counter-regional faults are observed, dipping back towards the continent, forming large grabenal mini-basins with antithetic regional faults. Geological environments associated with extensional tectonics Areas of extensional tectonics are typically associated with: Continental rifts Rifts are linear zones of localized crustal extension. They range in width from somewhat less than 100 km up to several hundred km, consisting of one or more normal faults and related fault blocks. In individual rift segments, one polarity (i.e. dip direction) normally dominates, giving a half-graben geometry. Other common geometries include metamorphic core complexes and tilted blocks. Examples of active continental rifts are the Baikal Rift Zone and the East African Rift. Divergent plate boundaries Divergent plate boundaries are zones of active extension as the crust newly formed at the mid-ocean ridge system becomes involved in the opening process. Gravitational spreading of zones of thickened crust Zones of thickened crust, such as those formed during continent-continent collision tend to spread laterally; this spreading occurs even when the collisional event is still in progress. After the collision has finished the zone of thickened crust generally undergoes gravitational collapse, often with the formation of very large extensional faults. Large-scale Devonian extension, for example, followed immediately after the end of the Caledonian orogeny particularly in East Greenland and western Norway. Releasing bends along strike-slip faults When a strike-slip fault is offset along strike such as to create a gap e.g. a left-stepping bend on a sinistral fault, a zone of extension or transtension is generated. Such bends are known as releasing bends or extensional stepovers and often form pull-apart basins or rhombochasms. Examples of active pull-apart basins include the Dead Sea, formed
Extensional tectonics