Erosional denudation of the British Columbia Coast Ranges as determined from fission-track ages of detrital zircon from the Tofino basin, Olympic Peninsula, Washington
In this study, we use up-section changes in fission-track (FT) ages of detrital zircons to infer the tectonic evolution of the source region that fed sediments to the Tofino basin, a Tertiary forearc basin in northwestern Washington State and southwestern British Columbia. We have dated 50 grains from each of eight samples from a 6-km-thick section that ranges in age from 40 to 19 Ma. The detrital zircons have not been reset and thus preserve information about the thermal and denudation history of the source region from which the sediments were derived. Each of our eight samples contains a distinct set of grain-age populations or peaks, indicating that the source region contained several FT source terrains. The number of peaks and their lag time (peak age minus depositional age) contain thermochronologic information about these source terrains. In this study, there is little evidence of active volcanism in the source region; thus the FT ages reflect cooling during denudation of the source regions. The oldest sample in our suite, from the upper Eocene Lyre Formation, shows a dominant peak at ca. 40 Ma, coincident with cooling ages of denuded metamorphic rocks of the Leech River Schist on southern Vancouver Island. This interpretation is supported by paleocurrent directions indicating a source in the vicinity of Vancouver Island, the development of a late Eocene unconformity across much of southern Vancouver Island and western Washington, and the predominance of lithic detritus in this part of the section. Samples from strata above the Lyre Formation are feldspathic, and they presumably reflect erosional denudation of a plutonic source terrain. Peaks in samples from the lower part of this feldspathic interval have widely scattered Jurassic to mid-Cretaceous FT ages; young zircons are notably absent. Up-section, peak ages in samples from the upper part of the feldspathic section form a dominant peak that gets progressively younger with time. While the age of the dominant peak becomes younger in younger samples, the lag time for this peak remains constant at ∼40 m.y. This peak can be recognized in a compilation of zircon FT ages from modern exposures of the Canadian Coast Mountains. We infer that zircons in our samples were derived from progressive erosional denudation of the Coast Plutonic Complex, which had, based on our results, a constant rate of denudation of ∼250 m/m.y. The scatter of ages in the older samples from the feldspathic interval may indicate the unroofing of a high-level sedimentary basin (containing the Cretaceous Nanaimo and Burrard Groups) that used to overlie parts of the Coast Plutonic Complex. Our study, along with others, shows that up-section studies of FT ages of detrital zircons can be effective in resolving the long-term denudation history of continental source regions.
Geologic interpretation of magnetic and gravity anomalies in the Strait of Juan de Fuca, U.S.–Canada
A gravity and magnetic survey of the Strait of Juan de Fuca and adjacent Pacific continental shelf was conducted to define the tectonic framework in this 20 to 35 km wide seaway and its relation to that of Vancouver Island and the Olympic Peninsula. The offshore extensions of large onshore faults are delineated by linear magnetic and gravity anomalies. One of these, the Leech River fault of southern Vancouver Island, marks the northern limit of oceanic-type basaltic basement present in western Washington and Oregon. This fault probably continues southeast-ward from Vancouver Island across the strait to near the northeastern coast of the Olympic Peninsula, and westward across the strait to the continental shelf off Cape Flattery. The Calawah fault, which extends northwestward from near Cape Flattery onto the Pacific shelf, terminates the Leech River fault. Northwest of the Leech River fault on the shelf, the Calawah fault probably is the contact between oceanic and continental crustal types. The gravity an...
Tectonic history of the eastern edge of the Alexander terrane
Rocks exposed west of the Coast Plutonic Complex in southern southeast Alaska form an imbricate thrust belt that overprints the tectonic boundary between two of the largest allochthonous crustal fragments in the North American Cordillera, the Insular and Intermontane composite terranes. In the Alexander terrane (Insular composite terrane), lower Paleozoic metavolcanic and metasedimentary rocks (Descon Formation) and dioritic plutons are unconformably overlain by Lower Devonian clastic strata (Karheen Formation). These rocks are overlain locally by Upper Triassic basalt, rhyolite and marine clastic strata (Hyd Group). Upper Jurassic and Lower Cretaceous metavolcanic and metasedimentary strata of the Gravina sequence unconformably overlie the Alexander terrane. The Gravina sequence forms a structural package over 15 km thick and records intermittent arc volcanism along the eastern edge of the Alexander terrane. The Gravina sequence is structurally overlain by upper Paleozoic and lower Mesozoic metamorphosed basaltic strata, marble, and argillite (Alava sequence), and locally by lower Paleozoic supracrustal rocks and orthogneiss (Kah Shakes sequence). Together, these constitute the Taku terrane which we correlate with the Intermontane composite terrane. Local unconformity of Gravina sequence strata over the Alava sequence demonstrates that the Gravina sequence overlapped an earlier structural boundary between the Intermontane and Insular composite terranes. The rocks were deformed in the mid-Cretaceous by west-vergent thrusting that was was broadly coeval with arc magmatism. Deformation involved emplacement of west-directed thrust nappes over the structurally intact and relatively unmetamorphosed Alexander terrane basement. Mid-Cretaceous tonalite, granodiorite, and quartz diorite intrude rocks of the thrust belt and are locally affected by the deformation. Mid-Cretaceous deformation occurred during two episodes that were contemporaneous with the emplacement of large sill-like plutons. Older structures record ductile southwest-vergent folding and faulting, regional metamorphism, and development of axial-planar foliation. The second-generation structures developed during the later stages of southwest-directed thrust faulting, which juxtaposed rocks of contrasting metamorphic pressures and temperatures. Structural, stratigraphic, and geochronologic data indicate that the two phases of regional thrusting in southeast Alaska occurred between 113 Ma and 89 Ma. Rocks in the western part of the thrust belt were uplifted regionally by 70 Ma. Deformation involved the collapse of a marginal basin(s) and a magmatic arc, and overprinted the older tectonic boundary between the Insular composite terrane and the late Mesozoic western margin of North America (at that time the Intermontane composite terrane). Contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism, and thus records intra-arc tectonism. Late Paleocene to early Eocene igneous activity and extensional (?) deformation subsequently affected the thrust belt.