Evidence for large-scale Laramide tectonic inversion and a mid-Tertiary caldera ring fracture zone at the Lightening Dock geothermal system, New Mexico
Stratigraphy in a recent intermediate depth (2100 ft) geothermal test hole is interpreted along side previous deep (7001 to 7404 ft depth) exploration holes to study large-scale structural controls for the Lightning Dock geothermal system and refine the stratigraphic and structural characterization of the shallow reservoir. Laramide compression and large-scale tectonic inversion of a northwest-trending Late Jurassic-Early Cretaceous extensional structure at the Lightning Dock geothermal site represents deformation that may exceed Neogene extensional processes in magnitude and duration and provide potential for a significant volume of shattered rock. Also, volcanic stratigraphy supports the hypothesis of Elston et al. (1983) for a midTertiary caldera ring fracture zone in the vicinity the geothermal area. Northeastand north-trending Neogene normal faults that cross cut older structures provide additional preparation of fractured ground. A north-trending Pleistocene normal fault may reopen older fractures that are favorably oriented and allow concentration and upward flow of deep-seated geothermal fluids. Because the Lightning Dock geothermal system is “blind” and has no surface manifestations, a better understanding of structures buried beneath Neogene alluvial basin fill may have application in geothermal exploration to find similar “blind” geothermal systems elsewhere in southern New Mexico and southeastern Arizona. FIGURE 1. Shaded relief topographic location map of the Lightning Dock geothermal area.
Arc abandonment as a cause for passive continental rifting: Comparison of the Jurassic Mexican Borderland rift and the Cenozoic Rio Grande rift
Two rift systems, one of late Mesozoic age and the other of Tertiary age, in the southern Cordillera of North America formed along the inner flanks of former continental arcs. Both rift systems were initiated when arc magmatism abandoned its former inboard extent as a result of retrograde motion of the subducted slab. Similarities in stratigraphy and geochemistry preceding and during crustal extension of each rift system suggest a three-phase magmatic-depositional model for the formation of passive continental rifts above a foundering subducted slab. Continental arc magmatism associated with normal subduction weakens the continental crust during phase 1. Phase 2 involves incipient retrograde motion of the slab, or slab foundering, initiating mantle return into the wedge-shaped volume between slab and overlying continental lithosphere. This causes crustal extension, lithospheric melting, and deposition of conglomerate in nascent rift basins stratigraphically above and adjacent to the extinguished arc. Caldera-related silicic volcanism defines an ignimbrite flare-up, accompanied by extrusion of lithosphere-derived basalt. Mafic volcanism, block faulting, and extensional sedimentary-basin formation continue after the end of silicic volcanism. During phase 3, decompression partial melting of convecting asthenosphere creates basalts with ocean-island chemical affinities intercalated with alluvial or marine sedimentary rocks in extensional basins.
On the use of the Hardening Soil Small Strain model in geotechnical practice
This article highlights the importance of using advanced constitutive soil models in numerical modelling as daily geotechnical practice. In this context, the Hardening Soil (HS) model is presented as the most advanced soil model implemented in the ZSoil finite element code. The article recalls the basic features of the HS model and it explains their role in numerical simulations. The role of pre-failure non-linear behaviour of soils is also discussed. An application of the HS model is illustrated on typical geotechnical problems such as retaining wall excavation, tunnel excavation and undrained loading followed by consolidation.