| Abstract |
Active transtensional orogens within continental lithosphere likely host elevated geothermal potential. Many existing electricalgrade geothermal resources (e.g. Walker Lane, Salton Trough) are found in regions influenced by both shear and tensile stresses (i.e. transtension). The tectonic controls related to the geothermal potential of transtensional areas are fundamentally different than that of pure continental extension. In extensional regions driven solely by gravitational buoyancy forces, factors such as flexural forces, crustal thickness variations, and lower crustal flow of ductile material into the extending region limit total strain and strain rate on normal faults. Where strain rate is low, each of these processes produces a negative feedback on the ability for non-vertical normal faults to develop high strain rates or accommodate large amounts of strain. As a result, regions of pure continental extension commonly form an area of distributed, rather than localized, dilational deformation. In contrast, transtensional regions inherently contain strike-slip faults with higher strain rates that absorb some fraction of the driving plate boundary forces. Because the negative feedbacks from buoyancy forces do not affect sub-vertical strike-slip faults, their presence can allow for elevated strain and strain rates in a more focused zone. Where these strike-slip structures are kinematically linked and adjacent to extensional structures (e.g. releasing bend or step-over), an elevated extensional strain rate and focused lithospheric thinning can occur in the intervening region of extension. This rapid and localized extension can more efficiently thin the lithospheric column, exhume hot lower crustal material, and promote extrusive and/or shallow intrusive volcanic activity. These processes can elevate the brittle-ductile transition and steepen the geothermal gradient, providing the necessary heat to a high-enthalpy geothermal system (e.g. Cerro Prieto, Coso). Transtensional structures, such as fault intersections and terminations, may also intimately control and help to focus the pathways of fluids in the upper crust. In summary, the oblique forces across a transtensional plate boundary may be more efficient at rapidly thinning the lithosphere than regions of continental extension, and thus have a higher potential for development of electrical-grade geothermal resources. |