DETACHMENT FAULTS AS DYNAMIC LINKS BETWEEN DEEP AND SHALLOW LITHOSPHERE (Invited Presentation)

Detachment faults inform the thermal state and rheological behavior of the lithosphere at the time of their formation. Field observations and numerical modeling show that lithospheric extension is expressed in various ways, from distributed thinning with multiple fault/shear zone systems, leading to boudinage of lithospheric layers, to localized shearing on detachment faults and shear zones. In continental and oceanic settings, the localization of extension on discrete shear systems that accommodate tens to 100s of kms of extension relies on the feedback between stretching of shallow brittle crust and flow of deep ductile crust. Detachments that bound oceanic core complexes develop in magma-starved spreading centers where ductile gabbro and serpentinite provide a low-viscosity layer that flows and therefore localizes continued shearing on the detachment fault. In the absence of deep-crust or upper-mantle flow, deformation would spread out in the upper crust; detachment faulting would be distributed and limited. In continents, detachments also develop where the deep crust is hot and can flow. In these cases, too, a single detachment system localizes strain, bringing deep and shallow crust into contact. Localization is enhanced by motion of the deep, low-viscosity crust that is rapidly transferred upward and intrudes as a gneiss dome. Ascent of the deep crust is driven by the pressure gradient generated by extension in the shallow crust. This process is well expressed in gneiss domes that contain fragments of deep crust. These high-P remnants (granulite, eclogite) indicate that deep crust was rapidly exhumed from the base of the orogenic crust (>45 km) or from zones of continental subduction (>100 km). Although detachment faults did not exhume these rocks from deep to shallow levels, the formation and evolution of the faults is integral to how the entire lithosphere deformed under extension, ultimately leading to exhumation of deep crust. Coeval development of sedimentary basins in the zone of strain localization (detachment) also influences the dynamics of the entire lithosphere, and, in cases involving dense basin fill, results in deep burial of shallow crustal material. Detachment faults are therefore ‘more than structures’: they are dynamic sites of vertical and lateral motion in the lithosphere.