DETACHMENT FAULTS AND FOOTWALL MYLONITES: ARE THEY COGENETIC? (Invited Presentation)

The recognition of the Cenozoic age and extensional origin of Cordilleran metamorphic core complexes (mccs) of the western United States revolutionized our understanding of the geometry of extensional fault systems in a variety of tectonic settings. A common premise built on field observations and thermochronology of mccs is that during their evolution, footwalls are progressively cooled during exhumation, and active fault rocks localize upwards through time as the shear zone narrows during its viscous to frictional evolution. Accordingly, the structurally lowest, higher-temperature mylonites are predicted to be cogenetic with the detachment fault, and record a deeper and early part of the progressive extensional strain history. It is increasingly recognized that this evolutionary model is overly simplistic, and that older shear zones are commonly captured and reactivated by detachment faults: cogenetic mylonites can be limited to lower-temperature (commonly greenschist facies) conditions, and cogenetic mylonites may also be absent. Exceptions include mccs with migmatitic or synextensional intrusive cores. Footwall mylonites that are seemingly kinematically compatible with Miocene detachments can record significantly older Paleogene extensional events, such as in the polyphase Eocene, Oligocene, and Miocene mccs of the northern Great Basin. Miocene detachments in the Death Valley, Colorado River trough, and Mojave regions commonly reactivate Late Cretaceous (Laramide) extensional shear zones. These relationships underscore the influence of lithologically-controlled rheological stratification and anisotropies resulting from prior deformation on the resulting geometries of many extensional detachment fault systems. We review examples of shear zone capture and reactivation from the western US and present new data from the Tucki Mountain mcc in the northern Panamint Range that documents Late Miocene reactivation and capture of a Late Cretaceous shear zone. At Tucki Mountain, greenschist-facies mylonites within the immediate footwall of the main detachment are demonstrably Late Cretaceous in age and in detail are kinematically distinct from the Late Miocene-Pliocene detachment and brittle extensional overprint. In light of the commonality of pre-detachment, older ductile flow preserved in mcc footwalls, care must be taken in the evaluation of potential syn-detachment, footwall accommodation, including isostatically-driven flow of low-viscosity crust.