DIVERSE PSEUDOTACHYLITES IN THE FOOTWALL OF THE WHIPPLE DETACHMENT FAULT: IMPLICATIONS FOR SEISMOGENESIS IN THE MIDDLE CRUST

The Whipple detachment fault in E. California is a classic example of a large-displacement (~40 km), low-angle normal fault formed during Miocene Basin and Range extension. The footwall of this fault exhibits a range of mid-crustal rocks deformed near the brittle-ductile transition, including mylonites, cataclasites, breccias and pseudotachylites, which provide insight into mid-crustal rheology from steady-state to seismic strain rates. Here we focus on a diverse array of pseudotachylites discovered in the Whipple footwall that have not been previously described. We examine the structural contexts, morphologies, and compositions of the pseudotachylites and discuss their implications for seismogenesis on continental low-angle normal faults. Veins that we interpret to be pseudotachylites occur as planar, anastomosing, and reservoir-like injections found along the margins of dikes, along mini-detachments kinematically linked to the Whipple fault, and within a few tens of centimeters below the silicified, erosionally resistant "microbreccia ledge" of the main detachment. The orientations of the vein generation surfaces are dominantly shallowly E-dipping, subparallel to the detachment fault itself; some occur on higher angle normal faults that sole into low angle shear zones. Veins were not found cutting the microbreccia ledge itself, suggesting that comminution and silicification post-dates pseudotachylite formation. In thin section, the veins exhibit a range in composition and degree of preservation. Some contain lath-shaped spherulites, others contain opaque, microcrystalline matrices with relict flow banding and embayed, primarily quartz clasts. Some pseudotachylite veins grade into cataclasites at their margins, suggesting cataclasis was precursory to vein formation, whereas others cut pristine mylonites with no evidence of earlier brittle deformation. Those that cut pristine mylonites contain clasts with dynamically recrystallized quartz grains with diameters of 5-7 μm. This suggests a minimum shear stress for formation of the pseudotachylite veins of 60-80 MPa based on quartz paleopiezometry. These data provide evidence that the low-angle Whipple fault and its splays were seismically active at mid-crustal depths.