Fault formation in porous sedimentary rocks at high strain rates: First results from the Upheaval Dome impact structure, Utah, USA

Previous theoretical work has suggested that porous sedimentary rocks subjected to sufficiently high strain rates may not form deformation band damage zones (DBDZs). This hypothesis is evaluated by an examination of faults within the porous Navajo Sandstone at the Upheaval Dome impact structure in Canyonlands National Park, southeastern Utah, where high strain-rate conditions are known to have occurred. We found no evidence for DBDZ formation along the accessible fault planes at Upheaval Dome. Instead, a layer of pulverized quartz grains within the Navajo Sandstone occurs adjacent to the fault planes. Similar material has been observed in association with dynamic rupture in crystalline and sedimentary rocks along the San Andreas fault in southern California and in metamorphic rocks along the Bosman fault in South Africa. Measured grain sizes obtained from the pulverized material collected at Upheaval Dome are consistent with strain rates of similar to 1-3 x 10(3) s(-1). Pulverized sedimentary rocks along the San Andreas fault imply strain rates of similar to 10(-2) to 10(1) s(-1). Strain rates along the faults at Upheaval Dome are well above the average values associated with intraplate tectonics but are consistent with, or faster than, seismic slip rates along faults such as the San Andreas fault. Our results support the hypothesized rate dependence of deformation in porous rocks.