Fault formation in porous sedimentary rocks at high strain rates

Abstract

Previous theoretical work suggests that faults in porous sedimentary rocks subjected to high strain rates do not form from deformation band damage zones (DBDZs) as typically observed in porous rocks. This hypothesis is evaluated by an investigation of faults mapped within the porous Navajo Sandstone at the Upheaval Dome impact structure in Canyonlands National Park, southeast Utah, where high strain rate conditions are known to have occurred. We found no evidence for DBDZ formation along the fault planes at Upheaval Dome. Instead pulverized quartz grains within the Navajo Sandstone are revealed adjacent to several fault planes. This material has previously been observed and documented in association with dynamic fracturing in crystalline and sedimentary rocks along the San Andreas Fault in southern California and in metamorphic rocks along the Bosman fault in South Africa. Additional work has shown that the grain sizes of pulverized rock can be related to the specific strain rate conditions under which pulverization occurred. Measured grain sizes obtained from the pulverized material collected at Upheaval Dome are found to be associated with strain rates of approximately 10^0 and 10^3 s^-1. Previously reported grain sizes for pulverized sedimentary rocks along the San Andreas Fault imply strain rates of approximately 10^-2 to 10^1 s^-1. Strain rates at Upheaval Dome are well above the average values associated with intraplate tectonics (10^-20 to 10^-17 s^-1), but are consistent with, or somewhat faster than, seismic slip rates along faults such as the San Andreas Fault.