Pulverization Within Sandstone Associated With Faulting at High Strain Rates at the Upheaval Dome Impact Structure, Utah

We report an investigation of faults in porous sedimentary rocks subjected to high strain rates. We focused on lower-displacement faults cutting Navajo Sandstone within the ring syncline of Upheaval Dome impact structure in Canyonlands National Park, Utah, where high strain rate conditions are known to have occurred. Results of our field investigation revealed grains within the Navajo Sandstone were pulverized adjacent to the fault planes. Previous work has defined pulverized rock as rock that has been shattered in place with no evidence of shearing and as having a powdery texture that crumbles under slight pressure applied by hand. Pulverized rock has previously been identified in crystalline and sedimentary rocks associated with the San Andreas Fault. The diameter of individual pieces of pulverized material can be related to the specific strain rate under which pulverization occurred. We find that pulverized material collected at Upheaval Dome has an average grain size of 42 microns, and referencing the relationship derived by Grady and Kipp [1987], is therefore associated with strain rates of ~ 106 s-1. According to previous work, grain sizes of 350 to 1,100 microns have been observed in pulverized sedimentary rocks associated with dynamic fracturing along the San Andreas Fault and relate to strain rates of ~ 104 and 105 s-1. Strain rates of these magnitudes are well above the values typically associated with tectonic rates (10-11 s-1and below) and laboratory testing rates (between 10-7 s-1 and 10-5 s- 1) refining the importance of high strain-rate processes associated with faulting of porous sandstones.