QUATERNARY FAULTING AND INTRAPLATE PALEO-EARTHQUAKES IN THE WIND RIVER BASIN (WYOMING)

Although typically associated with Late Mesozoic-Early Cenozoic Laramide tectonics, the Wind River Basin (Wyoming) contains numerous active faults as demonstrated by fault scarps in Late Quaternary alluvium. Yet, the role of these faults in the context of slow, present-day intraplate deformation is not well understood. We present preliminary results of recent efforts to document structural styles, rates of faulting, and paleo-earthquake characteristics. Owing to low slip rates, surface expressions such as fault scarps can have subtle landscape expressions. Along the northern Margin of the Wind River Basin, the WNW-ESE striking Stagner Creek Fault is expressed as fault scarps in numerous Late Quaternary geomorphic surfaces. This study assess recent faulting on the Birdseye Creek section of the Stagner Creek Fault system using (1) quantitative analysis and modeling of the fault scarps in Late Quaternary surfaces, and (2) shallow seismic reflection imaging of the shallow fault geometry. A high-resolution digital surface model of the study area was constructed using low-altitude digital photogrammetry. A notable result is identification of fault scarp within the Holocene-age surface – a previously undocumented result that changes the estimate of timing for the last surface-rupturing event. The 30 cm height of the scarp suggests a paleoseismic event of magnitude 6.0 – 6.2. Two shallow seismic reflection profiles were obtained using a rolling-spread technique with geophone spacing of 2 meters, and shot spacing of 6 meters. A fixed-spread seismic refraction line was also obtained for use in creation of a seismic velocity model for the area. This velocity model showed a simple two-layer structure representing ~10 meters of alluvium on top of Wind River Formation bedrock. Shallow seismic reflection profiling suggests the controlling fault is a north-dipping reverse fault. Due to the high-angle nature of this reverse faulting, its proximity to the adjacent Owl Creek Mountains, and the tectonic setting of the Wind River Basin, this faulting is likely the result of reactivated Laramide structure.