INSIGHTS INTO GEOMETRY AND EVOLUTION OF EXTENSIONAL BASINS IN THE WESTERN U.S. FROM COMPARISON OF GEOLOGICALLY- AND GEOPHYSICALLY-DEFINED LOCATIONS OF BASIN-BOUNDING FAULTS

Comparison of the locations of normal faults defined by surface geologic mapping and by geophysical methods provides insight into the geometry and evolution of extensional basins in the western U.S. Throughout the Basin and Range province and Rio Grande rift, the surface strand of the basin-bounding normal fault, expressed either as a range-front fault or isolated Quaternary scarp, is 1-2 km rangeward of the main basin-bounding fault defined by gravity data. Examples involving range-front faults include the fault along the west side of Dixie Valley, the Frenchman Mountain fault in Las Vegas Valley, and the Bare Mountain Fault in Crater Flat, all in Nevada. Other examples involving Quaternary scarps include the Big Chino and Verde faults in the transition zone in Arizona, strands of the Cache and Wasatch faults in Utah, and the Hubbell Springs fault along the eastern boundary of the Rio Grande rift in north-central New Mexico. In most cases, the gravity-defined strand has greater buried displacement than that inferred from uplift of bedrock in the adjacent mountain range. Because of the non-unique nature of gravity interpretations, a number of explanations have been proposed for the location discrepancy, such as low-angle fault geometry, slide blocks, or the presence of high-density alluvium abutting the fault zone. However, other datasets, such as magnetic and seismic, suggest that the margins of many of these extensional basins consist of multiple, stepped normal faults, with faulting migrating with time rangeward away from the basin centers. An exception is an older fault rangeward of both the Quaternary scarp and the gravity-defined basin-bounding fault in the Rio Grande Rift. The generally observed mismatch in fault locations suggests a period of quiescence between the major basin-forming Miocene extension and Quaternary extension. Possible mechanisms for the evolution of faulting include rotation of the stress field, basin tilting leading to fault orientations that are no longer favorable to continued displacement, influence of underlying pre-existing basement structure, or influence of the semi-free surface at the basin edge. Models that predict ground shaking, groundwater flow, and migration of deep geothermal fluids may need to account for the buried basinward location of the main basin-bounding fault.