INSIGHTS INTO GEOMETRY AND EVOLUTION OF EXTENSIONAL BASINS ALONG THE WASATCH FAULT AND EAST AND WEST CACHE FAULT ZONES, UTAH AND IDAHO, U.S.A

Gravity data in northern Utah and southern Idaho reflect the complex 4-D development of basins along the northern Wasatch fault zone (NWFZ), west of the Wellsville, Malad, and Bannock Ranges and eastward in Cache Valley. Analysis of older gravity data and 2,716 new measurements in northern Utah and southern Idaho: 1) delineates major buried intrabasinal faults with seismic potential in SW and N Cache Valley; 2) shows that the E-dipping West Cache fault zone (WCFZ) produced hangingwall basins comparable to those above master faults of the W-dipping East Cache fault zone (ECFZ) and NWFZ; and 3) highlights > 65% overlap between three right-stepping en echelon faults of the WCFZ. Inversion of the isostatic residual gravity map for basement depth was guided by seismic-reflection lines and geologic sections constrained by oil-well data, and by removal of gravity effects within basement. Gravity gradients suggest that faults in the WCFZ are steeper than those of the ECFZ north of 41^o 45'. A basin 2 - 3 km deep extends the terminus of the NWFZ ~10 km NW from its traditional end near Malad City, ID, to a point due west of Elkhorn Peak, ID. Subbasins in Cache Valley are at least as wide and as deep as those in the hangingwall of the NWFZ. Late Quaternary slip rates across the Brigham City segment of the NWFZ exceed those across adjacent parts of the WCFZ and ECFZ by factors of 10, yet the subbasins have comparable depths. Thus, displacements across the NWFZ, WCFZ, and ECFZ in the past likely were episodic, with similar long-term slip rates. Buried transverse bedrock ridges intersect most segment boundaries of the NWFZ, but intersect few of the WCFZ and ECFZ. One ridge is near the south termini of the Dayton-Oxford and Lewiston faults within Cache Valley. The W-dipping Basin-and-Range faults mainly strike N, yet deeper parts of the subbasins trend NNW. This NNW trend may reflect older faults related to WSW slip of the Bannock detachment system (BDS) between ~10.3 - 4 Ma. This extends the known influence of the BDS > 15 km into Utah. A drop in isostatic gravity of 25 mGals along the southern and central segments of the ECFZ confirms a large normal fault with ~ 4.5 km of throw. This throw decreases abruptly along the northern Utah and Idaho segments of the ECFZ, and steps 9 - 12 km W to the concealed Lewiston fault. The latter is 26 km long, and forms a step of 2 - 3 km in the basement.