Three-dimensional variations in extensional fault shape and basin form: The Cache Valley basin, eastern Basin and Range province, United States
Seismic reflection profiles, drill-hole data, and geologic maps delimit the form of normal faults and Tertiary sedimentary rocks in the southern half of the Cache Valley basin in northern Utah. Dips of faults and sedimentary rocks were estimated from time-migrated reflection profiles by using the stacking velocities for the data. At the southern end of the basin, the East Cache fault zone is listric; it shows 50°W dips near the surface and ≈20°W dips at depth. The fault zone has been the site of ≈5.6 km of net dip slip. Tertiary rocks dip 18°E–25°E and exhibit a rollover geometry above the fault zone. No faults are interpreted on the western side of the basin. In the central part of the basin in Utah, the East Cache fault is a single fault that dips at least 45°W near the surface and is the site of 4.5–6.4 km of net dip slip. Here, the basin is bounded to the west by the West Cache fault, which is the site of at least 1 km of net slip that increases northward to 2 km of net slip. Slip on the East Cache fault resulted in planar, east-dipping, older Tertiary rocks near the bottom of the basin. Younger Tertiary strata, with southwest, west, and northwest dips, reflect complex tilting due to slip on the West and East Cache faults. Anticlines in the Tertiary basin-fill deposits are present in the central part of the basin and may reflect changes in normal-fault geometry at depth. Northward, dip slip on the East Cache fault zone decreases to 2.5 km. The basin is broad, shallow, and filled with nearly flat lying Tertiary rocks. This area, near the north-south midpoint of the basin, is bounded by the West and East Cache faults, but slip on the West Cache fault appears to diminish northward. A north-trending reflection profile tied to both drill-hole data and the east-trending seismic profiles indicates that the basin is deeper in the southern end. The along-strike changes in fault geometry, the amount of fault-slip, the subsurface form of the basin-filling sedimentary rocks, and the form of the basin indicate a complex history of faulting and deposition during its formation. This study and other recent ones from the Basin and Range province indicate that such complexities may be typical of many Tertiary basins in the region.