"STRESS SWITCHING" ALONG THE LIMA RESERVOIR FAULT IN YELLOWSTONE'S WAKE

A zone of high seismicity, within the Centennial Tectonic Belt that forms the northern wake of the Yellowstone hotspot, is characterized by focal mechanisms with N-S-extension, NE-SW-extension, and NE-trending right-lateral strike-slip faults (also with N-S-extension). It extends from near the Madison fault at Madison Canyon southwestward across the Centennial Valley past the E-terminus of the Lima Reservoir fault and the W-terminus of the Centennial fault. A sequence of slip-vectors for eight paleo-surface-ruptures during the last 45 ka was identified in new trenches across the Lima Reservoir fault zone near its E-terminus. This sequence of eight events shows that regional “stress switching” plays a role in the transition from N-S-extension related to the Yellowstone hotspot to NE-SW-extension related to the northern Basin and Range in Yellowstone’s wake. Older normal-faulting events with N-S-extension (6-8) are consistent with extension related to the Yellowstone hotspot. “Stress switching” begins about 12-10 ka with oblique-reverse-slip events (5 and 3) which are related to the Centennial shear zone (CSZ), but which are also consistent with N-S-hotspot-extension. “Stress switching” also occurs with onset of Basin and Range normal faulting (events 4, 2, and 1), exhibiting NE-SW-extension. Because some sections of the Lima Reservoir fault zone have a NE-trend, they also exhibit right-lateral slip related to the CSZ during Basin and Range extension on NW-trending sections of the fault zone. The recurrence interval for normal faulting events with N-S-extension appears to be 10-15,000 years whereas that for normal faulting events with NE-SW-extension is about 1-3,000 years.