INTERACTION BETWEEN THE NORTHERN BASIN AND RANGE AND THE YELLOWSTONE STRESS FIELDS NEAR THE RED ROCK FAULT, SOUTHWESTERN MONTANA

The N45W-trending Red Rock fault (RRF) bounds the Tendoy Range and is typical of active faults in the northern Basin and Range where NE-SW extension is primarily accommodated by earthquakes like the Ms=7.3, 1983 Borah Peak, Idaho earthquake. Trench and geomorphic data show that the 15-km-long Sheep Creeks segment (SCS) of the RRF experienced both mid-Holocene and late Pleistocene surface ruptures whereas surface rupture on the 11-km-long Timber Butte segment (TBS) last occurred during the late Pleistocene. Late Holocene fans adjacent to the SCS are preferentially incised into older fan heads, emerge lower down on fan surfaces, and many trend NNE, oblique to the RRF. The preferred fault-plane solutions (FPS) for the Mb=5.3, 20 August 1999 Red Rock Valley earthquake (108, 55, 95) and its largest aftershock indicate NNE-SSW extension on a WNW-trending normal fault beneath the Red Rock River basin. This extension direction matches the NNE-trend of many late Holocene fans, suggesting northward tilting of the RRF hanging wall occurred during the mid-Holocene event. Nine of 20 well constrained FPS near the Red Rock River basin exhibit NE-SW extension and 9 others exhibit NNE-SSW extension. NNE-SSW extension and northward tilting are both consistent with the Yellowstone stress field (YSF) which increasingly influences seismicity and neotectonics eastward through the Centennial Valley to the Hebgen Lake region. We propose that northern Basin-and-Range extension produces large, infrequent earthquakes that rupture the combined 26-km length of the active SCS and TBS of the RRF, whereas YSF-related earthquakes rupture more westerly striking, sub-basin faults and induce surface rupture only on the SCS. Thus the interaction of the YSF and the stress field of the northern Basin and Range explains (1) differential slip rates both on active segments of the RRF and on other nearby faults, (2) northward tilting of the SCS hanging wall during the mid-Holocene event, and (3) deeper, fault-bounded, sediment-filled, sub-surface troughs beneath the Red Rock River basin as revealed by gravity data. An inferred N-trending, sub-basin fault intersects the RRF at the SCS-TBS boundary at Little Water Canyon and accommodates differential displacement between segments.