NEOTECTONICS ALONG THE RED ROCK FAULT, SW MONTANA

The Red Rock Fault, an East dipping range front normal fault in SW Montana, displays significant youthful offset (<4 ka) and marked segmentation. Characterization and radiocarbon age calibration of pedogenic carbonate development stage in soils within offset footwall fans and terraces surfaces provide relative ages of recent offset at a number of locations along strike. Mapping of these footwall surfaces and their hanging wall fan equivalents provide a holistic view of recent offset history. Within the southernmost of three recognized segments, a 4 ka offset fan surface at the mouth of Chute Canyon is consistent with previous findings including a trenching study at the mouth of Little Sheep Creek (~5 km south of Chute), which indicated ~3 ka most recent offset. However, at Garr Canyon (3 km north of Chute) similarly aged fans show no offset and a much older fan surface, dated at 10.5 ka, is offset at the mouth of Big Sheep Creek (6 km north of Chute), suggesting the activity of a separate central segment. Within the northern segment, a relatively old offset fan is prominent at the mouth of Dry Canyon while multiple generations of younger (dated at ~3.5 ka) fans show no offset. This relationship is observed at several other localities in the north segment, confirming older ages of the last surficial rupture. Preliminary topographic analysis confirms the proposed concentration of youthful activity in the southern segment. The observed along strike activity gradient is likely related to ongoing tectonism associated with the thermal uplift, incipient crustal loading, and subsidence following the passage of the Yellowstone hot spot, as the southern extent of the Red Rock fault lies only 30 km north of the Snake River Plain.