PALEOSEISMOLOGY OF THE CLAN ALPINE FAULT, WEST-CENTRAL NEVADA

As part of a Basin and Range (B&R) paleoseismic transect, we are studying major extensional Quaternary faults from Salt Lake City to Reno between latitudes 39° and 41° N. We recently trenched the southern part of the Clan Alpine fault (CAF) about 5 km NW of Cold Springs Station. The fault bounds the east margin of the Clan Alpine Mountains, which has an impressive front that suggests recent tectonism. A slip rate of 0.15 mm/yr has been reported for the CAF based on scarp height/age estimates. Conversely, conspicuous fault scarps mark only a fraction of the fault.

Although most of the CAF is in a BLM Wilderness Study Area, we trenched a 7- to 8-m high partly buried scarp on the main fault and a largely buried (<3-m-high) antithetic fault scarp 1.6 km downslope. Exposures of the main fault revealed two colluvial wedges above alluvial-fan deposits that we estimate to be about 130 ka. The faulting produced ca. 4 m offset (5-6 m scarp) on a nearby same-age alluvial fan. The penultimate fault-scarp colluvium gave a preliminary luminescence (OSL) age of 31.1±2.3 ka, which suggests that the penultimate event was about 30-35 ka. Indirect evidence of older faulting is recorded by >7 m of surface offset in the next older sequence of alluvial-fan deposits (about 250 ka).

The antithetic fault shows direct evidence for 3 faulting events (about 6 m offset). The penultimate fault-scarp colluvium gave a preliminary OSL age of 27.3±1.9 ka, which suggests that the penultimate event was ca. 25-30 ka. The upthrown block has relict Bt/Bk soil horizons (100¿ kyr) in debris-flow deposits that directly overly fan deposits, which we estimate to be about 130 ka. The antithetic fault records more events than the main fault; thus it may be a transitional, intravalley link between the east-dipping CAF and the west-dipping Desatoya fault.

Although cosmogenic exposure dating will help determine the age of faulted landforms, we suspect that the CAF has a low slip rate (0.03-0.05 mm/yr) and long recurrence intervals (>10 kyr). Thus, the range front's expression may be an artifact of lithology and/or faster slip earlier in the Quaternary or Pliocene. The much lower slip rate suggested here for the CAF has important implications for the general activity rate of many of the major normal faults in the B&R province.