Paleoseismology of the Clan Alpine Fault
Michael Machette, Kathleen Haller, Koji Okumura, Cal Ruleman, and Shannon Mahan
Comparison of time of faulting, GPS velocity data, and fault slip rates for the area adjacent to U.S. Highway 50 (track of GPS network).
As part of a transect to study the geologically young activity of faults across the Basin and Range (B&R) province (Fig. 1), we are studying major extensional young faults from Salt Lake City, Utah, to Reno, Nevada, between 39° and 41° N.
In September 2001, we initiated a study of the southern part of the Clan Alpine fault (CAF), which is about 5 km NW of Cold Springs Station, Nevada, on U.S. Highway 50. The fault bounds the Clan Alpine Mountains on the west and the Edwards Creek Valley on the east (Fig. 2). Preliminary geological studies suggested the fault has a slip rate of 0.15 mm/yr; the fault creates an impressive range front, although conspicuous fault scarps mark only a small portion of the entire range front. To evaluate the times of recent surface breaks on the fault, we excavated a trench across a 7-8 m high partly buried scarp (Fig. 3) on the main fault and a largely buried (<3 m high) antithetic fault scarp 1.6 km toward the center of the basin. The sedimentary layers exposed in the trenches revealed evidence for four faulting events on the main fault since deposition of the alluvial-fan sediments in the trench. We estimate that these sediments are about 130,000-250,000 years old. Luminescence dating of fault-scarp colluvium gave ages of 9,300±7000 years and 27,600±2,600 years (Fig. 4), which suggests that the most recent earthquake occurred at about 10,000 years ago and the penultimate earthquake occurred at about 30,000 years ago.
Indirect evidence of two older faulting events is recorded in old alluvial-fan deposits that are thought to have an age of 130,000 to 250,000 years. The antithetic (basinward) fault shows direct evidence for three surface-faulting earthquakes (about 6 m of cumulative vertical displacement).
Simplified map of south wall of the western trench (CAW) across the Clan Alpine fault, west-central Nevada
The fault-scarp colluvium gave luminescence ages of 10,000±1000 and 28,000±2000, which suggests that most recent event occurred at about 10,000 years ago and the penultimate event occurred at about 30,000 years ago. The times of these events is very similar to those recorded in the trench across the main scarp. Sediments on the upthrown block have a well-developed soil (estimated to be 100,000 years old) that is formed in coarse debris-flow deposits that directly overly distal fan (or lake) deposits, which we estimate to be 130,000 years old. The antithetic fault records more events in the past 130,000 years than the larger, main fault; thus, the antithetic fault may be a transitional, intravalley link between the southern part of the east-dipping CAF and the northern part of the west-dipping Desatoya fault that forms the eastern margin of the Edwards Creek Valley.
We suspect that the CAF has both a low slip rate (0.03-0.08 mm/yr) and a long average recurrence interval (equal to or greater than 20,000 years). Thus, the range front's prominent expression may be an artifact of resistant rock being exposed in the uplifted mountains and/or a remnant of when the fault had a faster slip rate, possibly in Quaternary or Pliocene time. Our study indicates that the CAF has a slip rate that is 2-5 times lower than previously reported; this conclusion has important implications for the general activity rate of many of the major normal faults in the B&R province and the Central Nevada Seismic Belt (Fig. 2), which lies just to the west of the Clan Alpine Mountains. Detailed geological studies such as the one described here provide the best information on recent fault activity, earthquake recurrence, and slip rates.
Machette, M.N. Haller, K.M., Okumura, K., Ruleman, C.A., Debray, S., and Mahan, S., 2002 Preliminary paleoseismic history of the Clan Alpine fault, west-central Nevada Geological Society of America Abstracts with Program, v. 34, no. 4, p. A-3
Machette, M.N. Haller, K.M., Ruleman, C.A., Mahan, S., and Okumura, K., 2005 Evidence for late Quaternary movement on the Clan Alpine fault, west-central Nevada Trench logs, location maps, and sample and soil descriptions U.S. Geological Survey Scientific Investigations Map 2189 - http://pubs.usgs.gov/sim/2005/2891/