DEXTRAL DISPLACEMENT ON THE HONEY LAKE FAULT ZONE, NORTHERN WALKER LANE, NORTHEAST CALIFORNIA AND WESTERNMOST NEVADA: PRELIMINARY CONSTRAINTS INFERRED FROM OLIGOCENE ASH-FLOW TUFF STRATIGRAPHY

GPS geodetic measurements indicate that the northern Walker Lane (NWL) accommodates ~20% of relative motion between the North American and Pacific plates. The Honey Lake fault zone (HLFZ) is one of four major, NW-striking, right-lateral faults comprising the NWL. Regional relations suggest that this strike-slip fault system is younger than ~5 Ma. The Holocene slip rate along the HLFZ has been estimated at 1.1 to 2.6 mm/yr (Wills and Borchardt, 1993), but cumulative slip has not been previously constrained. Multiple late Oligocene ash-flow tuffs were erupted from calderas in central Nevada and deposited in paleovalleys in western Nevada and northeast California. These paleovalleys provide potential piercing lines by which to gauge offset across the strike-slip faults in the NWL.

Ash-flow tuffs were examined in the Diamond and Fort Sage Mts, which lie on the west and east sides of the HLFZ, respectively. Five ash-flow tuffs, ranging in age from 31.1 to 25.1 Ma and totaling ~245 m in thickness, were distinguished in a paleovalley in the Black Mountain area of the Diamond Mountains. Three of these tuffs, totaling ~200 m in thickness, also occupy a paleovalley in the Fort Sage Mts. Based on the similar sequences of tuffs, the paleovalleys in the Diamond and Fort Sage Mts are, permissibly, offset segments of a once continuous late Oligocene topographic depression. On the basis of remanent magnetizations from the 25.1 Ma Nine Hill Tuff (NHT), vertical-axis rotations appear to be negligible, at least on the southwest side of the HLFZ. Remanent magnetizations from the NHT in the Diamond Mountains (2 sites, 16 samples, D=338.9°, I=50.1°, a95=5.7°) overlap the reference direction for the NHT (D=335.1°, I=57.5°; Deino, 1989) at the 95% confidence level. Estimates of offset on the HLFZ hinge upon the overall orientation of this paleovalley. Regional relations suggest a W to WNW trend, whereas tuff exposures in the Diamond Mountains locally imply a more northerly trend. If N-trending, ~50 km of dextral offset are required on the HLFZ, whereas a more easterly trend implies far less offset (~10-15 km). The smaller estimate is more compatible with Quaternary slip rates and inferred timing of strike-slip faulting. Anisotropy of magnetic susceptibility in the tuffs is being evaluated to better define the overall orientation of the paleovalley.