PRELIMINARY PALEOSEISMOLOGY OF THE SANTA ROSA RANGE FAULT ZONE, HUMBOLDT COUNTY, NEVADA

The Santa Rosa Range fault zone (SRRFZ) bounds the western margin of the Santa Rosa Range and the eastern margin of the Quinn River Valley in the Basin and Range province of northern NV and southeastern OR. We excavated a trench across an 8.5-m-high scarp on the westernmost of two strands that form a 1.5-km-wide left step in the range front, 5 km southeast of Orovada, NV. The scarp crosses an alluvial-fan complex that emanates from several canyons along this part of the SRRFZ. Luminescence dating indicates that the faulted fan is at least as old as the Eetza cycle (MIS 6) of pluvial Lake Lahontan.

Scarp profiling and luminescence dating were used to determine a long-term slip rate at the Orovada trench site. Topographic profiles indicate about 7 m of surface offset across the trenched scarp. An older untrenched scarp just uphill from the trench has about 4 m of surface offset, and the eastern range-front strand, located east of the trench site at the mouth of McConnell Creek, offsets the same fan surface about 3 m. Luminescence dating of loess deposits overlying the fan alluvium exposed in a soil pit in the footwall yielded ages of 140-160 ka, so the combined long-term average slip rate across both range-front traces at the latitude of the trench site is no more than 0.1 mm/yr.

We used the presence of multiple colluvial wedges and intervening buried soils to infer at least four surface-rupturing earthquakes on this strand of the SRRFZ since fan deposition. The trench exposed a thick sequence of silty colluvial deposits and buried soils faulted against fan alluvium. Luminescence dating of the colluvial sediments yielded the following preliminary ages for these events: most-recent event--7-11 ka; penultimate event--90-110 ka; third event--130-140 ka; fourth event-->140 ka. The trench was not deep enough to expose fan deposits in the hanging wall so we cannot rule out the possibility of additional faulting events, but nearby stream exposures indicate we probably exposed nearly all of the post-fan sediment in the hanging wall. We used the thickness of colluvial wedges (1-1.5 m) to estimate displacements of 1-2 m/event.