PRELIMINARY RESULTS OF A TRENCH INVESTIGATION OF THE SANTA ROSA RANGE FAULT ZONE, HUMBOLDT COUNTY, NEVADA

In June 2001, we excavated trenches at two sites near Orovada, Nevada on the Santa Rosa Range fault zone (SRRFZ), one of the most prominent normal fault systems in the northern Basin and Range province. A trench on a small (0.5-m-high) piedmont fault scarp 7.5 km SW of Orovada that offsets shorelines of the youngest (Sehoo) highstand of Lake Lahontan collapsed during excavation and had to be abandoned. However, geomorphic relations clearly indicate the youngest event on this trace post-dates the 13-ka age of the shoreline. A second trench was excavated on an 8.5-m-high scarp on the range front fault, 5 km SE of Orovada. The latter trench site is located on the western of two strands that form a major left step in the range front. The trench exposed a thick sequence of silty colluvium faulted against alluvium in a large fan that emanates from McConnell Creek. We interpret at least four surface-rupturing earthquakes on this strand of the SRRFZ since deposition of the McConnell Creek fan. All four colluvial wedges exposed in the trench are 1-1.5 m thick; abundant eolian sediment is present in all colluvial deposits, so event displacements are probably 1-2 m. Topographic profiles indicate about 8 m of surface offset across the scarp.

The age of the faulted fan deposits is unknown, but a pit excavated in the upthrown block exposed two packages of loess and an intervening stage II Bk soil horizon, directly overlying slightly weathered fan sediment. Our preferred interpretation is that the fan is related to a pre-Sehoo (Eetza) highstand that was subsequently buried by loess deposits following desiccation of the Eetza and Sehoo lake levels. If this scenario is correct, then the buried fan surface formed at least 130 ka, the approximate age of the youngest prominent Eetza shoreline in the region. Pending luminescence ages will provide age estimates of the loess deposits overlying the fan sediment. Our scarp-profile data indicate total vertical offset of about 8 m on the western (trenched) strand, and another 3 m on the eastern (untrenched) strand, 1.5 km east of the trench site at the mouth of McConnell Creek. If the fan sediments are at least 130 ka, then the combined long-term average slip rate across both range-front traces is <0.1 mm/yr.