PRELIMINARY PALEOSEISMOLOGY OF THE STEENS FAULT ZONE, BOG HOT VALLEY, NEVADA

The 200-km-long Steens fault zone forms the most topographically prominent fault-bounded escarpment in the Basin and Range province of southeastern Oregon and northern Nevada. The down-to-the-east normal fault is marked by Holocene fault scarps along the southern one-third of the fault from the vicinity of Pueblo Mountain to the southwestern margin of Bog Hot Valley near Denio, Nevada. We excavated a trench across a right-stepping fault zone that extends south from the southern end of the Pueblo Mountains and traverses the floor of Bog Hot Valley. This site was chosen because of the presence of well-preserved fault scarps on lacustrine deposits thought to be suitable for luminescence dating, and the proximity of two nearby geodetic stations that straddle the fault zone.

The trench was at an elevation of 1292 m, and exposed an east-dipping fault zone and an adjacent graben in well-stratified lacustrine silt, sand, and gravel. The site is about midway between two paleo-shorelines (1310 m and 1280 m) of pluvial Lake Alvord, which during the last-glacial maximum extended 120 km from the northern end of the Alvord Desert to the southern end of Bog Hot Valley. The late Quaternary history of Lake Alvord is poorly known and is complicated by the occurrence of at least one overflow event that may have lowered the lake level from the 1310 m shoreline to the 1280 m shoreline sometime in the late Quaternary. Luminescence ages on the lacustrine deposits are pending, but our limited soils data (maximum stage I-II Bk horizon development) are more consistent with a latest Quaternary (Lahontan Sehoo-equivalent) age than with correlation to an older lacustrine cycle.

The trench exposed distinct fault-scarp colluvial wedges and intervening soils that are clear stratigraphic and structural evidence of three post-lake surface-faulting earthquakes. Fault reconstructions indicate that these earthquakes, from oldest to youngest, were accompanied by about 1.1 m, 1.1 m, and 2.2 m of vertical displacement. Radiocarbon and luminescence ages indicate the youngest earthquake occurred shortly before 4 ka; ages of the previous two earthquakes are pending, but must have occurred in the interval between 4 ka and desiccation of Lake Alvord below the elevation of the trench site.