Slip rate of the West Tahoe Fault and the age of glacial deposits near Lake Tahoe, California

Abstract

The West Tahoe Fault is the 45 km long, primary range-bounding fault of the Sierra Nevada at the latitude of Lake Tahoe, California. It is a N-NW striking, east dipping normal fault that has a pronounced terrestrial Holocene scarp extending from near Meyers, CA to Emerald Bay, where it continues offshore along the western margin of the lake until it approaches Tahoe City. The West Tahoe Fault displaces late Pleistocene moraines and glacial deposits along much of its extent. Terrestrial Cosmogenic Nuclide (TCN) surface exposure ages of sixteen boulders were collected from faulted moraines at Cascade Lake. Eight ages from the last-glacial Tioga moraine range from 13.9 to 23.4 ka, and seven of those ages average 21.2 ± 1.2 ka, similar to Tioga moraines elsewhere in the Sierra Nevada. The eight ages from the penultimate Tahoe moraine are scattered from 14.5 to 120.5 ka. Treatment of the oldest measured age of 120.5 ± 11.5 ka as the minimum age of deposition of the older Tahoe moraine is based on the assumption that morainal boulders contain little cosmogenic inheritance and scatter is largely the result of boulder surface erosion and exhumation, as well as the conclusions of other glacial studies regarding the ~140 ka timing of the Tahoe glaciation elsewhere in the Sierra Nevada. Vertical displacements measured from lidar on the crests of the Tioga and Tahoe moraines are 32 ± 12 and 59 ± 10 m, respectively. Dividing these displacements by the ages of the moraines, gives maximum vertical separation rates of 1.5 ± 0.7 and 0.5 ± 0.1 mm/yr since emplacement of the Tioga and Tahoe moraines respectively. At face value this implies a rapid increase in slip rate with time, but because nowhere else in this region is found evidence for rapid changes in slip-rates during the late Pleistocene, and as the post-Tahoe rate is more similar to previous estimates, it is suspected that this post-Tioga rate is an apparent rather than real rate, and is likely an artifact of morainal deposition over a pre-existing fault scarp. Dividing the post-Tahoe vertical separation rate by the sine of dips ranging from 50° to 70° results in a slip rate of 0.6 ± 0.2 mm/yr. Dividing an average surface slip of 1.3 m (based on the empirical relation between fault length and coseismic surface slip in Wesnousky, 2008) by this slip rate allows for an average return time for ~M7 earthquakes of ~2-4 ka on the West Tahoe Fault.