THE GEOLOGICAL SOCIETY OF AMERICA
New geologic mapping, tectonic geomorphologic, and cosmogenic radionuclide geochronologic data provide the fi rst numerical constraints on late Quaternary vertical slip and horizontal extension rates across the southern Sierra Nevada frontal fault zone, California. This fault zone exposes numerous NNW-striking, east-facing normal fault scarps that offset seven distinct Quaternary alluvial surfaces and a rockslide deposit. Beryllium-10 cosmogenic radionuclide surface-exposure dating of these
... dating of these surfaces provides surface abandonment model ages of 123.7 ± 16.6 ka, 60.9 ± 6.6 ka, 25.8 ± 7.5 ka, 4.4 ± 1.1 ka, and 4.1 ± 1.0 ka on alluvial fan surfaces, and 18.7 ± 3.9 ka on the rockslide deposit. These age constraints, combined with measurements of vertical surface offset across fault scarps, yield preferred late Pleistocene to Holocene vertical and horizontal extensional slip rates of 0.2-0.3 mm/yr and 0.1-0.2 mm/yr. Vertical slip-rate estimates in this study are comparable to late Pleistocene vertical slip-rate estimates across other prominent range-front normal faults within the Basin and Range Province. These geologic and geochronologic results indicate that the eastern escarpment of the southern Sierra Nevada has remained tectonically active throughout the late Quaternary. Combining our data with slip data from the Owens Valley and Lone Pine faults implies that slip along the eastern escarpment of the Sierra Nevada block is partitioned into three components: dominantly dextral slip along the Owens Valley fault, intermediate oblique slip along the Lone Pine fault, and subordinate normal slip along the Sierra Nevada frontal fault zone. These observations are consistent with global positioning system (GPS) data, which indicate that dextral shear-strain accumulation dominates today along the western boundary of the Eastern California Shear Zone and Basin and Range Province.