© 2006 Geological Society of America

New geologic mapping, tectonic geomor-phologic, and cosmogenic radionuclide geo-chronologic 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 numer-ous NNW-striking, east-facing normal fault scarps that offset seven distinct Quaternary alluvial surfaces and a rockslide deposit. Beryllium-10 cosmogenic radionuclide sur-face-exposure dating of these surfaces pro-vides 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 Pleis-tocene 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 Pleisto-cene vertical slip-rate estimates across other prominent range-front normal faults within the Basin and Range Province. These geo-logic 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 Val-ley 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 indi-cate that dextral shear-strain accumulation dominates today along the western bound-ary of the Eastern California Shear Zone an