QUATERNARY EXHUMATION RATE CENTRAL PANAMINT RANGE, CALIFORNIA FROM U-PB ZIRCON AGES

Seven samples from four tuffaceous layers ranging from 5 cm to 3 m thick were collected from an 180 m alluvial section near Ballarat and dated using the SHRIMP-RG. The alluvial section underlies a late Pleistocene abrasion surface and is faulted at its base near the Panamint Range front where it apparently both structurally overlies and is truncated by one of the major range bounding faults along an east step in the Panamint fault zone. One sample collected in the lower part of the alluvial section contains abundant Pleistocene zircons ranging in age from 0.6 to 1.1 Ma with a statistically significant weighted mean age of 0.9 Ma. These ages overlap with the Long Valley magmatic events and provide a minimum age for the deposition for the section. They also could reflect contribution from tephra units such as Upper Glass Mountain tuffs. A minimum long term vertical slip rate of ~ 0.345 mm/yr for the Panamint Valley fault zone can be estimated from the separation of dated horizons in the alluvial section and previous work on 36Cl dating of salt horizons from nearby core (Jannik and others, 1991). Detrital zircon populations indicate that tuffs interbedded in alluvial deposits in Panamint Valley are Pleistocene age and include reworked clasts from Precambrian basement, Mesozoic plutons, and Tertiary volcanics. The majority of zircons appear to be mixture of reworked sediment derived from ~1.3 Ga Precambrian basement rocks, plutons from the Panamints ranging in age between ~154 and 80 Ma, Miocene and Pleistocene volcanic sources. Some samples include smaller sub-populations of Permian and Triassic aged zircons. Zircons of Miocene age range from ~ 6 to 35 Ma and may have been derived from units such as the Immigrant Canyon Tuff, ~8 Ma and the Coso Ash, ~6 Ma. Extensive Tertiary volcanics in the southern Panamints could also be a source, however Tertiary volcanics are not present in the watersheds that presently feed this alluvial deposit suggesting material has been denuded from the range. Preliminary results suggest an inverse age relation of reworked Tertiary zircons within the stratigraphic section, with younger tuffs in the alluvial section dominanted by older reworked Tertiary zircons.