TWO-STAGE EXHUMATION OF THE SOUTHERN SIERRA NEVADA/ TEHACHAPI MOUNTAINS FROM FISSION-TRACK AND (U-TH)/HE ANALYSES

New zircon and apatite fission track (FT) and (U-Th)/He analyses of nine samples from a NW-SE transect across the southern Sierra Nevada/Tehachapi Mountains reveal two major stages of cooling and exhumation over the past 70 million years. The thermochronometric methods used document the cooling history of samples from ~250 to 40°C in a region that is affected by three active faults: the White Wolf thrust fault in the northwest, and a down-to-the-south normal fault, as well as the Garlock fault, a major left-lateral strike-slip fault, in the southeast. In the northwestern part of the transect, three samples yielded ages of ~70 to 50 Ma for all three methods. Two samples from the central part of the transect yielded similar zircon and apatite FT ages and younger apatite He ages of 33 and 19 Ma. The four samples from the southeastern part of the transect yielded zircon FT ages of 52 to 46 Ma, apatite FT ages of 20 to 18 Ma, and apatite (U-Th)/He ages ranging from 16 to 8 Ma. Thermal models were obtained from these data (which included fission track lengths) using HeFTy (Ketcham et al., 2005). The thermal models are consistent with two major stages of cooling and exhumation, the first from 70 to 50 million years ago, during the waning stages of the Sierra Nevadan orogeny, and the second from 20 million years ago to the present, during the initiation of extension in the Basin and Range. The second event is more strongly evident in data from the southeastern part of the range, indicating that the range has tilted to the northwest, and that substantially more exhumation has occurred in the southeast (3-5 km) than in the northwest (~1 km) over the last 20 million years. The initiation of the second event at 20 Ma is coincident with previous estimates for the inception of the Garlock fault but the amount of exhumation that has occurred is more consistent with that expected from the footwall of a major normal fault. Based on these data, we therefore suggest that extension began on the normal fault on the southeastern side of the Southern Sierra Nevada/Tehachapi mountains at 20 Ma.