POST 20 MA EXHUMATION OF THE SOUTHERN SIERRA NEVADA/ TEHACHAPI MOUNTAINS, FROM FISSION-TRACK AND (U-TH)/HE ANALYSES

Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He (He) analyses of nine samples from a NW-SE transect across the southern Sierra Nevada/Tehachapi Mountains reveal two major stages of exhumation over the past 70 million years. The data document the cooling history of the transect from ~250 to 40°C in a region affected by three active faults: the White Wolf thrust fault in the northwest, and the left-lateral Garlock fault, as well as a relatively unimportant (?) down-to-the-south normal 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 progressively younger ages of 68 - 19 Ma. The four samples from the southeastern part of the transect yielded ZFT ages of 52 - 46 Ma, AFT ages of 20 - 18 Ma, and He ages of 16 - 8 Ma. Thermal models were obtained from these data (which included AFT 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 m.y.a., during the waning stages of the Sierra Nevadan orogeny, and the second from 20 m.y.a. 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, leading us to suggest that the Garlock fault may have initially been a normal fault.