UPLIFT AND EXHUMATION OF THE CENTRAL TEHACHAPI MOUNTAINS DERIVED FROM JOINT APATITE (U-TH)/HE AND FISSION-TRACK ANALYSES

New apatite (U-Th)/He (He) and fission track (FT) analyses were obtained from six samples along a NW-SE transect of the central Tehachapi Mountains (Tejon Ranch Conservancy) in an effort to better constrain the late-stage uplift and exhumation of the Tehachapi Mountains. He and FT analyses are commonly used to date the timing of exhumation at shallow crustal levels, interpret the structural evolution of orogenic belts, and they are particularly useful when paired. In the central Tehachapi Mountains, the active left-lateral Garlock fault lies ~4 km to the north of the southern mountain front, and is subparallel to the overall trend of the Tehachapi Mountains. Four samples were collected from north of the Garlock fault and 2 from the south. He ages obtained from 5 samples in this study ranged from 8.9 to >70 Ma, with the 2 youngest ages (8.9 and 12.2 Ma) occurring in the highest elevation samples just to the north of the active Garlock fault. Apatite FT analyses of these two samples yielded significantly older ages of 32 and 65 Ma. In a previous study 20 km to the east, Blythe and Longinotti (2013) obtained similar (9-13 Ma) He ages from the highest peaks, but significantly younger (11 - 20 Ma) apatite FT ages. They interpreted those data to indicate two phases of cooling related to uplift and exhumation: the earlier phase ended at ~70 Ma, and a more recent phase began at ~20 Ma and may have accelerated at ~10 Ma, as the Garlock fault was activated. These new data from the central Tehachapi mountains are different from those to the east in 2 respects: 1) there is no indication of a cooling event beginning at ~20 Ma, and 2) the total amount of cooling and exhumation from ~13 Ma to the present is less than that seen to the east. These new data should provide new insights into the overall tectonic evolution of the Tehachapi Mountains. Thermal modelling using HeFTy is underway to better constrain the uplift and exhumational history of this region.