POST 10 MA TILTING AND EXHUMATION OF THE SOUTHERN SIERRA NEVADA/TEHACHAPI MOUNTAINS FROM APATITE (U-TH/HE) ANALYSES

Nine new apatite (U-Th)/He ages for samples from the southernmost Sierra Nevada/Tehachapi Mountains reveal late Cenozoic exhumation and tilting. The samples were collected along a northwest-trending transect, traversing a block bounded to the north by the White Wolf blind thrust fault and to the south by the Garlock left-lateral strike-slip fault. Both faults are active. He ages range from 66 Ma to 7.4 Ma, with the samples located farthest to the northwest yielding the oldest ages. He ages become increasingly younger to the south, as the sample elevation increases. This age/elevation relationship is consistent with progressively increasing amounts of exhumation (and surface uplift) to the south. In a previous study (Khalsa and Blythe, 1999), these same samples were analyzed using apatite fission-track thermochronometry. Apatite fission track ages ranged from 72 to 16 Ma, and were consistently older than the corresponding He age. He and fission track age patterns, as well as apatite fission-track length modeling, are best explained by northward tilting of the fault-bounded block, with greater exhumation of the higher elevation southern samples, those closest to the Garlock fault. These new apatite (U-Th)/He ages are best interpreted as indicating significant vertical motion (1-2 km) along the Garlock fault in the last 10 Myr. We are in the process of collecting zircon fission track analyses from these samples to further constrain the exhumation history of the area.