CRUSTAL EXHUMATION IN A TRANSPRESSIONAL SETTING, THE SAN EMIGDIO MOUNTAINS, CALIFORNIA

The San Emigdio Mountains, in the western Transverse Ranges of California, are an apparent example of an archetypical, transpressional structural system, bounded to the south by the San Andreas strike-slip fault, and to the north by the active Pleito-Wheeler Ridge-White Wolf thrust system. Young (~4-6 Ma) apatite (U-Th)/He ages obtained in the southern portion of the range adjacent to the San Andreas fault, however, are difficult to reconcile with existing structural models of the San Emigdio Mountains as a thin-skinned thrust system. Restoration of apatite (U-Th)/He ages with depth below a well exposed regional unconformity indicates that plutonic rocks in the San Emigdios represent a continuous crustal section, as much as 10 km in total thickness, that has been exhumed along the San Andreas fault system.

The (U-Th)/He ages indicate an exhumation rate of ~0.4 mm/yr for the late Miocene and early Pliocene, with continued uplift after ~4 Ma. The proximity of this exhumed section to the San Andreas fault is consistent with previous observations of crustal exhumation along steeply dipping strike-slip faults, including other portions of the San Andreas system (Spotila et al. 1998) and along the San Gregorio-Hosgri fault system (House et al., 2000), but is significantly greater in scale. Apatite (U-Th)/He ages from the Mt. Pinos region, on the opposite side of the San Andreas from the San Emigdio Mountains, yield early to mid-Miocene ages, and thus no evidence of significant crustal exhumation. Rheologic heterogeneities and pre-existing structural fabrics are two potential causes for the localization of such extreme crustal exhumation in this transpressional system.