Low-temperature thermochronology of the San Gabriel and San Bernardino Mountains, southern California: Constraining structural evolution

Recently, apatite fission-track and (U-Th)/He analyses have been used to constrain the low temperature thermal history of the San Gabriel and San Bernardino Mountains of the central Transverse Ranges of southern California. In this paper, we use these data to estimate the timing of initiation of exhumation (uplift and erosion) on fault blocks within both ranges. We calculate average exhumation rates for different time intervals and reconstruct the evolution of both ranges and adjacent fault blocks over the past 12 m.y. using the Matti and Morton (1993) reconstructions as a base. With these reconstructions we are able to evaluate the variations in response (indicated by exhumation rate) to tectonic forces as the San Gabriel Mountains moved past the San Bernardino Mountains. In general, the San Bernardino Mountains have a much simpler exhumational and structural history than the San Gabriel Mountains, with the most recent phase of exhumation and uplift of geomorphic surfaces beginning ca. 2.5 Ma. This time is long after the most recent phases of exhumation began on fault blocks within the San Gabriel Mountains. There, distinct variations in fault-block exhumation patterns began ca. 12 Ma, when the southern strand of the San Gabriel fault became active. We conclude that although some similarities in exhumational history between the two ranges are evident beginning ca. 3-2.5 Ma, the differences in exhumational patterns between the two ranges indicate that they are not reacting similarly to regional tectonic stresses. The exhumational patterns observed suggest that the San Jacinto Mountains-Peninsular Ranges province is acting as an indentor, controlling when faults have turned on and off and the loci of highest exhumation rates.