EXHUMATIONAL HISTORY OF THE SAN JACINTO MOUNTAINS, FROM NEW APATITE FISSION TRACK ANALYSES

New apatite fission track (AFT) analyses from the San Jacinto Mountains are used to constrain the exhumational history of the fault-bounded block. AFT ages are commonly interpreted as recording the time of cooling through a ~110°C closure temperature. In a previous study, George and Dokka (1994) obtained AFT ages of 79-74 Ma from the San Jacinto Mountains, with no correlation between age and elevation seen. The six new AFT ages obtained in this study were substantially younger, ranging from 56 to 29 Ma, and did show a correlation between age and elevation. Three samples from an elevational transect on the eastern side yielded ages of 54, 40.8, and 29.1 Ma with respect to elevations ranging from 2622 to 227 m. A sample collected from an elevation of 1240 m from the northern front of the San Jacinto Mountains yielded an AFT age of 56.6 Ma but is not interpreted as it appears to be in a large landslide block (F. Jordan, pers. comm., 2009). Two samples from fault blocks on the western side of the San Jacinto Mountains yielded AFT ages of 56.8 and 44.0 Ma. The age patterns obtained in this study were different from those obtained by George and Dokka (1994) and corresponded closely to the apatite (U-Th)/He analyses of Wolf et al. (1997), which represent cooling through a lower closure temperature of ~70°C. We interpret these new AFT data and the existing (U-Th)/He analyses to represent a later, less rapid phase of cooling and exhumation (~40°C of cooling or 1.3 km of exhumation over ~3-5 Myr at ~55 Ma), than had been interpreted by George and Dokka. The younger AFT ages at lower elevations may be partially reset; we will use AFT track length analyses to better interpret these younger ages.