BURIAL AND EXHUMATION HISTORY OF THE SAN JUAN CANYON AND SILVERADO CANYON AREAS OF THE NORTHERN SANTA ANA MOUNTAINS, SOUTHERN CALIFORNIA: EVIDENCE FROM APATITE FISSION-TRACK THERMOCHRONOLOGY

The Northern Santa Ana Mountains (NSAM) are bound by the Whittier-Elsinore Fault system to the northeast and form the Los Angeles Basin's southeast boundary. The southwest-dipping, homoclinal NSAM include granitic, sedimentary, and volcanic rocks that span the late Jurassic to late Miocene. Whereas intrusive and depositional ages of rocks in the NSAM are well documented, maximum burial and exhumation timing have not been constrained. To constrain the burial and exhumation of the Silverado and San Juan Canyon areas of the NSAM, apatite fission-track (AFT) dating was used to reconstruct their thermochronologic history. The intrusive age for the western Peninsular Range Batholith in the NSAM is ~120 to ~105 Ma and AFT ages for granitic samples are 100 to 109 Ma. Granitic sample AFT ages relative to intrusive ages indicate rapid cooling through the apatite partial annealing zone (PAZ). However, mean track lengths of 12.3 ± 0.2 microns indicate the granitic samples may have resided near the PAZ prior to final exhumation. AFT ages of sedimentary samples from the ~90 M.y. old Baker Canyon Member are 75 to 70 Ma with mean track lengths of 11-12 microns. Baker Canyon track-length distributions indicate that burial by the younger Late Cretaceous to late Miocene sediments that comprise the bulk of the southwest-dipping homocline was not sufficient to reset the Baker Canyon AFT ages. AFT ages of the Baker Canyon sandstone are ~ 30 M.y. younger than the nearby NSAM granitic rocks. Our preliminary interpretation is that the NSAM were not the dominant Baker Canyon sediment source and that the source was located farther east in the Peninsular Range Batholith where AFT ages are similar to Baker Canyon AFT ages. If the main source of the Baker Canyon sediments was located east of the NSAM, then the NSAM may not have been a topographic high in the Late Cretaceous. The bulk of the NSAM uplift occurred after deposition of the Late Cretaceous Baker Canyon Member and perhaps after deposition of the southwest tilted Miocene sedimentary sequence.