LATE CRETACEOUS-EOCENE DENUDATION OF THE PENINSULAR RANGES BATHOLITH OF SOUTHERN AND BAJA CALIFORNIA

Late Cretaceous-Eocene denudation of the Peninsular Ranges batholith (PRB) overlapped with flat slab subduction along the southwestern margin of North America. Two-dimensional, thermo-kinetic models that predict closure age distributions of detrital minerals from erosion of synchronously intruded and differentially exhumed arc crust have been applied to new U-Pb and Ar-Ar detrital thermochronologic data from Late Cretaceous-Eocene PRB forearc strata. The localities span a 750 km strike length along the PRB. Following upon initial calculations presented in Lovera et al. (1999), we have introduced a more realistic description of topographic development and erosion history. Additionally, Cretaceous pluton emplacement is constrained by new zircon U-Pb age measurements. Detrital age distributions statistically calculated from the model at different deposition times are compared to measure age distributions to constrain the erosion history of the PRB. We employ a generalized Kolmogorov-Smirnov statistic (KS) that takes into account age uncertainties to establish the degree to which model output match the measured age distributions. Namely, evaluation of the null hypothesis is based upon the maximum separation of the Gaussian kernel probability functions (D) instead of the discrete cumulative-frequency distributions as in the traditional K-S method. The studied rocks include: Santa Ana, San Diego, and San Miguel Island areas (northern PRB), El Rosario embayment (central PRB), and the Vizcaino peninsula (southern PRB). Detrital zircon populations indicate that Late Cretaceous detritus supplying all three areas was overwhelmingly derived from 125-90 Ma crystalline basement and was progressively dominated by 100-90 Ma granitoids from the eastern PRB. We find that the northern and central PRB was characterized by variable denudation rates (up to 1 km/m.y. or more prior to 90 Ma and from 80-70 Ma). While denudation rates in the southern batholith were also initially high prior to 90 Ma, much lower rates (<0.2 km/m.y.) persisted thereafter. We attribute this north-south contrast to details of the Laramide flat subduction. Peneplanation of the northern batholith and transition from local to more easterly, Mojave/Sonoran sources was initiated by the Late Paleocene in the northern PRB and persisted to the Late Eocene.