COUPLED MAGMATISM-EROSION IN CONTINENTAL ARCS IN CONTROLLING CRUSTAL GROWTH: CASE STUDY OF THE LATE CRETACEOUS PENINSULAR RANGES BATHOLITH AND ASSOCIATED FOREARC BASIN, SOUTHERN CALIFORNIA

Continental magmatic arcs are characterized by voluminous magmatic flare-ups accompanied by rapid arc unroofing and sedimentation in the forearc basin. Such magmatism and erosion may be dynamically linked and may influence the long-term evolution of crustal thickness. To evaluate the contribution by magmatic addition and erosion on crustal thickness, we conducted a case study in the Peninsular Ranges batholith (PRB) in southern California, where Late Cretaceous (125-75 Ma) emplacement of granitic plutons coincided with a major pulse of arc-derived sediments into the adjacent forearc basin in late Cretaceous-early Eocene. We compiled zircon U-Pb ages in the PRB plutons, and estimated magmatic addition rates from exposed areas of plutons within different age groups. To track the eroded arc materials, we collected sandstone samples from the PRB forearc basin. We measured U-Pb ages in detrital zircon by laser ablation ICPMS and major element content in detrital hornblende by electron probe microanalysis, and calculated emplacement depths of eroded plutons from Al-in-hornblende thermobarometer. Erosion rate for each sample was then estimated through dividing the peak “detrital” emplacement depth by the lag time between peak detrital zircon age and depositional age. Both magmatic addition and erosion rates are within the order of 0.1-1 km/Myr. Magmatic addition peaked at 100-90 Ma, followed by long-lasting erosion with decreasing erosion rates during 90-50 Ma. Mass balance and isostatic modeling suggest that due to high magmatic influx, up to 20 km excess crustal growth and 3 km elevation increase can be achieved shortly after peak magmatism. Rapid erosion was driven by the magma-induced uplift and lagged behind peak magmatism with a 4 Myr response time. Continuation of erosion long after the cessation of magmatism resulted in gradual smoothing of the topography and almost complete removal of the excess crustal thickness by the time of late Eocene. With comparable magmatic and erosion rates, fast erosion during magmatism may have significant controls on the depth of pluton emplacement and heat transfer. Thus this coupled magmatism-erosion may also play an important role in the evolution of thermal state, metamorphism and rheology of the mid-upper crust in the continental arcs.