EPSL: Coupled magmatism-erosion in continental arcs: reconstructing the history of the Cretaceous Peninsular Ranges batholith, southern California through detrital hornblende barometry in forearc sediments

Hehe Jiang, Cin-Ty A. Lee

Continental magmatic arcs are characterized by voluminous flare-ups accompanied by rapid arc unroofing and sedimentation in the forearc basin. Such magmatism and erosion may be dynamically linked and influence the long-term evolution of crustal thickness. To evaluate these effects, we conducted a case study in the Peninsular Ranges batholith (PRB) in southern California, where mid-Late Cretaceous (125-75 Ma) emplacement of felsic plutons coincided with a major pulse of arc-derived sediments into the adjacent forearc basin. We compiled zircon U-Pb ages in the PRB plutons and estimated magmatic addition rates from exposed areas of plutons with different ages. To obtain erosion rates, sandstone samples of known depositional age from the PRB forearc basin were investigated. Major element compositions of detrital hornblendes were determined by electron probe microanalysis and used to calculate emplacement depths of eroded plutons using Al-in-hornblende barometry. These results were combined with laser ablation ICPMS based U-Pb ages of accompanying detrital zircons to estimate the integrated erosion rate by dividing the detrital hornblende emplacement depth by the lag time between peak detrital zircon age and depositional age. Both magmatic addition and erosion rates are between 0.1-2 km/Myr. Magmatic addition peaked at 100-90 Ma, followed by a long, protracted period of erosion between 90-50 Ma. Mass balance and isostatic modeling suggests that due to high magmatic influx, more than 30 km integrated crustal growth and 5 km elevation increase was achieved shortly after peak magmatism. The data and models suggest that erosion was driven by magma-induced crustal thickening and subsequent surface uplift, with an erosional response time of 3-6 Myr. Prolonged erosion after the cessation of magmatism resulted in gradual smoothing of the topography and significant removal of the excess crustal thickness by late Eocene time. The short erosional response times inferred from this study suggest that erosion and magmatism are intimately linked, begging the question of whether the thermal state, metamorphism and rheology of crust in continental arcs is controlled in part by the interplay between erosion and magmatism. We speculate that syn-magmatic erosion, through its effects on the thermal structure of the crust, may also play a role in modulating the depth of pluton emplacement.

Earth and Planetary Science Letters, Volume 472, 15 August 2017, Pages 69-81. doi: 10.1016/j.epsl.2017.05.009

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