POLYCYCLIC METAMORPHISM AND EXHUMATION OF SUBDUCTION COMPLEX ROCKS, CEDROS ISLAND, BAJA CALIFORNIA

High-pressure blocks from subduction mélanges are key records of the subduction interface but remain difficult to interpret due to uncertainty around their exhumation mechanisms and preservation. We present new mapping, outcrop-scale observations, and geochronology from superb exposures of subduction zone assemblages at Punta Prieta Ridge on Cedros Island, Baja California, to investigate the history of subduction recorded in the rocks and how they were exhumed and preserved. The rocks of Punta Prieta Ridge are exposed in the footwalls of high- and low-angle normal faults that carry Cretaceous forearc basin strata and attenuated mantle sections of the Cedros Island Ophiolite in their hangingwalls. The footwall rocks are subduction zone assemblages that are organized into distinct thrust sheets that decrease in metamorphic grade and degree of strain structurally downward. High-pressure amphibolite blocks from the structurally highest nappe yield ⁴⁰Ar/³⁹Ar ages between 172 and 164 Ma and are hosted in a schistose blueschist-facies volcaniclastic-matrix which yields a zircon U-Pb maximum depositional age (MDA) of ca. 88 Ma. Structurally underlying non-schistose siliciclastic-matrix mélange horizons in the lower nappes contain lawsonite- and epidote-blueschist facies blocks, which yield white mica/glaucophane ⁴⁰Ar/³⁹Ar ages of 149 and 144 Ma, respectively. These results and the presence of blueschist-facies clasts in low-grade conglomerates of the lower nappes support multiple cycles of erosional unroofing of the subduction complex and recycling of high-pressure rocks back into the subduction trench.