UCLA

Abstract High‐pressure rocks from subduction complexes are key records of the physical and chemical processes that operate on the subduction interface, but interpretation of these records requires accurate structural understanding of where they formed in the subduction zone and the mechanisms by which they were exhumed. We present new geologic mapping, outcrop‐scale observations, and geochronology from subduction‐zone assemblages at Punta Prieta Ridge on Cedros Island, Baja California (Mexico), to investigate the history of subduction, exhumation, and structural assembly of these rocks. 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 organized into distinct nappes that decrease in metamorphic grade and degree of strain structurally downward. Garnet‐amphibolite and blueschist blocks within the subduction complex yield 40 Ar/ 39 Ar cooling ages between 172 and 144 Ma and are hosted in siliciclastic rocks which yield detrital zircon maximum depositional ages between 92 and 72 Ma. Based on field evidence and age relationships, we interpret the occurrence of older higher‐grade blocks in younger clastic matrix to be depositional rather than the result of tectonic mixing. We then present a model for their exhumation and assembly via multiple cycles of extensional unroofing of the subduction complex and sedimentary recycling of high‐pressure rocks back into the subduction trench. We conclude by comparing the record of subduction on Cedros Island to other parts of the Cordillera. Key Points Subduction complex rocks are exhumed in the footwalls of normal faults beneath forearc basin strata and attenuated mantle High‐pressure blocks record 40 Ar/ 39 Ar ages from 172 to 144 Ma and are hosted in much younger siliciclastic matrix (ca. 92 to 72 Ma) Blocks were assembled via multiple cycles of extension and sedimentary recycling back into the subduction trench