Trench retreat recorded by a subduction zone metamorphic history

Upper amphibolite-facies metamorphism in subduction zone rocks may occur under exceptional tectonic settings. Differentiating competing mechanisms for its occurrence requires carefully integrated, high-resolution thermobarometric and geochronologic studies of mélange rocks with well-defined field relationships. We present new pressure, temperature, and age data from the classic Cretaceous Catalina Schist in southern California (USA) that allow us to establish a plausible model for its high-temperature metamorphic history. Our results indicate that garnet-amphibolite blocks in the structurally highest amphibolite-facies mélange preserve evidence of three stages of tectonic evolution: (1) prograde lawsonite eclogite-facies metamorphism that peaked at 2.4–2.7 GPa with temperatures >580 °C during fixed-trench subduction (120–115 Ma); (2) post-peak epidote eclogite-facies metamorphism followed by amphibolite-facies metamorphism at 1.4–1.3 GPa with temperatures of 740–790 °C during trench retreat (115–105 Ma); and (3) isothermal decompression (1.3 GPa to <1.0 GPa at temperatures of ~780 °C) and cooling during trench advance and slab-flattening subduction (ca. 105–100 Ma). Our model implies the presence of a continuous Cordilleran subduction system in the Cretaceous, which had varying tectonic regimes through episodes of trench retreat/advance and slab shallowing/steepening that, in turn, dictated the development of the Cordilleran arc system.