GOING DEEPER – INSIGHTS INTO THE TECTONICS OF SUBDUCTION ZONES FROM THE CATALINA SCHIST (CA) (Invited Presentation)

The Catalina Schist (CA) is famed for its subduction-related mélange and mineralogical and geochemical evidence for extensive fluid-rock interaction, but it also records tectonic processes and the physical and thermal nature of the subduction interface. The terrane presents many thermal-tectonic puzzles, including unusually high temperatures recorded by the amphibolite-facies units, evolution of the subduction geotherm over a few million years from ~17-20°C/km previously inferred for these older amphibolite-facies rocks to ~10-12°C/km recorded by younger blueschist-facies rocks, and a mostly isofacial block-in-matrix structure found within mélange zones preserved throughout the terrane (unlike other mélanges). Recent thermobarometry and geochronology of mélange blocks reveal several new insights into these puzzles. Trace element thermometry and elastic barometry indicate the amphibolite-facies rocks developed at higher pressures and therefore colder geothermal gradients than inferred from traditional thermobarometry– reducing the high-T gradient to ~14-16°C/km. Heterogeneities in peak temperature within the seemingly uniform high-grade rocks indicates mélange mixing on spatial scales ≤12 km along the interface. Garnet Sm-Nd geochronology reveals a relatively narrow range in peak metamorphic ages, suggesting that the mixing occurred over at least seven million years. Finally, we have assessed both spatial and temporal variabilities in peak metamorphism for relatively unstudied ‘exotic’ blocks found in lower-grade units which indicate mixing on a slightly larger scale (~20-30 km) and in one case provides evidence for an earlier deeper subduction event. This work provides insight into broader questions about the nature of the subduction interface, the thermal evolution of a subduction system, processes of tectonic mixing within the subduction interface, and the impacts of these processes on geochemical cycling and deep seismic phenomena.