NEW CONSTRAINTS ON THE THERMAL STRUCTURE OF THE CATALINA SCHIST

Subduction zones often exhibit large-scale, internally coherent rocks occurring in conjunction with chaotically mixed, block-in-matrix mélange structures, which are thought to represent the interface between the downgoing and overriding plates. The relationship between coherent rock and the mélange and its implications for subduction dynamics are not well understood. The Catalina Schist paleosubduction zone on Santa Catalina Island, California preserves an amphibolite-grade mélange that overlies a km-scale region of coherent amphibolite. Field and petrographic evidence show that even though the unit is coherent, there are mineralogical changes across it that could be indicative of a thermal gradient. Furthermore, garnet Lu-Hf ages from the coherent amphibolite are older than garnet Sm-Nd ages from the mélange, suggesting that the coherent amphibolite may have been subducted to greater depths than the overlying mélange. Structural and mineralogical observations from transects across the coherent amphibolite are used to construct a pseudo-stratigraphic column of the unit, providing a detailed picture of a downgoing slab at ~35-40 km depths. Zirconium-in-rutile thermometry and quartz-in-garnet elastic barometry are used to determine the peak temperature and pressure conditions of the coherent amphibolite and assess whether thermobarometric gradients are present across the unit. The results are compared to previous estimates of the peak metamorphic conditions experienced by blocks in the amphibolite mélange. This analysis suggests that the coherent amphibolite may have experienced a different metamorphic history than its associated mélange, which raises the possibility for large-scale differential movement of material at and below the subduction interface. This work contributes to our understanding of the subduction zone interface at depths at which processes such as metamorphic fluid flow and transient slow slip phenomena are known to occur.