Petrochronologic constraints on the thermal evolution of the Catalina Schist

The role of transient tectonic processes (eg. nascent subduction, slab rollback, delamination, subduction erosion, etc.) in the formation of many subduction-related terrains is poorly understood. Constraining the relationship between these processes and exhumed terrains is crucial for interpreting how the terrains relate to subduction overall. The Catalina Schist (Santa Catalina Island, California) is interpreted to be a fossilized subduction interface. It consists of km-scale sub-horizontal thrust sheets ranging from lawsonite-albite to upper amphibolite facies. While the thermal gradients recorded by the lower grade units ( 10-12˚C/km; 1) are broadly consistent with those observed in other exhumed terrains [2] and with benchmark thermal models for young oceanic crust [3], the anomalously high temperature recorded by the amphibolite facies units is difficult to reconcile with steady-state subduction. Mélange blocks throughout the terrain record different peak metamorphic conditions that may be related to long-term or punctuated changes in the thermal structure of the slab. Four blocks in particular appear to span the range of high-T thermal gradients recorded by the terrain and are the focus of this study. The peak metamorphic conditions of these blocks were estimated using Zr-in-rutile thermometry and quartz-in-garnet barometry and range from 625˚C at 1.45 GPa to 716˚C at 1.30 GPa. This corresponds to thermal gradients ranging from 12.7 to 16.2˚C/km. Sm-Nd garnet geochronology was used to determine the timing of these conditions. Preliminary data for the samples yield promising results (¹⁴⁷Sm/¹⁴⁴Nd ratios between 1.07 and 4.12). Ages range from appx. 115 to 152 Ma, falling around previous age estimates using Lu-Hf on garnet [4] and U-Pb on titanite [5]. Additional geochronologic data being collected for the samples will provide precise constraints on the thermo-tectonic evolution of the Catalina Schist and elucidate the role of transient tectonic processes in its formation.

[1] Grove and Bebout (1995) Tectonics [2] Penniston-Dorland et al (2015) EPSL [3] Syracuse et al (2010) Phys. Earth Planet. Inter. [4] Anczkiewicz et al (2004) EPSL [5] Awalt et al (2013) GSA Abstracts with Programs