A mélange of subduction ages: Constraints on the timescale of shear zone development and underplating at the subduction interface, Catalina Schist (CA, USA)

Abstract

The presence of mélange at the subduction interface influences numerous geochemical and geophysical processes. However, the relationship between the timescales of mélange development, deformation, and resultant mass transport is poorly understood. Here, we use Sm-Nd garnet geochronology to elucidate the timing of peak metamorphism for five garnet amphibolite tectonic blocks from the amphibolite-facies mélange zone of the Catalina Schist (Santa Catalina Island, CA). Ages range from 108 to 115 Ma and do not appear to correlate with the peak metamorphic temperature recorded by each block (between 640 and 740°C). The lack of correlation between age and peak temperature favors the tectonic mixing model previously proposed for the unit. These ages overlap with previous estimates of 111 to 114 Ma for peak metamorphism of the mélange zone but are predominately younger than an estimate for the structurally lower coherent amphibolite unit of c. 115 Ma. Hornblende 40Ar/39Ar and U-Pb rutile ages from previous studies suggest that the unit rapidly cooled to 500-550°C before 105 Ma. Collectively, these results demonstrate that mélange formed over at least 7 Myr from 115 to 108 Ma and was followed by rapid cooling to 500-550°C between 108 and 105 Ma. The structural and chronologic relationship between the mélange zone and underlying lower-grade units indicates that the rapid cooling occurred in conjunction with underplating. The age discrepancy between the mélange zone and the underlying coherent amphibolite unit may indicate that the two units were juxtaposed either during or after the underplating event.