A Melange Of Subduction Ages: Constraints On The Timescale Of Shear Zone Development And Underplating At The Subduction Interface, Catalina Schist (Ca, Usa)

The presence of melange at the subduction interface influences numerous geochemical and geophysical processes. However, the relationship between the timescales of melange 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 melange zone of the Catalina Schist (Santa Catalina Island, CA). Ages range from 108 to 116 Ma and do not appear to correlate with the peak metamorphic temperature recorded by each block (between 640 and 740 degrees 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 111-114 Ma for peak metamorphism of the melange zone but are predominately younger than an estimate for the structurally lower coherent amphibolite unit of ca. 115 Ma. White mica Ar-40/Ar-39 ages from previous studies suggest that the units cooled asynchronously to 400-425 degrees C by 106 to 97 Ma at rates between 18 and 43 degrees C/Ma. Collectively, these results demonstrate that melange formation occurred over at least 8 Myr from 116 to 108 Ma and was followed by cooling. The structural and chronologic relationship between the melange zone and underlying lower-grade units indicates that the cooling occurred in conjunction with an underplating event between 109 and 108 Ma. The age discrepancy between the melange zone and the underlying coherent amphibolite unit may indicate that the two units were juxtaposed either during or after the underplating event.