Assessing P-T variability in melange blocks from the Catalina Schist: Is there differential movement at the subduction interface?

In subduction-related tectonic melange, thermobarometry on individual blocks can in principle constrain the scale of lithological mixing along the subduction interface. Previous thermobarometric investigation of the tectonic amphibolite facies melange unit in the Catalina Schist, Santa Catalina Island, California, USA, has suggested relatively limited mixing among blocks (<= 12 km). Here we further investigate scales of mixing among metamorphically disparate ('exotic') blocks within epidote-amphibolite and lawsonite-blueschist facies melange of the Catalina Schist using field and petrographic observations, Zr-in-rutile thermometry, and quartz-in-garnet elastic barometry. A new statistically based method is presented for calculating elastic barometry maximum pressures. The exotic blocks record peak metamorphic temperatures between 580 and 735 degrees C and peak pressures between 1.16 and 1.65 GPa. Temperatures primarily fall in between those recorded by rocks in the amphibolite facies and epidote amphibolite facies units (643-735 degrees C and 553-596 degrees C respectively). The pressure estimates encompass those recorded by blocks from the amphibolite facies melange (1.34-1.44 GPa), although the exotic blocks record a much larger range of pressures. The large range of recorded temperatures and pressures suggests that blocks within the epidote amphibolite unit were sourced from and mixed along a 20-30 km region of the subduction interface while an exotic block from the lawsonite blueschist facies unit appears to have been sourced from at least 70 km deeper than the unit it is hosted in. Metre- to kilometre-scale variations in matrix mineral rheology likely control strain partitioning at the interface and permit differential transport of melange blocks over variable length scales.

Automated summary

Thermobarometric analysis on tectonic melange in California reveals different blocks with metamorphic temperature and pressure differences, suggesting mixing from varying depths along the subduction interface possibly up to 20-30 km.