COMPUTING RATES AND DISTRIBUTIONS OF ROCK RECOVERY IN SUBDUCTION ZONES

Bodies of rock that are detached (recovered) from subducting oceanic plates, and exhumed to Earth's surface, become invaluable records of the mechanical and chemical processing of rock within subduction interface shear zones. While exhumed shear zone rocks with high-pressure (HP) mineral assemblages provide insights into the nature of rock recovery, various interpretations concerning thermal gradients, recovery rates, and recovery depths arise when directly comparing the rock record with numerical simulations of subduction. Constraining recovery rates and depths directly from the rock record presents a major challenge, however, as small sample sizes of HP rocks make statistical inference weak. As an alternative approach, this study implements a "soft" clustering classification algorithm to identify rock recovery in numerical simulations of oceanic-continental convergence. Over one million markers are traced and classified from 64 simulations representing a large range of presently active subduction zones on Earth. Marker pressure-temperature (PT) distributions are compared across models and with the rock record to address the following three questions: How do recovery rates vary among subduction zones? How is recovery distributed along the plate interface, and how does the distribution change across subduction zone settings? Numerical experiments show recovery pressures (depths) correlate strongly with convergence velocity and moderately with oceanic plate age, while PT gradients correlate strongly with oceanic plate age and upper-plate thickness. Recovery rates strongly correlate with upper-plate thickness, yet show no correlation with other initial conditions. Likewise, PT distributions of recovered markers show variable compatibility with the rock record depending on the collection of natural samples and suite(s) of numerical experiments. A significant gap in predicted marker recovery is found near 2 GPa and 550 ˚C, coinciding with the highest density of exhumed HP rocks. Implications for such a gap in marker recovery include a numerical model that fails to mimic certain recovery mechanisms, inconsistent thermal gradients among numerical experiments and exhumed HP rocks, or a relative overabundance of rocks studied from around 2 GPa and 550 ˚C (scientific bias).