Collaborative Research: Constraining the thermal conditions of the subduction interface by integrating petrology and geodynamics

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Subduction zones are places on Earth where one of Earth's tectonic plates dives beneath another. They are the location of many societally-relevant hazards, including the generation of Earth's deadliest earthquakes, such as the 2011 Tohoku earthquake and associated tsunami, and volcanic eruptions such as those at Mt St Helens (1980) and Mt Pinatubo (1991). The processes that lead to these earthquakes and volcanoes are ultimately dependent on the thermal structure of subduction zones - that is how hot it is at great depths. Metamorphic rocks exhumed from ancient subduction zones contain unique records of the temperatures that they witnessed as they traveled down a subduction zone before being exhumed. Geodynamic models and geophysical observations provide estimates of the thermal structures of present-day subduction zones. Interestingly, the rock record suggests significantly warmer conditions than those predicted for modern subduction zones. The proposed work will investigate the reasons for this discrepancy, which may lie in the way we interpret conditions from the rocks, the way they are exhumed, or even in how we compare model predictions and the rock record.This proposal aims to address the rock-model discrepancy through two key questions: 1) Does our current interpretation of the rock record accurately represent the thermal structure of the associated ancient subduction zone? and 2) Did rocks get exhumed from ancient subduction zones that were hotter on average than modern subduction zones? To address Question 1 this team will determine P-T conditions for metamorphic rocks exhumed from five well-characterized localities that represent a range of thermal structures using relatively new analytical methods: trace element thermometers (e.g. Zr-in-rutile) and mineral inclusion barometers (e.g. quartz-in-garnet barometry). To address Question 2, they will develop geodynamical models of the ancient subduction zones represented by the exhumed rocks. These models will incorporate the effects of region-specific subduction dynamics, such as variations in slab age and subduction rate with time, subduction initiation, ridge subduction, and slab breakoff, when applicable. The model-predicted P-T conditions along the subduction interface will be compared with the newly produced P-T estimates to re-evaluate their disparity. The two-pronged approach of combining petrological observations and geodynamical modeling allows quantitative exploration of the thermal evolution of subduction zones. Subduction-related metamorphic rocks are the only direct samples of material from the plate interface; evaluating the P-T conditions using the latest thermobarometric approaches will provide the team with a more accurate and precise way to untangle the complex history they have experienced. Through geodynamical modeling, the effects of individual parameters on the thermal structure of subduction zones can be isolated, and this targeted approach will allow evaluation of possible explanations for the warm conditions that are recorded by exhumed rocks. The application of the two-pronged approach to the selected ancient subduction localities will allow researchers to determine whether the disparity can be reconciled. The results of this work have important implications for many processes, including geochemical cycling of volatiles, construction of continental crust, and the conditions that lead to arc volcanism. This work will engage graduate students and early career scientists in new collaborations between scientists of different disciplines (petrologists and geodynamicists) and at different institutions both in the US and abroad. EarthCache (TM) sites will be created in California (Franciscan and Catalina) as part of the project. These sites will engage the public in geoscience through the popular activity of geocaching and will disseminate information about subduction zone geology through information tied to direct observations of geologic features exhumed from subduction zones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.