Collaborative Research: Deciphering Subduction Dynamics: Case Study of the Catalina Schist

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Subduction zones, which occur where two tectonic plates converge and one plate is thrust down into Earth's mantle, are the location of many important processes on Earth, including the generation of some of Earth's deadliest earthquakes and chains of volcanoes. At the interface between the subducting plate and the mantle significant heat transfer and physical and chemical exchange occur. To better understand how crustal and mantle materials move and interact within subduction zones, it is critically important to study rocks exhumed from this process. The proposed work will constrain how material moves within subduction zones by determining the temperature and pressure histories that these exhumed rocks experienced while in the subduction zone itself. For this work the principle investigators will analyze the chemical composition of the minerals rutile and titanite in rocks from a classic subduction locality, the Catalina Schist of southern California. They will then compare the recorded histories with histories predicted by different models for material behavior within subduction zones. In addition to the scientific goals of the project, this award is supporting the eduction of graduate and undergraduate students in an STEM discipline, is contributing to broadening of underrepresented groups in science, and the development of undergraduate course curricula in mineralogy and petrology. The project is facilitating a new collaboration between the partnering institutions involved in this effort, and is contributing direct exchange of scientific methods and analytical standards between trace element/geochronology laboratories. The results that derive from this project are being disseminated to the scientific community through research publications and presentations, and the data are being archived in appropriate community databases. This project will exploit new developments in thermobarometry and geochronology to better understand subduction zone tectonics, using exposures of Catalina Schist on Santa Catalina Island, California, as a type example. The work will provide new, high-precision Pressure-Temperature-time (P-T-t) constraints that critically test tectonic models with unprecedented geographic scope and precision. The proposed research addresses broad geodynamic questions, e.g. to what degree does flow in the subduction channel change temporally and affect subduction thermal evolution? In addition, the principle investigators will continue to develop petrologic and chronologic tools that broadly advance petrogenetic research in other fields. This research will address two main issues in the tectonic evolution of subduction zones: (1) What is the scale of melange flow as melange matrix progressively metamorphoses? (2) What tectonic processes produced distinctive high-Temperature subduction-related rocks? Numerous different models have been generally proposed. Four basic sets of data will be collected: (1) Samples will be collected over a well-mapped area of melange and along three key transects. (2) Electron microprobe analysis will provide Pressure-Time estimates and identify specific titanite and rutile grains and domains for trace element analysis. (3) Laser-ablation Inductively-coupled-mass spectrometry (LA-ICPMS) on a sub-grain scale (50-100 micron spots) will constrain temperature and time, respectively. (4) Raman spectroscopy of mineral inclusions will constrain entrapment pressures on the prograde Pressure-Temperature path. The resulting data will be used to test tectonic models by characterizing the degree of Pressure-Temperature-time homogeneity (scale of mixing) within a melange zone and Pressure-Temperature-time trends across structure. All data will be permanently archived with MetPetDB.