Collaborative Research: Crustal Overturn in Continental Margin Arcs During Magmatic Surges

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Subduction zones, where one lithospheric plate slides beneath another before returning to the mantle, results in melting of rock at depth and the rise of these melts to form long chains of volcanoes and subvolcanic plutons that together are called magmatic arcs. Because these arcs and surrounding host rocks are hot and undergoing tremendous stresses due to the plate motions they become extensively deformed forming orogenic belts. The evolution of continental margin orogenic belts and their associated magmatic arcs has a non-steady state ?tempo? of subduction, orogeny, magmatism, exhumation, and erosion/redeposition. This research project examines the details of this tempo particularly during magmatic surges (periods when large volumes of magma are formed and added to the crust) and what role was played by the tectonic transport of easily melted crust to regions at depth where melt occurs in subduction zones. The goal is to test the hypothesis that during the mid-Cretaceous (105-85 Ma) high magma surge episode much of the arc in the southern Sierra Nevada and Salinian blocks was emplaced within a crustal scale convection system. Preliminary studies of pendants around 105-85 Ma plutons indicate that, during rise of magmatism, metasedimentary and effusive metavolcanic host rocks were convectively overturned and underwent rapid downward displacement and high-strain flow. The research will expand and synthesize a number of data sets (e.g., field studies, geochronology, barometry, vertical displacement, strain fields, magma fluxes) plus perform extensive forward numerical, thermomechanical modeling capable of incorporating magma ascent, viscoelasto- plastic host behavior, large strains, nonlinear rheologies, and realistic phase transitions to understand the physics of the process.Volcanic eruptions dramatically impact human society and play a huge role in both the formation of our atmosphere and in climate change. Additionally crustal deformation in orogenic belts leads to short-term events such as earthquakes and long-term events such as mountain building that also impact our society. The recognition that arc crust, at times, undergoes overturn thus incorporating crustal materials into rising magmas will dramatically change the understanding of magmatic systems, the geochemical and geochronologic analyses of these systems, and rates of crustal growth. This research will improve understanding of these systems and the GIS based databases produced on geochronology, thermobarometry, vertical displacement rates, strain and strain rates, and magma fluxes will be made available to the public. A large number of undergraduates from several institutions will be involved in the research project through the University of Southern California Team Research program