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
Project outcomes In this project, we investigated mechanisms of vertical rock transport under chains of volcanoes known as "magmatic arcs" and the effect this mass transport has on the overall budget of magmatism. Magmatic arcs form at convergent plate margins above subducting plates. The area studied here is the California arc – the eroded remnants of a giant Andean-like subduction system that operated along the western margin of North America during the Mesozoic (250-80 million years ago). The California arc is dismembered by more recent tectonics and exposed throughout the state of California, with the largest intact area being the Sierra Nevada mountain range and the deepest exposed rocks being found in the Santa Lucia Mountains, along the California Coast ranges, south of Big Sur. Our study was designed to focus on these two areas, the central Sierra Nevada because of represents the classic section through an arc, and the Big Sur area because of the depth of exposure. We found that rocks belonging to the near surface environment during arc magmatism (upper crustal rocks like volcanic or sedimentary materials) can be quickly buried to depths in excess of 20 km at rates of several mm/year. As such, these rocks can be partially melted and incorporated into the younger magmas of the arc. This process is referred to as downward flow and can account for up to about 20% of the mass balance of some arc magmas. Downward flow is a compensation mechanism triggered by the uprising of magmatic bodies from deeper in the crust and a manifestation of convective mass transport in arcs. While important, it does not rival other mechanisms of transporting near surface rocks to great depths, such as thrusting along major faults or subduction and tectonic underplating of sedimentary materials. The biggest contribution of this project to questions outside its discipline is that we provide new constraints on mass transport in the continental crust and specifically in areas where arc magmatic rocks form. Andean arcs contain some of the largest metal concentrations on the Planet. Our work can help better understand the budget of various elements in arc magmatism, including the economically relevant ones. This collaborative project trained several graduate and undergraduate students and was also used as research topic in upper level graduate classes. Students of various socio-economic backgrounds were involved in this project. Training of students ranged from field mapping techniques to teaching laboratory techniques in the geochemistry and isotopic facilities at Arizona. Several scholarly manuscript were published or are in the process of being published in high profile scholarly journals and all new data are made publicly available to other researchers in our community.
