Collaborative Research: Impact of Magmatic Underplating on the Evolution of Lower Continental Crust

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Spectacular volcanic eruptions are visible evidence for the fundamental role of magmatism in shaping Earth’s continents. But how does magmatism create and alter the continents at great invisible depths? Earth scientists have long inferred that mafic magmas rise from the mantle and pond near the bottom of the crust in a process called “underplating”. This process produces “hot zones” in the deep continental crust which lead to melting, magma generation, and magma mixing. Few localities exist on Earth where hot zones have been uplifted and exposed at the surface for direct observation. This project explores the development and evolution of magma in an ancient hot zone of underplating, now exposed at the surface in northern Saskatchewan. This team hypothesizes that the region once existed above a major zone of underplating near the base of the continental crust. Students and scientists will collect data to understand the timing and duration of magmatism, metamorphism, and densification of the deep crust. They will develop models to better understand complex images of the lower continental crust detected by geophysicists throughout North America. The project will provide insight into the processes that lead to the formation of magmas of varying composition, the mechanisms that weaken or strengthen the deep crust, and ultimately, the processes that facilitate formation and stabilization of continental crust on Earth.Magmatic underplating is a first order petrologic and tectonic process that is central to many models for the development and evolution of continental crust. However, remote studies of the deep crust via geophysics or xenoliths commonly lack the resolution to unequivocally constrain the impacts of underplating. This project explores the hypothesis that ”intraplating”, i.e., the emplacement of mantle-derived magma into the lower continental crust above the underplate, is critical for magmatic processes (assimilation, mingling, hybridization), deformation (weakening and crustal flow), and subsequent densification +/- foundering or delamination of lower continental crust. Key insights will be gleaned from direct field observations of the most deeply exhumed section of the 20,000 km2 Athabasca granulite terrane (1.1-1.6 GPa). These exposures are inferred to have experienced 650 m.y. of lower crustal residence prior to exhumation. They represent a rare opportunity to explore crust-mantle interaction and long-term evolution of lower continental crust due to intraplating, metamorphism, melting, and densification in three distinct tectonic settings. Deliverables include: (i) constraining the timing and duration of magmatic intraplating and metamorphism in each setting, (ii) identifying and quantifying densification mechanisms, (iii) constraining the influence of metasedimentary gneisses on the composition, density, and seismic velocity structure of intraplated lower continental crust, and (iv) using field-based observations and data to develop more robust geophysical models of underplated lower continental crust. Critical geophysical goals include: (1) evaluating the degree to which the settings of underplating and intraplating can be distinguished in seismic surveys, and (2) evaluating the degree to which the integrated geophysical properties in each tectonic domain can inform more robust interpretations of the pattern of seismically fast lower crust detected beneath parts of North America. Broader impacts include: (i) Two Ph.D. and 2-4 B.S. students will carry out research projects; (ii) A virtual international workshop for Earth scientists and graduate students focused on the origin and evolution of lower continental crust will be hosted; (iii) PIs and students will convene a Lower Crustal Field Forum where participants will visit and explore the Athabasca granulite terrane and discuss general implications for lower continental crust; (iv) PIs and students will engage in Earth science outreach as part of field work in the vicinity of Stony Rapids, Saskatchewan. The goal is to involve interested teachers and students in our virtual workshop and field forum, giving them opportunities to share their own insights on the culture, people, and landscape along the edge of the Athabasca granulite terrane.This project is jointly funded by three EAR programs (Petrology & Geochemistry, Geophysics, and Tectonics) as well as the Established Program to Stimulate Competitive Research (EPSCoR).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.