Collaborative Research: Probing feedbacks between thermal structure, petrologic transformation, and rheologic evolution within dynamically evolving subduction zones

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Subduction zones – places where one tectonic plate sinks beneath another – are responsible for the generation of deadly earthquakes, explosive volcanoes, global chemical cycling into the deep earth, and tectonic plate movements. The thermal structure of a subduction zone (i.e., the temperature of different parts of the subduction zone at depth) exerts a first order control on the strength and mechanics of an individual subduction zone and also on what materials and volatiles (e.g., water) are transported down to the deep earth within subducting plates. Together, these temperature-dependent mechanical and chemical processes dictate the occurrence of subduction zone hazards such as earthquakes and volcanism. Thus, a longstanding goal of subduction research is a quantitative understanding of subduction zone thermal structure. Because these zones are 100s of km thick and 1000s of km long, we cannot directly measure their thermal structure. However, we can create detailed numerical simulations (subduction models) that predict thermal structure and allow us to investigate how it evolves and influences these mechanical and chemical processes. These models are guided by a broad range of tectonic observables in active subduction zones and by studies of subducted rocks that have been exhumed back to the surface. These data illuminate a range of thermal, chemical (petrological), and mechanical (rheological) feedbacks that operate over the lifetime of a subduction zone but are typically omitted from thermal subduction zone models. For instance, chemical reactions (e.g., metamorphism) in subducting plates are not only highly-temperature dependent, but also likely to affect the thermal structure of subduction zones. This is because different metamorphic rocks have different strengths and densities which, in turn, affect the subduction properties (convergence velocity between the two plates, dip angle of the subducting plate) that ultimately control subduction zone temperature. Motivated by these dynamic interactions, we will develop a suite of subduction models that directly incorporate these thermal-chemical-mechanical feedbacks. This modeling approach will allow us to probe how, and how rapidly, subduction zone thermal structure evolves, and also to characterize how this thermal variability impacts plate boundary strength and chemical cycling in these important tectonic zones. In addition to supporting undergraduate, graduate, and postdoctoral researchers, this project will also benefit society and the geoscience community through a combination of education, outreach, and scientific in-reach in the following ways: (1) we will develop an online lab activity for introductory geology classes to expose beginning geoscientists to computational methods, (2) we will host an in-reach subduction zone workshop at the University of Washington, and (3) we will reach out to the public by developing a digital exhibit on subduction zones at The Beneski Museum of Natural History (Amherst College).To capture dynamic and time-evolving subduction behavior for Earth’s range of subduction settings, we will fully integrate geodynamic, petrologic, and rheological components into our modeling framework. Petrologic modeling will reveal the loci of slab devolatilization and density transformations through time. A suite of experimentally and geologically constrained rheologies will be used to calculate the time-evolving crustal viscosity structure. Both components will be fully integrated into the geodynamic modeling component (i.e., a time-dependent subduction model) so that calculated petrological phases, densities, and viscosities are dictated by, and also affect, the thermal evolution of the geodynamic model. After iteratively increasing the complexity of models (so as to preserve physical intuition as the number of model components grow), we will run models for parameter combinations corresponding to each subduction system on Earth. This will enable us place bounds on the properties of Earth’s slabs (temperature, dehydration systematics, density, viscosity), in space and time, and address three targeted questions relating to the co-evolution of slab thermal structure, dehydration, and mechanical properties: What evolutionary phase of subduction is associated with the most water transport to the deep mantle? What is the mechanical control on the so-called “decoupling depth” at subduction zones? And, lastly, what is the dominant control on the bi-modal timing of subducted rock exhumation?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.