Experimental constraints on the rheology of the mantle lithosphere at the base of the seismogenic zone

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The largest and most damaging earthquakes occur along plate boundaries where lithosphere is subducted beneath the continents. Geoscientists have discovered that a wide range of fault slip processes occur in these regions, and significant progress has been made understanding these events by integrating geophysical observations with models and constraints from lab experiments. However, there are still gaps in our understanding of mechanical properties at the relevant conditions. Previous work at these conditions has been hindered by relatively poor resolution of the stresses being imposed on the sample during an experiment. The investigators have made new modifications to their high-pressure equipment that will lead to significant improvement in the ability to measure stress and characterize the fault slip behavior that occurs within the subduction zone environment. The results will provide new constraints necessary to understand how earthquake slip initiates and to explain why there is a limited depth range over which the earthquakes occur. The project will fund two Ph.D. students (one completing a thesis and another starting a thesis during the duration of the project). Outreach will be coordinated with the Department’s DEEPS STEP (STEM in Providence Public Schools) program. Engagement in rock mechanics, with its strong links to engineering, has the potential to bridge a gap to URM STEM students, who remain dramatically underrepresented in Earth Sciences.This project involves experiments to: (a) quantify frictional behavior at high-temperature under both dry and wet conditions; (b) investigate low temperature plasticity of olivine aggregates at high pressure conditions near the brittle-plastic transition, which is also relevant for constraining the extrapolation of rate-and-state friction laws to mantle conditions; (c) constrain the role of dehydration reactions on the frictional stability of the lithosphere; and (d) determine frictional behavior and flow law parameters for alteration phases at high pressure to constrain the strength of altered lithosphere (and stress state prior to dehydration). The improvements to the investigator's apparatus enhances the ability to: (a) conduct creep tests, which will allow acquisition of better data to constrain flow law parameters; and (b) resolve transient deformation, which significantly improves the ability to quantify rate-and-state friction parameters at the high pressure conditions where seismicity occurs. The results will provide constraints for a wide range of scientists interested in the diverse fields of metamorphism, geodynamics, seismology, the physics of earthquakes, and fluid transport.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.