Experimental Constraints on the Rheology and Seismicity of Subducting Lithosphere and the Slab-Wedge Interface

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To understand the spatial and temporal distribution of earthquakes in subduction zones, as well as the processes responsible for the wide spectrum of fault slip behaviors on the slab/wedge interface, it is important to constrain the rheological properties of altered lithosphere, how they evolve during dehydration reactions, the rheology of reaction products, and the feedbacks between metamorphic reactions and transport properties of the downgoing slab. Primarily motivated by surface heat flow measurements in subduction zones, current mantle wedge corner flow models use various boundary conditions to impose decoupling between the subducting slab and overlying mantle wedge. One hypothesis is that a layer of weak serpentinite (a hydrated magnesian silicate) above the slab accommodates the majority of the deformation. Serpentine has also been hypothesized to play an important role in the origin of the double seismic zone observed in numerous subduction settings, and on the origin of intermediate and deep seismicity along the slab. In other settings, dehydration of lawsonite (a hydrated calcium-aluminum silicate) has been hypothesized to promote seismicity. In this case, seismicity is interpreted to result from high pore fluid pressures that arise during dehydration, and subsequent embrittlement of the rock. Examining the frictional behavior of fault materials over a wide range of sliding velocities is essential for understanding the dynamics of stress evolution and slip during earthquakes. Geophysical observations suggest that sheet-structure minerals (e.g., serpentine, talc and other clays) may control the strength and frictional behavior of creeping sections of faults. Previous friction experiments on serpentinite document velocity-strengthening behavior at plate-tectonic displacement rates, consistent with this hypothesis. However, high velocity friction experiments on serpentinite document dramatic weakening and dehydration at slip velocities above ~ 0.1 m/s. Our experiments demonstrate an approximately 1/V dependence of friction on velocity above a characteristic weakening velocity Vw ~ 0.1 m/s, consistent with theoretical predictions for flash heating and subsequent weakening of asperity (uneven) contacts. Extrapolation of these results to mantle conditions suggests that slow-slip events and/or slow earthquakes could be nucleated by seismic ruptures into serpentinized regions. Primarily motivated by new results from experiments in our lab that both challenge and inform some of these hypotheses, we propose to study (a) the rheological properties of antigorite at high pressure and temperature; (b) the role of dehydration reaction kinetics on the mechanical evolution of serpentinites and blue-schists; (c) the rheology of slab dehydration reaction products at conditions appropriate for intermediate depth earthquakes and (d) the frictional properties of serpentinites at seismic and infra-seismic slip rates.Parts of the project will form the basis of a Ph.D. thesis for a Brown University student and the senior thesis chapter for Helen Doyle and another undergraduate student. The results of the experiments will provide constraints for a wide range of scientists with interests in the diverse fields of metamorphism, geodynamics, seismology, the physics of earthquakes, and fluid transport, crossing the boundaries between the new GEOPRISMS initiatives. We will make all of our results available including digital records of all raw data files - via on-line supplements and our lab websites.