Mechanisms and Efficiency of Ductile Strain Localization Below Major Continental Strike-slip Faults: Numerical Experiments Incorporating Laboratory-derived Rheologies

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This project will investigate the long-term deformation and strain evolution due to major strike-slip faults in the continental crust. In particular, the project will use numerical models to evaluate the efficiency of various strain-softening mechanisms, such as thermo-mechanical coupling, grain-size reduction, and mylonitic fabric, and assess the degree to which these promote or inhibit strain localization, individually and in combination, in response to long-term fault slip. This will be accomplished using finite element models that will incorporate realistic geotherms, far-field loading rates and loading histories, depth-dependent compositions, and constitutive relationships inferred from laboratory experiments. The simulations will investigate conditions under which permanent shear zones may develop in an initially unstrained ductile substrate. The magnitude and distribution of deviatoric stresses in the ductile lower crust and upper mantle will be evaluated, and inferences made about the long-term strength of continental lithosphere as a function of temperature regime, composition, deformation rate, total displacement, and other relevant factors. Observables that will be brought to bear on the model predictions include grain size distributions from the exposed mid-to-lower crustal shear zones, inferences of deviatoric stress from petrologic and micro-structural data, seismic structure and anisotropy below active fault zones, and geodetic observations of transient and secular deformation due to major strike-slip faults. The respective data and models will be used to test the hypothesis that the distributed viscoelastic flow and localized shear in the ductile substrate represent end member behavior of fault zones with different degree of maturity, with localized shear prevalent on high-slip rate, high total offset (e.g. plate boundary) faults, and diffuse deformation dominating for immature faults.The degree to which strain is localized in the ductile part of the lithosphere below major faults is a major unresolved question in continental tectonics. Two classes of models have been proposed: one postulating a broadly distributed viscous deformation in the lower crust and upper mantle (the "thin lithosphere" model), and another one postulating extension of localized shear well below the brittle-ductile transition (the "thick lithosphere" model). Understanding the mechanics of lithospheric shear zones is essential for a number of problems in continental tectonics, including the long-term strength of the Earth's crust and upper mantle, stress transfer from the relative plate motion to seismogenic faults, and, ultimately, seismic hazards. Geological and geophysical evidence has been presented in support of both the "thin" and "thick" lithosphere models, possibly indicating differences in deformation styles between various locations, tectonic settings, deformation rates, and total displacements. If such variability exists, it is of interest to establish the main controlling factors and governing mechanisms on the observed deformation styles. Realistic models of long-term deformation informed by the experimentally determined ductile properties of rocks will bear on the long-standing debates such as the block-like versus diffuse deformation in the continental interiors, the effective strength of the continental lithosphere, and the mechanisms of transient deformation following large earthquakes.