Constraints on the Rheology of the Lithosphere in Western North America from Observationally Driven Dynamic Models

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The problem to be studied in this research involves the three dimensional dynamics of western North America. The objective is to incorporate the effects of 3-D global mantle circulation, along with detailed lithosphere structure. The project will incorporate latest results of lithosphere and upper mantle structure, obtained from EarthScope's USArray data, into the models. The models will yield predictions of strain rate, surface motion of lithosphere, and upper mantle structure. Benchmarking with 3-D models indicates that reliable structural information leads to reliable estimates of the stresses responsible for the active deformation field, independent of precise knowledge of absolute values of effective viscosity. Given constraints on the structure, we will investigate the range of models that satisfy surface observations of velocity, lithosphere deformation indicators, and World Stress Map stress orientations. This modeling places a bound on the magnitudes of depth-integrated stresses acting within the lithosphere and, as such, places bounds on the lithosphere rheology. We propose to use final model constraints on stress magnitudes and depth integrated lithosphere effective viscosities, together with heat flow measurements, strain rates, and crustal thickness estimates to investigate the bounds on rheological flow parameters for the lower crust and upper mantle beneath Western North America. Finally, we propose to investigate 3-D regional lithosphere dynamic models to address the influence of rheologically weak zones within the middle crust, their impact on depth-dependent horizontal flow, along with effects on coupling of mantle flow.Stresses acting within the Earth's lithosphere are responsible for strain that is ultimately released in earthquakes along faults. Over geologic time scales, this permanent strain results in mountain building and basin formation. High-speed computers now make it possible to investigate the magnitude, scale, and distribution of processes that lead to these stresses and strains within the lithosphere. These processes include mantle convection, along with forces associated with topography and variations in thickness and density of the Earth's crust. Seismological observations provide constraints on variations of thickness and density of the crust, along with images of structure within the Earth's convecting mantle. These images, or velocity variations, yield information on the density variations that drive mantle convection. This project will use a vast amount of new constraints (provided by the NSF funded EarthScope project) on structure beneath western North America to place bounds on the magnitude and distribution of stresses acting within the lithosphere. Models, which involve finite-element solutions of force-balance, will incorporate the effects of three-dimensional mantle flow, along with lithosphere topography and structure. Models will be further constrained by surface observations of plate motions (obtained from high precision GPS), and heat flow measurements. This research will help delineate the magnitude and distribution of the state of stress within the lithosphere. This result on the state of stress, including its link with the specific driving mechanisms, will provide a fundamental framework for investigations of earthquake source physics and the earthquake cycle in general. This work will also provide important insights into how continental deformation evolves over longer time scales. Finally, project will provide technical training to scientists who will enter the work force with improved skills involving problem solving and communication, applicable in both industrial and academic settings.