Collaborative Research: Constraining Fault Displacement Histories and Lithospheric Dynamics using Geology and Geophysics

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The goal of this project is to improve our understanding of how crustal-scale structures relate to lithospheric-scale dynamics and present-day crustal strain fields using the southern San Andreas fault zone as a natural laboratory. Specifically, the research is concentrating on a determination of the time-scales over which displacement rates vary on the various faults composing the San Andreas fault zone near San Bernardino, California. A new modeling technique based on the theory of smoothing splines is being developed to infer instantaneous fault displacement rates from available displacement rate averages from geodetic and geological studies. Time-variable displacement rates are estimated subject to the condition that the total displacement across a set of faults is conserved through time. Preliminary results indicate appreciable variations in displacement rate for the San Andreas and San Jacinto fault zones on million, 100 thousand, and 10 thousand year time-scales. Despite large variations through time, existing data are consistent with the notion of a constant total displacement rate. The contribution of eastern California shear zone faults is also being explored. The birth of the San Jacinto fault correlates with the development of a major restraining bend in the San Andreas fault. Instantaneous displacement rate estimates indicate that the San Andreas fault rate slowed by some 80 percent coincident with the formation of the San Jacinto fault. These fault formation and displacement rate variation data have implications for lithospheric dynamics. An important part of this project is the use of nonlinear 2D and 3D numerical models that include brittle elastic-plastic and visco-elastic rheologies to understand possible dynamic processes underlying the observed displacement rate changes. Numerical experiments are being used to explore the parameters that control fault zone formation and evolution, including an assessment of the relative importance of crustal thickening, variations in normal stress on oblique fault segments, and the viscous strength of the lower crust and upper mantle.