COLLABORATIVE RESEARCH: Uplift and Exhumation Along the San Andreas Fault Zone: An Empirical Study of Transpression

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Transpression occurs along 42% of the San Andreas fault system. Its influence onthe behavior of the transform fault is thus of first-order importance and may represent oneof the most important complications in developing a comprehensive understanding ofhow strike-slip faults and plate boundaries behave. With this in mind, the PI's propose acollaborative study that will systematically and quantitatively integrate geologicalconstraints on the manner in which transpressional deformation is accommodated atseveral sites along the San Andreas fault. A functional understanding of transpression would make patterns of vertical deformation easy to predict where primary boundary conditions, such as the degree of plate motion obliquity and the mechanics of the main transform fault, are known. However, the material properties of the San Andreas fault are debated, and considerable evidence suggests that secondary factors (structural and lithologic heterogeneity, erosivity) play a significant role in shaping the geologic response to transpression. As a result, a general understanding of how transpressional plate motion becomes manifest as deformation in the crust has not been developed. One of the PI's objectives is to quantitatively evaluate the effects of external factors on deformation partitioning, so that we may ultimately evaluate models of transpressional deformation that are predicated on assumptions of the mechanical properties of the San Andreas fault. Their approach will be to use the spatial and temporal distribution of vertical motion, manifested in bedrock uplift (surface uplift + exhumation), as a representation of transpression. They will combine thermochronologic constraints ((U-Th)/He dating) on the age and magnitude of exhumation with geomorphic and structural constraints on surface uplift, erosion, and the architecture of deformation that drives this bedrock uplift. They will use these complementary techniques to determine how patterns of vertical motion are spatially distributed with respect to the San Andreas fault (i.e. near vs. far field) and its obliquity to plate motion. The PI's objective is thus to develop a functional relationship between transpressive deformation and these key variables. This will require developing controls on how the chronology of vertical motion relates to the emergence of transpression at each site as well as controls on the effects of independent boundary conditions (e.g. geological, structural, climatic) along strike. They will focus on four sites (Carrizo Plain/Temblor Range, San Emigdio Mountains, northern San GabrielMountains, and Coachella Valley), each of which represents a major gap in understanding of transpression along the San Andreas fault. The integrated results fromthese sites, when synthesized with previous studies, will provide the full range inboundary conditions and transpressive parameters needed to create specific tests ofdifferent models of transpressional deformation. A key aspect of this proposal is the collaboration that will be employed in attaining our goal. The PI's have extensive knowledge of southern California geology and the analytical techniques to be employed. Spotila was among the first to apply this type of integrated approach to understanding portions of the Transverse Ranges. House has actively pursued new applications of the thermochronologic technique that will be employed in this study. Brady provides a robust foundation in structural geology and geophysics that will link uplift estimates with individual structures. Post-doctoral associate Niemi will add an additional expertise in Cenozoic stratigraphy, structural geology, and field mapping. This number of PI's will also enable us to begin work at all field sites concurrently and represents a science team strong enough to tackle this significant problem.