ROCK UPLIFT ALONG THE SAN ANDREAS FAULT AND IMPLICATIONS FOR PROCESSES OF TRANSPRESSION

Unlike oceanic transforms, continental strike-slip faults are typically fixed along irregular continental margins or to heterogeneous zones of weakness and do not always form or remain parallel to plate motion. The San Andreas fault, a type example of continental strike-slip systems, is transpressive along more than half its length. Understanding transpression is thus fundamental to understanding the behavior of continental transforms. Models of transpression make predictions for the spatial distribution of convergent deformation, which can be tested using the proxy of rock uplift. Based on extensive existing data and new observations, we have analyzed patterns of rock uplift (surface uplift based on geomorphic clues and exhumation based on thermochronometry) along the entire San Andreas fault. Rock uplift is generally concentrated in the near-field of the fault zone. Unless associated with a specific secondary structure in the broad borderlands, rapid rates of rock uplift are confined to within ~10 km of the main strike-slip fault trace. This is consistent with models of strain partitioning and wrenching, rather than models based on stress decoupling between a purely strike-slip main fault and pure shear distributed across wide (~50-100 km) borderlands. Rock uplift also relates to the degree of obliquity between the fault and plate motion. The highest rates of rock uplift are observed where obliquity exceeds 20 degrees, consistent with predictions of pure-shear dominated transpression. However, these trends are obscured by local structural complexity, such as fault stepovers or intersections (e.g. San Emigdio Mountains or San Gorgonio Pass). Unexpectedly high and low rates of rock uplift also seem to relate to local erodibility. Variations in rock type (e.g. resistant metasediments to weak serpentine) and precipitation (~10-200 cm/yr) along the length of the fault may affect rock uplift via potential feedbacks with erosional mass transport. This implies a linkage between transpressive deformation and surficial processes, similar to that more typically associated with convergent plate boundaries.