Near-field response to transpression along the southern San Andreas fault, based on exhumation of the northern San Gabriel Mountains, southern California

Abstract

Near-field mountain building is common along transpressive faults, but how vertical deformation relates to oblique plate motion versus local structural complexity has not been universally established. We have investigated the mechanisms of exhumation along the southern San Andreas fault in the northern San Gabriel Mountains to examine the effects of pure-shear-dominated transpression (obliquity >20°). Although rugged topography suggests near-field uplift, low-temperature thermochronometry indicates minimal exhumation. Apatite (U-Th)/He ages are >10 Ma along narrow ridges within the fault zone, implying near-field rock uplift is not pervasive along this oblique plate boundary. Ages farther from the fault are younger (∼5 Ma) and suggest a deformation event occurred during the transition of activity from the San Gabriel fault to the modern San Andreas strand. These results reveal a heterogeneous crustal response to transpression and imply that secondary structures or geometrical complexities play a primary role in near-field bedrock uplift along transpressive faults. Copyright 2007 by the American Geophysical Union.