Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface

To investigate the architecture of transpressional deformation and its long -term relationship to plate motion in southern California, we have studied the deformation pattern and structural geometry of orogeny within the San A ndreas fault system. The San Bernardino Mountains have formed recently at t he hub of several active structures that intersect the San Andreas fault ea st of Los Angeles. This mountain range consists of a group of crystalline b locks that have risen in association with transpressive plate motion along both high- and low-angle faults of a complex structural array. We have used a deeply weathered erosion surface as a structural datum to constrain the pattern of vertical deformation across fault blocks in and adjacent to this mountain range. By subtracting the hanging wall and footwall positions of this preuplift horizon we have determined vertical displacement along two m ajor thrust faults. We conclude that one fault, the North Frontal thrust, h as played a more significant role in raising the large fault blocks and can explain the uplift of all but a few crustal slivers. On the basis of the p attern of displacement associated with this thrust fault we have also infer red fault zone geometry beneath the range. Rather than simply steepening in to a high-angle fault zone or flattening into a decollement, the thrust fau lt may have a complex, curviplanar geometry. The pattern of rock uplift als o enables us to calculate the total motion accommodated by this orogeny. We estimate that >6 km of convergence.(5% of the total plate motion in the la st 2 Myr) has occurred. This horizontal shortening is associated spatially with the 15-km-wide restraining bend in the San Andreas fault zone near San Gorgonio Pass. The entire range may thus have risen because of a small geo metric complexity in the San Andreas fault rather than the obliquity of far -field plate motion.