TRANSPRESSIVE BLOCK EXHUMATION ALONG THE SAN ANDREAS FAULT IN THE NORTHWESTERN SAN GABRIEL MOUNTAINS

The relationship between oblique plate motion and mountain building at transform margins is difficult to constrain. Although mountain belts often relate to convergent fault bends and may fit with existing models of transpression, a functional relationship between plate motion and vertical deformation along entire transpressive strike-slip faults has not been developed. The predominance of oblique convergence and rugged topography in southern California makes the southern San Andreas fault an excellent setting for studying this problem. The northwestern San Gabriel Mountains (nSGM) are an elongate crystalline block located between the San Gabriel and San Andreas faults that has not been previously examined from the context of transpressive orogenesis. This range lies northwest of the well-studied main part of the San Gabriel Mountains, but is separated from it by the structural low of Soledad Canyon. Rugged, fault-parallel ridges in the nSGM suggest bedrock uplift may increase towards the more than 20° oblique San Andreas fault, as expected for pure-shear dominated transpression and as observed at Yucaipa Ridge in the San Bernardino Mountains. We have used apatite radiogenic helium dating, sensitive to exhumation from ~2 km depth, to quantify the exhumation pattern of this range. Apatite helium ages in the nSGM decrease to the southeast, subparallel to the San Andreas fault. Old ages (23-33 Ma) occur near rounded, weathered ridges of Liebre Mountain in the northwest while younger ages (4-8 Ma) occur to the southeast as far as the Sierra Pelona. This pattern does not fit with a simple model in which rock uplift increases towards the core of the main fault zone. The age pattern instead implies the range is a broad fault block tilted to the northwest. Because young ages occur on both ridges and valleys in the southeast, rapid exhumation and tilting may have occurred in the late Miocene. This exhumation must be explained with local structures, rather than the well known range-bounding thrust faults to the south, because the nSGM are isolated from other crystalline blocks of the Transverse Ranges. Although it is not yet known what structures are responsible for uplift of the nSGM, these observations imply a complex and heterogeneous mode of transpressive orogenesis.