Exhumation History of the Little San Bernardino Mountains, California: Implications for Constraining the Timing and Change of Regional Transtension to Transpression

Vertical deformation along the 1100 km transform boundary between the North American and the Pacific plates in California is difficult to clearly quantify at different spatial and temporal scales. Only in localized areas, where plate obliquity creates a locally converging boundary segment, are the manifestations of vertical deformation readily observable. A restraining bend along the southern segment of the San Andreas Fault (SAF), between the Yucaipa Ridge and the Little San Bernardino Mountains (LSBM), represents an area where 27 degrees of plate obliquity create a locally convergent segment of the boundary. Understanding how deformation is accommodated adjacent to the SAF is critical for understanding how slip and related deformation are partitioned in areas where major strike-slip faults form restraining bends. Exhumation rates and timing for the LSBM are constrained by 15 apatite fission-track (AFT) and 7 (U-Th)/He (AHe) ages along a 23 km, N-S (perpendicular to the SAF) transect in western Joshua Tree National Park. AHe ages are ~4-5 Ma at distances of 5-12 km north from the SAF, but abruptly increase at >12 km to 20 to 40 Ma. AFT ages increase gradually from 7 to 20 Ma at 3-12 km, but also increase abruptly to 48-65 Ma at distances of >12 km to the north and toward the Pinto Mountain Fault. Detailed annealing and diffusion modeling of the AFT and AHe data show that the rates of cooling, and hence exhumation, decrease with distance from the SAF. Implied exhumation rates range from 0.42 - 0.62 mm/yr nearest the SAF to 0.14 - 0.21 mm/yr at approximately 12 km from the SAF. Approximately 12 km from the SAF, the rates of exhumation decrease substantially (0.06 to 0.10 mm/yr) and remain consistent for the remainder of the transect. The location of change in exhumation rate at 12 km is interpreted as a major structure acting as a pivot point for the northerly tilt of the LSBM. Young AFT and He ages nearest the SAF likely represent the onset of a transitional phase when regional transtension changed to transpression, concomitant with the development of the restraining bend.