Uplift and erosion of the San Bernardino Mountains associated with transpression along the San Andreas fault, California, as constrained by radiogenic helium thermochronometry

Apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the San Andreas fault. By documenting aspects of the low-temperature (40°1 00°C) thermal history of the tectonic blocks of the San Bernardino Mountains in southern California, we have placed new constraints on the magnitude and timing of uplift. Old helium ages (64-21 Ma) from the large Big Bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the Late Cretaceous period. These ages imply that the surface of the plateau may have been exposed in the late Miocene and was uplifted only -1 km above the Mojave Desert in the last few Myr by thrusting on the north and south. A similar range in helium ages (56-14 Ma) from the higher San Gorgonio block to the south suggests that its crest was once contiguous with that of the Big Bear block and that its greater elevation represents a localized uplift that the Big Bear plateau did not experience. The structure of the San Gorgonio block appears to be a gentle antiform, based on the geometry of helium isochrons and geologic constraints. Young ages (0.7-1.6 Ma) from crustal slices within the San Andreas fault zone indicate uplift of a greater magnitude than blocks to the north. These smaller blocks probably experienced 2_3-4 km of uplift at rates 2_1.5 mm/yr in the past few Myr and would stand 2_2.5 km higher than the Big Bear plateau if erosion had not occurred. The greater uplift of tectonic blocks adjacent to and within the San Andreas fault zone is more likely the result of oblique displacement along high-angle faults than motion along the thrust fault that bounds the north side of the range. We speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike-slip system. Transpression thus appears to have been accommodated by both vertical displacement within the San Andreas fault zone and thrusting on adjacent structures. Copyright 1998 by the American Geophysical Union. Paper number 98TC00378. 0278-7407 /98/98TC-00378$12.00