Near‐field transpressive deformation along the San Andreas fault zone in southern California, based on exhumation constrained by (U‐Th)/He dating

Low‐temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the San Andreas fault zone in southern California has experienced recent, rapid exhumation. Eight apatite (U‐Th)/He ages from a 1‐km‐relief section along Yucaipa Ridge in the San Bernardino Mountains range from 1.4 to 1.7 Ma. The minimal change in age with elevation implies exhumation of ∼5–7 mm yr −1 , sustained for at least several hundred thousand years. Three titanite helium ages from the ridge are much older, ranging from 57 to 82 Ma. These show a steep gradient with elevation, representing either an exhumed, partial retention zone or slow cooling through much of the Tertiary. These data imply that a total exhumation of ∼3 to 6 km has occurred since 1.8 Ma. It is uncertain whether this exhumation terminated as early as 1 Ma or has continued up to the present at a decelerated rate. We surmise that this exhumation represents rock uplift in the absence of major surface uplift, in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief. The record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion. Such rapid, large‐magnitude exhumation within the strands of the San Andreas fault zone is important for models of transpressional tectonics. It is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain. However, it is inconsistent with a stress‐partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike‐slip fault. In addition, a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust. These models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far‐field borderlands, rather than concentrated in the near field. Alternatively, the exhumation of Yucaipa Ridge may have been driven by the nearby restraining bend in the San Andreas fault at San Gorgonio Pass, in which case it represents local fault geometry rather than accommodation of far‐field plate motion.