Quantifying vertical slip evolution in active strike-slip fault systems is essential for reconstructing fault slip histories and fault interactions at plate boundaries, but linking geologic and earthquake-cycle deformation remains challenging. We address this problem using the San Felipe fault (SFF) in southern California (United States), which developed at ca. 1.5 Ma during reorganization of the San Andreas fault system. We compare high-spatial-resolution apatite (U-Th)/He (AHe) thermochronometry data with an Interferometric Synthetic Aperture Radar (InSAR)− and Global Navigation Satellite Systems (GNSS)−based model of surface uplift rates across Yaqui Ridge, a crystalline block bounded by SFF strands. Individual AHe dates are as old as ca. 66 Ma, but a subset of dates from samples adjacent to SFF strands are ca. 1.5−0.8 Ma across a range of effective U concentration (eU). AHe date−eU patterns and models of cooling converted to slip rate are consistent with a pulse of localized vertical slip of 2−3 mm/yr at SFF initiation, followed by a slowdown to 1 mm/yr. Similar ca. 1.5 Ma exhumation signals along the southern San Andreas fault system reveal a regionally coordinated vertical deformation pulse during the growth of new faults like the SFF. Comparison of AHe data with elastic fault models of present-day surface deformation across Yaqui Ridge suggests accelerated vertical slip at the inception of the SFF occurred coseismically, requiring higher earthquake frequency and/or magnitude. Our study reveals how young (≤1 Ma) AHe dates can be compared with geodesy to resolve the evolution of rates and mechanisms of vertical fault slip in active strike-slip fault systems worldwide.
Armstrong et al. (Wed,) studied this question.