Dissected, concordant summit surfaces are often treated as geomorphic isochrons to gauge rock uplift. Yet, a geomorphic surface appearing to be concordant need not have everywhere formed at the same time. We assess the synchroneity of formation of geomorphic surfaces in California’s Gabilan Range and Mesa (USA), a set of low-relief highlands adjacent to a restraining bend of the San Andreas fault. Digital elevation models reveal a knickpoint at ∼610−660 m above mean sea level (amsl) in the northern Gabilan Range that partitions a gently tilted, elevated plateau. Detrital cosmogenic 10Be from six basins shows plateau erosion rates of 9−31 m/m.y. whereas canyon rates reach 64−87 m/m.y. Combining the ∼600 m knickpoint relief with these contrasts indicates that rapid incision—and thus accelerated rock uplift—began at 14−8 Ma. These Miocene ages for the northern Gabilan erosional surface uplift pre-date the geomorphically continuous ≤2.8 Ma Paso Robles−capped surface farther south. Uplift in the northern Gabilan Range coincides with a nonconformity below oil productive basal transgressive sandstone in the giant San Ardo oil field, late-Miocene fan-delta deposition in the San Joaquin basin, and the emergence of the San Andreas Big Bend. The Gabilan Range and Mesa therefore likely comprises diachronous fragments assembled over the past ∼10 m.y. Our study demonstrates that summit accordances require independent chronology before being used as tectonic datums and suggests that present uplift—and associated seismic hazard—on the northern Gabilan Range is modest relative to adjacent Coast Range segments.
Hilley et al. (Thu,) studied this question.