Along-strike variation in catchment morphology and cosmogenic denudation rates reveal the pattern and history of footwall uplift, Main Gulf Escarpment, Baja California
Topography is expected to record tectonic, climatic, and rock strength controls on long-term denudation rates in active margins. We test this hypothesis in the Sierra San Pedro Martir, Mexico—the footwall of the normal-faulted, western margin of the Gulf of California rift system—by relating topographic metrics with 10 Be-derived catchment-averaged denudation rates. Denudation rates and topographic metrics record along-strike gradients in rock uplift relative to base level that increase asymmetrically from fault tips to maxima within the northern half of the range. Surface uplift of an Eocene erosional surface and slope-break knickpoints found at increasingly higher elevations in the northern segments of the Sierra San Pedro Martir fault system suggest that range asymmetry is due to a recent northward acceleration in the rate of rock uplift relative to base level. By characterizing the relationship between channel steepness and cosmogenic denudation rates, we extrapolate millennial-scale denudation rates to million-year time scales and estimate ages for the transient increase in rock uplift rates and the initial onset of normal faulting. We infer that the Sierra San Pedro Martir fault system initiated during the middle Miocene (ca. 16–14 Ma) in the center of the range and ca. 12–8 Ma near the fault tips. Recent increases in rock uplift rates during the late Pliocene (ca. 3–2 Ma) coincide with lithospheric rupture in the Delfin basins to the east and represent a westward migration of strain from hanging-wall detachments to the Sierra San Pedro Martir fault system. Age estimates are consistent with independent geologic constraints and show that pairing of carefully selected cosmogenic denudation rates with topographic analysis can be used to extract tectonic signals from topography over million-year time scales.