RESOLVING A TRANSIENT EROSIONAL RESPONSE TO TECTONIC UPLIFT IN THE SAN BERNARDINO MOUNTAINS, CALIFORNIA, BY COMBINING LUMINESCENCE THERMOCHROLOGY AND COSMOGENIC RADIONUCLIDES

The San Bernardino Mountains (SBM) include some of the fastest uplifting tectonic blocks in the world due to the local transpressional forces associated with the southern big bend of the San Andreas fault (SAF). Exhumation rates in the SBM have previously been constrained with apatite (U - Th)/He ages (AHe) and cosmogenic radionuclide ¹⁰Be (Be-10) catchment-averaged denudation rates, both of which show an increase in exhumation rates nearer the SAF to the south, with the highest rates observed for the Yucaipa Ridge tectonic block (YRB). Despite the difference in methodological averaging times, AHe and Be-10 yield similar exhumation rates of about 0.2 – 3.5 mm/yr.

What is less understood is how erosion evolves within the uplifting landscape, including the relative importance of mass wasting and fluvial erosion in shaping the topography. If hillslope erosion is set by fluvial downcutting incision and occurs through frequent mass wasting events, topographic relief may have remained constant in the Late Quaternary. However, if fluvial incision has migrated upstream, the locus of mass wasting events may have varied spatially also, causing a lagged erosional response of hillslopes compared to river incision In this case, we may expect recent variations in exhumation rate over millennial timescales and disparate patterns of topographic relief in landscapes.

To address this question, we use topographic analysis, new and published basin-averaged erosion rates and new ultra-low-temperature thermochronology results to examine how the rapidly exhuming YRB and the adjacent San Gorgonio tectonic block are eroding on timescales from 10^3 to 10^5 yr. We observe that the landscape is in topographic disequilibrium and shows topographic and geochronological evidence of erosion migrating upstream. The response of hillslopes and fluvial erosion is discussed. We suggest that the combination of Be-10 catchment-averaged erosion rates with in situ luminescence thermochronology provides valuable information about the dynamics of landscape response to extreme tectonic uplift rates.