Testing a geomorphic mechanism for stalling of escarpment retreat using cosmogenic radionuclides and river grain size measurements

Abstract

Escarpments are stepped topographic asymmetries defined by low-relief uplands and lowlands separated by a steep, high-relief landscape. Escarpments have long been studied and debated, as the topographic asymmetry is interpreted to imply landscape disequilibrium and drainage divide migration away from the escarpment face. Passive margin Great Escarpments that formed during continental rifting have been particularly heavily debated due to their prominent topographic appearance and enigmatic longevity, motivating the development of some of the first-generation numerical landscape evolution models. Prevailing wisdom and numerical models suggest that due to the across-divide topographic asymmetry, these features migrate quickly over geologic time, a theory that is generally unsupported by cosmogenic-derived millennial-scale erosion rates. Low-temperature thermochronology suggests that escarpments erode at rates of roughly 1 km/Myr early in their development. Cosmogenic radionuclide-derived erosion rates suggest rates closer to 10 to 100 m/Myr on more recent time scales, based on comparable to at most 3x higher rates on the escarpment side of the divide than the upland, suggesting stalled or slow escarpment retreat. This discrepancy has led to research attempting to reconcile the stark topographic asymmetry of escarpments and apparently slow millennial-scale erosion rates with landscape evolution models that demonstrate sustained, rapid retreat rates through geological time. While many ideas have been proposed, none appear to satisfactorily explain why modern erosion rates predict such slow rates of escarpment retreat.In this study, I use cosmogenic radionuclide-derived erosion rates from in-situ 10Be and 36Cl in addition to channel hydraulics, grain size measurements, and numerical modeling to quantify the role of sediment caliber, an unaccounted-for geomorphic control in current interpretations, in arresting escarpment migration in Puerto Rico. This mechanism has the potential to explain why, despite strong topographic asymmetry, the erosion rates remain comparable across the divide over geologic time. My results show that despite dramatic topographic asymmetry across the Puerto Rican Escarpment, erosion rates are comparable, indicating little to no escarpment motion. These results suggest differences in bedrock erodibility across the escarpment divide, which I show can be explained by coarser grain sizes shielding river beds from erosion along escarpment-draining rivers. I argue that erodibility is lower on the steep escarpment face due to infrequent mass-wasting events, which load bedrock channels with coarse immobile sediment, while the upland is supplied with finer-grained sediment. I also demonstrate that bedrock channels in Puerto Rico adjust their topography to accommodate transport of the intermediate grain size given the average stream flow condition, allowing the preservation of coarse sediment on the escarpment face, which in turn slows and lowers erosion rates by armoring bedrock channels, arresting retreat.