CONTRASTS IN DENUDATION RATES, CLIMATE, SUBDUCTION DIP ANGLES, AND PLATE CONVERGENCE RATES ALONG THE SOUTHWESTERN MEXICAN COAST

Erosion shapes landscapes and plays a crucial role in the interplay of climate and tectonics in Earth system dynamics. Quantifying erosion rates and how they vary with factors such as climate and tectonic setting is therefore important for understanding landscape evolution as well as Earth’s long-term climate evolution. Here we use the southwestern Mexican coast as a natural laboratory for studying effects of climate and tectonics on erosion rates. Convergence rates, subduction dip angles, and average climate indices all vary sharply along the Middle America Trench (MAT), where the Cocos Plate subducts beneath the North American Plate. Convergence rates increase from ~2.0 cm/yr in the north, in the state of Jalisco, to 7.5 cm/yr in the south, in Chiapas. Average precipitation also increases to the south, spanning a greater than four-fold range across diverse ecosystems including desert lowlands and high-altitude tropical rainforests. The change in subduction dip angle is more localized, steepening to the south from ~15° to ~45° at the Tehuantepec Fracture Zone, which is near the middle of the MAT. We identified a series of catchments that span these natural gradients in tectonics and climate and measured concentrations of cosmogenic ¹⁰Be in sediment from streams and in soils from ridge tops. This permits us to study how millennial-averaged erosion rates vary with tectonic and climatic factors. In Jalisco, in the north, we find that denudation rates are higher on ridge tops and lower within the surrounding catchments, consistent with relief decline. Conversely, in Chiapas, in the south, denudation rates are lower on a ridge top than in the surrounding catchments, consistent with relief growth. Preliminary results suggest that erosion rates are lower in the north, where plate convergence rates are also lower, suggestive of tectonic control of erosion rates. We discuss how our results advance understanding of linkages among climate, tectonics, and Earth surface processes.