Unraveling Flat Mountain Tops in the Coastal Cordillera of Central Chile: Based on Erosion Rates, Knickpoints, and Uplift Mechanisms

The Coastal Cordillera of Central Chile, recognized as the world's longest coastal mountain range, exhibits notable variations in erosion rates, mean precipitation, vegetation cover, and topography along its expanse. Serving as a natural laboratory, this region facilitates an in-depth exploration of the intricate interplay between tectonics and climate, owing to its distinct climate gradient and unique subduction margin features. Moreover, subduction and migration of the aseismic Juan Fernandez Ridge (JFR) from northern latitudes to its current position (~ 33.5°S) establish distinct subduction erosion conditions in the north and accretion conditions in the south of the ridge. This has implications for the tectonic deformation style of the forearc, potentially influencing the style and timing of uplift.

Across the region, numerous high elevation – low relief surfaces, often surrounded by knickpoints resembling flat mountain tops, offer valuable insights into the temporal aspects of knickpoint formation hence uplift processes, which might reflect the history of the ridge subduction.  Using geomorphometric indices such as steepness, chi, and knickpoint zones along rivers, we conduct a comprehensive analysis of these surfaces. Initial morphological assessments reveal no obvious trend in the distribution of these surfaces along the strike, although their size diminishes from north to south. Additionally, we used in situ cosmogenic ¹⁰Be nuclides to quantify erosion rates at five different flat mountain tops, thereby determining the knickpoint initiation time. Erosion rates are lower above knickpoint than the ones below knickpoints as expected.  Consistently low erosion rates (0.004 mm/yr – 0.07 mm/yr) prevail across the region. Considering the substantial height of these surfaces (approximately 1.5-2 km), the initiation time of the knickpoints might show the history before arrival of the JFR in the south, whereas in the north they might be comparable with the passage of the JFR. However, by incorporating paleoclimate and geodynamic conditions overtime into the landscape evolution model, we anticipate obtaining more precise results for comparison. In conclusion, the Coastal Cordillera of Central Chile undergoes complex interactions among tectonics, seismology, and climate. A nuanced understanding of these processes contributes significantly to broader insights into convergent plate boundaries and the geological evolution of forearcs.