Quantifying cut-and-fill terrace cycles since the Middle Pleistocene in the Patagonian Steppe, Argentina

The rapid response of Patagonian topography to ice-mass changes, facilitated by the presence of a slab-window, offers an ideal setting for investigating the interplay between surface processes, climate dynamics, and solid Earth rheology. This study focuses on glacio-fluvial terrace sequences of the Ro Santa Cruz and Ro Shehuen (50S), which are fed by glacial meltwater from the Southern Patagonian Icefield and extend for over 200 km along the entire length of the river. Recent research in Patagonia demonstrates that glacio-fluvial gravels from terraces formed in the vicinity of glacier outlets can be reliably correlated to glacial terminations. Thus, these cut-and-fill terrace sequences provide a geomorphic archive uniquely positioned to directly correlate landscape responses with periodic climate forcing. Yet, the spatially extensive preservation of these terraces over 100s of kilometers likely reflects the influence of geodynamic processes active over continental length-scales. Radiometric dating of basalts overlying the oldest terrace generations documents eastward-draining paleo-valleys by 3.2 Ma. New surface exposure-dating of terraces using in situ cosmogenic 10Be and 21Ne reveal onset of net incision at ~1 Ma, with individual terrace ages well-correlated to Patagonian glaciations and global cold periods. We attribute terrace abandonment and incision following glacial cycles to a drop in sediment supply relative to water discharge, likely influenced by the formation of a proglacial lake (Lago Argentino). While the onset of net incision aligns with the decline of the greatest ice extent in Patagonia and the Mid-Pleistocene Transition (MPT), terrace ages and geometry underscore the need to link net incision to regional geodynamic processes. Sub-parallel, vertically offset terrace profiles require a regional base-level fall of 100 m since 1 Ma, while terrace age-elevation relationships show a temporally non-uniform regional incision history. These observations cannot be explained by climatically-forced sediment supply variation, but likely relate to the evolution of the mantle underlying the slab window. Our study highlights the complex interplay between climate-driven factors and regional geodynamics in shaping the fluvial landscape of southern Patagonia.