Pleistocene landscape evolution of Southern Patagonia: Insights from 10Be Dating of Fluvial Terraces

Southern Patagonia hosts uniquely well-preserved fluvial cut-and-fill terraces, formed along rivers fed by glacial meltwater, recording the onset of a regional phase of net incision. However, the timing and driving mechanism of incision remain debated. Published thermochronometric dating and modelling suggest increased exhumation in the last 1–3 Myr. Radiometrically dated basalt flows establish the existence of a eastward-draining paleo-valley with flows going existing terraces by 3.2 Ma. To better constrain the timing of Pleistocene river incision and landscape evolution in southern Patagonia, we present new cosmogenic ¹⁰Be exposure ages of terraces near Tres Lagos and the upstream reaches of the Río Santa Cruz (50ºS). Preliminary terrace surface exposure ages at Tres Lagos are between 70 ka–1.02 Ma, whereas upstream Río Santa Cruz terraces are between 390 ka and 1.04 Ma. The terrace age sequence shows that a phase of net incision started ~1 Myr after the widespread emplacement of basalts, concomitant with enhanced climatic forcing following the Mid-Pleistocene Transition. Our new exposure ages are in agreement with dated fluvial terraces of other Patagonian rivers, where ages range from 400 ka–1 Ma (47ºS; Tobal et al., 2021). Moreover, our record of Pleistocene landscape evolution is similar to other records throughout the Andes, where the timing of fluvial incision has been linked to enhanced climatic forcing after ~1 Ma. Our results point to a strong influence of the Mid-Pleistocene Transition on landscape evolution at a continental scale, notably including southernmost South America.