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<p>Glacial and fluvial landforms record the recent history of Earth’s surface, and hold information on the climatic or tectonic processes that shape the landscape. Southern Patagonia hosts uniquely well-preserved fluvial cut-and-fill terraces. A record of fluvial incision since 1.5–4 Ma is preserved from K-Ar dated basalt flows atop relict paleosurfaces, and published regional thermochronometric dating and modelling suggest an increased phase of exhumation in the last 1–3 Ma. However, few constraints exist on the onset of river incision, which might provide clues as to possible drivers of regional landscape change. To constrain the timing of Pleistocene incision and landscape evolution in southern Patagonia, we present new cosmogenic <sup>10</sup>Be exposure ages of surface cobbles and amalgamated pebbles from fluvial terraces in the Tres Lagos region 50ºS) and the Río Santa Cruz. Locally, dated basalt flows set a maximum age of ~2.2 Ma for the Tres Lagos terraces, and between 2.2 and 1.7 Ma in the Condor Cliffs region of the Río Santa Cruz. Preliminary <sup>10</sup>Be ages for terrace surfaces in the Tres Lagos region reveal ages between 45–845 ka. Ages of upstream fluvial terraces of the Río Santa Cruz reveal ages between 290–830 ka. The sequence of terrace ages shows that the phase of net incision started ca. 1 Ma after widespread emplacement of basalts, concomitant with enhanced climatic forcing following the Mid-Pleistocene Transition. Ages are also in agreement with the incision history recorded in dated fluvial terraces of other Patagonian rivers, notably the Río Deseado, where ages range from 400 ka–1 Ma (47ºS; Tobal et al., 2021). We argue that the combined results suggest that this net-incisional phase was widespread, therefore unlikely to result from local tectonic drivers, hence probably climatically driven. Our record of Pleistocene landscape evolution is similar to other records throughout the Andes, where the timing of fluvial incision has been linked with the transition to enhanced climatic forcing after ~1 Ma (e.g., Central Andes). Our results point a strong influence of the Mid-Pleistocene Transition on landscape evolution on a continental scale, and notably also in the southernmost regions of South America.</p>

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