Influence of the post-Miocene tectonic activity on the geomorphology between Andes and Pampa Deprimida in the area of Provincia de La Pampa, Argentina

The genesis of the relief between the Andes and the Pampa Deprimida plain between 36° and 39°S has never been considered. The region is intermediate between two contrasting geomorphic styles, the meridian-oriented highs and depressions of the Sierras Pampeanas to the north and the eastwards sloping northern Patagonian mesetas to the south. From geophysical data, it coincides with an intermediate zone between a flat-slab subduction zone to the north and a normal subduction zone to the south.From west to east (68° to 64°W), four units follow each other: the easternmost portion of the Sub-Andean piedmont, the depression of the Río Chadileuvú, a Plateau, and a high scarp separating it from the Pampa Deprimida lowland. The Plateau is the southernmost portion of the Brazilian shield. Geomorphological and sedimentological analyses led us to the following conclusions: 1. the Andes uplift created a large piedmont reaching the Pampa Deprimida and including the Plateau which between the Pliocene and the Middle Pleistocene was shaped in a series of stepped levels covered by Andean fluvial sediments; 2. the meridian-oriented Rio Chadileuvú depression is of tectonic origin, younger than the Middle Pleistocene, and breaks the continuity between the piedmont and the Plateau: this depression could be an incipient foreland basin; 3. the eastern scarp is a fault scarp, probably Upper Pleistocene in age, due to a faster activity of the fault zone between the craton and the Macachín Trough. This young morphotectonic activity coincides with the change from a west–east Patagonian pattern to a north–south orientation of the relief typical of the Sierras Pampeanas, but younger than them. The river network was affected by this evolution. During the Upper Miocene, a palaeo-Río Negro flowed to the north–east, then shifted southwards. The Río Colorado entered the Pampa region during the Upper Pliocene creating a set of stepped fluvial accumulation terraces, while the piedmont was drained by eastward streams. Following the formation of the Chadileuvú depression, a north–south drainage, largely endoreic, replaced the Sub-Andean flows. A capture of the Río Chadileuvú by the Río Colorado is going on due to the incision of the Río Colorado in the south.The regional landscape is therefore directly related to Late Pleistocene tectonic activity, whereas climate changes explain the different sedimentary characteristics of the surficial deposits.