Transcontinental retroarc sediment routing controlled by subduction geometry and climate change (Central and Southern Andes, Argentina)

Central Argentina from the Pampean flat-slab segment to northern Patagonia (27-41°S) represents a classic example of a broken retroarc basin with strong tectonic and climatic control on fluvial sediment transport. In this provenance study, we combine framework petrography and heavy-mineral data to trace multistep dispersal of volcaniclastic detritus first eastwards across central Argentina for up to ~1500 km and next northwards for nearly another 1000 km along the Atlantic coast. Compositional signatures reflect different tectono-stratigraphic levels of the orogen uplifted along strike in response to varying subduction geometry as well as a different character and crystallization condition of arc magmas through time and space. In the presently dry climate, fluvial discharge is drastically reduced to the point that even the Desaguadero trunk river has become endorheic and orogenic detritus is dumped in the retroarc basin, reworked by winds, and temporarily accumulated in dune fields. At Pleistocene to early Holocene times, instead, much larger amounts of water were released by melting of the Cordilleran ice sheet or during pluvial events. The sediment-laden waters of the Desaguadero and Colorado rivers then rushed from the tract of the Andes with greatest topographic and structural elevation, fostering alluvial fans inland and flowing in much larger valleys than today toward the Atlantic Ocean. Sand and gravel supply to the coast was high enough not only to promote rapid progradation of large deltaic lobes but also to feed a cell of littoral sediment transport extending as far north as the Río de la Plata estuary.