Geological evidences of the Zonda palaeo-river megafan, Central Andes of Argentina (33°S)

The Subtropical Andean foreland basin sedimentation of Argentina was marked by fluvial megafans during the Late Quaternary. However, at the latitude 33°S, where an active orogenic front is advancing across the foreland basin, the regional Quaternary fluvial dynamics are still overlooked. This is mainly due to thick aeolian mantle overlying these fluvial deposits preventing classical field geological studies. In this research, remote sensing and morphometric indices analysis coupled with field data allowed us to tackle to detect an ancient fluvial megafan. This Distributive Fluvial System was associated with the Zonda palaeoriver which was related to the temporary confluence of the Mendoza and Tunuyán rivers during part of the Quaternary. Landforms linked to the megafan were detected using ALOS PALSAR DEMs and SRTM. The Zonda megafan covers an area of ca. 7096 km² and is characterized by a distributive pattern of wide fluvial palaeochannels interspersed with aggradational lobes and terminal splays. Scarce exposed outcrops show fine-grained fluvial sediments containing pumice clasts. Geochemical analysis showed that these clasts originate from the Maipo volcano (∼450–150 ka. BP). Furthermore, the span life of the Zonda paleoriver until the Late Pleistocene was constrained by exploratory AMS dating. Based on our findings, we propose that the evolution of the Zonda fluvial system is driven by a complex interaction between climatic and tectonic forcings.