Pierre Lacan

Geomorphic evidence for Plio-Quaternary compression in the Andean foothills of the southern Neuquén Basin, Argentina

The southern central Andes foothills are made of imbricate thick- and thin-skinned fold and thrust belts. They were formed during Cretaceous and Miocene compressive periods by successive eastward propagation episodes of the orogenic front. We analyze remnant terraces and alluvial deposits of the Neuqun River and its tributaries to study the Plio-Quaternary tectonic regime in the southern Neuqun Basin. Topographic profiles of remnant terraces show a crustal-scale (tens of kilometers) anomaly above the Chihuido South anticline. This anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline. Another anomaly in the terrace profiles, correlated with a knickpoint in the current Neuqun River, is observed farther east, near Aelo. We interpret these anomalies to be the consequence of a Plio-Quaternary uplift of the Chihuidos and Aelo anticlines. Folding and surface faulting of Plio-Quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the Miocene orogenic front and of the predominantly thin-skinned Agrio fold-and-thrust belt. From these data we infer that the southern Neuqun Basin is still shortening under a compressive stress regime and that the Andean orogenic front is migrating farther to the east. Extensional features in the study area correspond mainly to thin-skinned gravitational gliding, resulting from crustal uplift. Thus, we argue that the tectonic compressive regime is preserved in the southern Neuqun Basin throughout the Plio-Quaternary, though the slab may have steepened during this period.