Crustal collapse in the Andean backarc since 2 Ma: Tromen volcanic plateau, Southern Central Andes (36°40′–37°30′S)

Analysis of seismic lines and gravity data shows the existence of Quaternary extensional depocenters beneath thick covers of < 1 Ma-old volcanic rocks in the Tromen volcanic plateau backarc region (36°40′–37°30′S). Dating and mapping of pre- and post-extensional volcanic units and structure in this area indicate that the main phase of crustal collapse developed during the 1.7–0.7 Ma time interval. However, Late Quaternary reactivations of the extensional structure show that this process is still going on, perhaps with decreasing intensity. Moreover, identification of “mechanical” rift zones and a potentially related main thermal anomaly leads to propose that an east-dipping detachment exists beneath the Late Cretaceous–Late Miocene fold and thrust belt, controlling the crustal collapse at these latitudes. The southernmost Central Andes (35°–37°30′S) have undergone a period of shallow subduction from ~ 13 to ~ 5 Ma, that led to expansion of the arc toward the foreland, generation of ductile–fragile crustal transitions, and subsequent foreland imbrications more than 550 km away from the trench. This framework shifted to a normal Andean subduction type after ~ 5 Ma, and the arc front re-established in the present western position. The consequences of this readjustment were: a) widespread volcanic eruptions of intra-plate melts in the eastern backarc (foreland plateau flows), and b) a major trough formed between the arc front and the foreland plateau basalts (Las Loicas trough). This extensional basin controlled the emplacement of crustal melts as well as primary mantle-derived products, well represented in the Tromen volcanic plateau.