REVISITING THE RETROARC FORELAND BASIN MODEL FOR THE ALTIPLANO BASIN

The Central Andean Plateau (CAP) is taller, wider, has thicker crust, and has greater retroarc shortening than anywhere else in the Andes. It also hosts some of the thickest Cenozoic strata in the Andes. Models proposed to account for the evolution of the Altiplano Basin include an eastward migrating fold-thrust belt and foreland basin, backarc extension, and a hinterland setting. Any of these could be modified by slab flattening or lithospheric delamination. However, the detailed, plateau-scale Cenozoic basin history needed to test proposed tectonic models has been unavailable. Here we synthesize new detrital zircon maximum depositional ages (MDAs), measured stratigraphic sections, and sediment provenance analysis of non-marine strata to develop a model of the Eocene–early Miocene evolution of the Altiplano-Puna plateau region. Stratigraphic sections show that the northernmost Altiplano records a classic foreland basin succession of depozones, while other sections along strike lack this diagnostic foreland basin signature. Stratigraphic correlations based on the new zircon MDAs show a Paleocene–Eocene unconformity/condensed section followed by resumption of rapid sediment accumulation. Rapid sediment accumulation resumed asynchronously from north to south: at 46–43 Ma at 15–16°S, at 36 Ma at 18°S, and as young 19 Ma at ~23°S. Provenance data indicate that Eastern Cordilleran detritus appeared in Altiplano strata at progressively younger ages to the south. The southward progression of this basin reorganization proceeded in lockstep with initiation of exhumation in the Eastern Cordillera, and also a lull in the magmatic arc followed by widespread volcanic flare-up. We present a model in which the observed stratigraphic patterns and hiatus, along with the magmatic lull and flare-up, orogenic widening, and high-magnitude shortening, are upper plate responses to shallowing and resteepening of a late Paleocene–early Miocene flat slab beneath the Altiplano-Puna plateau which drove a change from foreland to hinterland basin deposition, rather than the result of deposition from classic foreland basin. Hydration and weakening of the upper plate lithosphere facilitated later development of the suite of unique features which characterize the CAP.