Basin formation, magmatism, and exhumation document southward migrating flat-slab subduction in the central Andes

Abstract

The Central Andean Plateau is the largest plateau formed in a non-collisional setting and is taller, wider, has thicker crust, and has greater retroarc shortening than any other region of the modern Andes. It also hosts some of the thickest Cenozoic strata in the Andes, which were deposited synchronously with punctuated widening and narrowing of the magmatic arc. Multiple hypotheses have been advanced to explain these unique characteristics, yet the detailed, plateau-scale Cenozoic basin history needed to test these hypotheses remains unavailable and therefore has not been integrated into the established records of deformation and magmatism. Here we synthesize new detrital zircon maximum depositional ages (MDAs) and sediment provenance analysis of non-marine strata from ∼15°–16.5°S with existing records of sediment accumulation, deformation, and magmatism from ∼14°–24.5°S to develop a model of the Eocene–early Miocene evolution of the Altiplano-Puna plateau region. Stratigraphic correlations based on the new MDAs show a Paleocene–Eocene unconformity/condensed section followed by resumption of rapid sediment accumulation. Rapid sediment accumulation resumed at 46–43 Ma at 15–16°S, at 36 Ma at 18°S, and as young 19 Ma at ∼23°S. Provenance data presented herein 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 hiatus, along with the magmatic lull and flare-up, orogenic widening, and high-magnitude shortening, are upper plate responses to shallowing and resteepening of an late Paleocene–early Miocene flat slab beneath the Altiplano-Puna plateau. In the proposed model initial slab flattening was driven by late Paleocene–early Eocene subduction of buoyant oceanic crust: potentially a Manihiki Plateau Conjugate. Subsequently, southward shallowing and resteepening of a late Eocene–early Miocene flat slab beneath the Altiplano-Puna Plateau was driven by subduction of an asperity on the Nazca Plate which we tentatively identify with the Juan Fernandez Ridge. The geographic coincidence of all of these features suggests that the flat slab hydrated and weakened the upper plate lithosphere, thereby facilitating later development of the suite of unique features which characterize the Central Andean Plateau.