DIACHRONOUS DEFORMATION, MAGMATISM, AND BASIN FORMATION IN THE CENTRAL ANDEAN PLATEAU DUE TO FLAT SLAB SUBDUCTION

Within the Andes, the Central Andean Plateau (CAP) is anomalously wide; up to 600–800 km between the trench and the eastern deformation. The CAP also hosts the thickest crust in South America, reaching 70 km in southern Peru. The CAP has experienced vertical-axis rotations resulting in formation of the Bolivian orocline and greater shortening than regions to the north or south. The Bolivian orocline, in turn, intercepts the South American Low-Level Jet, producing a pronounced north-south gradient in precipitation.

Multiple hypotheses, including both upper and lower plate effects, have been advanced to explain these characteristics. The large scale implicated by many of these mechanisms requires equally large-scale observations. However, correlating basin stratigraphy over these distances has been hampered by a lack of datable material, particularly in Paleogene clastic strata. Here we synthesize new detrital zircon-based stratigraphic ages for non-marine strata from ~15°–16.5°S with existing records of sediment accumulation, deformation, and magmatism from ~14°–26°S to develop a model of the Eocene–early Miocene evolution of the plateau. We present 42 new maximum depositional ages (MDAs) based on 13,171 new U-Pb ages from six stratigraphic sections.

MDAs define a period of slow or zero sediment accumulation in the Paleogene which was followed by rapid sediment accumulation in the late Paleogene–Neogene. The age of the resumption of rapid sediment accumulation youngs systematically to the south at a rate of ~0.35°/Myr. Following the unconformity/condensed section, rapid sediment accumulation initiated at 46–43 Ma at 15-16°S but as young as 36 Ma at 18°S and 19 Ma at ~23°S. Both the southward advance of the unconformity and onset of rapid sediment accumulation proceeded in lockstep with the initiation of exhumation in the Eastern Cordillera and also a magmatic lull and widening of the magmatic arc followed by magmatic flare-up. We propose a model in which southward shallowing and resteepening of an Eocene–early Miocene flat slab beneath the CAP resulted in the stratigraphic hiatus, along with magmatic migration, lulls and flare ups, orogenic widening, tectonic rotations, and high-magnitude shortening. All of these provided both geographic and geological boundary conditions for later development of the CAP.