REGIONAL CHRONOSTRATIGRAPHY HIGHLIGHTS ASYNCHRONOUS FORELAND-TO-HINTERLAND TRANSITION IN THE CENTRAL ANDES’ ALTIPLANO BASIN

Tectonic models of the central Andes of southern Peru and western Bolivia propose that the Altiplano Basin (AB) transitioned from a retroarc foreland position to a hinterland position as shortening jumped ~400 km inboard from the Western Cordillera to the Eastern Cordillera (EC). The spatio-temporal progression and tectonic drivers of this structural event have been debated. Recent detrital zircon (DZ) work on the northern AB has suggested EC development progressed southward; however, whether this applies to the remainder of the AB has not been tested. Detailed regional reconstructions of syntectonic basin geometry and sediment sources through time can be powerful tools for testing model predictions, but this has been largely precluded for the AB by sparse depositional age constraints.

We present new DZ U-Pb results and sedimentological observations from across-strike transects in the central and southern AB. We use these data to test model predictions that EC orogenic wedge growth should be recorded in the adjacent AB as an influx of EC-sourced sediment and increased flexural subsidence on the basin’s eastern side. For the central AB transect, DZ maximum depositional ages (MDAs) indicate mid-Eocene—Oligocene subsidence rates were higher in the east than west, consistent with flexure in a hinterland setting induced by a growing EC. Paleocurrents, clast counts, and DZ source mixture modeling suggest the EC was well-developed by the late Eocene, later here than further north. In contrast to the central AB, mid—late Eocene subsidence rates in the southwestern AB accelerated earlier than in the southeastern AB, as expected for an eastward-migrating foredeep with a tectonic load to the west. However, paleocurrents, clast counts, and DZ mixture models are consistent with results further north and indicate the EC became a significant sediment source for the southeastern AB in the late Eocene, as expected from published thermochronology and studies of time-equivalent stratigraphy in the adjacent EC. Overall, our results support a southward-migrating foreland to hinterland transition along the AB’s entire length. The spatial diachroneity suggests that a fairly localized process, such as a southward-sweeping flat slab segment and/or change in plate interface properties, caused the jump in deformation and set up the Central Andes’ anomalous width and high shortening magnitude.