DECONVOLVING MULTIPLE SUBSIDENCE MECHANISMS IN RETROARC CORDILLERAN GEODYNAMICS THROUGH BASIN ANALYSIS OF THE ALTIPLANO BASIN

Synorogenic basins provide a spatio-temporally extensive perspective on lithosphere-scale processes that drive vertical crustal movement. High-resolution basin analysis in the central Andes informs the ongoing debate on the role and relative importance of upper- versus lower-plate processes at continental subduction margins, a particular problem where flat-slab subduction is involved. Stratigraphy of the Altiplano region spans Late Cretaceous–Cenozoic central Andean orogenesis. While recent sedimentology and paleoaltimetry data have improved the four-dimensional picture of vertical crustal movement in the northern Altiplano (southern Peru and NW Bolivia), limited chronostratigraphy for Paleogene non-marine strata has precluded a comprehensive subsidence analysis for the entire Altiplano.

We report U-Pb detrital zircon geochronology from newly measured/remeasured stratigraphic sections and integrate these data with published chronostratigraphy. Preliminary tectonic subsidence analysis of nine locations spanning the Altiplano Basin from ~13°S to 25°S shows variation in the timing, rate, and magnitude of basin development. Across these latitudes, stratigraphy records Late Cretaceous subsidence followed by a Paleocene decrease in subsidence rates. A subsequent N to S-progressing onset of rapid Paleogene subsidence in the northern and central AP can be partially attributed to coeval crustal shortening and tectonic loading-induced flexure. Subsidence rates decrease for the northern Altiplano by ~24 Ma but remain high for the central Altiplano until <10 Ma. Geohistories generally do not conform to idealized subsidence curves for particular drivers, suggesting superposition of mechanisms such as lithospheric flexure, extensional/transtensional faulting, lithospheric densification, and/or dynamic topography. This ongoing work will extend coverage of age constraints on central and southern Altiplano strata (SW Bolivia) for a more complete picture of basin-forming tectonic-geodynamic processes in cordilleran systems.