TRACKING EXHUMATION AND OROGENIC TAPER IN THE CENOZOIC CENTRAL ANDES OF SOUTHERN PERU USING DETRITAL ZIRCON DOUBLE DATING

Tectonic models of the central Andean Altiplano Basin (AB) suggest that subsidence in the Late Cretaceous–late Paleogene was driven by flexure due to loading by an eastward-propagating, retroarc fold-thrust belt in the adjacent Western Cordilleran (WC). Exhumation of the adjacent Eastern Cordillera (EC) began in the Eocene, driven by flat subduction of the Nazca Plate. This model predicts that exhumation and sediment derivation from the WC in the Late Cretaceous and Paleogene would have been joined by EC-derived sediment in the Eocene.

We test these predictions by integrating published provenance data with new detrital zircon double dating (coupled U-Pb and (U-Th)/He) to unravel the spatial and temporal evolution of exhumation of the orogen and its relationship to orogenic taper. We use the U-Pb data (n=133) to identify two main groups of sediment sources: group I (900-1200 Ma ages) and group II (400-700 Ma ages). Group I is attributed to early Paleozoic strata in the Altiplano or the Proterozoic massif below the Altiplano, whereas group II is potentially late Paleozoic–Mesozoic strata in the Altiplano and/or the Paleozoic strata in the EC. Lag time for group 1 increases from 65 to ~35 Ma and decreases after 35 Ma, suggesting tectonic quiescence followed by accelerated exhumation. For group II, lag time decreases rapidly in the latest Cretaceous but then increases again in the Paleogene, similar to group I. After 35 Ma, lag time and absolute thermochronological age for group II increase dramatically suggesting introduction of a new, non-reset sediment source with similar U-Pb ages.

Published provenance data indicate that sediment provenance is dominated by WC/Altiplano sources except for a pulse of EC detritus that arrives between 55 and 45 Ma. This is consistent with group I and II lag times of 10–20 Myr in the latest Cretaceous, indicating of rapid exhumation in the WC/Altiplano. We interpret the Paleocene–Eocene increase in lag time as a period of tectonic quiescence in the WC/Altiplano as shortening migrated to the EC, thereby placing the orogen in a subcritical state. Counterintuitively, subsidence rates in the AB decreased during EC exhumation, pointing to subcrustal support. Rebuilding taper resulted in reinitiation of exhumation in the WC/Altiplano after 35 Ma, affecting both previously exhumed regions and new, non-reset sediment sources.