255-4 Exhumation history in the Central Andes Using Detrital Zircon Double Dating: Implications of Slab Geometry vs Orogenic Wedge Evolution

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record

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Authors:

Abstract:

The central Andes between ~14ºS and 22ºS features exceptional orogenic width and crustal thickness relative to other regions despite sharing broadly the same subduction history, suggesting upper plate controls on orogenesis. The anomalous orogenic width was established in the Eocene when deformation migrated into the Eastern Cordillera (EC). We examine two hypotheses for the driving mechanisms behind Paleogene evolution of the central Andes by reconstructing the exhumation history of basin-bounding ranges and its link to the orogenic taper. The first postulates that exhumation was driven by flat-slab subduction of the Nazca Plate, which reactivated an inherited lithospheric heterogeneity in the EC, leading to its exhumation. The alternative hypothesis proposes that deformation within the Western Cordillera (WC) orogenic wedge increased the taper angle, advancing the deformation front and initiating EC exhumation.

We test these hypotheses by integrating maximum depositional ages (MDAs), provenance data, and double-dated detrital zircon (U-Pb and (U-Th)/He) thermochronology from stratigraphic sections at ~16°S on the western and eastern sides of the northern Altiplano Basin (AB) to determine changes in source-region exhumation patterns. Depositional ages in both sections span the Late Cretaceous-late Eocene. We identified two main groups of sediment sources for both sections based on U–Pb ages: early Paleozoic strata in the Altiplano or the underlying Proterozoic massif and late Paleozoic to Mesozoic strata within the Altiplano and/or from Paleozoic strata in the EC. Lag-time analysis from the western section suggests a constant source of rapid exhumation from 66.7 to 35 Ma. After 35 Ma, an increase in Paleozoic zircons (~258-535 Ma; U–Pb ages; 13.7%) marks the introduction of a new sediment source. The eastern section shows rapid exhumation from ~45 to 40 Ma coupled with an introduction of a new sediment source at ~45 Ma (~460-533 Ma; U–Pb ages; 17.9%). A Paleogene eastward advance of the deformation front in the east coupled with constant, rapid exhumation in the west contradicts predictions based on a single, unified orogenic wedge. In that case, transition from an oversteepened to more stable wedge geometry should be accompanied by a decrease in exhumation rate in the hinterland. We therefore conclude that the eastward advance was enabled by an Eocene flat-slab subduction episode during which the WC and EC acted as mechanically independent orogenic wedges. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8332

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