INVESTIGATING CORDILLERAN MARGIN DEVELOPMENT USING DETRITAL THERMOCHRONOLOGY IN THE CENTRAL ANDES OF SOUTHERN PERU

The link between subduction zone dynamics, orogenesis, and surface processes remains unclear. In the central Andes, debate persists regarding the onset of Andean shortening, and whether the fold-thrust belt deformation pattern reflects cyclical feedbacks of the upper plate or are driven by changes in subduction zone geometry. This study presents new results from detrital zircon U-Pb geochronology and (U-Th)/He thermochronology double dating to 10 horizons from the Cusco stratigraphic section of southern Peru to determine upsection lag time trends (the difference between depositional and cooling ages). The Cusco stratigraphic section (59–7 Ma) in the Central Andes preserves much of the sedimentological archive of Cenozoic Andean orogenesis and spans a proposed shallow subduction phase at 45–27 Ma. Sediment provenance results indicate sediment sourced from the Western and Eastern Cordilleras, and new detrital thermochronologic results likely represent cooling ages from both regions. 194 new (U-Th)/He ages were returned and 4 stratigraphic samples were partially reset and offer no information on lag time. Detrital (U-Th)/He results from non-reset samples reveal a dominant age mode at 80 Ma, consistent with other datasets that argue for ca. 80 Ma onset of Andean shortening. Lag times of non-reset samples decrease upsection from 41 to 9 Ma, demonstrating that the Eastern and Western Cordillera were shortening simultaneously, and reveal a previously undocumented phase of distributed deformation across this segment of the Andes immediately following slab shallowing. Such distributed deformation may indicate a protracted phase of subcritical wedge state. Near the top of the Cusco section, there is an increase in lag time from 9–7.4 Ma. The upsection increase in lag time follows the deformation jump inboard to the Subandean Zone and may reflect a shift to supercritical wedge state. We integrate new detrital thermochronologic results with structural datasets to explore the predictions of cordilleran model development, such as the Cordilleran Cyclicity Model, and models that emphasize the role of changing slab geometry. These results demonstrate how changing slab geometry can disrupt Cordilleran cycles and emphasize linkages between deformation and surface processes with the geometry of the subduction zone.