RESOLVING THE GEODYNAMIC EFFECTS OF SPREADING RIDGE SUBDUCTION IN THE PATAGONIAN ANDES USING MULTICHRONOMETER THERMOCHRONOLOGY

The subduction of oceanic spreading ridges and subsequent emplacement of slab windows has been proposed to modify regional stress regimes, thermal structure, and lithospheric thickness of the overriding plate with implications for erosion rates and landscape evolution. However, local structural and lithospheric heterogeneities make it difficult to definitively isolate the effects of spreading ridge subduction in modern ridge subduction settings. We use thermochronologic modeling that integrates apatite and zircon (U-Th)/He and apatite fission track datasets to reconstruct a 20 m.y. orogen-scale pattern of the subduction of the Chile Ridge spreading center along the western margin of the southern Patagonian Andes (47°S-54°S) throughout the Neogene. Modeling results identify a northward migrating signal of rock cooling from 20 Ma to 5 Ma throughout the arc batholith and retroarc that predate the subduction of the Chile Ridge spreading center by 2-5 Myr. We suggest that this cooling trend is related to topographic uplift and associated exhumation as a response to oblique ridge subduction and crustal thickening along the leading edge of the subducting ridge. Our findings indicate simultaneous isostatic uplift across volcanic arc/batholith and thrust belt domains >300 km from the subduction zone. The absence of this signal <160 km from the trench may indicate a thermo-mechanical threshold limiting crustal thickening and/or uplift along the subduction margin. This uplift predates subduction of the Chile Ridge itself by ~ 2-5 m.y. and is recognized as a cooling trend in time-temperature histories constrained by thermochronology. Cooling and interpreted isostatic uplift cease within 10-20 m.y. following slab window formation suggesting that heat from asthenospheric upwelling does not produce measurable thermal resetting in the upper crust above the slab window.