Marked spatial gradient in the topographic evolution of the Andes spanning the Chilean flat-slab transition: evidence from stable isotope paleoaltimetry and zircon double dating

Abstract:

The major changes in the subduction angle of the Nazca plate are often hypothesized to have important consequences for the tectonic evolution of the Andes. Temporal and spatial patterns of topographic growth and exhumation are indicators that should help elucidate any linkages to subduction angle. Here, we combine observations from stable isotope paleoaltimetry with detrital zircon double dating between 30 and 35°S to demonstrate a consistent increase in surface and rock uplift in the Andes south of 32°S. The stable isotope data are from Miocene pedogenic carbonates collected from seven different basin sequences spanning different tectonic and topographic positions in the range. Paleoelevations between 1 km and 1.9 km are calculated using modern local isotope-elevation gradients along with carbonate-formation temperatures determined from clumped isotope studies in modern soils. Present day, low elevation foreland localities were at their present elevations during the Miocene, while three of the intermontane basins experienced up to 2 km of surface uplift between the end of deposition during the late Miocene and present. Detrital zircon (U-Th-Sm)/He and U-Pb double dating in three modern drainage basins (Tunuyán, Arroyo Grande and Río de los Patos) reveals clear Miocene exhumation signals south of the flat slab with no recent exhumation apparent at 32°S. The exhumation pattern is consistent with paleoaltimetry results. Interestingly, the maximum inferred surface uplift is greatest where the crust is thinnest, and the timing of the observed changes in elevation and exhumation has not been linked to any documented episodes of large-magnitude crustal shortening in the eastern half of the range. The spatial pattern of surface uplift and exhumation seems to mimic the Pampean flat slab’s geometry, however, it could be equally well explained by eastward migration of a crustal root via ductile deformation in the lower crust and is not related to flat-slab subduction.