Across-strike asymmetry of the Andes orogen linked to the age and geometry of the Nazca plate

Abstract:

The spine of Andes – the trace of the highest mountain topography – weaves back and forth, in places near the coastline, in others farther inland. Its position is thought to be partially influenced by orographic rainfall distribution that controls where most exhumation takes place within the orogenic wedge. Orographic rainfall along the Andes coincides with distinct topographic forms of mountains and higher erosion rates in some cases which has been argued as a climatic control of the across-strike asymmetric distribution of mass of the Andes. However, the distribution of forces within the overriding plate is expected to be strongly linked to the age and geometry of the subducting plate, with an older subducting plate causing a steeper slab and higher stress near the trench. Here, we assess the competition between these tectonic and climatic end-member models of mountain range asymmetry. Orogen asymmetry is computed as the trench to mean peak topography distance normalized by the trench to retro-arc deformation front distance (0.5 is symmetric) and then compared to the age and radius of curvature of the subducting Nazca plate using previously published data. Climatic influence is captured in the pro-wedge to retro-wedge ratio of average decadal rainfall rates. A partial regression analysis demonstrates that the asymmetric shape of the Andes orogen is most strongly explained by the age and radius of curvature of the subducting Nazca slab as predicted by geodynamic, force-balance models, leaving orographic rainfall to account for little remnant, unexplained variance. We confirm this lack of climatic control using compiled cosmogenic nuclide erosion rate data from the Octopus database. Noting that topographic form indeed covaries with rainfall rates, we demonstrate that this does not require different erosion rates by showing a weak relationship between erosion rates and rainfall rates (R² of 0.15). Hillslope gradient explains 44% of the observed variance in erosion rates in the Andes across order of magnitude changes in rainfall rates. Our results suggest that mountain range migration might be a common component of orogenesis but for reasons different than previously thought. Cyclical variations in Andean orogeny might also accompany lateral migrations of mountain ranges.