Spatial distribution and quantification of surface uplift in the South-Central Andes and Precordillera.

Abstract:

The southern Central Andes are a natural setting to study the distribution of surface uplift from the core of an orogen (henceforth high Andes) to its adjacent fold and thrust belt (henceforth Precordillera). The magnitudes and distributions of surface uplift between 28°S and 35°S and across a change in slab angle have only sparsely been constrained through stable isotopes, increasing southward from nule near 28°S to ~2 km near 34°S. How surface uplift compares between the high Andes and the Precordillera remains unclear. Here, we used TopoToolbox and the Topographic Analysis Kit to map of river steepness (ksn) as a proxy for the spatial distribution of uplift using a digital elevation model. We extracted and analyzed river profile data for 20 basins along the High Andes and 13 basins in the Precordillera. We obtained an average best fit concavity index upstream of knickpoint clusters to reconstruct longitudinal river profiles for each basin, from which we were able to quantify surface uplift along and across strike. We also used a simple stream-power based landscape evolution model in Landlab to test how large a smoothing window on ksn is necessary to suppress the effects of high lithological resistance. Accounting for this lithological effect, uplift in the Precordillera increases southward from 82 m to 835 m, with ksn-inferred highest uplift rates in the Central Precordillera where the San Juan River crosses the belt. In the high Andes, uplift interestingly follows the latitudinal trend of constant mean topography, with an average of ~1.5 - 2 km from 28°S to 35°S. This signal is consistent on both sides of the Andes, which is in agreement with previous studies of erosion rates. The high Andes and Precordillera have, therefore, differently preserved the along strike uplift patterns, which is either related to timing of uplift (younger near 34°S), variable erosional efficiency (higher near 30°S), or a combination of both. Sediment accumulation rates in the adjacent foreland and stable-isotope surface uplift proxies in wedge-top basins both increase southward and are younger in the south, suggesting that these geologic proxies and the Precordillera uplift are related. These observations may point to a diachronous uplift pattern that is accommodated in the Precordillera but not in the high Andes north of 32.5°S.