Measuring dynamic topography in South America

DÁVILA, F.M., AVILA, P., MARTINA, F., CANELO, H., NÓBILE, J.C, COLLO, G., SANCHEZ-NASSIF, F., EZPELETA, M.

Andean Tectonics

Elsevier

Año: 2019; p. 1 - 34

The understanding of the modern observed topography, as well as basin and landscape evolution, requires of a good comprehension of isostatic and dynamic forces (in addition to paleoclimate changes) acting at different time and space scales. Here we review different topographic features of South America using the residual topography concept at regional and local scales. Residual topography is the remaining value that results from the balance between modelled isostatic elevations and measured elevations. This is considered the best measurement of dynamic topography. We incorporate local geological observation to test the dynamic topography from modern and ancient examples using local subsidence analyses as well as paleo-elevations calculations from previous studies and from paleo-lithospheric thicknesses estimations based on geochemical studies on basalts. Three transects across the northern-central, southern-central and southern Andes (Peruvian Andes-Solimoes-Amazonas, Precordillera-Pampas and Patagonian areas, respectively) were inspected, particularly in areas affected by the subduction of oceanic ridges, where subsidence and/or uplifting might been influenced by dynamic forces. Our results show contradictory results and conflicts from north to south among models and with respect to previous studies. While the pericratonic and cratonic areas of Peru and Brazil, further east from the leading edge of the Nazca ridge influence, show positive residual topographies (dynamic uplift), local subsidence studies evidence a residual subsidence driven by sublithospheric forces. The opposite occurs in Patagonia. Most geological interpretations and our subsidence models suggest the need of dynamic subsidence, which does not account for the negative residual topography models. The southern-Central Andes estimations, in turn, show relatively good matching between subsidence models and residual topographies approaches in the pericratonic areas (in the Pampas), which support dynamic subsidence likewise the dynamic topography modelling. But strong disagreements arise along the flat-slab segment, where dynamic uplifting are expected from dynamic topography models. We suggest that the regional elevation state is mainly rule by the lithospheric and crustal structure and composition, supported by dynamic forces that can laterally change and generate local morphological features like isolated basins or promontories. Future and ongoing studies will allow giving answers to the apparent contradictions.