Chapter 3 - Measuring dynamic topography in South America

Abstract

The understanding of modern topography, and basin and landscape evolution, requires clear comprehension of isostatic and dynamic forces (in addition to paleoclimate changes) over different temporal and spatial scales. Here we review different topographic features of South America using the residual topography concept at regional and local scales. Residual topography, the difference between modeled isostatic elevations and measured elevations, is considered the best measure of dynamic topography. We incorporate geological observations to test the dynamic topography for modern and ancient examples using local subsidence analyses, paleoelevation calculations from previous studies, and paleolithospheric thickness estimates based on geochemical studies of 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 subduction of oceanic ridges, where subsidence and/or uplift may be influenced by dynamic forces. Our results show contradictory results among models and with respect to previous studies. The pericratonic and cratonic areas of Peru and Brazil, farther east from the leading edge of the Nazca ridge, show positive residual topographies (dynamic uplift) and local subsidence studies support residual subsidence driven by sublithospheric forces. The opposite occurs in Patagonia. Most geological interpretations and our subsidence models suggest the need for dynamic subsidence, which does not account for negative residual topography. In turn, estimates for the southern Central Andes show a relatively good match between subsidence models and residual topography in pericratonic areas (Pampas), which support dynamic subsidence and topography. But strong disagreements arise along the flat-slab segment, where dynamic uplift is expected from models of dynamic topography. We suggest that regional elevation is mainly ruled by lithospheric and crustal structure, along with bulk composition, as supported by dynamic forces that can change laterally and generate local morphological features such as isolated basins or promontories. Ongoing and future studies will help resolve these apparent contradictions.