Constraints on South American crustal evolution and composition from crustal Poisson's ratio and thickness measurements

Using a receiver function stacking procedure and data from 37 broad-band seismic stations installed along a 3000-km EW profile across central South America, we have determined crustal Poisson's ratio (σ ) and thickness across the entire continent along 20-degree South. Similar to previous studies using various techniques/datasets, we have found a thick crust beneath the central Andes. In addition, we have found that the crustal Poisson's ratio in this area is higher-than-normal (0.27), suggesting a mafic average crustal composition. In contrast, the two sites located on the immediate east of the surface expression of the Andes are found to have low σ (about 0.23) and normal crustal thickness (36 km), suggesting a felsic composition. Under the assumption that the pre-shortening crust beneath the present-day Andes was similar to that beneath those two sites, the dramatic contrast in σ suggests underplating of mantle-derived mafic materials. Such an underplating could be achieved by thermal thinning of a weak basal lithospheric layer. Obviously, simple tectonic shortening of the felsic crust would not be able to form the inferred mafic Andean crust. This conclusion is consistent with geothermal and geochemical observations, which suggested the existence of mantle-derived rocks in the crust. Crust beneath the Parana basin and the Sao Francisco craton are found to have a normal thickness (38-44 km) with a medium-to-high σ (0.26). Possible correspondence between the observed spatial variation of crustal thickness and σ with the occurrence of the extensive flood basalt and other major tectonic events will be presented.