Lithospheric Structure of the Pantanal Wetlands, an Active Quaternary Basin in Mid-Plate South America: Results of the 3-Basins Project

The Pantanal Wetlands in SW Brazil is a presently subsiding basin between two cratonic domains, Amazonian to the north and Atlantic platform to the south. The driving force behind the Pantanal subsidence is not well understood: it could be related to long-range flexural effects of the Andean orogeny, or to dynamic topography due to upper mantle convection. The strongest low-velocity anomaly at lithospheric/asthenospheric stable South America occurs beneath the Pantanal. To help understand the mechanisms of basin development, 40 temporary seismic stations and 10 GNSS stations have been installed since 2016 in the Pantanal, Chaco and Paraná basins under the FAPESP-funded “3-Basins Project”. We present the first results of the crustal and upper mantle structure. A belt of thin crust (~35 km) is found beneath the eastern part of the Pantanal Basin between normal/thick crust (40-44 km) of the Paraná Basin to the east and a cratonic crust (~40 km thick) to the west. The belt of thinned crust seems to continue south beneath the Chaco-Paraná basin in Paraguay and NE Argentina. Both P- and S-wave teleseismic tomography show strong low-velocity anomalies (100-300 km depth) following the Araguaia/Paraguay foldbelt, passing beneath the Pantanal. Some global tomography models had traced the Nazca slab in the bottom of the transition zone beneath the Pantanal. Both regional P- and S-wave tomography, including the 3-Basins stations, confirm the geometry of the Nazca slab: it is stagnant near the 660 km discontinuity to the south of the Pantanal Basin, and it plunges directly to the lower mantle to the north of the Basin. Preliminary receiver function analysis of the 410 and 660-km discontinuities indicate an influence of the cold, stagnant Nazca slab in lowering the 660-km discontinuity under the Pantanal/Chaco basin. This change of dip (contortion?) of the Nazca slab may disturb the upper mantle convection and cause downward flow beneath the Basin. Analysis of SKS splitting indicates strong anisotropy beneath the Pantanal and flow around a cratonic block beneath the Paraná Basin. Distribution of intraplate earthquakes in two main seismic zones correlates with thin lithospheric spots.