Crustal and Lithospheric Structure of the Pantanal Wetlands, an Active Quaternary Basin Surrounded by Cratons in South America.

The Pantanal Wetlands in Brazil is presently subsiding between two cratons, Amazon to the north and Atlantic platform to the south. The driving force behind this subsidence is not known: it could be long-range flexural effects of the Andean orogeny, or dynamic topography from mantle convection. The strongest asthenospheric low-velocity anomaly in stable South America occurs beneath the Pantanal. To understand the mechanisms of basin development, 40 seismic stations were installed (FAPESP “3-Basins Project”). Our first results are: A belt of thin crust (~35 km) is found beneath 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. Both P- and S-wave teleseismic tomography show strong low-velocity anomalies (100-300 km depth) beneath the Pantanal. Global tomography had traced the Nazca slab in the bottom of the transition zone beneath the Pantanal. Both regional P- and S-wave tomography confirm the geometry of the Nazca slab: stagnant near the 660 km discontinuity, south of the Pantanal Basin, and plunging to the lower mantle, north of the Basin. This change of slab dip may disturb upper mantle convection and cause downward flow beneath the Basin. Receiver function analyses of the 410 and 660-km discontinuities show that the cold, stagnant Nazca slab lowers the 660-km discontinuity under the Pantanal/Chaco basin. SKS splitting indicates strong anisotropy beneath the Pantanal and flow surrounding the Amazon craton and a cratonic block beneath the Paraná Basin.