Basement reactivation in a sub-Andean foreland flexural bulge: The Pantanal wetland, SW Brazil

The role of sub-Andean foreland peripheral bulges in controlling sedimentar y sequence deposition and Quaternary geomorphology is discussed in the case of the Pantanal wetland, SW Brazil. This shallow (0.5 km) and wide (>200 k m) depression was formed because of uplift and flexural extension in the br ittle upper crust which mechanically reactivated the Neoproterozoic Paragua i fold/thrust belt. Depth to the basement and structural information within the Pantanal wetland were obtained from gravity, reflection seismics, dril l hole, and digital topography data. The upper part of the basin is filled with Pliocene-Quaternaly unlithified sands. The age of the oldest sediments is unknown, and normal faults are mostly observed on the western border of the basin. A two-dimensional flexural model composed of a semi-infinite th in elastic plate for the Brazilian shield lithosphere with an effective ela stic thickness of 125-150 km, its western edge at 67.5 degrees W under the Altiplano and loaded on a free end by the Andes topography, predicts the fo rebulge to be presently located over the Pantanal wetland. Flexural modelin g was constrained using gravity data extending from the Pacific Ocean to th e east of the Pantanal wetland. The width of the bulge is 700 km, and its a mplitude is similar to 310 m. The total amount of extension and subsidence observed in the Pantanal basin is interpreted as due to a cumulative strain since late Oligocene when the Brazilian shield started its westward migrat ion, together with compression and uplift of the Central Andes. The main ph ase of the wetland subsidence is very likely related to the last compressio nal pulse in the Andes, during the upper Pliocene-lower Pleistocene (simila r to 2.5 Ma), as inferred from field study of fault kinematics of the Centr al Andes.