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

Abstract

The role of sub‐Andean foreland peripheral bulges in controlling sedimentary sequence deposition and Quaternary geomorphology is discussed in the case of the Pantanal wetland, SW Brazil. This shallow (0.5 km) and wide (>200 km) depression was formed because of uplift and flexural extension in the brittle upper crust which mechanically reactivated the Neoproterozoic Paraguai fold/thrust belt. Depth to the basement and structural information within the Pantanal wetland were obtained from gravity, reflection seismics, drill hole, and digital topography data. The upper part of the basin is filled with Pliocene‐Quaternary 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 thin elastic plate for the Brazilian shield lithosphere with an effective elastic thickness of 125–150 km, its western edge at 67.5° W under the Altiplano and loaded on a free end by the Andes topography, predicts the forebulge to be presently located over the Pantanal wetland. Flexural modeling was constrained using gravity data extending from the Pacific Ocean to the east of the Pantanal wetland. The width of the bulge is 700 km, and its amplitude is ∼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 migration, together with compression and uplift of the Central Andes. The main phase of the wetland subsidence is very likely related to the last compressional pulse in the Andes, during the upper Pliocene‐lower Pleistocene (∼2.5 Ma), as inferred from field study of fault kinematics of the Central Andes.