Neotectonics and Seismicity in the southeast edge of the Pantanal Basin (Mato Grosso do Sul, Brazil)

The largest expression of neotectonics in the State of Mato Grosso do Sul is documented in the Pantanal Sedimentary Basin, considered a N-S tectonic trench, with 400 km in length, 250 km in width and a sedimentary thickness of > 400 m. The Pantanal Basin is located in the west of the Mato Grosso do Sul state, occupying an area of 135,000 km². The Pantanal Plain is surrounded by the Maracaju-Campo Grande and Taquari-Itiquira plateaus to the east, Guimarães and Parecis to the north, Urucum-Amolar to the west and Bodoquena to the south. The region is surrounded by plateaus in rocks that range from the Precambrian to the Cenozoic. Historically, the Pantanal Basin is considered one of the seismic regions of Brazil. Intraplate earthquakes mainly result from ruptures along zones of preexisting weaknesses, located close to structural inhomogeneities, which concentrate efforts that, added to regional efforts, are capable of generating earthquakes. The main structural features identified are expressed by joints, faults and fault zones. Two main sets of fault types were recognized: normal and transient. The normal faults are those related with the NE-SW opening of the basin, while the transcurrent faults are younger and characterized by an E-W direction. The direction of the binary E-W and dextral movement enable the development of NW-SE discontinuities. The drainage network and the structure of the landforms are strongly controlled by these structural discontinuities, associated with the collapse of blocks. This allows the development of areas with greater subsidence and accommodation space for Holocene sediments in the basin, such as the so-called “Brejão do Negro”, which proved to be a frequently flooded area, marked by a hidden fluvial lake due to a floating mat of macrophytes. The Rio Negro lineament is an important seismic-tectonic feature in the southeastern portion of the Pantanal Basin, functioning as an E-W shear zone, with current neotectonic movement, forming a pull-apart rhombohedral structure. The earthquakes in the Pantanal Basin present values between 0.6 to 5.4 mb. In the brittle shear region comprised by the Negro lineament (area comprised by the Brejão do Negro), the magnitudes are higher, usually between 3.0 and 4.0 mb. Recent earthquakes recorded in this basin are the result of uniform E-W compression stresses. These lineaments impose tectonic landforms, being indicated by subsidence, escarpment, fluvial avulsions and captures, elbows, rectilinear lake edges, among others, associated with evidences neotectonic activity in the Pantanal Basin. From surveys focused on the identification of structural discontinuities and seismological activity, two tectonic events were identified for the area: the first being distensive, associated with the opening of the basin, and the second of transcurrent character.