THE NEOPROTEROZOIC-EARLY PALEOZOIC TECTONIC EVOLUTION OF THE NORTHEAST OF BRAZIL: AN INTEGRATED GEOLOGICAL-GEOPHYSICAL ANALYSIS OF THE PRE-SILUIAN BASEMENT OF PARNAÍBA BASIN

Abstract

THE NEOPROTEROZOIC-EARLY PALEOZOIC TECTONIC EVOLUTION OF THE NORTHEAST OF BRAZIL: AN INTEGRATED GEOLOGICAL-GEOPHYSICAL ANALYSIS OF THE PRE-SILURIAN BASEMENT OF PARNAÍBA BASIN. The Phanerozoic sedimentary sequences of Parnaíba basin (PB) occupy at least 600.000 km2 of the NE region of Brazil. A regional Pre-Silurian unconformity marks the erosive planar base of the PB, and indistinctly cuts key tectonic units for the understanding of the Neoproterozoic evolution of western Gondwana (WG) in this region. The PB also preserves one of the largest Ordovician-Silurian sedimentary records of WG, represented by the Serra Grande Group. A comparative analysis of geophysical and geological datasets is presented here supporting a new proposal for the tectonic configuration of the pre-Silurian basement of PB, as well as presenting some insights related to the tectono-sedimentary controls of PB during the Early Paleozoic. Using integrated seismic interpretation and gravity modelling, constrained by an updated grid of the Moho depth, well data and a compilation of recent geophysical studies, two main basement blocks in the center of the basin were identified. They represent pre-Brasiliano inliers, surrounded by Brasiliano mobile belts, and assigned to two major crustal building blocks of western Gondwana. The Grajaú block belongs to the Amazonian-West Africa block, and is characterized by low gravity anomaly, thicker crust (41-45km), transparent seismic pattern of the basement and a high velocity lower crust. The Teresina block belongs to the Central African block and is characterized by slightly thinner crust (39-41km), higher values of gravity and magnetic anomalies and by the presence of a mid-crustal reflectivity (MCR), observed in seven seismic lines and here interpreted as a remnant of a paleosuture zone between both blocks. Along a NE-SW 500-km seismic and gravity profile, the MCR was interpreted as crustal-scale thrust faults verging westwards and also defined the Barra do Corda mobile belt, which was formed by the closure of the Goiás-Pharusian ocean, in between the Grajaú and Teresina blocks. This belt deforms the eastern margin of the Ediacaran Riachão foreland basin (RB), observed in seismic and well data beneath the SW portion of PB. The western margin of RB is bounded by eastwards verging thrust faults, interpreted as a zone of back-thrusts and thinned crust (~36km) in the eastern prolongation of the Araguaia belt beneath PB. To the east, the limit between the Teresina block and the Borborema Province is marked by the NE-SW Transbrasiliano Fault Zone (TBFZ), along which Neoproterozoic mylonites were recovered from wellbores, and a system of narrow pull-apart basins is interpreted in the seismic data, possibly during the Cambro-Ordovician. Field observations in the NE portion of PB described conglomerates in a scarp-related alluvial fan system composing the basal Ordovician-Sulurian sequence of PB (Ipu Formation). As also suggested in the seismic data, these deposits were possibly controlled by tectonic reactivations along the TBFZ. Finally, the isopach maps of the Paleozoic sequences of PB indicate that the basement configuration played a role in the basin tectono-stratigraphy. The depocenter of PB from Early Paleozoic to Early Mesozoic lied upon the Teresina Block and then, during the Mesozoic, it migrated to the west, towards the Grajaú block.