ALBERT

Description: The Borborema and BeninNigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks 〉2.0 Ga; in NE Brazil, 2.63.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Serido Group of NE Brazil (〈0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceara Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating northsouth continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaiba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi4{degrees}50 Lineament in Hoggar, may represent a cryptic suture.

Description: Cratonic basins comprise a significant component of the Earth's continental crust and surface geology. Their subcircular form and large areas of flat-lying, largely undeformed sedimentary rocks characterize the central regions of many continents, and are also a significant habitat for water, mineral and petroleum resources. These basinal regions have been extensively studied, yet there is little consensus on the driving mechanism of their subsidence or their greater tectonic context. Here we present the results of an integrated basin analysis of the Paleozoic–Early Mesozoic Parnaíba cratonic basin of NE Brazil. The analysis integrates existing geological and geophysical data, and a new deep-crustal geophysical dataset, to determine the deep structure of the basin and the underlying crust and mantle. Several major features have emerged from this which constrain the basins genesis: (1) continental–shallow-marine stratigraphy characterized by an exponentially decreasing tectonic subsidence with a relatively long time constant of the order of 70–90 myr; (2) a complex Proterozoic–Early Paleozoic basement that comprises at least three major crustal blocks defined by seismic facies and conductivity contrasts with no evidence of an extensive rift system beneath the basin; (3) a mid-crustal fabric that appears to define the top of a dense and seismically fast lower crust ( V p 6.7–6.8 km s –1 and V s 3.7–3.8 km s –1 ) and upper mantle ( V p 8.2–8.4 km s –1 ) directly beneath the basin, and which correlates with a sediment-corrected Bouger gravity anomaly high of +40–60 mGal; (4) a Moho that is generally as deep or deeper beneath the basin (40–45 km) than its surrounding region (34–40 km), and which appears stepped at the terrane boundaries; (5) a relatively conductive crust and upper mantle beneath the basin, and relatively resistive crust along the boundaries of the basement blocks; and (6) igneous events immediately before and after formation of the cratonic megasequence and a geochemically enriched mantle beneath the basin that sourced two major episodes of Mesozoic igneous intrusions. These latter events are responsible for the development of an atypical gas-prone petroleum system dependent on local magmatic events for heat generation and trapping configurations. The data describing these features are presented and discussed, and their implications used to draw conclusions about the formation of the Parnaíba Basin specifically and cratonic basins more generally.

Description: The basaltic Mosquito and Sardinha formations in the Parnaíba Basin are related to the opening of the Atlantic Ocean at the Triassic–Jurassic boundary and in the early Cretaceous, respectively. The Mosquito Formation consists of tholeiitic flows with both high-Ti (〉1.5 wt%) and low-Ti (〈1.5 wt%) compositions and the petrogenetic characteristics of enriched mantle reservoirs. The Mosquito Formation basalts have an initial 87 Sr/ 86 Sr isotopic composition of 0.70296–0.70841 and a low Nd isotopic composition (0.512245–0.512677) associated with an enrichment in large ion lithophile and high field strength elements relative to primitive mantle compositions. The Sardinha Formation is composed of high-Ti and low-Ti tholeiitic dykes with subordinate alkali basalts. The Sardinha Formation rocks have trace element and isotopic features associated with enriched mantle end-members. The initial isotopic compositions range from 0.702859 to 0.706703 and 0.512184 to 0.512671 for 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, respectively. The concentrations of large ion lithophile and high field strength elements are elevated relative to primitive mantle values. Although the Mosquito and Sardinha formations share some similarities, they can be differentiated by their unique petrographic characteristics and trace element concentrations. These differences allow the discrimination of the basaltic magmatism in the Parnaíba Basin and their association with large igneous provinces, such as the Central Atlantic Magmatic Province or the Paraná–Etendeka Magmatic Province. Supplementary material: Microprobe analyses for clinopyroxene (Table A1), plagioclase (Table A2) and olivine (Table A3) for the Mosquito and Sardinha formations, along with mixing calculation parameters for the geochemical model (Table A4) are available at: https://doi.org/10.6084/m9.figshare.c.3985437

Description: The Parnaíba Basin is a Phanerozoic intracontinental basin within the South America plate, lying on top of and within Precambrian terranes. The Parnaíba Basin Analysis Program wide-angle reflection–refraction (WARR) lies east–west and is 1150 km long profile crossing the basin and its margins. The WARR results show that the crust and uppermost mantle along the profile consist of the Amazonian Craton and Borborema Province, and the Grajaú and Teresina domains comprising the Parnaíba block hidden below the sedimentary cover of the basin itself. The lithospheric characteristics of the Parnaíba block and their differences from the adjacent Precambrian Amazonian Craton and Borborema Province elucidate some aspects of the present day existence of the sedimentary basin covering it. Important elements include the presence of a high mantle velocity and high-velocity lowermost crustal region, interpreted as linked to the intrusion of mafic material into the crust underlying the Grajaú domain, and indications that the crust in this area has been intruded since its consolidation in the Neoproterozoic. It is tentatively proposed that magmatism is related to the inferred thinning of the lower crust of the Teresina and Borborema segments of the profile, with this, in turn, linked to Cretaceous extensional tectonics and the opening of the South Atlantic Ocean. Supplementary material: Datasets and ray-tracing modelling for all 20 shot gather seismic sections from the WARR experiment of the Parnaíba Basin. Available at https://doi.org/10.6084/m9.figshare.c.4058582

Description: The basaltic Mosquito and Sardinha formations in the Parnaíba Basin are related to the opening of the Atlantic Ocean at the Triassic–Jurassic boundary and in the early Cretaceous, respectively. The Mosquito Formation consists of tholeiitic flows with both high-Ti (〉1.5 wt%) and low-Ti (〈1.5 wt%) compositions and the petrogenetic characteristics of enriched mantle reservoirs. The Mosquito Formation basalts have an initial 87 Sr/ 86 Sr isotopic composition of 0.70296–0.70841 and a low Nd isotopic composition (0.512245–0.512677) associated with an enrichment in large ion lithophile and high field strength elements relative to primitive mantle compositions. The Sardinha Formation is composed of high-Ti and low-Ti tholeiitic dykes with subordinate alkali basalts. The Sardinha Formation rocks have trace element and isotopic features associated with enriched mantle end-members. The initial isotopic compositions range from 0.702859 to 0.706703 and 0.512184 to 0.512671 for 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, respectively. The concentrations of large ion lithophile and high field strength elements are elevated relative to primitive mantle values. Although the Mosquito and Sardinha formations share some similarities, they can be differentiated by their unique petrographic characteristics and trace element concentrations. These differences allow the discrimination of the basaltic magmatism in the Parnaíba Basin and their association with large igneous provinces, such as the Central Atlantic Magmatic Province or the Paraná–Etendeka Magmatic Province. Supplementary material: Microprobe analyses for clinopyroxene (Table A1), plagioclase (Table A2) and olivine (Table A3) for the Mosquito and Sardinha formations, along with mixing calculation parameters for the geochemical model (Table A4) are available at: https://doi.org/10.6084/m9.figshare.c.3985437

Description: Results of P -wave traveltime seismic tomography in central Brazil unravel the upper-mantle velocity structure and its relationship with the tectonic framework. Data were recorded between 2008 and 2012 at 16 stations distributed over the study area, and were added to the database used by Rocha et al. to improve the resolution of anomalies, and to image the surrounding regions. The main objective was to observe the upper-mantle boundary zone between the Amazonian and São Francisco cratons, represented by mobile belts, inside the Tocantins Province, and to study the lithosphere related to the collision between these two cratons during the Neoproterozoic. A set of low-velocity anomalies was observed crossing the study area in the NE–SW direction, in agreement with the main trend of the Transbrasiliano lineament. The region where the anomalies are located was interpreted as the zone separating the Amazonian and São Franciscan palaeoplates. There is a good correlation between the low-velocity anomalies and the high seismicity of this region, suggesting that it is a region of weakness, probably related to lithospheric thinning. High velocities were observed under the Amazonian and São Francisco cratons. A model is proposed for the lithospheric subsurface in central Brazil, emphasizing the boundary zone between the main palaeoplates in the study area. After merging both databases, the low-velocity anomalies in the central part of the study area suggest tectonic partitioning of the lithosphere. Synthetic tests show that the tomography results are robust.