Abstract
The Phanerozoic intracontinental Parnaíba Basin in northeast Brazil lies atop crust composed of Archaean to Mesoproterozoic cratonic blocks and Neoproterozoic mobile belts. Recently, active and passive source geophysical surveys characterised the crustal structure beneath the basin. We use information from published active-source seismic and new, coincident receiver function (RF) data to obtain Vp/Vs ratios for sedimentary and crustal structure and make inferences about crustal compositions and tectonic evolution. In our approach, sedimentary and crustal Vp/Vs ratios are adjusted to match common conversion point (CCP) images of RFs and known Moho and basement geometry. We use a P-wave model from published wide-angle reflection/refraction (WARR) seismics, and structural features from a deep seismic reflection (DSR) profile. CCP images of the primary RF conversions were used to model the crust, whilst conversions of multiples were used for the sediment-basement interface. The maximum uncertainties in Vp/Vs are estimated to be 0.15 for the basin and 0.03 for the crust. Vp/Vs ratios in the basin were estimated between 1.7 and 2.2. Lower values correlate with the exposure of older units primarily in the east of the basin, whilst higher values coincide with exposed younger units of the Parnaíba Basin. The obtained crustal Vp/Vs ratios between 1.73 and 1.81 support the previously published segmentation of the crust. In particular, we identified three regions of elevated Vp/Vs ratios, which can be related to proposed Neoproterozoic suture zones underlying the Parnaíba Basin, as well as high velocity lower crust beneath. The high Vp/Vs ratios can be explained by mafic compositions, for example metamorphosed or intruded crust, or fluids and sedimentary rocks entrained into highly deformed crust, typical for modifications related to suture zones. These new deep geophysical models provide important and complementary evidence for crustal amalgamation and the formation of the Parnaíba Basin.
Highlights
The intracontinental Parnaíba Basin in northeast Brazil formed in the Phanerozoic as a consequence of lithospheric subsidence related to postorogenic thermal events and regional lithospheric extension after the Late Neoproterozoic/Cambrian Brasiliano-Pan African orogenesis (Goes and Feijo, 1994; Vaz et al, 2007)
Sedimentary and crustal Vp/Vs ratios in the Parnaíba Basin were obtained by combining common conversion point (CCP) stacking of new receiver function (RF) data with coincident existing deep seismic reflection (DSR)-wide-angle reflection-refraction (WARR) profiles
CCP imaging using conversions of RF-multiples exhibits a good agreement with the DSR and WARR images (Fig. 6a) and is able to reproduce some of the internal stratigraphic layering of the basin, as well as basement complexity
Summary
The intracontinental Parnaíba Basin in northeast Brazil formed in the Phanerozoic as a consequence of lithospheric subsidence related to postorogenic thermal events and regional lithospheric extension after the Late Neoproterozoic/Cambrian Brasiliano-Pan African orogenesis (Goes and Feijo, 1994; Vaz et al, 2007). Castro et al, 2016, 2014; Lima et al, 2017) and surface geology, in recent years, the crustal and sedimentary structure was subject to detailed geophysical surveys These surveys were concentrated along an approximately 1400 km long E-W oriented profile across the basin, including deep seismic reflection. We present here an analysis of published DSR and WARR images in combination with newly obtained RF data from the Parnaíba Basin to retrieve information on Vp/Vs ratios for the sedimentary package and crystalline crust and discuss their relation to possible rock com positions and tectono-magmatic evolution. We obtain Vp/Vs ratios for the sedimentary basin and the crystalline crust by matching basement and Moho interfaces in the CCP images with information from the published DSR and WARR data (see Data and Methods section for details). High crustal Vp/Vs ratios can be related to crustal lithologies typically found in collision and suture zones, or crustal intrusions, and variations of sedimentary Vp/Vs ratios can be related to differently compacted sedi mentary rocks
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