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.
- Composition and structure of the continental crust
- Intracontinental basin
- South America
- Vp/Vs ratios