Delimiting the Neoproterozoic São Francisco Paleocontinental Block with P-wave traveltime tomography

Abstract

SummaryThe São Francisco Paleocontinental Block (SFPB) represents part of the Congo-São Francisco Paleocontinent (CSFP), amalgamated around 2 Ga. In the Neoproterozoic, a branched continental rift system evolved to ocean basins around most edges of the SFPB that remained only partially linked to the Congo Paleocontinent by means of the Bahia-Gabon Continental Bridge. After the Brasiliano—Pan-African orogeny, two relatively preserved CSFP sectors formed the São Francisco and Congo cratons, surrounded by Neoproterozoic orogenic belts. Recent results of upper mantle P-wave seismic tomography allowed us to suggest a delimitation in lithospheric depths of the Neoproterozoic SFPB, which comprise the São Francisco Craton, and that this would have been connected with the Congo Paleocontinent along the Araçuaí Belt. It is characterized by high-velocity anomalies and its boundaries with other blocks are marked by low-velocity anomalies at lithospheric depths. We tested the resolution of the tomographic results through synthetic models obtained by a ray tracing scheme using the observed ray configuration. We observe that the lateral resolution is adequate, but the method used was not able to set the depth reached by the SFPB. Our results indicate that the SFPB area in lithospheric depths is larger than the surface area ascribed to the São Francisco craton, and thus, the SFPB basement deeply extends beneath neighboring orogenic regions, suggesting that these Neoproterozoic mobile belts, such as Araçuaí Orogen and the Brasilia Fold Belt, reworked the continental crust. We observe a low-velocity anomaly in the SFPB central region, corresponding to the Pirapora aulacogen. Our results have a good spatial correspondence with the low Bouguer anomalies used to define the SFPB in previous studies. The limits of the SFPB are consistent with deviation of the mantle flow, as suggested by SKS fast polarization.

A Thermo‐Compositional Model of the Cratonic Lithosphere of South America

“…The low temperatures and positive thermal density variations combined with negative compositional anomalies indicate the existence of deep depleted cratonic roots under both the São Francisco and eastern Paranapanema Cratons (Figure 13, GG’). Negative compositional anomalies and related increased Mg# demonstrate a larger subsurface extent of the São Francisco Craton than visible at the surface (Figure 9), confirming the propositions of previous works (Assumpção et al., 2017; Rocha et al., 2019). The center of low temperatures in the eastern Paranapanema Craton appears to be shifted toward the southern São Francisco Craton with increasing depth.…”

“…Negative compositional anomalies and related increased Mg# demonstrate a larger subsurface extent of the São Francisco Craton than visible at the surface (Fig. 9), confirming the propositions of previous works [Assumpção et al, 2017;Rocha et al, 2019]. The center of low temperatures in the eastern Paranapanema Craton appears to be shifted towards the southern São Francisco Craton with increasing depth.…”

Decratonization by rifting enables orogenic reworking and transcurrent dispersal of old terranes in NE Brazil

“…The lithological boundary of the SFC and BP is defined in many geological maps (Fig. 1 A), however seismic data indicate that the lithospheric cratonic signature at 100 km depth extends further north ending within the central portion of BP 35 , 36 (Fig. 1 C).…”

Lithospheric Architecture of the Paranapanema Block and Adjacent Nuclei Using Multiple‐Frequency P‐Wave Seismic Tomography

“…The models from previous studies are also included for comparison. SFPB is the São Francisco Paleocontinental Block (Rocha, Azevedo, et al., 2019), LAC is the Luiz Alves Craton (Cordani et al., 2009). TBL is the Transbrasiliano Lineament.…”