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Abstract
Abstract. The depth-dependent anisotropic structure of the lithosphere under the Borborema Province in northeast Brazil has been investigated via harmonic stripping of receiver functions developed at 39 stations in the region. This method retrieves the first (k=1) and second (k=2) degree harmonics of a receiver function dataset, which characterize seismic anisotropy beneath a seismic station. Anisotropic fabrics are in turn directly related to the deformation of the lithosphere from past and current tectonic processes. Our results reveal the presence of anisotropy within the crust and the lithospheric mantle throughout the entire province. Most stations in the continental interior report consistent anisotropic orientations in the crust and lithospheric mantle, suggesting a dominant northeast–southwest pervasive deformation along lithospheric-scale shear zones developed during the Brasiliano–Pan-African orogeny. Several stations aligned along a northeast–southwest trend located above the (now aborted) Mesozoic Cariri–Potiguar rift display large uncertainties for the fast-axis direction. This non-azimuthal anisotropy may be related to a complex anisotropic fabric resulting from a combination of deformation along the ancient collision between Precambrian blocks, Mesozoic extension and thermomechanical erosion dragging by sublithospheric flow. Finally, several stations along the Atlantic coast reveal depth-dependent anisotropic orientations roughly (sub)perpendicular to the margin. These results suggest a more recent overprint, probably related to the presence of frozen anisotropy in the lithosphere due to stretching and rifting during the opening of the South Atlantic.
Highlights
Understanding intraplate deformation and its relationship to deep geodynamic processes such as sublithospheric flow is critical for improving our understanding of the evolution of continents
This non-azimuthal anisotropy is likely related to complex fossil anisotropic fabrics, resulting from a combination of deformation along the ancient collision between Precambrian blocks, the Mesozoic extension, and thermomechanical erosion/mantle dragging by sublithospheric flow
Anisotropy parameters were examined for each station at two depth-window ranges: (1) crust (Fig. 6a), which was assumed to be located at a depth of between 0 and 33 km, in agreement with the 32–40 km range estimated by Luz et al (2015b) under the Borborema Plateau and 30–33 km under the surrounding basins; and (2) lithospheric mantle, which was taken to be at a depth between 33 and 100 km (Fig. 6b)
Summary
Understanding intraplate deformation and its relationship to deep geodynamic processes such as sublithospheric flow is critical for improving our understanding of the evolution of continents. Brasiliano– Pan-African deformation is well represented through the network of shear zones that pervade the Borborema Province (Vauchez et al, 1995; Neves et al, 2000). Julià: Seismic anisotropy of the Borborema Province or P-wave tomography (Simões Neto et al, 2019), and SKS splitting (Bastow et al, 2015) have greatly contributed to further our understanding of the relationships between inherited Precambrian structures, Mesozoic extensional processes, and episodes of post-breakup volcanism and uplift. Our results show the presence of non-azimuthal anisotropy above stations located along the aborted Cariri–Potiguar rift This non-azimuthal anisotropy is likely related to complex fossil anisotropic fabrics, resulting from a combination of deformation along the ancient collision between Precambrian blocks, the Mesozoic extension, and thermomechanical erosion/mantle dragging by sublithospheric flow
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