Numerical modeling of flexural stresses in the upper crust of NE Brazil – Implications for stress regimes in intraplate settings

Abstract

We model the contribution of the stresses generated by the topography and density heterogeneities in the stress field of NE Brazil and the interplay between these and regional stresses. Our model consists of a horizontally layered elastic slab (the lithosphere) that floats above an inviscid fluid (the asthenosphere), where the surface load is generated by the topography and bathymetry, and the internal loads are calculated from the Bouguer anomalies. The deviatoric flexural stresses are calculated in the Fourier domain. Our results showed local flexural stresses with magnitudes comparable to those of the plate-wide stresses, and mainly controlled by the long-wavelength Bouguer anomalies. Scenarios for the total stress field were calculated by superposing different values for an E-W-oriented regional compression. We predict the stress axes orientations and stress regimes for the equatorial margin and the Pernambuco Shear Zone (PESZ) region. However, we highlight the importance of spreading stresses, not considered here, in rotating the maximum horizontal compression (SHmax) predictions. The prediction of stress regime and stress axes orientation in PESZ using low to zero regional compression suggest that a decoupling of the shallow stresses from the plate-wide stresses might occur in this region. Another possible explanation found was that the spreading stresses might counterbalance the regional stresses. We conclude that the superposition of principally flexural and plate-wide stresses explains reasonably the observed stress regimes and SHmax orientations in a great part of NE Brazil. Furthermore, flexural stresses are playing an important role in the reactivation of the PESZ, possibly controlling the stress field.