Evidências geofísicas de domeamento na estruturação profunda da porção central da Faixa Seridó (NE-Brasil)

Abstract

The Seridó Belt (SB) is a Neoproterozoic schist belt in Borborema Province, northeast Brazil. It was deformed and metamorphosed in the context of the Pan-African/Brasiliano orogeny. During this tectonic event, the crust was partially melted, particularly at the connection between the Patos shear zone and SB, thus altering its rheological properties and, consequently, influencing its deformation. Among the occurrences of rocks associated with partial melting, the Santa Luzia anatetic dome (c. 575 Ma) is particularly noteworthy. The aeromagnetic map of SB shows an expressive long-wavelength (˃25 km) regional anomaly in the south-central part of the area. This anomaly is sigmoidal in shape and its central axis rotates from NNE-SSW to E-W near the Patos shear zone. Local anomalies with small wavelengths (<25 km) and large amplitudes (up to 2200 nT) overlap this regional anomaly. The limits of the magnetic anomalies often coincide with regional shear zones, thus revealing that the shear zones exert important tectono-structural control on the anomaly sources. Comparison with gravity data shows that both types of anomalies share the same common anomalous sources, being more magnetic and less dense than the surrounding crust. On the other hand, correlation with surface geology shows that the geophysical anomalies often occur in association within homogeneous magnetite-bearing migmatites and granites. We adopted two modelling approaches to analyse the available geophysical datasets. First, a three-dimensional (3D) automatic constrained inversion of the magnetic data was performed and, second, a two-and-a-half-dimensional (2.5D) interpreter-guided joint model was constructed based on magnetic and gravity datasets from three profiles. Through this work, we show the existence of a regional domical structure in the south-central part of SB. This large-scale gneiss dome is composed of four elongated internal structures, geophysically modelled as four anomalous bodies, besides the surrounding material. Together, these form a crustal structure with a long axis in the NE-SW direction that rotates to the E-W direction following the Patos shear zone. The Santa Luzia dome is a local small-scale example of this crustal-scale structure. The metasediments of the Seridó Group occur along synforms surrounding the internal structures that form the regional dome. In particular, because of the remarkable role of the Patos shear zone in shaping this crustal dome, the conception that the Patos shear zone was a very important tectonic constraint during the Pan-African/Brasiliano orogeny.