Strain localization and rheological weakening along mid-crustal anisotropies as recorded in the Itapetim and Tendó shear zones (northeastern Brazil)

Abstract

The Central Borborema Province (CBP) in NE Brazil hosts well-preserved mid-crustal shear zones that formed during the Pan-African orogeny in a strike-slip tectonic setting. These structures are key to investigate the strength-controlling processes in high-temperature continental shear zones, which are often obliterated from the geological record during late exhumation. We have analyzed mylonites and ultramylonites from the Itapetim and Tendó shear zones (ISZ and TSZ), two major NE-trending shear zones in the CBP, using a combination of field mapping, microfabric analysis and mineral chemistry. Both the ISZ and TSZ are primarily composed of granitic mylonites with discrete, thin ultramylonitic layers. In mylonites, K-feldspar porphyroclasts form a load-bearing framework that deforms through microfracturing, dislocation-glide-controlled deformation and grain boundary migration. Plastic strain is preferentially accommodated in discontinuous monomineralic quartz ribbons alternating with plagioclase-rich layers. The ultramylonitic layers are spatially and genetically associated with quartz veins and aplitic dykes, in which fluid-rock interactions induce strain softening. This process leads to the development of myrmekite-derived K-feldspar neoblasts and biotite-rich layers, further enhancing rheological weakening. The data gathered in this study suggest that strain is preferentially localized along lithological boundaries, resulting in the development of sinistral shear zones that conform to the overall tectonic setting of the central block of the Borborema Province. Our findings demonstrate strain compatibility between these NE-trending sinistral shear zones and the major E-W dextral lineaments that bound them, offering valuable insights into the mechanisms of rheological weakening in the post-collisional stage of continental orogens.