Paleostress Fields And 3-D Structure Of Poliphase Shear Zones In The Transition Craton-Orogenic Belt: Examples From The Neoproterozoic Of Southeastern Bahia, Brazil

Abstract

The IICSZ is a N45°-trending, 30km wide, intracratonic shear zone, extending for some 150km through the SSE portion of the Bahia State. The IICSZ is closely related to dykes and syenites of the Alkaline Igneous Suite of Southern Bahia and, to the Southwest, it is interrupted by the N140°-trending, Potiragua Shear Zone (PSZ), that establishes the tectonic limit of the Neoproterozoic Araçuaí Fold Belt and the Archaean-Proterozoic São Francisco Craton. The PSZ dips to SW and the IICSZ to NW, though the latter swaps northeastwards into a symmetric, positive, flower structure. A paleostress analysis based on the rientation of thousands of fault planes and fractures found in dykes and host rocks, coupled with the analysis of several kinematic indicators suggest that both shear zones evolved during a N-S compression and were later, perhaps progressively, reactivated by a E-W compressional tectonic event. Paleostress fields in both the IICSZ and PSZ were controlled by the orientation of the far-field stress, disturbances in field stress around re-activated shear zones, 3D-geometry of shear zones, tension concentration ('channeling') along shear zones, position of secondary faults and fractures, and orientation of shear zones, in relation to both the limit of the Araçuaí Fold Belt and the São Francisco Craton, and the site of intersection between the IICSZ and the PSZ (where the tension vectors converged to).