Abstract:
In the Camamu rift basin-CB, E of state of Bahia, idealized models of mineral symmetry fabrics are compared with orientation patterns of the: (i) 201 planes of principal foliation (SP) and 538 measures of stretching mineral lineation (LX) in basement rocks, and (ii) 2,530 structural lineaments terrain digital model images and 12,421 planes of faults and fractures. The principal structure observed were as follows: (i) fault/fracture sets are parallels to N00o-30o-trending regional foliations (SP), and orthogonal (N90o-trending) to diagonal (N120o-trending) to SP and LX, (ii) a combination between regional extension (N-S/E-W and NE-SW/NW-SE) and country rock mineral symmetry fabric orientations facilitated the N00o- to 30o-, N90o- and N120o-trending fault set formation, (iii) the N-S strike and subvertical E- and W-dips of the SP facilitated the continental split associated to the Mesozoic South Atlantic opening. Where SP was sub-horizontal LX becomes an important influence that “induced” fault propagation directions. So not only SP but a SP+LX combination was important to geometric construction of the fracturing fabric at the western border of the CB. Finally (iv) the lithospheric load release due to the erosion of the thick crustal layer that exposing granulite facies rocks may have resulted in the production of weakness planes and lines reactivated during rifting event. A combination of monoclinic-triclinic fracturing fabrics was observed where S-C shear planes were not to close or where polyphasic deformation history occurred and (pseudo) orthorhombic fabric was observed where SP and LX became subparallel under extreme mineral stretching and rotation. Consequently not only the far-field stress orientations but also the basement rocks mineral symmetry fabrics were crucial to the evolution of the basement and Camamu Basin’s fracturing patterns.
Keywords:
fracturing pattern; rift-type basins; mineral symmetry fabric; basement structural heritage