INITIATION AND PROPAGATION OF SHEAR ZONES IN A HETEROGENEOUS CONTINENTAL LITHOSPHERE

Continental plates represent theologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities. Such a deformation pattern is displayed by the Borborema shear zone system in northeastern Brazil. It involves continental-scale, curvilinear, E-W trending right-lateral transcurrent shear zones that branch off from a major NE trending, right-lateral strike-slip deformation zone formed at the northern termination of the Sao Francisco craton and that finally terminate in transpressive metasedimentary belts. We suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain (craton) and theologically weaker domains (basins). The effect of a low-viscosity domain on the deformation of a continental plate and, in particular, the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling. The low-strength domain induces an enhanced strain localization, and the geometry of the shear zone is significantly modified. It is either split, forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low-viscosity domain, or completely rotated toward the weak block. The perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation, which is controlled by its initial viscosity contrast relative to the surrounding lithosphere, its orientation relative to the convergence direction, and its distance from the shear zone initiated at the termination of the stiff block. The interaction between imposed boundary conditions (tectonic forces and plate geometry) and the internal structure of the plate may give rise to highly heterogeneous strain fields, as exhibited by the Borborema shear zone system, in which intraplate theological heterogeneities induce the development of branched or sinuous shear zones. A heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity, with shear zones bordering nearly undeformed blocks, and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories.