Termination of a continental-scale strike-slip fault in partially melted crust: The West Pernambuco shear zone, northeast Brazil

Abstract

Continental-scale strike-slip faults rooted in the upper mantle have been recently interpreted in deep seismic profiling. These shear zones transect the entire crust, and it may be assumed that the deformation pattern associated with their propagation at different depths was strongly influenced by the rheological properties of the rocks. The continental-scale West Pernambuco shear zone provides information on the deformational behavior of a hot and partially melted crust that has undergone large-scale shearing. This late Precambrian shear zone terminates eastward in a fanlike structure, passing to a wide transpression zone at a high angle to the shear direction. The main fault branches into numerous splay faults over a distance of about 50 km. Severe perturbation of the mylonitic flow pattern commonly occurs at branch points where subsidiary shear zones diverge from the main zone. In these areas, problems of strain compatibility are raised by the sharp bending and decrease in dip of the foliation surface. We propose that this unusual deformation pattern is a result of the low strength of the continental crust at the considered level of observation.