Pre-collisional anastomosing shear zones in the Kohistan arc, NW Pakistan

Abstract

AbstractDuctile strain localization commonly forms a pattern of shear zones anastomosing around lenses of less deformed rock. Initiation and development of anastomosing shear zones are studied through description of the structures and deformation history of plutonic rocks that form the lower crust of the Kohistan arc. Structures and textures developed in these rocks result from primary magmatic to solid state regional strain, overprinted by anastomosing shear zones. The primary strain was mainly acquired during magmatic emplacement at 100-90 Ma. Strain localization took place continuously from magmatic emplacement to solid state deformation during cooling of the plutons and formed three successive sets of shear zones. Set 1 is composed of associated discrete Riedel and thrust shear zones developed above solidus conditions during southwestward thrusting. Continuous deformation from solidus to amphibolite facies conditions between 100 and 83 Ma formed the second set of shear zones. The lower amphibolite facies set 3 shear zones are diffeentiated by larger strains recorded in the thicker mylonitic zones and enlargement of the spacing between shear zones during cooling. The anastomosing pattern of shear zones described here probably represents arc-related deformation during subduction of the Tethys oceanic lithosphere below the Kohistan arc.