Complex flow in lowest crustal, anastomosing mylonites: Strain gradients in a Kohistan gabbro, northern Pakistan

Abstract : Strain localization in near paleo-Moho metagabbros of the Kohistan Arc, northern Pakistan, produced anastomosing shear zones. The two-dimensional (2-D) analysis of strain gradients in planes parallel to the general flow direction reveals simple shear strain >7 combined with about 50% surface loss. The pargasite-garnet assemblage of the mylonite has a density 2.8% higher than the protolith, which is insufficient to account for the measured surface loss. Furthermore, major and trace element compositions of the gabbro and the mylonite indicate isochemical deformation. Average V p and acoustic impedance measured at room temperature and up to 300 MPa increase from gabbro through the gradient zone to mylonite; they are consistent with density measurements. The three-dimensional analysis of shape-preferred orientation and lattice-preferred orientation of sheared and synkinematic minerals indicates that the 2-D surface loss reflects sideways and lengthwise material transfer. Sideways and lengthwise material transfers take place in widening and lengthening mylonites, respectively. The anastomosing shear zone pattern impels this complexity of the regional flow. We conclude that shear zones with a thinning component are likely representative of deep crustal flow in other tectonic environments.