Shear strain localisation in the upper mantle and the lower to middle crust of the Kohistan arc (Pakistan).
Résumé
Strain localisation in near paleo-Moho metagabbros of the Kohistan Arc Complex, N-Pakistan, produced anastomosing shear zones. Strain gradients formed by passive simple shear of the magmatic assemblage (minimum gamma=7) combined with an apparent volume loss (about 50%). Major and trace elements analyses of the protolith gabbro and the mylonite indicate isochemical conditions. The pargasite-garnet assemblage of the mylonite has a density 2.8% higher than the protolith, which is insufficient to account for the measured volume loss. Average Vp and acoustic impedance measured at room temperature and up to 300 MPa increase from protolith through the gradient zone to mylonite, and are consistent with density measurements. The orthorhombic symmetry of the 3D Shape Preferred Orientation (SPO) and the Lattice Preferred Orientation (LPO) of pargasitic amphiboles combined with isochemical conditions and apparent volume loss in sections normal to the vorticity vector are best interpreted by sideways material transfer and strain variations at borders of the protolith lenses. Deformation sub-phases result from local changes in the direction of incremental strain axes during a single shear event. The complex, locally spinning flow pattern does not directly reflect the regional shear deformation but instead the relative displacement and/or rotation between comparatively stiff lenses. This complexity is likely representative of deep crustal flow in other tectonic environments.