MICRO-MESO STRUCTURAL DEFORMATION AND TECTONIC IMPRINTS OF THE HAZARA-KASHMIR SYNTAXIS ON LOWER SWAT, NORTHERN PAKISTAN.

Abstract

The rocks exposed across the study area show the effects of three successive deformations phases D1, D2, D3 are recorded, during which fabrics S1 S2, and S3 established by detailed microscopic and mesoscopic structural analyses Studies of samples depict the metamorphosed and distorted medium to high grade Indian plate cover sequence that occurred during the Himalayan orogeny. Two well-developed mesoscopic folds D2 and D3 which postdate the middle amphibolite metamorphism, were recognized in the study area which trends in NW-SE and NE-SW direction. NW-SE trending D2 folds developed during the NE-SW horizontal bulk shortening followed by NE-SW trending D3 folds, which are developed during NW-SE shortening. The newly established NW-SE trending S2 crenulation cleavages microscopically and mesoscopically boudins as well as mineral stretching lineations are overprinted by D3. We interpret that the D2 and D3 folds were turned counterclockwise during the tectonic event that resulted in the evolution of Hazara-Kashmir syntaxis following the main phase of Indian plate-Kohistan Island arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicate prograde metamorphism related with D2. The inclusion of muscovite and biotite in garnet porphyroblast indicates that the Barrovian metamorphic conditions predate D2 and D3. Garnet, staurolite and calcite porphyroblasts grew before D2 deformation because the sophisticated S2 cleavage related with D2 in the matrix wraps around these porphyroblasts. Garnet and staurolite porpyroblast grew before because the conditions initially favored the development of high-grade metamorphic minerals in the region. The inclusion trail in garnet and staurolite are truncated by S2 cleavage in the matrix. Under a microscope, the major matrix foliation is clearly outlined by, muscovite, biotite calcite, and quartz. Petrographic and field observations depict that D2 overturning reflects the same tectonic event that produced the HKS and D3 predates the HKS counterclockwise rotation. D3 related folds did not rotate during the HKS tectonic event, because the NE limb of D2, which contain D3 folds, transported top to NW around the D2 fold axis.