Petrographic and microstructural analysis of granitic rocks from the Indus syntaxis region of northern Pakistan
Peter B. Sak and Kevin R. Pogue
The Indus syntaxis is a basement-cored north-trending anticlinorium located in the Indus River valley of northern Pakistan between the village of Besham and Tarbela Dam. Precambrian gneiss exposed in the core of the syntaxis 13 km north of Tarbela Dam is the southernmost outcrop of Indian Plate basement in the Himalayan foothills of Pakistan. Basement gneiss is overlain by graphitic schist and marble of the Proterozoic Gandaf (Salkhala) Formation followed by quartzite and schist of the Proterozoic and Cambrian(?) Tanawal Formation. The metamorphic rocks are intruded by granitic rocks belonging to three suites. The oldest suite is Proterozoic(?) monzonite which only intrudes basement gneiss. Megacrystic granodiorite of the Late Cambrian to Early Ordovician Mansehra Granite intrudes the Tanawal Formation along the eastern limb of the syntaxis. Intruding the western limb of the syntaxis are syenite, alkaline granite, and carbonatite of the Late Carboniferous to Permian Ambela Granitic Complex. Previous studies assigned all granitic rocks on the western limb of the syntaxis to the Ambela Granitic Complex. However, the majority of these rocks are petrographically identical to the Mansehra Granite. Exposures of Mansehra-type granodiorite extend northward along the western limb of the syntaxis and eventually connect with the Choga granite gneiss. The Mansehra Granite and Choga and Swat gneisses are interpreted as remnants of a continuous and regionally extensive sheet-like intrusion that was breached by erosion of the Indus River and its tributaries.
Petrofabric analysis of mylonitized granitic rocks from the Tarbela Lake region indicate left-lateral shear within the syntaxis. Sinistral motion accounts for the juxtaposition of Gandaf Formation schist exposed on the western shore of Tarbela Lake with correlative slate of the Hazara Formation exposed on the eastern shore.