A geologic reconnaissance of the Indus syntaxis, northern Pakistan
Kevin R. Pogue, Peter B. Sak, Wali Ullah Khattak
The Indus syntaxis is an elongate north-striking faulted anticlinorium centered in the valley of the Indus River between the northern Gandghar Range and the Besham area. At its southern extremity near Tarbela dam, northeast-striking rocks in the Gandghar Range are separated from northwest-striking rocks across the river by a complex fault zone marked by pervasively sheared and mineralized graphitic schist (figure 1). Calkins (1975) mapped this fault zone as the Darband fault. Recent activity along the Darband fault is indicated by offset Indus River gravels in drill cores and tilted terraces near the dam site (Yeats and Hussain, 1989). The Gandghar Range southeast of the dam site is composed of low-grade Proterozoic metasediments in two structural blocks separated by the Baghdarra fault. The southeastern block exposes the slates and phyllites of the Manki Formation overlain by a thick limestone interval that is divided between the Shahkot, Utch Khattak, and Shekhai Formations. A relatively thin section of Tanawal Formation quartzite overlies the limestone. The oldest rocks in the northwestern block are graphitic schist and minor marble exposed along the Baghdarra fault. The schist has a gradational contact with the overlying Manki Formation. A thick interval of Tanawal Formation quartzite is separated from the underlying Manki Formation by limestone of the discontinuous Sobrah Formation. North of the Indus River, the oldest rocks are graphitic schist and marble that were mapped by Calkins (1975) as Salkhala Formation. Due to the ambiguity of the term "Salkhala" with regard to lithologic content, unit boundaries, and type section in Kashmir, these rocks will be referred to as Gandaf Formation in this study. The Gandaf Formation is overlain unconformably by the Tanawal Formation. A relatively complete Paleozoic and Mesozoic section overlies the Tanawal Formation in the Peshawar Basin to the west.
North of Tarbela Dam, the strike of rocks on either side of the syntaxis abruptly swings from east-west to north-south near Tarbela Lake. Rocks in the core of the syntaxis exposed along the lake shore consist of quartzo-feldspathic gneiss intruded by granite bearing characteristic blue-gray microcline. The upper part of the gneiss contains discontinuous intervals of marble. At many localities the basal gneiss unit and granite is pervasively sheared. Based on its stratigraphic position, grade of metamorphism, and lithologic similarity, the basal unit is correlated with Indian plate basement rocks of the Besham Group. The gneiss passes gradationally into a northward-thinning interval of Gandaf Formation schist and marble. The Gandaf Formation is overlain by an enormous thickness of quartzite and quartz-mica schist of the Tanawal Formation. On the east side of the syntaxis, the Tanawal is intruded by porphyritic granodiorite of the Mansehra Granite. On the west side of the syntaxis, similar rocks belonging to the Ambela Granitic Complex intrude the Tanawal. The belt of granitic rocks on the west side of the syntaxis narrows and becomes increasingly mylonitized to the north. Where the belt crosses the Barundu River it is a 2 km-wide mylonitic gneiss. It is presumed that this narrow belt of granitic rock continues northward to join the Choga granitic gneiss. East of Tarbela Lake, the Tanawal Formation is unconformably overlain by lower Paleozoic dolomite and quartzite in the Sherwan synclinorium. The youngest rocks of the synclinorium are burrowed feldpathic quartzites similar to the Ordovician Misri Banda Quartzite of the Peshawar Basin. West of Tarbela Lake, the Tanawal Formation is overlain by a thick sequence of marble with interbedded amphibolite that may correlate with parts of the Peshawar Basin Paleozoic section.
North-striking fault zones are exposed at many locations along the shoreline of Tarbela lake in the core of the Indus syntaxis. These fault zones are of three types; mylonitic shear zones in the Ambela and Mansehra Granites, brittle fault zones marked by breccias and sheared graphitic schist, and active faults that are expressed primarily as topographic lineaments. Variations in stratigraphic throw along strike indicate large components of strike-slip on some of these faults. Transpression related to oblique collision of the Indian plate is the favored model for the development of the syntaxis.
References Cited
Calkins, J. A., Offield, T. W., Abdullah, S. K. M., and Ali, S. T., 1975, Geology of the southern Himalaya in Hazara, Pakistan and adjacent areas: Geological Survey of America Professional Paper 716-C.
Yeats, R.S. and Husaain, A., 1989, Zone of Late Quaternary deformation in the southern Peshawar basin, Pakistan, in Malinconico, L.L. and Lillie, R.J., eds., Tectonics of the Western Himalaya: Geological Society of America Special Paper No. 232, p. 265-274.