Precambrian stratigraphy and the foreland-hinterland transition in the Himalayan thrust belt of northern Pakistan
Kevin R. Pogue, Joseph A. DiPietro, Wali Ullah Khattack
Recent investigations in the Indus syntaxis between Tarbela Lake and Besham have revealed critical relationships that permit the reconstruction of the pre-Himalayan configuration of the Precambrian of northern Pakistan which, in general, consists of Proterozoic and Archean(?) crystalline basement rocks overlain by a Proterozoic sedimentary and metasedimentary cover sequence. Both the thickness and metamorphic grade of the cover sequence increase to the north. South of the Main Boundary Thrust (MBT) Upper Proterozoic evaporites (the Salt Range Formation) unconformably overlie Proterozoic gneiss and granite (Gee, 1989). Between the MBT and the Panjal-Khairabad thrust (PKT), the Proterozoic sedimentary cover is represented by siltstone, argillite, slate, and limestone of the Hazara Formation and its western equivalent, the Dakhner Formation. Gypsum intervals in the Hazara Formation imply that the Hazara and Salt Range Formations are lateral equivalents. The crystalline basement for the Hazara and Dakhner Formations is not exposed. Between the PKT and Tarbela lake, the oldest exposed Precambrian rocks are slate and phyllite of the Manki Formation which are interpreted as the metamorphosed equivalent of the Hazara/Dakhner Formation. The Manki Formation is overlain by stromatolitic limestone of the Shahkot, Utch Khattack, and Shekhai Formations. These limestones thin northward to be replaced by quartzite and argillite of the Tanawal Formation. Near Tarbela Lake, the Tanawal Formation overlies marble and graphitic schist of the Gandaf Formation which is interpreted as a higher-grade equivalent of the Manki Formation. Northwest of Tarbela lake, in Swat, higher grade equivalents of the Tanawal Formation are mapped as the Manglaur Formation. The Gandaf Formation thickens northward in the Indus syntaxis and the deeper levels expose intervals of gneiss and sheared monzonite interpreted as a Proterozoic intrusive complex (the Kotla complex). North of Tarbela Lake, Gandaf Formation lithologies are included in the Karora Formation. In the northern part of the Indus syntaxis, the Karora Formation overlies the Besham Complex. The Besham Complex consists of gneisses that record early Proterozoic metamorphism (Baig, 1990; Baig and Snee, 1989; Treloar and Rex, 1990) that are intruded by granitic rocks and mafic dikes of Proterozoic and Paleozoic age. Near Besham, the unconformity of the Karora Formation with underlying early Proterozoic gneiss of the Besham Complex is marked by a conglomerate. However, at other locations, granitic rocks of the Besham complex intrude the basal part of the Karora Formation, obscuring this unconformity. The entire Precambrian sequence north of Tarbela Lake is extensively intruded by Cambrian granodiorite (the Mansehra and Swat Granites), Late Paleozoic alkaline granitoids (the Ambela intrusive complex), and Permian mafic dikes and sills. Precambrian rocks exposed north of Besham in the Nanga Parbat area are divided between the Iskere Gneiss, Shengus Gneiss, and Haramosh Schist (Madin, 1986). Radiometric dating implies that these units are equivalents of the Besham Complex, Mansehra-type granites, and the Karora Formation (Zeitler and others, 1989).
The relationships outlined above indicate that while the demarcation between basement and cover within the Precambrian can be defined in the southern part of the Pakistan Himalaya, it is obscure to the north. North of the PKT, much of the Precambrian section was subjected to pre-Himalayan metamorphism and north of Tarbela Lake the entire section is extensively intruded by granitic plutons. Mechanically, these rocks would behave as basement, not as cover. Exposures of Proterozoic gneisses in the Indus syntaxis indicate that the Himalayan basal decollement must have stepped downward into crystalline basement south of Tarbela Lake. Involvement of the basement in thrusting probably first occurs at the PKT which forms the southern limit of exposures of extensively metamorphosed rocks. The PKT thus forms the boundary between the foreland and hinterland in the Himalayas of Pakistan.
References
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