Himalayan deformation and metamorphism of rocks south of the Main Mantle Thrust, Karakar Pass area, southern Swat, Pakistan Public Deposited

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  • Rocks south of the Main Mantle Thrust (MMT) in Swat, Pakistan consist predominantly of Precambrian to Paleozoic granitic gneisses and metasedimentary rocks that represent the deformed and metamorphosed northern margin of the Indian subcontinent. In the Karakar Pass area the rocks are divisible into three lithologic units: the Swat granite gneisses, the Alpurai schists, and the Nikanai Ghar marbles. The Swat granite gneisses may represent the basement upon which the Alpurai schists were deposited. The Nikanai Ghar marbles are interpreted to form a large fault-and-fold nappe, here named the Nikanai Ghar nappe, that has been juxtaposed against the Alpurai schists. The rocks have undergone at least three deformations (D1, D2, and D3) and have been metamorphosed to the green-schist to epidote-amphibolite facies (M1). The first deformation, Dl, took place synchronously with M1 at about 550 to 425 degrees C at a depth of approximately 20 km. It produced a pervasive regional foliation (S1) in both the Swat granite gneisses and metasedimentary rocks, which is generally parallel to the original lithologic layering but is locally axial-planar to small, tight to isoclinal F1 folds. D2 is associated with retrograde metamorphism (M2) and folded S1 into open to tight F2 folds that range in size from small crenulations to folds of kilometer scale. It locally caused the development of an S2 crenulation cleavage or axial-planar mineral foliation. Associated with D2 was the formation of the Nikanai Ghar nappe. D3 formed large-scale regional cross-folds. Detailed mapping in the Karakar Pass area, regional reconnaissance, and interpretation of existing regional geologic maps suggest that the Swat granite gneisses and Alpurai schists may form a large recumbent fold nappe. The structural and metamorphic characteristics of rocks south of the MMT may be similar to those of the Pennine Zone of the European Alps. 40Ar/39Ar age-spectrum dates from hornblende that crystallized syntectonically with D1-M1 give an age of 37-40 Ma for this episode. 40Ar/39Ar muscovite dates may limit the youngest age for D2 and nappe formation to about 30 Ma . Results of this study show the existence of early Himalayan-aged metamorphism in rocks south of the MMT. MMT thrusting, Barrovian-type metamorphism, and intense deformation began at least as early as late Eocene and probably continued until mid to late Oligocene.
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