Structure and metamorphism of the Chakdara area NW of Swat river, Pakistan
Abstract
Two major faults, the Kohistan and Kishora thrust, divide the study area into three tectonic terranes, the Indian shelf, Garai melange, and Kohistan arc. Stratigraphy, deformation, and metamorphism within each of these terranes developed independently until they were juxtaposed during Eocene-Oligocene collision. An important aspect of this study is the documentation of youngest deformation phases related with Himalayan orogen.
Three deformation phases (D1, D2, D3) are recorded in the Indian shelf and the melange matrix, each characterized by distinct fabrics and folding. S1 occurs as relict intrafolial in the S2 fabric as well as in the porphyroblasts and pressure shadows, in the Indian shelf rocks. F1 are locally preserved and are found associated with movement on the Kishora thrust. S2 is defined by the orientation of platy or elongated minerals and axial planes of folds. F2 folds are strongly developed and strike NNE and SSW. These are associated with southeastward movement on the Kishora thrust. S3 developed as minor crenulation cleavage when S2 was folded during later deformation D3, F3 strikes NNW and NS. It may probably be associated with the Kohistan thrust during the development of the Indus syntaxis. These deformation phases took place under conditions of amphibolite facies in the Indian shelf and greenschist facies in the melange matrix.
The melange blocks and the Kohistan rocks record different deformation events. Apparently they preserve pre-emplacement structures. Two deformation phases are recorded. During D1, S1 and F1 developed. S1 occurs as relicts in the S2 foliation. D1 occurred independently in the melange blocks and Kohistan arc. During D2 the S1 foliation was folded. D2 in the blocks is associated with the Kishora thrust while that in the Kohistan rocks is consistent with the D2 of the melange matrix and the Indian shelf. D1 and D2 took place under conditions of greenschist facies in the melange blocks and of amphibolite facies in the Kohistan arc.
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