Geo-tectonic framework of the Himalaya of N Pakistan

Abstract

In the Karakorum Range there is a structurally complicated Cretaceous are comprising the Kohistan sequence. On its northern side the Northern Suture consists of a mega-mélange and is bounded to the S by tightly folded pillow-bearing volcanics and sediments. To the S the Kohistan Plutonic Belt consists of (southwards): (a) early foliated and late post-tectonic tonalites and diorites, (b) aplites and pegmatites (up to 30% of rock volume), (c) basic dykes up to 10 m thick, (d) the Chilas Complex, a stratiform cumulate body over 300 km long and 8 km thick (chromite-layered dunites, gabbros and norites) with a low pressure granulite-facies mineral fabric of tectonic origin, (e) an amphibolite belt with a complex mixture of other rocks, and (f) the Jijal Complex, a 200 km 2 tectonic wedge of high pressure granulites and chromite-layered dunites. Cumulate graded units in the Chilas Complex show that it is folded by an isoclinal anticline (F 1 ). The mid-upper crust of the are is folded by a 50 km half-wavelength F 2 , syncline. The whole Kohistan sequence with its two phases of isoclinal folds was tilted during Himalayan collision so that the structures are now subvertical. The Southern Suture (Main Mantle Thrust) has a wedge of glaucophane schists. The Indian plate contains a basement of psammites and schists intruded by Cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales.