A re-evaluation of the stratigraphy and evolution of the Kohistan arc sequence, Pakistan Himalaya: implications for magmatic and tectonic arc-building processes

New field mapping and structural data, combined with published geochemical data, from the Kohistan arc in the NW Himalaya, enable a re-evaluation of the arc stratigraphy. Key lithological units and their relationships are more clearly defined, permitting the construction of a revised magmatic-tectonic history for the arc. The oldest units are transitional oceanic-type basalts, which form the basement to the subduction related sequence. Arc-type gabbroic sheets and plutons intrude the oceanic basalts: together these form the Kamila Amphibolite Belt. Metasediments and basaltic lavas were deposited, within an extensional basin, onto the Kamila Amphibolite Belt basement. This sequence, exposed across the arc, forms a distinct stratigraphic unit which is formally defined here as the Jaglot Group. Sediment-charged turbidity currents transported material into the basin, whilst submarine eruptions contributed the basaltic component. This period of extension culminated in the eruption of high-Mg boninites of the Chalt Volcanic Group which overlie the rocks of the Jaglot Group. The earliest granitoids of the Kohistan Batholith predate suturing and intrude the Jaglot and Chalt sequences. At c. 100 Ma Kohistan sutured to Asia. suturing being accompanied by thickening of the arc with the development of major intra-arc shear zones and a penetrative, regionally developed steep cleavage. At c. 85 Ma intra-arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the Chilas Complex which clearly intrudes the Kamila Amphibolite Belt to the south and the Jaglot Group to the north. The Chilas Complex has been regarded as part of the pre-suturing, juvenile arc sequence. Field evidence summarized here show this to be not so. Heat advection associated with emplacement of the Complex caused amphibolite facies regional metamorphism, melting of the lower arc and plutonism. Some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 Ma, before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 Ma. At least three phases of extension and rifting, each separated by short lived phases of compression, charac­terized arc evolution. Much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone.