A Review of the geology and tectonics of the Kohistan island arc, north Pakistan
Published:January 01, 2010
Michael G. Petterson, 2010. "A Review of the geology and tectonics of the Kohistan island arc, north Pakistan", The Evolving Continents: Understanding Processes of Continental Growth, T. M. Kusky, M.-G. Zhai, W. Xiao
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This paper summarizes some 30 years of more intense recent work and almost 100 years of geological observations in Kohistan. The paper is divided into two section: an earlier factual-based section with minimal interpretation, and a later section summarizing a range of ideas based on the data as well as presenting new thoughts and interpretations. Kohistan is a c. 30 000 km2 terrane situated in northern Pakistan. The great bulk of Kohistan represents growth and crustal accretion during the Cretaceous at an intra-oceanic island arc dating from c. 134 Ma to c. 90 Ma (Early to Late Cretaceous). This period saw the extrusion of c. 15–20 km of arc volcanic and related sedimentary rocks as well as the intrusion of the oldest parts of the Kohistan batholith, lower crustal pluton intrusion, crustal melting and the accretion of an ultramafic mantle–lower crust sequence. The crust had thickened sufficiently by c. 95 Ma to allow widespread granulite-facies metamorphism to take place within the lower arc. At around 90 Ma Kohistan underwent a c. 5 Ma high-intensity deformation caused by the collision with Eurasia. The collision created crustal-scale folds and shears in the ductile zone and large-scale faults and thrusts in the brittle zone. The whole terrane acquired a strong penetrative foliation fabric. Kohistan, now an Andean margin, was extended and intruded by a diapiric-generated crustal-scale mafic–ultramafic intrusion (the Chilas Complex) with a volume of 0.2×106 km3 that now occupies much of the mid–lower crust of Kohistan and had a profound impact on its thermal structure. The Andean–post-collisonal (c. 90–26 Ma) period also saw the intrusion of the stage 2 and 3 components of the batholith and the extrusion of the Dir Group and Shamran/Teru volcanic rocks. Collision with India at c. 55–45 Ma saw the rotation, upturning, underplating and whole-scale preservation of the terrane. The seismic structure of Kohistan has some similarities to that of mature arcs such as the Lesser and Greater Antilles and Japan, although Kohistan has a higher proportion of high-velocity granulites in the lower crust. The chemical composition of Kohistan is very different from that of average continental crust, although it is similar to an analogue obducted arc within Alaska (Talkeetna), suggesting that ‘mature’ continental crust undergoes a series of geochemical processes and reworking to transform an initial stage 1 ‘primitive arc crust’. Most of Kohistan is gabbroic in composition, particularly within the lower and middle crust. A high proportion of the ‘basement’ volcanic units is also basaltic to basaltic andesite with smaller proportions of boninite, andesite to rhyolite, ignimbrite and volcaniclastic material. Post Eurasian-collision ‘cover’ volcanic rocks are highly evolved, comprising predominant rhyolites, ignimbrites and related volcaniclastic rocks. Most lithological units throughout the crustal section have an arc-like geochemical composition (e.g. high LREE/HREE and LFSE/HFSE ratios) although some have oceanic (main ocean and back-arc) characteristics. Isotopic compositions indicate that the great bulk of igneous rocks have an ultimate sub-arc mantle source. In broad terms the Kohistan terrane represents a juvenile mantle extract addition to the Phanerozoic continental crust with a total volume of c. 1.2×106 km3 (equivalent to c. 1/50 the volume of the Ontong–Java Plateau or Alaska).
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The Evolving Continents: Understanding Processes of Continental Growth
This volume honours the career of Brian F. Windley, who has been hugely influential in helping to achieve our current understanding of the evolution of the continental crust, and who has inspired many students and scientists to pursue studies on the evolution of the continents. Brian has studied processes of continental formation and evolution on most continents and of all ages, and has educated and inspired two generations of geologists to undertake careers in studies of continental evolution. The volume is organized into six sections, including: oceanic and island arc systems and continental growth; tectonics of accretionary orogens and continental growth; growth and stabilization of continental crust; collisions and intraplate processes; Precambrian tectonics and the birth of continents; and active tectonics and geomorphology of continental collision and growth zones.