Polarity of the subduction system responsible for the Cretaceous Kohistan island arc terrane, N. Pakistan: geochemical and structural constraints
Abstract
The Kohistan island arc terrane of N. Pakistan has experienced multiple collisions, resulting in obliteration of the subduction system responsible for its creation. Indirect evidence, based on structural vergences and across-the-arc variations in trace elements (Ti, Zr, Nb, and P) have been combined to interpret the nature of the subduction zones responsible for Kohistan in terms of their location and polarity (i.e., dip direction). Geochemical investigations, from north to south, in the Chalt volcanics, the Chilas complex and the Kamila amphibolite belt indicate a systematic increase in high-field strength elements from north to south. These variations indicate an overall decrease in the degree of melting or degree of source-region depletion, which in either case, give a facing direction for the arc terrane towards north. The structural vergences in the northern parts of the Kohistan terrane are also consistent with a south dipping subduction zone located to its north. Using these observations, a tectonic model is proposed for the origin of Kohistan terrane on a south-dipping subduction zone located at its northern margin. The subduction switched to the southern margin of the arc terrane soon after the Kohistan-Karakoram collision with a reverse polarity (i.e., dip direction towards the north). The model has been tested in the light of modern examples of arc-continent collisions in SW Pacific Ocean.
References
Arculus, F.M.J. & Powell, R., 1986. Source Component mixing in the regions of arc magma generation. Jour. Geophys. Res., 91, 5913-5926.
Bard, J. P., 1983. Metamorphism of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Earth Planet. sci. 65, 133-144.
Beard, J. S., 1986. Characteristic mineralogy of arc related cumulate gabbros: implications for tectonic setting of gabbroic plutons and andesite genesis. Geology, 14, 845-851.
Carney, N. J. & MacFarlane, A., 1982. Geological evidence bearing on the Miocene to recent structural evolution of the New Hebrides arc. Tectonophysics, 87, 147-175.
Chen, Y., 1992. Evolution tectonique le long D’une. transversale entre inde et Siberie: apports de nouvelles donnees paleomagnetiques Cretaceous du Tibet du Tarim et de la Dzungarie. Ph.D. Thesis, Universite de Paris, VII, 306p.
Cooper, P. A & Taylor, B., 1985. Polarity reversal in the Solomon island arc. Nature, 295, 22-24.
Coward, M. P., Windley, B. F., Broughton, R. D, Luff, 1. W., Petterson, M. G., Pudsey, C. G., Rex, D. C. & Khan, M. A., 1986. Collision tectonics in the NW Himalayas. In: Collision Tectonics (M. P. Coward & A C Ries, eds.). Geol. Soc. London, spec. Pub., 19, 203-219.
DeBon, F., LeFort, P., Dautel, D., Sonet, J. & Zimmermann, J. L., 1987. Granites of Western Karakoram and northern Kohistan (Pakistan): A composite mid Cretaceous to Upper Cenozoic magmatism. Lithos, 20, 19-40.
Dickson, W. R. & Hatherton, T., 1967. Andesitic volcanism and seismicity around the Pacific. Science, 157, 801-803.
Hamilton, W., 1978. Tectonics of the Indonesian region. U. S. Geol. Surv. Prof. Pap. 1078, 345p.
Jan, M. Q. & Howie, R. A., 1981. The mineralogy and geochemistry of the metamorphosed basic and ultrabasic rocks of the Jijal complex, Kohistan, N. W. Pakistan. Jour. Petrol., 22, 85-126.
Jan, M. Q., 1988. Geochemistry of amphibolites from the southern part of the Kohistan arc, N. Pakistan. Mineral. 52, 147-159.
Khan, M. A., Jan, M. Q. & Weaver, B. L., 1993. Evolution of the lower arc crust in Kohistan, N. Pakistan: temporal arc magmatism through early, mature and intra-arc rift stages. In: Himalayan Tectonics (P. J. Treloar & M. P. Searle, eds.). Spec. Publ. Geol. Soc. London, 74, 123-138.
Khan, M. A, Stern, R. J Gribble, R. Windley, B. E, 1997. Geochemical and isotopic constraints on subduction polarity, magma sources, and paleogeography ofKohistan intra-oceanic arc, northern Pakistan Himalaya. Geol. Soc. London (in press).
LeFort, P., Michard, A., Sonet, J. & Zimmermann, J. L., 1983. Petrography, geochemistry and geochronology of some samples from the Karakoram Axial Batholith, Pakistan. In: Granites of Himalayas, Karakoram and Hindukush (F.A. Shams, ed.). University of Punjab, Lahore, 277-387.
McCaffery, R. & Abrs, G. A., 1991. Orogeny in arc continent collision: the Banda arc and western New Guinea. Geology, 19, 563-566.
Petterson, M. G. & Windley, B. F., 1985. Rb-Sr dating of the Kohistan arc batholith in the Trans Himalayan of the North Pakistan and tectonic implications. Earth Planet. Sci. Lett., 74. 45-57.
Petterson, M. G. & Windley, B. E., 1991. Changing source regions of magmas and crustal growth in the Trans-Himalayas: Evidence from the Chalt volcanics and Kohistan batholith, Kohistan, N. Pakistan. Earth Planet. Sci. Lett., 102, 326-341.
Pudsey, C. J., 1986. The Northern Suture, Pakistan: margin of a Cretaceous island arc. Geol. Mag., 123, 405-423.
Saunders, A., Tarney, J., Marsh, N. G. & Wood, D. A., 1980. Ophiolites as ocean crust or marginal basin crust: a geochemical approach. In: Proceedings of International Ophiolite Conference, Nicosia, Cyprus (A. Panayiotou, ed.), 193-204.
Sugimura, A., 1973. Multiple correlation between composition of volcanic rocks and depth of earthquake foci. In: The western pacific island arcs, marginal seas and geochemistry (P. Coleman, ed.). Western Australia University Press, 471-482.
Sullivan, M. A., Windley, B. E., Saunders, A. D., Haynes, J. & Rex, D. C., 1993. A palaeogeographic reconstruction of the Dir Group: evidence for magmatic arc migration within Kohistan, N. Pakistan. In: Himalayan Tectonics (P. J. Treloar & M. P. Searle eds.). Geol. Soc. London, Spec. Pub., 74, 139-160.
Tahirkheli, R. A K., Mattaure, M., Proust, E, & Tapponnier, P., 1979. The India-Eurasia suture in northern Pakistan: synthesis and interpretation of data on plate scale. In: Geodynamics of Pakistan (A. Farah, A & K. A. DeJong, eds.). Geol. Surv. Pakistan, 125—130.
Taylor, R. N., Murton, B. J. & Nesbitt, R. W., 1992. Chemical transects across intra-oceanic arcs: implications for the tectonic setting of ophiolites. Geol. Soc. London, Spec. Pub., 60, 117-132.
Tahirkheli, R. A. K., 1983. Geological evolution of the Kohistan island arc on the southern flank of the Karakoram-Hindukush in Pakistan. Bolletino Geofisica Teorica ed Applicata, 25, 351-364.
Treloar, P.J., Brodie, K. H., Coward, M. P., Jan, M. Q., Khan, M. A., Knipe, R. J., Rex, D. C. & Williams, M. P., 1990. The evolution of the Kamila shear zone, Kohistan, Pakistan. In: Exposed cross-sections of the continental crust (M. H. Salisbury & D. M. Fountain, eds.). Kluwer Academic Publishers, 175-214.
Treloar, P. J., Rex, F. C., Guise, P. G., Coward, M. P., Searle, M. P., Windley, B. F., Petterson, M. G., Jan, M. Q. & Luff, I. W., 1989. K-Ar and Ar-Ar geochronology of the Himalayan collision in NW Pakistan. Constraints on the timing of suturing, deformation, metamorphism and uplift. Tectonics, 8, 881-909.
Virdi, N. S., 1992. Cretaceous marginal basins of the Indus-Kohistan collision zone, development and evolution. In: Himalayan orogen and global tectonics (A. K. Sinha, ed.). Oxford & IBH Publishing co., New Dehli. 157-168.
Weaver, B. L., Tarney, J. & Windley, B. F., 1981. Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, south west Greenland: nature of the parent magma. Geochim. Cosmochim. Acta, 45, 711-725.
Yoshida, M., Zaman, H., & Ahmed, M. N., 1996. Palaeo position of Kohistan arc and surrounding terranes since Cretaceous time: the paleomagnetic constraints. Proceedings of Geoscience Colloquium, Geoscience lab, Geol. Surv. Pakistan, 15, 83-101.
