Imbrication and Unroofing of the Himalayan Thrust Stack of the North Indian Plate, Northern Pakistan
Abstract
The northern part of the Indian Plate in North Pakistan is dominated by a crustal scale south-verging thrust stack composed of a number of thrust nappes, each of which is stratigraphically distinct. Major nappes recognised in the Swat to Kaghan area of north Pakistan are the Besham, Swat, Hazara, Banna, Lower Kaghan and Upper Kaghan nappes. Metamorphism was synchronous with early ductile stages of Himalayan deformation, the metamorphic pile being subsequently disrupted during the development of the thrust stack which marks the last phase of southeasterly directed Himalayan thrusting. Within each nappe the metamorphic grade increases upwards, an overall inversion that represents post-metamorphic imbrication within individual nappes, synchronous with the main phase of nappe stacking, rather than an originally inverted metamorphic gradient. As a result of this "within-nappe" imbrication each thrust slice within any particular nappe contains rocks of a higher metamorphic grade than those in the slice below, with sharp metamorphic breaks across the imbricating thrusts as well as across the major shears that bound the individual thrust nappes. Exhumation and unroofing of the rocks within the thrust stack was rapid. The assembly of the stack triggered rapid erosion, recorded in the Oligocene and Miocene molasse basins, and regional extension that included significant northward extension within the Main Mantle Thrust zone. Cooling ages imply that peak metamorphism was completed by 40 Ma ago, or within 15 to 20 Ma of collision; that the post-metamorphic thrust stack had been assembled by 25 Ma ago, and that much of the subsequent exhumation was completed by 18 Ma ago.
References
Burg, J.P., Brunel, M., Gapais, D., Chen. G.M, & Liu, G;H., 1984. Deformation of leucogranites of the crystalline Main Central Thrust sheet in southern Tibet. J. Struc. Geol. 6, 535-542.
Bohlen, S.R., Wall, V.J. & Boettcher, A.L., 1983. Experimental investigations and geological applications of equilibria in the system FeO-TiO2-Al2O3-SiO2-H2O. Amer. Mineral. 68, 1049-1058
Bohlen, S.R. & Liotta, J.J., 1987. A barometer for garnet amphibolites and garnet granulites. J. Petrol. 27, 1025-1034.
Bossart, P., Dietrich, D., Greco, A., Ottiger, R. & Ramsay, J.G., 1988. The tectonic structure of the Hazara-Kashmir syntaxis, Southern Himalayas, Pakistan. Tectonics 7, 273-297.
Butler, R.W.H. & Prior, D.J., 1988a. Anatomy of a continental subduction zone: the Main Mantle Thrust in northern Pakistan. Geol. Rundshau 77, 239-255.
Butler, R.W.H. & Prior, D.J., 1988b. Tectonic controls on the uplift of the Nanga Parbat massif, Pakistan Himalayas. Nature 333, 247-250.
Calkins, J.A., Offield, T.W., Abdullah, S.K.M. & Ali, ST., 1975. Geology of the southern Himalaya in Hazara, Pakistan and adjacent areas. U.S. Geol. Surv. Prof. paper 716C, 29pp.
Coward, M.P. & Butler, R.W.H., 1985. Thrust tectonics and the deep structure of the Pakistan Himalaya. Geology 13, 4 17-420.
Coward, M.P., Butler, R.W.H., Cambers, A.F., Graham, R.H., Izatt, C.N., Khan, M.A., Knipe, R.J., Prior, D.J., Treloar, P.J. & Williams, M.P., 1988. Folding and imbrication of the Indian crust during Himalayan collision. Phil. Trans. Roy. Soc. Lond. Series A. 326, 377-391.
Coward, M.P., Butler, R.W.H., Khan, M.A., & Knipe, R.J., 1987. The tectonic history of Kohistan and its implications for Himalayan structure. J. Geol. Soc. Lond. 144, 377-391
Coward, M.P., Jan, M.Q., Rex, D.C., Tarney, J., Thirlwall, M.F. & Windley, B.F., 1982. Geotectonic framework of the Himalaya in NW Pakistan. J. Geol. Soc. Lond. 139, 299-308.
Coward, M.P., Windey, B.F., Broughton, R.D., Luff, I.W., Petterson, M.G., Pudsey, C.J., Rex, D.C. & Khan, MA., 1986. Collision tectonics in the NW Himalayas. h: collision Tectonics (M.P. Coward & A.C. Ries, eds.) Geol. Soc. Lond. Sp. Publ. 19, 203-219.
Fery, JM. & Spear, F.S., 1978. Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib. Miner. Petrol. 66, 113-117.
Fletcher, C.J.N., Leake, R.C. & Haslam, H.W., 1986. Tectonic setting, mineralogy and chemistry of a metamorphosed stratiform base metal deposit within the Himalayas of Pakistan. J. Geol. Soc. Lond. 143, 521-536.
Ghazanfar, M. & Chaudhry, M.N., 1985. Geology of the Bunja-Battakundi area, Kaghan Valley, District Mansehra,
Pakistan. Geol. Bull. Punjab Univ. 20, 76-1 03.
Ghamnfar, M. & Chaudhry, M.N., 1986. Reporting MCT in NW Himalaya, Pakistan. Geol. Bull. Punjab Univ. 21, 10-18
Herren, E., 1987. Zanskar shear zone: northeast-southwest extension within the Higher Himalayas (Ladakh, India). Geology 15, 409-413.
Hodges, K.V. & Crowley, P.D., 1985. Error estimation and empirical geothermobarometry for pelitic systems. Amer. Mineral. 70, 702-709.
Hodges, K.V., Hubbard, M.S., and Silverberg, D.S., 1988. Metamorphic constraints on the thermal evolution of the central Himalayan orogen. Phil. Trans. Roy. Soc. Lond. series A. 326, 257-280.
Hodges, K.V. & Spear, F.S., 1982. Geothermometry, geobarometry and the A12SiO5 triple point at Mt. Mooslauke, New Hampshire. Amer. Mineral. 67, 1118-1134.
Humayun, M., 1986. Petrology of the Swat amphibolites and the development of a "Lesser Himalayan" basin. Geol. Bull. Univ. Peshawar 19, 83-10.
Johnson, G.D., Johnson, N.M., Opdyke, N.D. & Tahirkhelli, R.A.K., 1979. Magnetic reversal stratigraphy and sedimentary tectonic history of the Upper Siwalik Group, Eastern Salt Range and Southwestern Kashmir. In: Geodynamics of Pakistan (A. Farah & K.A. DeJong, eds.), Geol. Surv. Pakistan, Quetta, 149-165.
Kazmi, A.H., Lawrence, R.D., Dawood, H., Snee, L.W. & Hussain, S.S., 1984. Geology of the Indus Suture Zone in the Mingora-Shangla area of Swat, N. Pakistan. Geol. Bull. Univ. Peshawar 17, 127-144.
Latif, M.A., 1974. A Cambrian age for the Abbottabad Group of Hazara, Pakistan. Geol. Bull. Punjab Univ. 10, 1-20.
Lawrence, R.D. & Ghauri, A.A.K., 1983. Observations on the structure of the Main Mantle Thrust at Jijal, Kohistan, Pakistan. Geol. Bull. Univ. Peshawar 16, 1-10.
Lawrence, R.D. and 15 others, 1989. Tectonics south of the suture, Northern Pakistan. 28th International Geological Congress. Washington, Abs. 2, 265.
Le Fort, P., 1986. Metamorphism and magmatism during the Himalayan collision. In: Collision Tectonics (M.P. Coward & A.C. Ries, eds.), Geol. Soc. Lond. Sp. Publ. 19. 159-172
Le Fort, P., Debon, F. & Sonet, J., 1979. The lesser Himalayan cordierite granite belt, typology and age of the pluton of Mansehra, Pakistan. Geol. Bull. Univ. Peshawar 13, 51 -62.
Maluski, H. & Matte, P., 1984. Ages, of Alpine tectonometamorphic events in the northwestern Himalaya (northern Pakistan). Tectonics 3, 1-18.
Newton, R.C. & Haselton, H.T.,1 981. Thermodynamics of the garnet-plagioclase-Al2SiOS-quartz geobarorneter. In: Thermodynamics of Minerals and Melts (R.C. Newton, A. Navrotsky, & B.J., Wood, eds.). Springer-Verlag, London, 125-145.
Royden, L.H. & Burchfiel, B.C., 1987. Thin-skinned N-S extension within the convergent Himalayan region: gravitational collapse of a Miocene topographic front. In: Continental Extension Tectonics, (M.P. Coward, J.F., Dewey, and P.L. Hancock, eds.). Geol. Soc. Lond. Sp. Publ. 28, 611-619.
Searle M.P. & Rex, A.J., 1989. A thermal model for the Zanskar Himalaya. J. Metam. Geol. 7, 127-134.
Searle, M.P., Windley, B.F., Coward, M.P., Cooper, D.J.W., Rex, A.J., Rex. D.C., Li Tingdon, Xiao Xuchang. Jan, M.Q., Thakur, V.C. & Kumar, S., 1987. The closing of Tethys and the tectonics of the Himalaya. Geol. Soc. Amer. Bull. 98, 678-701.
Seeber, L. & Armbruster, J., 1979. Seismicity of the Hazara arc in northern Pakistan: decollement vs. basement faulting. In: Geodynamics of Pakistan (A. Farah & K.A. DeJong, eds.). Geol. Surv. Pakistan, Quetta, 131-142.
Treloar, P.J., Broughton, R.D., Coward, M.P., Williams, M.P. & Windley, B.F., 1989a. Deformation, metamorphism and imbrication of the Indian Plate, south of the Main Mantle Thrust, North Pakistan. J. Metam. Geol. 7, 111-125
Treloar, P.J., Coward, M.P., Williams, M.P. & Khan, M.A., 1989b. Basement-cover imbrication south of the Main Mantle Thrust, North Pakistan. Geol. Soc. Amer. Sp. Publ. 232, 137-152.
Treloar, P.J. & Rex, D.C. Coaling and uplift histories of the crystalline thrust stack of the Indian Plate internal zones west of Nanga Parbat, Pakistan Himalaya. In prep.
Treloar, P.J., Rex, D.C., Guise, P.G., Coward, M.P., Searle, M.P., Windley, B.F., Petterson, M.G., Jan, M.Q. and Luff, I.W., 1989c. K/Ar and Ar/Ar geochronology of the Himalayan collision in NW Pakistan: constraints of the timing of collision, deformation, metamorphism and uplift. Tectonics 8, 881-909.
Williams, M.P., 1989. Geology of the Besham area, north Pakistan: deformation and imbrication in the footwall of the Main Mantle Thrust. (This Volume)
Williams, M.P., Treloar, PJ. & Coward, M.P., 1988. More evidence of pre-Himalayan orogenesis in northern Pakistan. Geol. Mag. 125, 651-652.
Yeats, R.S. & Hussain, A., 1987. Timing of structural events in the Himalayan foothills of northwestern Pakistan. Geol. Soc. Amer. Bull. 99, 161-176.
Zeitler, P.A., 1985. Cooling history of the NW Himalaya, Pakistan. Tectonics 4, 127-151.
