Mantle exhumation along the Tirich Mir Fault Zone, NW Pakistan: pre-mid-Cretaceous accretion of the Karakoram terrane to the Asian margin
Published:January 01, 2000
A. Zanchi, S. Poli, P. Fumagalli, M. Gaetani, 2000. "Mantle exhumation along the Tirich Mir Fault Zone, NW Pakistan: pre-mid-Cretaceous accretion of the Karakoram terrane to the Asian margin", Tectonics of the Nanga Parbat Syntaxis and the Western Himalaya, M. Asif Khan, Peter J. Treloar, Michael P. Searle, M. Qasim Jan
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The left-lateral strike-slip Tirich Mir Fault, Chitral, NW Pakistan, is associated with a belt of peridotites, metagabbros and gneisses named the Tirich Boundary Zone (TBZ), separating the Late Palaeozoic-Mesozoic units of the East Hindu Kush from the Palaeozoic successions of the Karakoram block. These rocks were metamorphosed up to upper amphibolite facies conditions, followed by a greenschist facies overprinting, and then thrust on to very low grade metasediments; they were finally intruded at shallow levels by the mid-Cretaceous Tirich Mir pluton. Ultramafic rocks along the fault zone include well-preserved spinel lherzolites and harzburgites (Tirich Gol, Barum valley, Arkari Gol), whereas schistose serpentinites occur in the Rich Gol. Whole-rock analyses and mineral chemistry of olivine, clinopyroxene, orthopyroxene and spinel from these peridotites show a depleted signature. Microstructural and petrological features suggest a mantle origin for these ultramafic bodies, which equilibrated at temperatures ranging from 1000–1100 °C. Peridotites are faulted against partially metamorphosed igneous bodies including hornblende-gabbros, hornblende cumulates and quartz-diorites. Metamorphic rocks of the TBZ, which lay south of the ultramafic-mafic complex, include quartzites, amphibolites, garnet-sillimanite (± kyanite ± K-feldspar)-biotite gneisses and mica schists, locally displaying migmatitic textures.
A sub-continental character of the peridotites indicated by low temperatures of equilibration and by the presence of a deep crustal sequence. These characters along with the absence of an ophiolitic sequence may suggest that the TBZ represents a fragmented crust-mantle boundary developed along a zone of attenuated continental crust. The TBZ is interpreted as a sheared lithospheric section of a Jurassic-Early Cretaceous orogenic complex, formed as a consequence of the accretion of the Karakoram terrane to the southern side of the Pamir belts, which were progressively accreted to the Asian margin.
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Tectonics of the Nanga Parbat Syntaxis and the Western Himalaya
The western syntaxis of the Himalaya is one of the most exciting frontiers of continental tectonis studies. The region around the mountain of Nanga Parbat has some of the highest peaks, deepest valleys and highest uplift, exhumation and erosion rates known on earth. Surrounding regions include the Hindu Kush and Karakoram mountains (Asian plate), the Kohistan island arc and the Ladakh and Zanskar ranges of the western Himalaya (Indian plate). This volume includes 24 papers on all these regions as well as five new fold-out maps of the eastern Hindu Kush, the Spontang Ophiolite region of Ladakh, part of the west margin of the Indian plate, the Indus syntaxis in Pakistan and the Bouguer gravity anomalies in Pakistan.