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SYNCONVERGENT, DUCTILE N-DIRECTED SHEARING ON THE WESTERN MARGIN OF THE nanga PARBAT SYNTAXIS, N. PAKISTAN

地学前缘 2000年第S1期7卷 128-129页

作者： Tom Argles 1, Mike Edwards 2, Gavin Foster 1(1 Department of Earth Science, The Open University, Walton Hall, Milton Keyn es MK7 6AA, UK, E\|mail: t.w.argles@open.ac.uk 2 Institut fur Geologie, TU Bergakademie Freiberg, Berhard\|von\|Cottastrasse

A key issue in orogenic research today is the recognition and explanation of normal faulting in the heart of collisional mountain belts. The active Himalayan system remains an ideal locality for studying this phenomenon, both as E—W synconvergent extension of the Tibetan plateau and normal motion on the South Tibetan Detachment System (STDS). However, these processes are difficult to correlate with the evolution of the northwest Himalaya, particularly the nanga Parbat syntaxis where a Neogene tectono\|thermal overprint partially obscures the early collisional history. An integrated programme of structural mapping, petrography, thermobarometry and isotopic dating is presented that places important constraints on both the early\| and pre\|Himalayan evolution of the syntaxis. These data include evidence for synconvergent, ductile extension predating syntaxis development, and improved isotopic correlation of the tectonic units with the familiar central Himalayan thrust sheets, building on the work of Whittington et al (1999).Recent studies have focused on the rapid exhumation of the nanga Parbat\|Haramosh Massif (NPHM) during the last 10Ma, and the related Neogene thermal effects dominating the core of the massif (e.g. Zeitler et al. 1982, 1993). However, the degree of both structural and metamorphic Neogene overprinting varies within the massif, becoming weaker away from the summit region. In addition, the considerable variation in rock\|type outside the gneissic core results in both strain partitioning and various degrees of metamorphic reworking. Thus several workers (e.g. Wheeler et al. 1995) could reconstruct elements of the early and pre\|Himalayan history from field relations and mineral assemblages virtually untouched by Neogene processes. The eastern margin of the massif, in contrast to the active western margin, has remained largely unchanged during the Neogene, except for essentially passive rotation on the limb of the major syntaxial antiform. The original,

关键词： synconvergent syntaxis extension Himalaya ductile exhumation nanga Parbat

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GEOLOGY AND MINERAL CHEMISTRY OF THE DEOSAI VOLCANICS, BALTISTAN, N. PAKISTAN

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地学前缘 2000年第S1期7卷 416-416页

作者： Syed Hamidullah(National Centre of Excellence in Geology, University of Peshawar, Pakistan)

The westerly extension of the Dras volcanics in the Deosai plateau of Baltistan, northern Pakistan, lying east of the nanga Parbat—Haramosh Massif, is comprised of agglomerates and tuffs together with flows consisting of basalt, andesite and some rhyolite. In the filed these volcanics are overlying the Ladakh batholith and both these basic and acidic suites of rocks carry the signatures of the nanga Parbat—related orogeny. The flows appear to have evolved from a basaltic magma, with opaque oxide, clinopyroxene, hornblende and plagioclase, respectively appearing on the liquidus. These have been metamorphosed under greenschist facies conditions and may contain abundant epidote, chlorite and secondary amphibole. Metamorphic impact seems to be stronger in the west, i.e., in the vicinity of nanga Parbat—Haramosh Massif, than in the East. An 40 Ar/ 39 Ar age of (125 4±6)Ma on hornblende phenocrysts in an andesite is in agreement with the Late Jurassic to Cretaceous age of the Dras volcanics, in India, and indicates that nanga Parbat related tectonics may have played a part in the growth of lower green schist facies assemblage of the volcanic rocks.

关键词： Ladakh Deosai Dras Ladakh Batholith nanga Parbat basalt volcanics

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