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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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LONG TERM HISTORY (DEXTRAL TO SINISTRAL AND DUCTILE TO BRITTLE) OF THE DAMXUNG—JIALI SHEAR ZONE IN SOUTHERN TIBET

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地学前缘 2000年第S1期7卷 155-156页

作者： M. A. Edwards, L. Ratschbacher, members of INDEPTH field geology group(Asian Tectonics Research Unit, institut für geologie, Technische Universitt bergakademie freiberg, Bernhard\|von\|Cotta Strasse 2, D\|09596 freiberg, Germany,E\|mail: edwards@geo

The Damxung—Jiali shear zone is a shear zone in Southern Tibet that trends W—E for>*** appears to have a varied history of both displacement sense and deformation *** have examined microscopic and field data (including fault plane populations) in the westernmost 40km of the shear zone (on the northern margin of the Damxung graben near to the route of the INDEPTH II deep seismic line). The shear zone outcrops along the southern flank of an W—E trending chain of hills in which all valleys run N—S, providing a series of sections of exposure through the shear zone. The shear zone is 3～5km wide and has varying degrees of deformation. It cuts across the main trends of, and does not seem to be restricted to any particular lithology. The principal fabric in the shear zone is steeply to moderately south dipping. The bulk of the shear zone is phyllite and lower grade schist with a consistent degree of both ductile and brittle strain. A mylonitized carbonate unit that is sometimes present within the phyllites may have locally provided a mechanical instability as indicated by the higher strain. Elsewhere a conglomerate unit contains carbonate clasts that are stretched (prolate) up to 30∶*** most spectacular unit forms lozenges of quartzifeldspathic rich rock (several 100m wide & 2～5km long) whose microfabric indicates deformation mechanisms (e.g., wholesale grainsize reduction) that operated around 400～500℃; the highest grade of the deformation fabrics. The lozenges are remnant evidence of higher temperature strain (at higher strain rate?) that is preserved only in the quartzifeldspathic rock. They are interpreted to be mega\|scale boudins resulting from their strength contrast with the weaker phyllites and schists during later lower temperature deformation. All the presently preserved sense of shear indicators are observed to be left\|lateral, and bulk shortening directions resolved using slip data from fault plane populations are consistent with this sinistra

关键词： Southern Tibet Nyenqengtanghla Damxung— Jiali shear zone high strain

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