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$Fayek & Kyser 2000 uraninite fractionations.pdf - Queen's University$

528 M. P. SEARLE ET AL. Fig. 6. Simplified, schematic section across the Manaslu Himalaya, showing key features and our proposed location of the Main Central Thrust. Shaded area represents the zone of partial melting with migmatites and leucogranites (crosses). The Manaslu leuocogranite is wholly within the Greater Himalayan Sequence, following Searle & Godin (2003), with the South Tibetan detachment (STD) wrapping around the upper level of the granite. The Ramgarh thrust marks the southern limit of Tertiary Himalayan metamorphism in western Nepal and we prefer to link this with the Main Central Thrust. The reported location of the Ramgarh thrust in central and eastern Nepal (Martin et al. 2005; Pearson & DeCelles 2005), however, does not coincide with the Main Central Thrust. In central and eastern Nepal the location of the Ramgarh thrust is almost entirely interpreted from lithological repetition; a possible fault surface has been observed only in the Tribeni area of eastern Nepal. A more southerly location for the Ramgarh thrust is supported by pervasive deformation documented by quartz c-axis fabrics throughout central Nepal (Bouchez & Pêcher 1981). Although the recrystallization of quartz under significantly high strain has been recognized in the Ramgrah thrust sheet, as mapped by Martin et al. (2005) and Pearson & DeCelles (2005), it is interpreted to be locally confined to the immediate hanging wall of the inferred thrust (Pearson & DeCelles 2005). However, Bouchez & Pêcher (1981) showed that quartz c-axis fabrics are preserved for more than 6 km farther south than the mapped position of the Ramgarh thrust. Both lithological and structural data fit better with a structurally lower, more southerly, Ramgarh thrust where it is coincident with the Main Central Thrust. Restored sections show that the Ramgarh, Dadeldhura and Main Central thrust sheets of DeCelles et al. (2001) all have Proterozoic sedimentary rocks, Ulleri augen gneiss and Cambrian–Ordovician sedimentary rocks and granites as protoliths. Hanging-wall–footwall cut-offs can be successfully matched in restored sections (Fig. 7). We therefore propose that the Main Fig. 7. Generalized restored section across the Nepal Himalaya showing the pre-thrusting trajectories of the Main Central Thrust and South Tibetan Detachment shear zones and faults. The shaded horizon represents the Upper Proterozoic sedimentary rocks of the Lesser Himalaya, and Greater Himalaya. Within the Greater Himalayan Sequence these include the metamorphosed rocks of the Nawakot Group above the Ramgarh thrust (Main Central Thrust) and the Bhimpedi Group within the Kathmandu nappe, above the Mahabharat thrust. Sillimanite-grade pelitic gneisses within the Greater Himalayan Sequence are interpreted as metamorphosed Upper Proterozoic sedimentary rocks of the Haimanta–Vaikrita Group. The base of the Tethyan Himalaya consists of similar Neoproterozoic sedimentary rocks showing that the Lesser, Greater and Tethyan Himalaya were all part of one contiguous Indian plate.
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 529 Central Thrust should be mapped along the Ramgarh thrust, and all rocks above that should be incorporated into the Greater Himalayan Sequence. The major thrust systems propagated southward with time from the Early Miocene motion of the Greater Himalayan Sequence metamorphic core (Hodges et al. 1996; Godin et al. 2001) to the ,15 Ma motion along the Ramgarh thrust (DeCelles et al. 2001; Robinson et al. 2006). During the Late Miocene ductile shearing along the Main Central Thrust–Ramgarh thrust ceased, and thrusting propagated downsection to the Lesser Himalayan brittle imbricate thrust system. At least 120 km of southward translation has been estimated across the Ramgarh thrust sheet (Robinson et al. 2006). Mahabharat thrust Several klippen or thrust sheets of high-grade metamorphic rocks and granites (e.g. Almora klippe; Kathmandu complex) overlie low-grade or unmetamorphosed rocks of the Lesser Himalaya to the south of the main Main Central Thrust, as mapped in Langtang and the Ganesh Himal (Fig. 1). The thrust beneath these klippen has been variously termed the Almora or Munsiari thrusts in India (Heim & Gansser 1939; Valdiya 1980), and the Mahabharat and Dadeldhura thrusts in Nepal (Stöcklin & Bhattarai 1980; Upreti & LeFort 1999; Johnson et al. 2001). The Mahabharat thrust beneath the Kathmandu klippe encircles the southern Kathmandu valley and links up with the Main Central Thrust in the NW and NE (Upreti & LeFort 1999; Johnson et al. 2001). Rocks above the Mahabharat thrust include Proterozoic Bhimpedi Group and early–middle Palaeozoic Phulchauki Group sedimentary rocks, which are intruded by Ordovician granites and augen gneisses. Metamorphism reaches kyanite grade at the base and isograds are right-way-up from kyanite through garnet and biotite to chlorite grade (Johnson et al. 2001). Along the Mahabharat thrust dynamic metamorphism has locally inverted the thermal gradient with formation of garnet–biotite mylonites and phyllonites. Beneath the Mahabharat thrust carbonaceous pelites of the Nawakot Group show an inverted thermal gradient from 468 8C to less than 330 8C, based on Raman spectroscopy of carbonaceous material (Beyssac et al. 2004; Bollinger et al. 2004). These crystalline complexes have been termed ‘Lesser Himalayan crystallines’ or ‘Outer Lesser Himalayan crystallines’, but in reality they are lateral equivalents to the Greater Himalayan Sequence above the Main Central Thrust. They share similar upper Proterozoic and lower Palaeozoic protoliths, similar Miocene metamorphism, and similar 21–18 Ma leucogranite pegmatite dykes as the main Greater Himalayan Sequence to the north (Johnson et al. 2001). We concur with the conclusions of Johnson et al. (2001) and Johnson (2005) that the Mahabharat thrust is the same structure as the Main Central Thrust, but along a more southerly, proximal position in the restoration. The Mahabharat thrust climbs up-section in the transport direction, from being along the base of the inverted metamorphic sequence at Langtang in the north, to along the isograd fold hinge at Kathmandu (Fig. 8). The youngest thrust splays off beneath the Kathmandu complex to link with the Ramgarh thrust to the south and west of the Kathmandu complex (Fig. 1). All these thrusts are part of the Main Central Thrust but are restored at different depths, progressively ramping up-section towards the south. The Darjeeling klippe is another structural outlier of Greater Fig. 8. Geometry of the Main Central Thrust zone in the Langtang–Kathmandu nappe region of central Nepal showing the relationship of the Mahabharat and Ramgarh thrusts to the metamorphic isograds. This geometry combines the folded isograd model of Searle & Rex (1989) with the channel flow model for the Greater Himalayan Sequence (Law et al. 2006; Searle et al. 2006) and with the Johnson (2005) structural model for the Mahabharat thrust and Kathmandu nappe. It also explains the structural location of the greenschist- and amphibolite-facies metamorphic rocks of the Ramgarh thrust sheet (Beyssac et al. 2004; Bollinger et al. 2004) structurally beneath the Mahabharat thrust. Shaded area shows the zone of partial melting (sillimanite + K-feldspar gneisses, migmatites), with the main leucogranites (crosses) concentrated along the north. Metamorphic isograds have been sheared (by a combination of pure shear and south-directed simple shear), and flattened along the South Tibetan Detachment ductile shear zone above (right-way-up metamorphic isograds) and along the Main Central Thrust ductile shear zone below (inverted metamorphic isograds).
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