Origin and Emplacement of the Spontang Ophiolite, Ladakh

M.P. Searle, R. I. Corfield, O.R. Green, R. Pedersen

The Spontang ophiolite in Ladakh is one of the few remnant ophiolite thrust sheets of Tethyan oceanic crust and upper mantle preserved in the Himalaya. Detailed structural mapping of the Spontang ophiolite and all the highly deformed Mesozoic and early Tertiary sedimentary rocks beneath (Photang thrust sheet) has been carried out. The ophiolite comprises mantle sequence harzburgites up through ultramafic and gabbroic cumulates, isotropic gabbros, sheeted dykes and pillow lavas. The lower MORB pillow lavas are overlain by a basalt-andesite volcanic arc (Spong Arc) sequence erupted above an intra-oceanic subduction zone. U-Pb dating of zircons from high-level gabbros show that the main ophiolite was formed at 177 ± 1 Ma and the overlying andesites contain magmatic zircons dated at 88 ± 5 Ma, indicating the timing of initial subduction and the beginning of the obduction process. The project will extend the studies from Spontang to the Purang Ophiolite in SW Tibet and also to the ophiolite slices along the Indus-Yarlung suture zone in Ladakh (Nidar) and south Tibet (Kiogar, Amlang-la ophiolitic mélanges).

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Timing of India-Asia collision in the Ladakh Himalaya

M.P. Searle, O.R. Green

This project has used detailed stratigraphy and biostratigraphy combined with structural mapping along the Indus Suture zone in Ladakh to infer the timing of India-Asia collision in the western Himalaya. We use the marine to continental transition and dating of final marine sediments along the suture zone and across the restored Zanskar shelf carbonates to constrain the timing of final closure of the Tethys Ocean that once separated India from Asia. Nummulitic limestones belonging to planktonic foraminiferal zone P8 (50.5 Ma) are the final marine sediments along the suture zone. These rocks are overlain by continental-derived clastics and conglomerates of the Chulung-la and Nurla Formations of the post-collision Indus molasse basin.

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Structural-metamorphic evolution of the Ladakh-Zanskar Himalaya

M.P. Searle, D.J. Waters, in collaboration with M.R. St-Onge (GSC, Ottawa), S.R. Noble, R.R. Parrish (NIGL)

The Greater Himalayan slab is the zone of deep crustal Barrovian facies metamorphic rocks, from chlorite-biotite up to kyanite, sillimanite + muscovite and sillimanite + K-feldspar and cordierite + garnet grade gneisses, migmatites and leucogranites forming most of the highest peaks along the Himalaya. The GHS is bounded along the top by the Zanskar Shear Zone, part of the South Tibetan Detachment (STD) series of north (or NE) - dipping low-angle normal faults, and below by the Main Central Thrust (MCT), characterized by an inverted metamorphic gradient and by a mylonitic zone of high strain. This project involves detailed structural mapping, metamorphic petrology and thermobarometry, geochemistry of leucogranites, U-Pb, 40Ar/39Ar and fission track dating, and metamorphic modeling including phase diagram and pseudosection construction and determination of P-T-t paths throughout the structural section.

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Structure and inverted metamorphism along the Main Central Thrust zone

M.P. Searle, D.J. Waters, M.J. Streule in collaboration with R.D. Law (Virginia Tech), L. Godin, K. Larsen (Queens, Canada).

An inverted metamorphic field gradient associated with a crustal scale thrust fault (Main Central Thrust) has been recognized along the Himalaya for over 100 years. Much debate recently has centered on the location of the high strain zone compared to the zone of inverted metamorphic isograds (from sillimanite-kyanite down section to biotite-chlorite). This project has used regional mapping, micro-structural analysis, quartz fabrics and thermobarometric data combined with U-Th-Pb and 40Ar/39Ar dating to place constraints on the location, magnitude and timing of motion along the MCT. Previous work was undertaken in the Kishtwar - Kulu and Gharwal regions of NW India. We are working particularly in the central and eastern Nepal region (Kali Gandaki, Marysandi, Burhi Gandaki, Trisuli, Dudh Kosi and Arun valley sections) and around the Kathmandu klippe.

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Low-angle normal faulting in the Himalaya: South Tibetan Detachment

M.P. Searle, D.J. Waters in collaboration with R.D. Law (Virginia Tech), M.J. Jessup (Knoxville, Tennesse), J.M. Cottle (NIGL)

The northern contact of the Greater Himalayan Sequence (GHS) of high-grade metamorphic rocks, migmatites and leucogranites is the South Tibetan Detachment (STD) system of low-angle, north-dipping normal faults. This fault forms the passive roof fault for the partially molten mid-crust layer that was extruding southwards from the thickened Indian plate Himalayan crust during the late Miocene. In the Everest region, two normal faults - the upper Qomolangma Detachment and the lower Lhotse Detachment (LD) bound the greenschist - lower amphibolite facies rocks of the Everest Series. This project relies on mapping, petrofabrics especially quartz C-axis, themobarometry and metamorphic modeling as well as U-Pb and 40Ar/39Ar dating to constrain the evolution of this important structure. As well as the Everest region, we have also studied the structure in Zanskar, Garhwal and western Nepal (Annapurna - Manaslu) regions.

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Plio-Pleistocene crustal melting in the Nanga Parbat syntaxis, Pakistan

M.P. Searle, D.J. Waters in collaboration with J. Crowley (Boise State), S. Bowring (MIT), J. Boyce and K.V. Hodges (Arizona State)

The Western Himalayan syntaxis around the peaks of Nanga Parbat (8125 m) and Haramosh (7398 m) in North Pakistan has revealed extremely young and ongoing high-temperature metamorphism and crustal melting that is unique to the Himalayan chain. Precambrian rocks of the lower Indian crust has been metamorphosed during the Late Eocene-Oligocene-Early Miocene in common with all the Greater Himalaya, but also record a Late Neogene regional sillimanite-cordierite metamorphism. Isotope dilution TIMS U-Pb dating has revealed monazite ages as young as 700-690 ka indicating Pleistocene crustal melting. Nanga Parbat today is characterized by active hydrothermal systems circulating around the massif, extreme uplift - exhumation rates, active bounding fault margins and extreme relief. This project combines structural mapping with metamorphic petrology, thermobarometry and modeling with precise U-Th-Pb dating of accessory minerals.

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Structure of the Annapurna - Manaslu Himalaya, west Nepal

M.P. Searle in collaboration with L. Godin, K.P. Larson (Queens, Canada)

The Annapurna range in west Nepal shows Palaeozoic-Mesozoic sedimentary rocks above the South Tibetan Detachment that show spectacular large-scale north-vergent backfolds. Middle crust metamorphic rocks are exposed south of the STD in the main range as well as domes (Chako dome, Nar valley) to the north. As a result of new mapping, the Manaslu leucogranite, dated 24-19 Ma, previously thought to intrude across the STD is now known to wholly underlie the STD (Phu Detachement) thereby placing better constraints on motion along the STD (<18 Ma). Recent structural analysis has also placed better constraints on the location and timing of motion along the Main Central Thrust and Ramgarh Thrust along the base of the GHS. This project involves detailed mapping, structural analysis, thermobarometry and metamorphic modeling.

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Structure of the Langtang - Shisha Pangma Himalaya, Nepal - Tibet

M.P. Searle, D.J. Waters in collaboration with R.D. Law (Virginia Tech) and S.M. Reddy (Perth, Australia)

The Langtang Himalaya north of Kathmandu provides a classic profile across the Greater Himalayan sequence metamorphic core of the Himalaya. The Main Central Thrust near the village of Syanru records a high strain zone corresponding to the zone of inverted metamorphic isograds from kyanite down to biotite-chlorite grade. Structurally above this sillimanite + muscovite and sillimanite + K-feldspar gneisses show the beginnings of partial melting over a wide region and approximately 7-10 km thickness of the middle crust. Numerous two mica + garnet + tourmaline leucogranites occur in the highest structural levels with the massive Shisha Pangma leucogranite forming a 4 km thick sill complex immediately beneath the south Tibetan Detachment. Numerous cordierite-bearing leucogranites have recently been discovered in the upper Langtang glacier region. Our present research is focused on quantifying strain using quartz preferred orientation across the MCT zone, P-T-t paths in the high-grade gneisses and melt zone and dating the cordierite-bearing leucogranites.

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Crustal anatexis and Channel Flow processes, Everest - Makalu massifs, Nepal, South Tibet

M.P. Searle, D.J. Waters, M.J. Streule in collaboration with R.D. Law (Virginia Tech), M.J. Jessup (Knoxville, Tennesse), J.M. Cottle, M. Horstwood, R. R. Parrish (NIGL)

In the core of the Greater Himalaya widespread in situ partial melting in sillimanite + K-feldspar gneisses resulted in formation of migmatites and Ms + Bt + Grt + Tur ± Crd ± Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4-6 kbar; 15-20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0.74-0.79) and heterogeneous indicating a 100% crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta - Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. In the Makalu region cordierite-bearing leucogranites are the youngest phase of magmatism along the GHS. This project combines regional 3D mapping of cliff sections and map areas with geochemistry, isotope chemistry, thermobarometry and U-Th-Pb dating.

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Structure, metamorphism and melting in the Eastern Himalaya (Kangchenjunga Himal - Sikkim - Bhutan)

M.P. Searle, D.J. Waters in collaboration with R.R. Parrish (NIGL)

The Kangchenjunga (8598 m) massif in eastern Nepal and Sikkim shows nearly 10 km thickness of leucogranite emplaced as giant sills dipping very gently to the north. These leucogranites are the thickest known along the Himalaya and correlate with a zone of partially molten middle crust imaged on deep crustal seismic sections from project INDEPTH. Such widespread melting is restricted to mid-crust levels and we use the excellent 3D exposures and northward tilt to examine a 25 km deep structural profile through the Himalayan crust. The project combines regional mapping with thermobarometry and U-Pb dating through the migmatite terrane and leucogranite sheets of the Kangchenjunga - Siniolchu massifs.

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