Thrusting, extension, and doming during the polyphase tectonometamorphic evolution of the High Himalayan Crystalline Zone in NW India
Introduction
The metamorphic core zone of the Himalayan orogen consists of a 5–40 km thick sequence of amphibolite facies to migmatitic paragneiss, with minor orthogneiss, metabasite and calcsilicate gneiss. This High Himalayan Crystalline Zone (HHCZ) thrusts over the low- to medium-grade metasedimentary series of the Lesser Himalaya along the Main Central Thrust (MCT), a major intra-continental thrust developed within the Indian plate margin during Early Miocene, since ca. 23 Ma (e.g. Frank et al., 1977, Hubbard and Harrison, 1989, Coleman, 1998) (Fig. 1). In numerous transects across the range, the HHCZ is separated from the overlying, low-grade sediments of the Tethyan Himalaya by the extensional structures of the South Tibetan Detachment System (STDS, Burchfiel et al., 1992). Geochronological and structural data indicate that STDS extensional movement initiated during Early Miocene, around 23 Ma (e.g. Hodges et al., 1992, Dèzes et al., 1999). Broadly contemporaneous movements along both the MCT and the STDS consequently reflect a tectonically controlled exhumation of the HHCZ.
For more than 1400 km along the range, in the central and eastern parts of the Himalaya, the HHCZ corresponds to a fairly monoclinal, NE-dipping slab up to 20 km thick, cropping out mainly in the frontal part of the orogen. This rather simple geometry contrasts significantly with what is observed in the NW part of the Himalaya of India, north of the Kulu Valley. In this later region, the amphibolite facies to migmatitic gneisses of the HHCZ are mainly exposed in a more internal part of the orogen, where they broadly form a 180 km long and 60 km large dome structure in the Himalaya of Zanskar (Fig. 1). These gneisses are almost completely surrounded by lower grade metasediments, and they are not in cartographic continuity with the similar high-grade rocks cropping out as a thick sheet in the frontal part of the belt, from the Kulu Valley to the SE. In contrast, between the Kulu Valley and the Chenab Valley, the hanging wall of the MCT in the frontal part of the orogen mainly consists of greenschist facies metasediments (chlorite to biotite zones) that will be referred to as the Chamba zone in the following discussion (Fig. 1). The Chamba zone is cartographically connected to the basal part of the Tethyan Himalaya, that consist of a several kilometres thick series of Neo-Proterozoic to Cambrian detrital sediments (graywackes, siltstones and pelites), referred to as the Haimantas (e.g. Griesbach, 1891, Frank et al., 1995). It is worth emphasizing that although the HHCZ unit is generally separated from the Tethyan Himalaya by extensional structures of the STDS, a gradual transition between the low-grade Haimantas and the high-grade paragneisses of the HHCZ is observed in several parts of the NW Himalaya. It has consequently long been recognized that the HHCZ paragneisses represent metamorphic equivalents of the Haimantas, and not the true basement onto which the Tethyan Himalaya sediments were deposited (e.g. Griesbach, 1891, Frank et al., 1973, Steck et al., 1993, Vannay and Steck, 1995).
In the Himalaya of Zanskar, the north-eastern contact between the HHCZ and the overlying low-grade sediments of the Tethyan Himalaya corresponds to the 150 km long Zanskar Shear Zone (ZSZ), a ductile extensional shear zone that accommodated a minimum slip of ca. 35 km during Early Miocene (ca. 23–19 Ma; Herren, 1987, Dèzes et al., 1999). Since the description of this spectacular tectonic setting, most geological studies have been focussed on the NE border and central part of the HHCZ of Zanskar (e.g. Honegger et al., 1982, Kündig, 1989, Stäubli, 1989, Dèzes et al., 1999, Searle et al., 1999, Walker et al., 1999, Stephenson et al., 2000). In contrast, only limited work has been done along the southern border of this unit (Pognante et al., 1990, Steck et al., 1999), and the tectonic and metamorphic transition between the HHCZ and the Chamba zone remains still poorly constrained. This transition zone appears to be characterized by a complex tectonic evolution, involving two phases of NE-directed and SW-directed nappe tectonics, as well as SW-directed extension (Steck et al., 1999). Moreover, the south-eastern end of the HHCZ of Zanskar is deformed by a large-scale dome structure, called the Gianbul dome, centred on a Early Miocene leucogranite.
The aim of the present study is to provide new constraints on the tectono-metamorphic evolution of the south-easternmost limit of the HHCZ of Zanskar, on the basis of detailed petrographic and thermobarometric investigations in the Miyar Valley. Together with comparable data for the NE limit of the HHCZ along the Gianbul Valley (Dèzes, 1999, Dèzes et al., 1999), these new results allow us to propose a reconstruction of the tectono-metamorphic evolution along a complete transect across the Gianbul dome.
Section snippets
Lithological and tectonic setting of the Miyar Valley section
The Miyar Valley, in the Upper Lahul region, represents a natural cross-section through the southern border of the HHCZ of Zanskar (Fig. 2, Fig. 3). This high-grade unit consists mainly of amphibolite facies to migmatitic paragneiss, separated from the chlorite to biotite grade Haimantas metasediments of the Chamba zone by a SW-dipping extensional shear zone called the Khanjar Shear Zone (Steck et al., 1999). Two types of intrusive granites are also observed along the studied transect. The Kade
Prograde metamorphic field gradient (M1)
The metapelites of the HHCZ in the Miyar Valley preserve a typical Barrovian metamorphic field gradient indicating a gradual increase of metamorphic conditions from SW to NE. Moving upsection along the valley, from the village Udaipur to the Gumba glacier upstream, a gradual succession of chlorite, biotite, garnet, kyanite+staurolite, sillimanite and migmatite zones can be observed (Fig. 2, Fig. 3). This continuous metamorphic field gradient indicates a gradual, although rapid, transition
Methodology
In order to quantitatively constrain the peak P–T conditions along the Miyar section, 16 garnet-bearing samples (14 metapelites and 2 metabasites) were selected for geothermobarometry. The mineral analyses are provided in Table A1, Table A2. No chemical zoning was observed in the plagioclases, muscovites, biotites and hornblendes analysed for thermobarometry. Except for one garnet preserving a weak growth zoning, the majority of the analysed garnets show flat composition profiles testifying to
Oxygen isotope thermometry
In order to obtain additional independent constraints on the peak temperatures along the studied section, 9 samples were analysed for oxygen isotope thermometry. The isotopic fractionation between pairs of mineral phases is a function of the temperature of equilibration, and it is independent of the pressure. For this study, we have analysed quartz–garnet, quartz–kyanite and garnet–sillimanite mineral pairs, inasmuch as these phases are the more likely to retain peak isotopic compositions even
Retrograde metamorphic evolution (M4)
In the Miyar section, the retrograde evolution in the footwall of the extensional Khanjar Shear Zone is well recorded in the metapelites of the kyanite zone near Yuling (Fig. 4). The initial stage of the retrograde history is characterized by the appearance of sillimanite, growing as a fine-grained fibrolite on biotite or at the expense of kyanite (Fig. 5c and d). Cordierite can be relatively abundant in some samples, where it grew as post-kinematic poikiloblasts, sometimes surrounding kyanite (
Gianbul Valley section
The NE half of the Gianbul dome has been investigated by Dèzes, 1999, Dèzes et al., 1999 in the Gianbul Valley. The main structure in the later section is the Zanskar Shear Zone, which marks the transition between the high-grade metamorphic rocks of the Gianbul dome and the low-grade sediments of the Tethyan Himalaya to the NE (Fig. 2, Fig. 3). The tectono-metamorphic history in the Gianbul section is the consequence of two main events: (1) an initial phase of crustal thickening related to the
Chronology of the tectonic events
The chronology of the pre-MCT tectono-metamorphic evolution in Zanskar is still poorly constrained. From the Miyar Valley to the Kulu Valley, the NE-verging structures associated with the Shikar Beh nappe are overprinted by the SW-verging structures associated with the thrusting of the HHCZ toward the foreland along the MCT (Steck et al., 1993, Vannay and Steck, 1995, Steck et al., 1999, Wyss et al., 1999). In the studied transect, the relative chronology of the Shikar Beh nappe emplacement
Synthesis
The petrographic and quantitative P–T results for the Miyar section (Fig. 4, Fig. 7) and for the Gianbul section (Fig. 9; Dèzes et al., 1999) provide information about the depth of burial of the studied samples, as well as about the thermal structure during the tectonic evolution of a complete transect across the Gianbul dome. On the other hand, the mapping and structural analysis of this transect (Dèzes, 1999, Dèzes et al., 1999, Steck et al., 1999) constrain its kinematic evolution. These
Discussion and conclusions
In the central and southeastern parts of the Himalaya, the HHCZ high-grade rocks were mainly exhumed in the frontal part of the range, as a consequence of a tectonic exhumation controlled by combined thrusting along the MCT and extension along the STDS detachments (e.g. Hodges et al., 1992, Vannay and Hodges, 1996, Wyss et al., 1999). In the NW Himalaya, however, the hanging wall of the MCT in the frontal part of the range consists mainly of low- to medium grade metasediments (Chamba zone),
Acknowledgements
We thank Emmanuel Marclay, Micha Schlup and Olivier Zingg for assisting with field work, Georges Mascle for discussion and helping with samples transport, Laurent Nicod for preparation of thin and polished sections, François Bussy for supervising microprobe work, Johannes Hunziker and Jorge Spangenberg for supervising isotopic work at the Stable Isotope Laboratory at the University of Lausanne, and Zachary Sharp, Viorel Atudorei and Matthieu Girard for isotopic analysis at the Stable Isotope
References (48)
- et al.
Magmatism and metamorphism in the Ladakh Himalayas (the Indus–Tsangpo suture zone)
Earth and Planetary Science Letters
(1982) - et al.
Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation
Nature
(2001) Thermobarometry using multi-equilibrium calculations: a new technique with petrological applications
Canadian Mineralogist
(1991)- et al.
Age of metamorphism in the Zanskar Tethys Himalaya (India)
Géologie Alpine, Mémoire H.S.
(1991) - et al.
The South Tibetan Detachment System, Himalayan orogen; extension contemporaneous with and parallel to shortening in a collisional mountain belt
Geological Society of America Special Paper
(1992) - et al.
The Kangmar Dome: a metamorphic core complex in southern Xizang (Tibet)
Science
(1990) Observations on the origins and ascent mechanisms of granitic magmas
Journal of the Geological Society of London
(1998)U–Pb constraints on Oligocene–Miocene deformation and anatexis within the Central Himalaya, Marsyandi valley, Nepal
American Journal of Science
(1998)Tectonic and metamorphic evolution of the Central Himalayan domain in southeast Zanskar (Kashmir, India)
Mémoires de Géologie (Lausanne)
(1999)- et al.
Synorogenic extension: quantitative constraints on the age and displacement of the Zanskar Shear Zone (NW Himalayas)
Geological Society of America Bulletin
(1999)
Tertiary Himalayan structures and metamorphism in the Kulu Valley (Mandi–Khoksar transect of the western Himalaya)—Shikar Beh Nappe and Crystalline Nappe
Schweizerische Mineralogische und Petrographische Mitteilungen
Relations between metamorphism and orogeny in a typical section of the Indian Himalayas; NW-Himalaya; S-Lahul, Kulu; Himachal Pradesh; first comprehensive report
Tschermaks Mineralogische und Petrographische Mitteilungen
Geology and petrography of Kulu—South Lahul area
Colloques Internationaux du Centre National de la Recherche Scientifique
Geological Map of the Kishtwar–Chamba–Kulu Region (NW Himalaya, India)
Jahrbuch der Geologischen Bundesanstalt
Geology of the Central Himalaya
Memoir of the Geological Survey of India, XXIII
Northeast–southwest extension within the Higher Himalayas (Ladakh, India)
Geology
Realistic propagation of uncertainties in geologic thermobarometry
American Mineralogist
Simultaneous Miocene extension and shortening in the Himalayan Orogen
Science
An internally consistent thermodynamic data set for phases of petrological interest
Journal of Metamorphic Geology
40Ar/39Ar age constraints on deformation and metamorphism in the Main Central Thrust Zone and Tibetan Slab, eastern Nepal Himalaya
Tectonics
Domal structures and high-grade metamorphism in the Higher Himalayan Crystalline, Zanskar Region, north-west Himalaya, India
Journal of Metamorphic Geology
Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names
The Canadian Mineralogist
Comparison of conventional and garnet–aluminosilicate quartz O isotope thermometry: insights for mineral equilibration in metamorphic rocks
American Mineralogist
Extensional tectonics in the Himalayan Orogen, Zanskar, NW India
Geological Society of London, Special Publication 74
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2014, Journal of Asian Earth SciencesCitation Excerpt :The injection of anatectic melt into SW-directed extensional shear bands (Steck et al., 1999) indicates that the development of these extensional structures is intimately associated with the migmatite and dome formation. Moreover, in the rocks of the kyanite zone, in the footwall of the MSZ, the successive crystallization of fine-grained fibrolite growing at the expense of kyanite (Fig. 5c), the crystallization of cordierite as post-kinematic poikiloblast surrounding kyanite (Fig. 5d), and, lastly, the crystallization of andalousite as stable aluminosilicate reveal that these rocks underwent retrograde metamorphic evolution characterizing a nearly isothermal decompression (Robyr et al., 2002). These observations indicate that the MSZ originally acted as a NE-directed synmetamorphic thrust along which the rocks now forming the HHC zone of Zanskar were underthrust below the Chamba zone before being reactivated as a SW-directed ductile zone of extension during the exhumation of the Gianbul dome.