The Namche Barwa syntaxis: evidence for exhumation related to compressional crustal folding

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Abstract

Geological observation in the eastern end of the Himalayas shows that the Asia/India suture is folded and continues southward into India and Burma where the continuation of the Transhimalayan plutonic complex can be identified. Metamorphic rocks derived from India occur structurally below the suture, in the core of a regional antiform. Isotopic and fission track dating are consistent with the geological interpretation. In addition, they establish cooling exhumation of rocks from c. 30 km depth within the last 4 Ma. Rapid exhumation of deep crustal levels in mountain systems is generally related to extension subsequent to thermal softening of a thickened continental lithosphere. However, in the Eastern Himalayas, we argue that exhumation is caused by c. 10 mm yr−1 erosion coeval with crustal scale folding. The general history of this syntaxis resembles the evolution of the western Himalayas syntaxis in Pakistan.

Introduction

The Himalayas terminate at both ends in syntaxes (Wadia, 1931), a distinctive feature of collisional belts (e.g. the Alps, the Caribbean) where orogenic structures seem strongly bent around a vertical axis. Although little documented, the eastern syntaxis of the Himalayas, named Namche Barwa after the 7756 m high peak that towers it, is traditionally regarded as symmetrical to the Nanga Parbat syntaxis, the western bend of the Himalayas in Pakistan (Wadia, 1931; Gansser 1991). Indeed, on topographic maps and space images of the area, the Namche Barwa peak seems to mark the vertical axis of a 180° bend of the Himalayan structural trends, giving rise to the U-turn of the Yalu Tsangpo (Brahmaputra) River. Geologists have therefore inferred that there would be a bent segment of the Palaeocene Tethyan suture zone around the Namche Barwa (Wadia, 1957; Gansser, 1966, Gansser, 1980, Gansser, 1991). Chinese exploratory studies have resulted in a geological map displaying a variety of undifferentiated migmatitic gneisses of Proterozoic age (Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences, 1988). Interpretation of Landsat and Spot satellite images extended our field observations to prepare a geological sketch map based on lithology distribution and structure orientation (Fig. 1). Analytical results include microstructural work to relate deformation and metamorphism, along with geochronology to constrain the timing and rates of development of this end of the Himalayas. In this contribution we wish to extend a report where first results and a hint at their interpretation in terms of a fast growing, crustal scale antiform have been introduced (Burg et al., 1997). The crustal antiform folds the Yalu Tsangpo segment of the Tethyan suture into a sharp “syntaxis” mirroring the western syntaxis of the Himalayas, around the Nanga Parbat mountain. Having documented the main tectonic units in the Namche Barwa area, we will discuss the southeastward continuation of the Tethyan suture into India and Burma.

Section snippets

Lithologies: Trans-Himalayan and Indian crustal segments

Paleozoic and Mesozoic quartzites, quartzphyllites, shales and marbles that screen calc alkaline plutons of the Transhimalayan plutonic belt (Gansser, 1980, Burg et al., 1983) wrap around the Namche Barwa area proper (Fig. 1). The sediments become penetratively deformed and metamorphosed towards contact with dykes and plutons and the metamorphic grade reaches regional anatexis in the northeastern, eastern and southeastern, structurally deeper regions. Between Nyingchi and Parlung (Fig. 1)

Folding

The large-scale structure is a 30 to 40 km wide antiform whose hinge lies near Doxiong-La. Its northern limb is dominated by large parasitic folds such as the tight, north- to north-east facing anticline that builds the Namche Barwa peak proper (Fig. 4). Dips of foliation planes, vergence of mesoscopic folds and the lithologies of pebbles that come indisputably from the Giala Peri peak suggest that it contains an antiform equivalent to the Namche Barwa anticline. The southern limb appears less

Metamorphic history

We concentrate our discussion on the core area and do not document the high temperature–low pressure metamorphic sequences of the Transhimalayan plutonic belt.

Retrograde greenschist overprint was detected only from local growth of actinolite, epidote and chlorite in cracks and fractures and occurs in the reworked serpentinite bearing mylonitic suture. These metamorphic temperatures range between 430–480°C and pressures between 5.5–6.5 Kbar (Wang, 1993). Because it is localised, this late

Geochronology

In the Transhimalayan rocks, conventional K–Ar ages derived from micas fall between 39 and 18 Ma, contrasting with dates younger than 14 Ma and as young as 1.2 Ma within the core migmatitic gneisses (Fig. 1). These ages refer to the times when the different crustal segments cooled to temperatures less than c. 300°C (Harrison et al., 1985) and point to young thermotectonic activity in the core of the Namche Barwa antiform. To corroborate these young low-temperature ages, we document zircon and

Rapid erosion of growing crustal folds

The samples used in the present work come from one short section within the core of the antiform, without any major faults between them (Fig. 1, Fig. 4). Therefore we assume that they all have undergone the same bulk history, related to that of the deepest parts of the antiform. Petrological calculations (Fig. 13) do not yield syn-decompression heating as expected from thermal-exhumation models with 1D heat transfer and linear erosion rates (England and Thompson, 1984; Davy et al., 1989). We

Discussion

Our field data, accurate dating and modelling establish that the Namche Barwa syntaxis is a fast-growing, post-collision arcuation of Himalayan trends. It is a large, dominantly northeast plunging antiform within the core of which Indian plate gneisses have been exhumed from below the Transhimalayan plutonic belt over the last 4 Ma. The youthful geomorphology suggests that uplift and concurrent exhumation are continuing to produce the Namche Barwa syntaxis, which involves a combination of

Acknowledgements

This work is supported by the ETH-Zürich (Projekt 1-20-888-94). P. Davy is funded by the French CNRS and Dia Zhizhong by the Chinese Academy of Sciences in Chendu. The authors would like to thank Suz-Chung Ko for valuable help in deciphering the Chinese literature and Yang Hsuanlin for his efficient assistance with field work and logistics. We would also like to thank D. Vance for sharing his experience in garnet Sm–Nd analytics with us, H. Derksen for carrying out mineral separation and J.

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