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Critical Comments on Trends in Himalayan Geology
For citation: Fuchs G., 2011, Critical comments on Trends in Himalayan Geology, available in www.ngs.org.np (website of Nepal Geological Society).
Critical comments on Trends in Himalayan Geology
1. STRATIGRAPHY AND TECTONICS IN THE LESSER HIMALAYA
For decades there was dispute whether the unfossiliferous rock complex was Palaeozoic or Precambrian. On the basis of rare fossil finds and age data it is now generally accepted to be Precambrian to Early Cambrian (UPRETI, 1996; BHARGAVA et al., 2011).
In my view not yet settled is the correlation of the thick carbonate complexes of the Lesser Himalaya. SAKAI (1983), UPRETI (1996) and others regard the Kerabari and Krol Carbonates of the frontal zone (Parauthochtonous Unit, FUCHS, 1975) as Neoproterozoic. The Surtibang-, Dhading-, Shali-, and Deoban Carbonates of the inner zones are considered to be Midproterozoic (BHARGAVA et al., 2011). From the great lithological similarity and associated series a correlation of the two above named complexes should not be excluded.
Certainly a difficulty in the Stratigraphy is the confusion in nomenclature. Most authors introduce new names and their own scheme. My aim was to use the already existing and well-known terms like Chail, Shali, Krol etc from the type localities in India also in Nepal (FUCHS, 1967, 1977 and 1982; FUCHS & FRANK, 1970). These units can be traced along the Himalaya and are not confined by political borders. But my suggestion was not accepted.
Attempts have been made to subdivide the Kuncha-Chail complex by means of some conspicuous quartzite horizons (e.g. Fagfog Quartzite, STÖCKLIN, 1980). The quartzites, however, are of rather limited lateral extent. Thick conglomerate-quartzite bodies pass into pure phyllite series over a short distance along the strike. Probably they represent delta deposits and are not suitable for stratigraphical subdivision.
Closely connected with stratigraphy are tectonic problems. HAGEN (1969) proposed a complicated system of nappes. But he did not characterize their rock content and how they were discerned. The knowledge of the primary succession is necessary.
In the Hiunchuli region of West Nepal we found one of the rare occurrences where the stratigraphic order is still preserved (FUCHS & FRANK, 1970). Careful studies of the formation boundaries showed that they were stratigraphic, and the sedimentary structures indicated upright order. From bottom to top we find:
- Green and white quartzites and phyllites (Chail, Kuncha).
- Red and white quartzites and metapelites (Khaira, Nourpul).
- Red and green slates and carbonates.
- Black phyllites (Shali Slates, Benighat).
- Blue and grey stromatolitic dolomites (Shali, Surtibang, Dhading).
This succession forms Chail Unit 1 (C1). After a thrust this sequence is repeated, but in reduced thickness W of Jangla Bhanjyang (C2), and above another thrust again phyllites and quartzites (Chail-Kuncha) follow representing C3. They are the topmost beds of the Lesser Himalaya immediately below the Main Central Thrust (MCT).
Towards the E the described successions pinch out and in the Kali Gandaki-Pokhara region we find only monotonous Kuncha-Chail Formation of vast extent. Carbonate marker horizons appear again in the Nawakot Complex W of the Kathmandu-Synclinorium (STÖCKLIN, 1980, map). These carbonates indicate thrusts at their top and make it probable that such tectonic planes exist also where the Chail-Kuncha rocks seem to form a uniform complex.
There are also authors who deny nappes in the Lesser Himalaya (ARITA et al., 1984). They accept the crystalline outliers as allochthonous, but the underlying Midland complex they take as autochthonous, affected only by vertical movements. That implies that the succession observed in the latter is stratigraphic. This assumption is disproved by our work (FUCHS, 1977) in the Galwa Window (HAGEN, 1969). In the core of this large dome we find stromatolitic dolomite with varicoloured carbonates and metapelites (C1). This unit is overthrust by Kuncha-Chail Formation succeeded by varicoloured conglomerates and quarzites, which in turn are overlain by thick dolomite (C2). Above another thrust Kuncha-Chail Formation follows (C3) forming the top of the Lesser Himalayan sequence under the MCT (FUCHS, 1977, Plate 1). The lowest of these units (C1) may be autochthonous, the upper units definitely are nappes (C2, C3); dragged by the overthrusting Crystalline Nappes the C3 shows the largest width of transport with outliers on the Mahabharat Range (S of Daban, Phalabang; FUCHS & FRANK, 1970, Plate 1). C3 has also wide extent in Ranimatta-Dailekh area N of Surkhet (FUCHS, 1977, Plate 1). The quartzites, phyllites and metabasics of the Kuncha-Chail Formation are also accompanied by granites. The latter intruded the Chails and are frequent in the Western Himalaya. Their ages cluster around 1800 Ma (FRANK et al., 1995).
Another problem is the Krol belt – is it a nappe or a frontal folded wedge of the Lesser Himalaya? I have introduced the term Parautochthonous Unit for it. It overthrusts the Tertiary Zone along the Main Boundary Thrust (MBT), but always has formed the southernmost part of the Lesser Himalaya. The frequent occurrence of Gondwana and Tertiary beds shows its proximity to the Indian Craton (Garhwal-, Tansing Synclines). There are no outliers or relics of a “Krol Nappe” N of the frontal range, at the base of the Crystalline klippes or the MCT. The superposition of the Chail Nappes is evident from the outliers of Satengal, Banali, Lansdowne and the Mahabharat Range (FUCHS & FRANK, 1970; FUCHS & SINHA, 1978). The Parautochthonous Unit is not metamorphosed or of only low grade. The assumption of a Krol Nappe implies that this unit would be intercalated between the greenschist metamorphosed Midland complex (Chail Nappes) and the amphibolite facies grade Crystalline Nappes. This was nowhere observed, rather an increase in metamorphic grade from lower to upper structural units (inverse metamorphism). Thus I regard the Krol belt as a folded wedge zone in the frontal part of the Lesser Himalaya.
2. CORRELATION CENTRAL CRYSTALLINE - CRYSTALLINE OUTLIERS
UPRETI (1995) hesitates to accept the Central Crystalline as a root zone of the crystalline klippes in the Lesser Himalaya, because of a series of differences.
Contrary I should like to stress that in eastern Nepal and Sikkim the root zone is still connected with the frontal parts of the Crystalline Nappe. In the western Himalaya the Crystalline Nappes with their sedimentary cover – the Kashmir and Chamba Synclinoria – can be traced to their roots in the High Himalaya Crystalline (HHC). It can be observed there that the highly migmatized gneiss complex becomes reduced towards the south and is replaced by less metamorphosed Salkalas. If we consider that the frontal portions of the Crystalline Nappe have been exposed to cooling when rocks of the root zone were still in the center of the regional metamorphism, differences are easily explained. Further it is a common experience that the basal portions of a nappe are frequently kept back by friction and the upper beds are enriched in the frontal parts (e.g. Kashmir Nappe). This may account for the smaller thickness of the gneiss-migmatite complex in the klippes.
Differences in the facies of the basal Tethyan sedimentary cover of the klippes (e.g. Kathmandu, Jaljala Dhuri) to the Tethyan Zone are explained by the fact that there are also facies changes within the Tethyan Zone. From the Nilgiri in the E to Kanjiroba in the W the silt content increases considerably. Similar changes in N-S direction can not be excluded.
Thus I see no necessity to introduce the term Mahabharat Thrust (UPRETI, 1995) at the base of the Kathmandu Klippe. We may safely take it as the MCT.
3. EARLY PALAEOZOIC OROGENY
At the beginning of my research in Nepal (FUCHS, 1967) I recognized the gradation from the Central Crystalline into the Cambro-Ordovician sediments of the Tibetan Zone by passing away of the metamorphism. Further I found a significant break in the sedimentary development at the Ordovician-Silurian boundary: Continental slope sedimentation (as we would say today) changed to a variety of platform facies. In context with the Ordovician unconformity in the W Himalaya (HAYDEN, 1904) I concluded the existence of a Caledonian Orogeny in the Himalaya, which was responsible for a different development of the Tibetan- and Lesser Himalaya (FUCHS, 1967). This idea was new and was not accepted by most geologists. In the following decades the importance of the Tertiary metamorphism was well substantiated and generally accepted (see LE FORT, 1975).
Most granites in the Crystalline, however, were dated as about 500 Ma. The interpretation of these Early Palaeozoic ages was that epirogenetic movements related with the Pan-African event have caused the granitic intrusions (LE FORT et al., 1986).
In recent years an increasing number of arguments for an Early Palaeozoic orogeny were proposed (FRANK et al., 1995; GEHRELS et al., 2003, 2006): Eoproterozoic-Ordovician metasediments were folded before the intrusion of the 500 Ma granites, related regional metamorphism reached at least garnet grade, exhumation and erosion of this complex followed. Further evidence is the angular unconformity in Spiti (HAYDEN, 1904), which was traced by all following workers in the Tethyan Zone of the NW-Himalaya. In my view the Ordovician Conglomerate correlates also with the Ralam Conglomerate between Martoli- and Garbyang Formations in Kumaun (FUCHS, 1967). Thus recent research substantiates my ideas proposed in 1967, but without reference to it.
BHARGAVA et al. (2011) suggest Early Palaeozoic thrust tectonics also in the Lesser Himalaya (Tons area). The thrust transport of ca 50 km, however, appears not convincing.
4. THE SOUTH TIBETAN DETACHMENT SYSTEM (STDS)
Since BURCHFIEL & ROYDEN (1985) discovered an extensional zone along the southern margin of the Tibetan Plateau many workers studied the STDS (see the references compiled by HODGES, 2000, p.338). I know personally the Sarchu- and Zanskar Faults in the NW-Himalaya, which belong to this system. There is a marked hiatus in the material and metamorphic grade on both sides of the faults, locally there is also cataclastic deformation. In maps the STDS is mostly shown as a continuous tectonic line. Actually it is a zone with faults on various levels replacing each other along the strike.
Apparently it has become a must among geologists to find the STDS, even in areas where it is not indicated by any observations. Mostly the STDS is assumed between the HHC and the Tethyan sedimentary sequence. In this way gradations and the primary connection of metamorphics and sediments are ignored. This was frequently the case at the base of the Dolpo-Manang Synclinorium, further in Kumaun, where GANSSER (1964) described passages, and in Spiti. The STDS is an extensional zone caused by differential movements of the Tibetan Plateau and the growing Himalaya. The sedimentary basins named above are part of the Himalayan block, thus the STDS is expected further N, in Nepal in the region N of Mustang-Palchung Hamga Himal.
My colleague and friend Dr. Manfred Linner prepared my manuscript for digital publication. Many thanks for this and for fruitful discussions.
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8th Nepal Geological Congress, November 27-29
October 28, 2016 (Nepal Tourism Board, Bhrikutimandap, Kathmandu)