| Tectonostratigraphic
division of Himalaya
Heim and Gansser (1939), and Gansser (1964) divided the
rocks of the Himalaya into four tectonostratigraphic zones
that are characterised by distinctive stratigraphy and physiography.
From north to south, these are the Sub Himalayan, Lesser
Himalayan, Greater Himalayan, and Tibetan Himalayan zones.
Terai
The Terai is the Nepalese portion of the Indo-Gangetic Plain
that extends from the Indian Shield in the South to the
Siwalik Fold Belt to the North. The plain is a few hundred
metres above sea level and usually 400 to 600 m thick. it
is composed of Recent of Quaternary alluvium, boulder, gravel,
silt and clay. Terai Plain is underlain by a thick, relatively
flat-lying sequence of Mid to Late Tertiary molasse (Siwalik
Group) which uncomformably overlies subbasins of early Tertiary
to Protorozoic sediments (Surkhet, Gondwana and Vindhyan
Groups) and igeneous and metamorphic rocks of the Indian
Shield (Agrawal, 1977; Acharya and Ray, 1982; Raiverman
et.al, 1983).
Sub-Himalaya (Siwaliks)
The Sub Himalayan Zone or the Siwaliks of Nepal extends
throughout the country from east to west in the southern
part. It is delineated by the Himalayan Frontal Thrust (HFT)
and Main Boundary Thrust (MBT) in south and north respectively.
The Siwaliks consist of very thick (4000 to 6000m) molasses-like
fluvial sedimentary deposits comprising a coarsening-upwards
sequence as a whole, which reflects the rising history of
the Himalayas (Gansser, 1964).
The Sub Himalayan zone is the 10 to 25 km wide belt of
Neogene Siwaliks (or Churia) Group rocks, that forms the
topographic front of the Himalaya. It rises from the fluvial
plains of the active foreland basin, and this front generally
mapped as the trace of the Main Frontal Thrust (MFT). The
Siwaliks Group consists of upward-coarsening successions
of fluvial mudstone, siltstone, sandstone, and conglomerate.
The Siwaliks Group in Nepal comprise of three units that
are known as lower, middle and upper members. These units
can be correlated with the Sub Himalaya of Pakistan and
of northern India (Burbank et al., 1996). Palaeocurrent
and petrographic data from the sandstone and conglomerate
indicate that these rocks were derived from the fold-thrust
belt, and deposited within the flexural foredeep of the
Himalayan foreland basin (Tokouka et al., 1986; DeCelles
et al., 1998)
Lesser Himalaya
The Lesser Himalayas lies in between the Sub-Himalayas and
Higher Himalayas separated by MBT and the Main Central Thrust
(MCT) respectively. The total width ranges from 60-80 km.
The Lesser Himalayas is made up mostly of the unfossiliferous
sedimentary and metasedimentary rocks; like shale, sandstone,
conglomerate, slate, phyllite, schist, quartzite, limestone,
dolomite etc. Ranging in age from Precambrian to Miocene.
The geology is complicated due to folding, faulting and
thrusting and these complications added by the unfossiliferous
nature. Tectonically, the entire Lesser Himalayas consists
of two sequences of rocks: allochthonous, and autochthonous-paraautochthonous
units; with various nappes, klippes and tectonic windows.
The northernmost boundary of the Siwaliks Group is marked
by the Main Boundary Thrust (MBT), over which the low-grade
metasedimentary rocks of the Lesser Himalaya overlie. The
Lesser Himalaya, also called the Lower Himalaya, or the
Midlands, is a thick (about 7 km) section of para-autochtonous
crystalline rocks comprising of low- to medium grade rocks.
These lower Proterozoic clastic rocks (Parrish and Hodges,
1996) are subdivided into two groups. Argillo-arenaceous
rocks dominate the lower half of the succession, whereas
the upper half consists of both carbonate and siliciclastic
rocks (Hagen, 1969; Le Fort, 1975; Stöcklin, 1980).
The Lesser Himalaya thrust over the Siwaliks along the MBT
to the south, and is overlained by the allochtonous thrust
sheets of Kathmandu and HHC along the MCT. The Lesser Himalaya
is folded into a vast post-metamorphic anticlinal structure
known as the Kunchha-Gorkha anticlinorium (Pêcher,
1977). The southern flank of the anticlinorium is weakly
metamorphosed, whereas the northern flank is highly metamorphosed.
Main Central Thrust Zone
The Main Central thrust (MCT) is the single largest structure
within the Indian plate that has accommodated Indian-Asian
convergence. It extends for nearly 2500 km along strike
and has been the site of at least 140 and perhaps more than
600 km of displacement (Schelling and Arita, 1991; Srivastava
and Mitra, 1994). Heim and Gansser (1939) defined the MCT
in Kumaon based on the difference in metamorphic grade between
low to medium-grade rocks of the Lesser Himalaya and higher-grade
rocks of the Greater Himalaya. However, the fault originally
defined by Heim and Gansser (1939) is not the MCT, but a
fault within Lesser Himalaya rocks (Valdiya, 1980; Ahmad
et al., 2000). This misidentification symbolizes the challenge
that workers have faced in locating the MCT. The metamorphic
grade within the Lesser Himalaya increases towards the MCT
and at higher structural levels.
In central Nepal, the metamorphic grade increases from low
(chlorite + biotite) to medium (biotite + garnet + kyanite
??staurolite) towards the MCT over a north-south distance.
The highest-grade rocks (kyanite and sillimanite gneisses)
are found within the MCT shear zone, i.e. upper Lesser Himalaya.
Arita (1983) places two thrusts (MCT I and MCT II) on each
side of the MCT shear zone.
Higher Himalaya
This zone extends from the MCT to Tibetan-Tethys Zone and
runs throughout the country. This zone consists of almost
10km thick succession of the crystalline rocks, commonly
called the Himal Group. This sequence can be divided into
four main units, as Kyanite-Sillimanite gneiss, Pyroxenic
marble and gneiss, Banded gneiss, and Augen gneiss in the
ascending order (Bordet et al., 1972).
The Higher Himalayan sequence has been variously named.
French workers used the term Dalle du Tibet (Tibetan Slab)
for this unit (Le Fort, 1975; Bordet et al., 1972). Hagen
(1969) called them Khumbu Nappes, and Lumbasumba Nappes.
Arita (1983) calls it the Himalayan Gneiss Group, and it
lies above the MCT II, or the upper MCT.
The HHC are mainly comprised kyanite- to sillimanite-grade
gneisses intruded by High Himalayan leucogranites at structurally
higher levels (Upreti, 1999a). Throughout much of the range,
the unit is divided into three formations (Pêcher
and Le Fort, 1986). In central Nepal (Guillot, 1999), the
upper Formation III consists of augen orthogneisses, whereas
the Middle Formation II are calcsilicate gneisses and marbles,
and the basal Formation I are kyanite- and sillimanite bearing
metapelites, gneisses, and metagreywackes with abundant
quartzite.
The gneiss of Higher Himalayan zone (HHZ) is a thick continuous
sequence of about 5 to 15 km (Guillot, 1999). The northern
part is marked by North Himalayan Normal fault (NHNF), which
is also known as the South Tibetan Detachment system (STDS).
At its base, it is bounded by the MCT. The protolith of
the HHC is interpreted to be Late Proterozoic clastic sedimentary
rocks deposited on the northern Indian margin (Parrish and
Hodges, 1996).
Tibetan-Tethys
The Tibetan-Tethys Himalayas generally begins from the top
of the Higher Himalayan Zone and extends to the north in
Tibet. In Nepal these fossiliferous rocks are well developed
in Thak Khola (Mustang), Manang and Dolpa area. This zone
is about 40km wide and composed of fossiliferous sedimentary
rocks such as shale, sandstone and limestone etc.
The area north of the Annapurna and Manaslu ranges in central
Nepal consists of metasediments that overlie the Higher
Himalayan zone along the South Tibetan Detachment system.
It has undergone very little metamorphism except at its
base where it is close to the Higher Himalayan crystalline
rocks. The thickness is currently presumed to be 7,400 m
(Fuchs et al., 1988). The rocks of the Tibetan Tethys Series
(TSS) consist of a thick and nearly continuous lower Paleozoic
to lower Tertiary marine sedimentary succession. The rocks
are considered to be deposited in a part of the Indian passive
continental margin (Liu and Einsele, 1994). |
