About 40 million years ago the
Indian Plate came in contact with the Eurasian Plate. Before that there was a
sea in between the two plates which is called the Tehtys Sea, when the two
continents got closer and closer the sea retarded and the land mass got lifted
up. After the contact the Indian Plate started getting subducted under the
Eurasian Plate, since the Indian Plate was much lighter than the Eurasian Plate
so it did not get sinked into the mantle. Since then this subduction and
thrusting has become a continuous process. The Himalaya was formed around 2
million years ago. The rocks of Himalaya are thrusting upward about 2 to 5 mm
per year and horizontally southward. It ranges about 2400 km from Punjab
Himalaya in the west to Aarnanchal Himalaya in the east. Nepal lies in the
central region of this Himalaya range. Since the formation of these Himalayan
range is comparatively younger than geology of other continents so the rock are
generally younger than on other parts of the world and are relatively weaker
and more unstable.
The continuous process of
collision of two plates has resulted in many thrusts and faults in Himalaya
region and these thrust and fault separate Nepal in different zones. These
zones and the thrusts that separate them are:
1.
Indo Gangetic Zone / Terai Zone
Separated by
Main Frontal Thrust (MFT)
2.
Siwalik Zone
Separated by
Main Boundary Thrust (MBT)
3.
Lesser Himalaya Zone
Separated by
Main Central Thrust (MCT)
4.
Higher Himalaya Zone
Separated by
South Tibetian Detachment System (STDS)
5.
Tibetian Tethys Zone
Separated from
the Eurasian Plate by Indus-Tsangpo Suture Zone (ITS)
1. Indo
Gangetic Plain / Terai Zone
It lies in the southern most part of Nepal and is separated
from the Siwalik Zone by Main Frontal Thrust (MFT) or also called Himalayan
Frontal Thrust. The Terai region gradually rises from 100m in South to 200m in
North. It mostly contains alluvial deposit (in average about 1500 m). It
contains alluvium from Pleistocene era to recent time. It lies continuously
from west to east except along Nepal India border at Chitwan and Rapti Valley
where it coincides with the Siwalik Zone This zone is also further divided into
three zones namely Bhabar Zone, Middle Terai or Marshy Zone and Southern Zone.
a.
Bhabar Zone
It
is well developed around the mouth of major rivers whereas in other parts it is
filled with debris derived from the Churia Hills.
b.
Middle Terai or Marshy Zone
It
consists of flat land with marshy nature and there exists artesian nature. The
alluvium deposit can be found from gravel, pebble, cobble, sand to yellow clay
deposits.
c.
Southern Zone
The
sediment deposits get finer as it moves southwards. It consists of fine sands,
silts and clays.
2.
Siwalik Zone
The Siwalik Zone lies just north
of the Indo-Gangetic Plain and lies on the foot hill of Lesser Himalaya. It is
bounded by MFT in the south and MBT to the north. Physiographic units Chure
Pahad and Dun valleys lie in this zone. The sediments in this region was
transported by rivers and accumulated due to uplift and denudation of the
Himalaya. It is generally covered with thick forest and youngest sedimentary
rock in the range. Sediments found are usually mudstone, sandstone,
conglomerate usually getting coarse upwards. The age of sediments ranges from
Middle Miocene to Early Pleistocene era (about 1-16 million years). It can be
sub-divided into three regions Lower Siwalik, Middle Siwalik and Upper Siwalik.
a. Lower
Siwalik
It generally
consists of interbedded fine grained mudstone and sandstone. The proportion of
mudstone is larger than sandstone. Sediments are derived from the Thethys zone
and Lesser Himalaya Zone in this region and were deposited by meandering action
of river system which used to flow from north-east to south-west direction at
that time. The age of this region is from Middle Miocene to Early Pliocene era
(about 16-10 million years).
b. Middle
Siwalik
Rocks in this
region are usually coarser, thick interbedded sandstone and mudstone with
larger proportion of sandstone. These rocks have acquired the nickname "pepper
and salt". The sediments in this region were derived from Higher Himalaya
region and deposited by braided river system which was flowing from North to
South Direction. It age ranges from Early Pliocene to Late Pliocene era (about 10-3
million years).
c. Upper
Siwalik
It is usually
characterized by very coarse grained rocks like cobble and pebble conglomerate
with lenses of mud and sand. Sediments were also deposited by braided river
system which was also flowing from North to South Direction as in Middle
Siwalik. The sediments were derived from Lesser Himalaya as well as from
Siwalik itself. It ages from Early Pleistocene to Late Pleistocene (about 3-1
million years).
3. Lesser
Himalaya Zone
The Lesser Himalaya is bounded by MBT in the south and MCT in the
north. The three physiographic units The Mahabharat Range, Midlands and frontal
parts or southern parts of the Fore Himalaya belong to this zone. This zone is
made mostly of sedimentary rocks and metamorphic rocks like Shale, Sandstone, Limestone,
Dolomite, Slate, Phyllite, Schist, Gneiss, Amphibolites, Quartzite, Marble. The
rocks in this region are highly folded and faulted and have resulted in complex
structures. Its age range is from Precambrian to Oligocene era.
4. Higher
Himalaya Zone
The Higher Himalaya Zone is bounded by MCT to south and STDS to north.
This region has extremely rugged terrain with very stiff slopes and deep cut
valleys. Generally all terrain level above 5000m from sea level can be
considered Higher Himalaya Zone. The world's tallest peak lies in this zone,
the Mount Everest but, not only the tallest but also highest number of peaks
above 8000m lie in this range. Rocks found in this region are high grade metamorphic
rocks, which are in succession of 10-12 km, these rocks are gneissies, migmatites,
schist, quartzite, marble, etc.
5. Tibetian
Thetys Zone
This zone lies north of the
Higher Himalaya Zone and is bounded by the STDS in south and separated by
Indus-Tsangpo Suture Zone (ITS) in north from the Eurasian plate. This zone is
composed of sedimentary rocks such as shale, limestone and sandstones aging
from Cambrian to Cretaceous. The rocks are highly fossiliferous. In Nepali
places like Manang, Mustang and Dolpa are name "Himal Pari Ko Gaun" (i.e.
Villages across the Himalaya) since they lie north of the Himalayan range.
The main structures that are
present in Nepalese Himalaya are these thrust zones, namely Main Frontal Thrust
(MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT), South Tibetian
Detachment System (STDS) and Indus-Tsangpo Suture Zone (ITS). The locations of
these thrust zones are also explained above paragraphs and schematic diagram is
given below showing different geologic zones of Nepal and Thrust Zones
separating them.
Thrusts are not lines but are
weakness plane. When the Indian Plate collided with the Eurasian Plate these
thrusts were formed and since Indian Plate is still getting sub-ducted into the
Eurasian Plate it is believed that still more thrusts are being created in the
southern direction. The rocks in these thrust zones are highly fracture or
crushed. The geodetic measurement has shown that MBT is quite active nowadays.
This movement along the MBT appears obvious due to geomorphic features like
pressure ridges.
Significance in Geotechnical Engineering
The knowledge of these zones and
structures are very important for a geotechnical engineer. Since
characteristics of certain zone can be favorable for certain purpose while unfavorable
for other. Like the Terai Zone is favorable for land use like cultivation and
settlement, the Siwalik Zone may be favorable for quarry site for gravel and
stone. Considering these diversity even the design specification for hill and
terai (plain) is different for infrastructures such as road and irrigation.
Structures present in the rocks are also very important for locating
construction site for the proposed structure. Geological Structures that are
commonly found in Nepalese Himalaya are:
·
Folds
·
Active Faults and Thrusts
·
Joint, Cracks and Fractures
·
Orientation and characteristics of the Bedding and Foliation
Plane
Almost all of these structures
are unfavorable for construction of infrastructure although orientations of
some of them are favorable. Folds are sometime regional and sometime local in
range of scale. Local folds are more risk than regional ones so care should be
adopted when construction has to be done on them. Active faults and thrusts
should be avoided while finding the location for construction of structure as
they have low bearing strength and rocks underneath them are highly crushed and
fractured. Joints cracks and fractures also present problem during
construction. They pose problem of stability and also of seepage during
construction of dams and reservoirs, so should be avoided as far as possible or
should be treated when option of avoiding is not available. Orientation and
characteristics of bedding plane and foliation plane plays major role in the
hill slope stability and during alignment of tunnels. It is one of the major
factors that govern the alignment of the tunnel. These planes are sometimes
considered favorable and sometimes unfavorable due to their orientation.
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