Himalayas (3)

This notes provide the information about tectonic divisions and evolution of Himalayas. movement of Indian plate has also taken up in brief.
Tectonic Division of Himalaya
Evolution of Himalaya
Movement of Indian Plate.

Resource description_ Rasoul Sorkhabi, The himalayan Journal, 2010

1. Amit K. Mishra
Assistant Professor
Department of Geology
Banasthali University
Himalayas
An introduction

2. •Digital Elevation Model showing the Himalaya, Karakoram, and Tibetan Plateau with locations of the major peaks.

3. •The Himalaya, located on the southern fringe of the Tibetan Plateau,
form a mountain arc (convex toward the south) about 2400 km long
and 250-300 km wide; they are bounded by two structural bends on
the northwest (Nanga Parbat, the Indus gorge) and northeast
(Namcha Barwa, the Tsangpo gorge). The Himalayas are the loftiest
and still rising mountains in the world.
• Indeed, the Tibetan Plateau and its surrounding mountains form the
largest topographic high on earth. All of the mountain summits taller
than 7000 m are located in this part of our planet, and several large
rivers, which rise in these highlands, provide fresh water for nearly
three billion people in Asia (almost half of world population).
•Sediments shed from the Himalaya have also formed extensive
agricultural plains in northern India, Pakistan, Nepal, and
Bangladesh which have supported civilizations for millennia.

4. •The Himalaya act as a topographic barrier to the summer-time
warm monsoon winds coming from the Indian Ocean and the
winter-time cold winds coming from Siberia, and have thus
caused cold dry climate in Tibet but torrential rainfall on the
valleys and plains to the south.
•Rich diversity of flora and faunas extending from the almost
sea-level plains upward the Higher Himalaya is also
significant for ecology. The Himalaya offer splendid peaks and
valleys for mountaineers, tourists and artists. Major
earthquakes that occasionally visit the Himalayan region can
indeed be devastating for people.
•Last but not least, these mountains are a natural laboratory
for geologists to study how mountains of similar type on Earth
form and influence the environment. For all these reasons,
scientific studies of the Himalaya are significant and
necessary.

5. The Anatomy of the Himalaya
•It is interesting that geologists sometimes
employ biological terms to describe
mountains; for example, ‘anatomy’ for
tectonic framework and structure of
mountains, or ‘evolution’ for the geologic
history and formation of mountains. To
understand how the Himalaya formed we
need to consider its structure the types and
ages of the rocks and the nature of faults that
have fashioned these mountains.

6. •Geological mapping in various parts of the
Himalaya over the past century has led the
geologists to divide these mountains into six
major longitudinal zones or northwest-
northeast trending belts. Although local
differences exist to some degree, the overall
consistency of these zones along the entire
length of the Himalaya is significant, and
points to a single tectonic explanation for the
mountain formation. The six zones of the
Himalayans are described below from north
to south.

8. •The term ‘Trans-Himalaya’ for a mountain range to the north
of the Indus and Yarlung-Tsangpo rivers was first used by
Alexander Cunningham in his book Ladak (1854), but it was
Sven Hedin who popularized the name in his book Trans-
Himalaya (1909-1912) which documents Hedin’s exploration
of this region.
•Geologically speaking, the Trans-Himalaya is made up of
granitic and volcanic rocks formed from 110-40 million year
ago (Ma). These igneous rocks intruded the metamorphic and
sedimentary rocks of the southern Tibetan block.
•The Trans-Himalaya is geographically divided into several
areas: Kohistan (to the west of Nanga Parbat), Ladakh
(between Nanga Parbat and the Karakoram strike-slip fault),
Kailash, Gangdese, Lhasa (all in southern Tibet), and Mishimi
(to the east of Namche Barwa).

10. •A ‘suture zone’ is a tectonic line along which two
continental plates collide and join each other.
•Prior to the collision, an ocean separates the two continents
but it gradually shrinks as the ocean floor subducts beneath
one of the continents. The collision thus occurs after the
intervening sea completely subducts and disappears, and the
moving continental plates crash into each other.
•The Indus-Tsangpo Suture Zone marks the boundary
between the Indian and Asian plates. It was so named by
the Swiss geologist Augusto Gansser in his Geology of the
Himalayas(1964) because the upper river valleys of the Indus and Tsangpo lie along this suture zone. Here, we find ocean-
floor volcanic rocks, subduction-related high-pressure
metamorphic rocks (greenschist rocks), and deep-sea
sediments of the Tethys Ocean.

11. •This zone consists of Cambrian through Eocene sediments (sandstone, shale and
limestone) which were deposited on the continental shelf of the Tethys Ocean. These
sedimentary rocks contain many kinds of fossils including the Cretaceous-age ammonite
fossils which are called ‘shaligram shila’ in Nepal and are used as amulets (dark stones
devoted to Lord Shiva) by the Himalayan Hindus.
•Since most of this zone lies within southern Tibet, it is also sometimes called the Tibetan
Himalaya.
•In northwest India, the Tethyan sediments are exposed in Kashmir, Zanskar, Chamba, and
Spiti. With a width of about 100 km, the Tethys Himalaya is the widest zone of the
Himalaya.
•The stratigraphic thickness of sediments in this zone reaches up to 12 km. Within the
Tethyan Himalaya, there is also a series of dome-shaped structures made up of Cambrian-
age (about 500-450 Ma) granite gneiss or Miocene-age (18-9 Ma) white granites
(leucogranites), collectively referred to as the North Himalayan Granite Gneiss Domes.
These rocks and structural domes have been uplifted by a thrust fault called the North
Himalayan Thrust, situated at their base.
•Further south, the Tethys Himalaya is separated from the Higher Himalaya by a major
normal fault called the South Tibetan Detachment, first mapped in the 1980s in southern
Tibet and Zanskar.

12. •The Higher Himalaya, the backbone of the Himalayan mountains, is
made up of 10-20 km-thick metamorphic rocks (schist and gneiss)
and granites situated at altitudes of 3000 m to over 8000 m.
• These rocks are Proterozoic-Cambrian age (2,000-500 Ma) and
belong to the continental crust of the Indian plate. In addition we
also find white granites (leucogranites) of Miocene-age (24-17
Ma) in the upper parts of the Higher Himalaya.
•The South Tibetan Detachment constitutes the northern boundary of
the Higher Himalaya where the highest summits of the Himalaya
(over 7000 m) lie.
•Along its southern boundary, the Higher Himalaya is thrust over the
Lesser Himalaya along the Main Central Thrust, so named by
Augusto Gansser who first mapped this structure in Kumaon
(reported in Central Himalaya: Geological Observations of the Swiss
Expedition, 1939).

13. •French geologists working in Nepal have
also called this zone the Tibetan Slab
because it forms the basement for the
thick pile of the Tethyan (Tibetan)
sedimentary rocks. Still other geologists
also called it the Central Crystalline
zone. ‘Crystalline’ refers to an igneous
or metamorphic rock, in which minerals
were crystallized in the rock itself; unlike a
sedimentary rock which contains minerals
derived from other rocks.

14. •To the south of the Higher Himalaya (the Main Central Thrust) lies
the Lesser Himalaya which consists of metamorphosed sedimentary
rocks (quartzite, marble, slate, phyllite, schist and gneiss) and
minor volcanic and granitic rocks of Proterozoic-Cambrian age
(2000-500 Ma).
•The stratigraphic thickness of the rock sequence in this zone is 10-
20 km. In a plate-tectonic setting, the Lesser Himalaya, like the
Tethyan Himalaya and the Higher Himalaya, belongs to the northern
margin of the Indian plate.
•Unlike the rich fossiliferous sediments of the Tethyan Himalaya, the
sedimentary rocks in the Lesser Himalaya barely yield fossils partly
because they are much older (dating back to a period when life
forms were not abundant or diverse) and partly because they have
been metamorphosed. Nevertheless, metamorphism in the Lesser
Himalaya was less severe (under lower temperatures) compared to
the Higher Himalayan metamorphic rocks.

15. •Within the Lesser Himalaya, geologists have
also mapped transported, isolated parts of
the Higher Himalayan (Central) crystalline
rocks; these are called the ‘outer crystalline
zone’ or the Higher Himalayan ‘klippe’. (
‘Klippe’ is a German word which literally
means ’cliff’). These rocks were transported
on the back of the Main Central Thrust onto
the Lesser Himalaya, but were later isolated
from their root zone in the Higher Himalaya.
The Almora klippe in Kumaun is a classic
example.

16. •With elevations of 2000-3000 m, the Lesser
Himalaya is home to many hilly resorts and
forests. In Nepal, the Lesser Himalayan zone
geographically corresponds to the
Mahabharat Range.
•The southern boundary of the Lesser
Himalaya is the Main Boundary Thrust
which has uplifted this zone atop the Sub-
Himalayan (Siwalik) rocks. The Main
Boundary Thrust was first mapped (and
named) in 1864 by Henry B. Medlicott, a
British geologist working for the Geological
Survey of India.

18. •The Sub-Himalaya (or the Outer Himalaya) geographically
corresponds to the Siwalik range (or the Churia range in
Nepal) � foothills ranging in elevation from 250-800 m.
•This zone is made up of 10-km thick succession of sandstone
and mudstone shed from the Himalayan mountains, and
carried and deposited by rivers, especially since the Miocene
(over the past 24 million years).
•These sediments have yielded abundant fossils of mammals
(elephants, horses, etc.), who then lived in the Himalayan
forests. (The Shivalik Fossil Park in Saketi, Himachal Pradesh
houses examples of these fossils and recreates the ancient
environment in which the animals lived.

19. •The Siwalik foothills were surveyed by
Captain Proby Cautley of the East India
Company in the 1840s-1850s, when he
was constructing the Ganga Canal in the
Doab region of north India. Cautley’s
geologist colleague Hugh Falconer
examined the rocks and fossils excavated
from the hills, which he called ‘Siwalik’,
after the pilgrimage site at Haridwar,
dedicated to Lord Shiva.

20. •The Siwalik foothills were uplifted approximately
one million years ago along high-angle reverse
faults (still active), called the Himalayan Frontal
Fault (or Thrust) and first mapped in India and
Nepal by the Japanese geologist T. Nakata in
1972. The Himalayan Frontal Fault marks the
boundary between the Siwalik Range and the
north Indian plains. Similar faults have also been
mapped in Pakistan by Robert Yeats in the 1980s.
The Himalayan Frontal Fault is a series of
‘blind’ faults (the fault plane is not visible at the
surface), and can be mapped from the topographic
features they have created. These faults are often
concealed under an apron of debris, called
Bhabhar, washed down from the hillsides.

21. •As the Himalayan Frontal Fault began
uplifting the Siwalik range, a series of river
valley were formed within the Sub-
Himalayan zone; these tectonic basins are
called dun (doon), and some well-known
examples include Dehra Dun in India and
Chitwan in Nepal.

22. India, Gondwana and Tethys
•About 500 million years ago, a supercontinent comprised by
Australia, Antarctica, India, Africa, and South Africa, was
assembled in the southern hemisphere. Geologists have called it
Gondwana, after the Gond tribes in central India, where the
sediments of this supercontinent were first studied in the late
nineteenth century.
•Within Gondwana, India was adjacent to Madagascar-Africa on
the west and to Antarctica on the east.
•The Paleo-Tethys Ocean bordered the northern margin of
Gondwana.
•During the Permian (275-260 Ma) a series of continental fragments
consisting of Turkey, Iran and Tibet drifted away from Gondwana,
thus opening the Neo-Tethys Ocean on their trails.
•When a continent splits apart, a large volcanic eruption affects some
part of it. The Permian-age Panjal Traps (basalts) exposed in the
Pir Panjal Range of Kashmir are believed to be the volcanic
outpouring of continental rifting between India and the Tibetan block.

23. •Gondwana largely remained intact until the late Jurassic when this
supercontinent started to fragment, part of a long global process
which eventually created the present-day continents and oceans.
•India together with Madagascar was drifted away from Africa in the
late Jurassic (about 165 Ma); afterwards, India was separated from
East Antarctica at about 135 Ma and began its northward journey
across the Neo-Tethys Ocean, at the same time as the Indian Ocean
was opening behind India.
•
Around 85 Ma, a further split occurred between India and
Madagascar.
•The
Cretaceous-Cenozoic boundary at 65 Ma marks an important
event in Earth’s history. The dinosaurs, ammonites and many other
species were wiped out probably because a huge meteorite struck
our planet and played havoc with the environment.
•At this time, India was passing over the Reunion hotspot which
caused a large volume of volcanic rocks � the Deccan traps � to
flood the central western India; this also caused the Seychelles-
Mauritius fragment to separate from India.

25. •As India drifted northward, the Neo-Tethys
ocean floor began to subduct beneath the
southern margin of Asia (along the
Karakoram and Tibet). This usually happens
because the oceanic crust (mainly basalt) is
heavier than the continental crust (mainly
granite), and if pushed the oceanic crust
subducts into an oceanic trench. The
subducting slab then partially melts
(dehydrates), and produces a large volume of
granitic and volcanic rocks in the form of an
island arc (Aleutian type) or a continental
margin igneous belt (Andes type).

26. •In the northwestern part of the Himalaya, we have evidence of
Cretaceous-age Kohistan-Ladakh island arc made up of
volcanic rocks similar to those found today on the Aleutian
Islands in the northern Pacific Ocean.
•At about 85 Ma or so, the Kohistan-Ladakh island collided
with the Karakoram margin as the oceanic floor between the
two subducted and closed. Therefore, in the northwestern
Himalaya there is a Northern Suture Zone between the
Karakoram and the Kohistan-Ladakh blocks; this suture zone
has also been called the Chalt Suture in Kohistan and the
Shyok Suture in Ladakh, after the two valleys where the Chalt
and Shyok volcanic rocks were mapped.
•After the Northern Suture formed, the subduction-related
igneous activity continued along an Andes-type continental
margin from Kohistan and Ladakh all along the southern
Tibetan block. The age of granites found in this Trans-
Himalayan belt have been determined by the radiometric
methods to range from 110 Ma through 40 Ma.

27. The India-Asia Collision
•When did India collide with Asia? Several types of data appear to point to
the early-middle Eocene period.
•First, reconstruction of the speed of the Indian plate in the geologic past
indicates that the plate motion drastically decreased in the Eocene. During
Cretaceous and Paleocene times (85-55 Ma), India moved northward at
rates of 15-20 cm per year but slowed down to only about 5 cm per year by
45 Ma. (These data come from examination of magnetic anomalies of rocks
on the Indian Ocean floor and from paleomagnetic analyses of rock
samples from the Trans-Himalaya by various scientists.)
•Second, the youngest granites in the Trans-Himalaya, produced by the
subduction of the Neo-Tethys Ocean floor beneath Asia, have been dated
by radiometric methods to be about 40 Ma. This indicates the ending of
subduction of Neo-Tethys sometime before 40 Ma.
•Third, the sedimentary record in the Himalaya show that after the lower-
middle Eocene (55- 45 Ma) the depositional environment changed from
purely marine to continental.

28. •In short, data of different kinds have led
many geologists to believe that at first the
northwest corner of the Indian plate
collided with Asia, probably at 55 Ma or
even 60-65 Ma (as some studies suggest);
then the Neo-Tethys ocean floor closed in
an oblique manner and India moved in
counterclockwise rotation until its
northeastern corner docked with Asia at
about 45 Ma. The Indus-Tsangpo Suture
Zone marks the line of this plate collision.

29. •Block diagram illustrating the structure of the Himalaya. STD is the South Tibetan Detachment—the
low angle normal fault that bounds the northern margin of the Greater Himalayan metamorphic
rocks and granites (magenta color); MCT is the Main Central thrust that places the metamorphic
rocks south over un-metamorphosed rocks of the Lesser Himalaya. MBT is the Main Boundary
thrust, the active southern margin of the Himalaya and the fault zone along which earthquakes are
triggered.

30. The Evolution of the Himalaya
•The Himalaya are products of 55 million years of tectonic
compression and structural deformation.
•Examination of geochronologic, structural, petrologic and
sedimentary data from various parts of the Himalaya indicates
that at least five major episodes or phases were milestones in
the geologic history of these mountains.
•Here a question may arise: While the motion of the Indian
plate has been steady and constant (about 5 cm per year)
over the past 45 million years, how is it possible that the uplift
of the Himalaya has not been at a uniform rate but in distinct
episodes?
To answer this, consider an earthquake prone area
undergoing tectonic stress; the stress builds up gradually over
time, and then the area ruptures along a fault which releases
the stress as it surpasses the strength of the rock. On a larger
scale too, it takes time for major tectonic events to occur.

34. •Even before the India-Asia continental
collision, tectonic deformation was taking
place in the leading margin of the Indian
plate, near the Neo-Tethys subduction
zone and the continental margin of
southern Tibet. However, the Himalayan
drama began after the collision.

35. •It is conceivable that the first mountain range to emerge was
the Trans-Himalayan igneous arc, and that the Indus and
Tsangpo were the first rivers to arise and transport sediments
in the Eocene period. Geochronologic analyses of granites
from the Trans-Himalaya indicate that these rocks rapidly
cooled and eroded at about 40 Ma. And, there are basins in
the southern front of the Trans-Himalaya filled with continental
sediments (for example, in Kargil, Kailash and Lhasa).
•Mapping in southern Tibet by An Yin reveals the existence of
a south-directed thrust fault which he has called the
Gangdese Thrust, probably part of a deep-seated structure
responsible for the uplift of the Trans-Himalaya. (Along most
of the Indus-Tsangpo Suture, geologists, as early as Ausgsot
Gansser in 1930s, have mapped northdirected backthrusts,
which are probably later and shallower structures compared
to the Gangdese Thrust.)

36. •Partly concurrent with and partly after the uplift of the
Trans- Himalayan range, tectonic deformation also
affected the Tethyan Himalaya. Interestingly, the
Tethyan sediments in most of the Himalaya have
escaped metamorphism (they are still sedimentary
rocks) but they are highly folded and faulted. The
emergence from the sea and folding of these
sediments probably occurred shortly after the India-
Asia collision and continued for millions of years. The
North Himalayan Thrust and the uplift of the granite
gneiss domes within the Tethyan Himalaya began
during this phase (40-35 Ma). The Eocene-age
tectonic deformation in the Himalayan region may be
called the ‘Eo- (early) Himalayan’ episode.

37. •Since the India-Asia collision occurred in a
diachronous (variable time), oblique manner, both
the Trans-Himalayan and Eo-Himalayan events
first began in the northwestern Himalaya (in
Pakistan and Ladakh) around 55-45 Ma, and then
affected the eastern parts of the Himalaya at 35
Ma.
•The fact that the Trans-Himalaya and the Tethyan
Himalaya were uplifted prior to the Higher
Himalaya is evident from the ‘antecedent’ (flowing
before) pattern of many of the Himalaya river (the
Indus, Satluj, Ganga, Kali Gandaki, Arun and
Brahmaputra); these river courses rise behind the
Higher Himalaya (predating its uplift) and cut deep
gorges through it to flow southward.

38. •There is ample evidence that during the early Miocene (24-17 Ma), the Higher Himalayan
rocks, which were then buried 20-25 km deep and metamorphosed and partially melted
under temperatures of 600-800 degrees Celsius and pressures of 6-10 kilobars, began a
rapid uplift along the Main Central Thrust.
•Concurrent with the activity of this fault, the Tethyan Himalayan zone was detached from
the Higher Himalaya along a normal fault, named the South Tibetan Detachment. This
fault, which runs parallel to the Main Central Thrust and forms the northern boundary of the
Higher Himalaya, caused tectonic extension (spreading), gravitational gliding and
back-
folding of sediments in the Tethyan Himalaya (the down-thrown block), while uplifting and
exhuming the Higher Himalayan rocks (the upthrown block).
•Moreover, a series of white granites (leucogranites) formed during 24-17 Ma were
emplaced and rapidly uplifted along the South Tibetan Detachment. These granites
constitute the core or the summit of the highest peaks in the Himalaya including Manaslu
and Annapurna in Nepal and Badrinath and Shivling in Kumaun. It is interesting that
these granites are located along the upthrown edge of the South Tibetan Detachment,
indicating that their formation (possibly by decompressional melting of deeply buried rocks)
and exhumation to the surface were related to the activity of this fault.

39. •Along the Main Central Thrust we do not find any
Miocene-age granites. Nevertheless, the Main Central
Thrust emerged as a major structure across which the
topography of the Himalaya markedly increases from
south to north. The slip on the Main Central Thrust is
more than 150 km as this thrust has transported the
Higher Himalayan metamorphic rocks onto the Lesser
Himalaya (where we find them as klippe outliers),
some 150 km distance their root zone.
•With the uplift of the Higher Himalaya beginning in the
early Miocene, increasing amounts of sands were
deposited not only in the Siwalik and Indus-Ganga
basins but were also transported by rivers to the
Arabian Sea (forming the submarine Indus Fan) and
the Bay of Bengal (the Bengal Fan).

40. •During the late Miocene (11-7 Ma), sedimentation rates in the
Siwalik basin drastically increased, and the mineral kyanite
which, in the Himalayan region is found only in the Higher
Himalayan metamorphic rocks (to the north of the Main
Central Thrust), first appeared in the Siwalik sediments.
These data indicate rapid erosion of the Himalaya at that time.
There is also limited geochronologic evidence from Pakistan
that the Main Boundary Thrust began its activity in the late
Miocene.
•Geochemical analyses of samples from the Indian Ocean
drilled cores and from the Siwalik ancient-soils indicate that
the Monsoon seasonality related to the Himalayan
topographic barrier also began in the late Miocene, indicating
that by 7 Ma, the Himalaya had attained sufficient heights.

41. •Some geologists including Augusto Gansser and K. S. Valdiya have
highlighted the significance of the Himalayan uplift over the past 2.5
million years or so, while certain geologists who confine their
research to the Miocene geochronology of the Himalaya have ignored or downplayed the Quaternary events as if the maximum
topography of the Himalaya was reached in the Miocene and since
then only erosion has shaped these mountains.
•Considering various lines of evidence I consider the Quaternary
period as an important renewed phase of tectonic uplift in these
mountains. The uplift of the Siwalik range along the Himalayan
Frontal Fault over the past one million years, geochronologic
evidence (fission-track ages of 1-3 Ma) of rapid erosion from various
parts of the Higher Himalaya (Nanga Parbat, Zanskar, Gangotri,
Khumbu Himal, Namche Barwa, etc.), presence of active faults and
large earthquakes, deep and narrow gorges in the Higher Himalaya,
uplifted terraces of young sediments, and enormous amounts of
coarse-grained fluvial sediments in the Quaternary record of the
Himalayan foreland basins all testify to the intense neotectonics in
this region.

42. The Himalaya’s Adventure
Continues
•Geophysical surveys by the seismic (sound wave) reflection and gravity methods indicate that the
crustal thickness beneath the Himalaya is about 70 km, which is twice that of a normal continental
crust (for example, the Indian Peninsula). In other words, the Himalaya are higher because these
mountains are sitting on an over-thickened continental crust, produced by compression, thrusting,
and folding.
•Geologists believe that all major faults in the Himalaya, including the Normal Himalayan Thrust,
the South Tibetan Detachment, the Main Central Thrust, the Main Boundary Thrust, and the
Himalayan Frontal Fault, are all joined at depth to a major shallow-dipping structure, called the
Main Himalayan Thrust or Decollement (a French word meaning Detachment).
•This structural model together with the available geologic data discussed in this article implies that
as the Indian plate has pushed northward, thrust faults have developed sequentially from north to
south and that as a new thrust fault has ruptured, the tectonic push has added some elevation to
the entire Himalaya. In other words, the present elevation of the Himalaya is a cumulative effect of
55 million years of continental collision, a magnificent drama of episodes which geologists have
painstakingly pieced together and are beginning to make sense of it.
•A period of 55 million years or so since the India-Asia collision
may seem a long time from human perspective, but by Earth’s
standards, the Himalaya are still young, rising mountains.