Geology of the Himalayas

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Fig 1: The Earth in the Early Permian (290 million years ago) when India was part of Gondwana and bordered to the north by the Cimmerian Superterrane. Paleogeographic reconstruction by Dèzes (1999), based on Stampfli & Borel (2002) and Patriat & Achache (1984).[a]
Cimmeridian Superterrane from Gondwana. Based on Stampfli & Borel (2002) and Patriat & Achache (1984).([1][2][3])[b]
Fig 3: The earth in the Cretaceous. The Cimmeridian Superterrane has accreted to Mega Laurasia, the oceanic crust of the Neotethys is subducted to the north along the Dras volcanic arc, the Shigatze Ocean opens as a consequence of back-arc spreading, India is separated from Africa and E. Gondwana and the Indian Ocean opens. Paleogeographic reconstructions based by Dèzes (1999), on Stampfli & Borel (2002) and Patriat & Achache (1984).
Fig 4: The northward drift of India from 71 Ma ago to present time. Note the simultaneous counter-clockwise rotation of India. Collision of the Indian continent with Eurasia occurred at about 55 million years ago. Source: www.usgs.org (modified)
Fig 5: Geologic - Tectonic map of the Himalaya, modified after Le Fort & Cronin (1988). Green is the Indus-Yarlung suture zone.
Fig 6: Geological Map of the NW Himalaya; for references, see image description or bibliography. HHCS: High Himalayan Cristalline Sequence; ISZ: Indus Suture Zone; KW: Kishtwar Window; LKRW: Larji-Kulu-Rampur Window; MBT: Main Boundary Thrust; MCT: Main Central Thrust; SF: Sarchu Fault; ZSZ: Zanskar Shear Zone.
Fig 7: Simplified cross-section of the north-western Himalaya showing the main tectonic units and structural elements by Dèzes (1999).
Lhasa Terrane (also called Karakoram-Lhasa Block/Terrane
) lies within the Transhimalayas in its east side. Bangong-Nujiang Suture Zone separates Qiangtang Terrane from the Lahsa Terrane

The geology of the Himalayas is a record of the most dramatic and visible creations of the immense

polar regions. This last feature earned the Himalaya its name, originating from the Sanskrit
for "the abode of the snow".

From south to north the Himalaya (Himalaya orogen) is divided into 4 parallel

Sivalik), Main Boundary Thrust (MBT), Lesser Himalaya (further subdivided into the "Lesser Himalayan Sedimentary Zone (LHSZ) and the Lesser Himalayan Crystalline Nappes (LHCN)), Main Central thrust (MCT), Higher (or Greater) Himalayan crystallines (HHC), South Tibetan detachment system (STD), Tethys Himalaya (TH), and the Indus‐Tsangpo Suture Zone (ISZ).[5] North of this lies the transhimalaya in Tibet which is outside the Himalayas. Himalaya has Indo-Gangetic Plain in south, Pamir Mountains in west in Central Asia, and Hengduan Mountains in east on China–Myanmar border
.

From east to west the Himalayas are divided into 3 regions,

Eastern Himalaya, Central Himalaya, and Western Himalaya, which collectively house several nations and states
.

Making of the Himalayas

Main article: Pre-collisional Himalaya

During Late

Palaeozoic, the Indian subcontinent, bounded to the north by the Cimmerian Superterranes, was part of Gondwana and was separated from Eurasia by the Paleo-Tethys Ocean (Fig. 1). During that period, the northern part of India was affected by a late phase of the Pan-African orogeny which is marked by an unconformity between Ordovician continental conglomerates and the underlying Cambrian marine sediments. Numerous granitic
intrusions dated at around 500 Ma are also attributed to this event.

In the Early

Neotethys ocean (Fig. 2). From that time on, the Cimmerian Superterranes drifted away from Gondwana towards the north. Nowadays, Iran, Afghanistan and Tibet
are partly made up of these terranes.

In the Norian (210 Ma), a major rifting episode split Gondwana in two parts. The Indian continent became part of East Gondwana, together with Australia and Antarctica. However, the separation of East and West Gondwana, together with the formation of oceanic crust, occurred later, in the Callovian (160-155 Ma). The Indian plate then broke off from Australia and Antarctica in the Early Cretaceous (130-125 Ma) with the opening of the "South Indian Ocean" (Fig. 3).

In the Late

Asian plates from very fast (18-19.5 cm/yr) to fast (4.5 cm/yr) at about 55 Ma[8] is circumstantial support for collision then. Since then there has been about 2500 km[9][10][11][12] of crustal shortening and rotating of India by 45° counterclockwise in the Northwestern Himalaya[13] to 10°-15° counterclockwise in North Central Nepal[14]
relative to Asia (Fig. 4).

While most of the oceanic crust was "simply" subducted below the Tibetan block during the northward motion of India, at least three major mechanisms have been put forward, either separately or jointly, to explain what happened, since collision, to the 2500 km of "missing continental crust".

Even though it is more than reasonable to argue that this huge amount of crustal shortening most probably results from a combination of these three mechanisms, it is nevertheless the last mechanism which created the high topographic relief of the Himalaya.

The Himalayan tectonics result in long term deformation. This includes shortening across the Himalayas that range from 900 to 1,500 km. Said shortening is a product of the significant ongoing seismic activity. The continued convergence of the Indian plate with the Eurasian plate results in mega earthquakes. These seismic events can reach greater than MW 8 and result in intense damage to infrastructure. The mid-crustal ramp in the Himalayas is a key geologic feature in the history for both long-term and short-term seismic processes linked to deformation and shortening. Over the last 15 Ma, the ramp has gradually moved south due to duplexing, accretion, and tectonic undercutting.[16]

The ongoing active collision of the Indian and Eurasian continental plates challenges one hypothesis for plate motion which relies on subduction.

Major tectonic subdivisions of the Himalaya

One of the most striking aspects of the Himalayan orogen is the lateral continuity of its major tectonic elements. The Himalaya is classically divided into four

tectonic units that can be followed for more than 2400 km along the belt (Fig. 5 and Fig. 7).[c]

Sub-Himalayan (Churia Hills or Sivaliks) tectonic plate

The Sub-Himalayan tectonic plate is sometimes referred to as the Cis-Himalayan tectonic plate in the older literature. It forms the southern

orogen
.

Lesser Himalaya (LH) tectonic plate

The Lesser Himalaya (LH) tectonic plate is mainly formed by Upper

tectonic windows
(Kishtwar or Larji-Kulu-Rampur windows) within the High Himalaya Crystalline Sequence.

Central Himalayan Domain, (CHD) or High Himalaya tectonic plate

The Central Himalayan Domain forms the backbone of the Himalayan orogen and encompasses the areas with the highest topographic relief (highest peaks). It is commonly separated into four zones.

High Himalayan Crystalline Sequence (HHCS)

Approximately 30 different names exist in the literature to describe this unit; the most frequently found equivalents are "Greater Himalayan Sequence", "

Leh-Manali Highway. It is now generally accepted that the metasediments of the HHCS represent the metamorphic equivalents of the sedimentary series forming the base of the overlying "Tethys Himalaya". The HHCS forms a major nappe which is thrust over the Lesser Himalaya along the "Main Central Thrust
" (MCT).

Tethys Himalaya (TH)

The Tethys Himalaya is an approximately 100-km-wide

sediments of the TH. Stratigraphic analysis of these sediments yields important indications on the geological history of the northern continental margin of the Indian sub-continent from its Gondwanian evolution to its continental collision with Eurasia. The transition between the generally low-grade sediments of the "Tethys Himalaya" and the underlying low- to high-grade rocks of the "High Himalayan Crystalline Sequence" is usually progressive. But in many places along the Himalayan belt, this transition zone is marked by a major structure, the "Central Himalayan Detachment System", also known as the "South Tibetan Detachment System
" or "North Himalayan Normal Fault", which has indicators of both extension and compression. See ongoing geologic studies section below.

Nyimaling-Tso Morari Metamorphic Dome (NTMD)

"Nyimaling-Tso Morari Metamorphic Dome" in the Ladakh region, the "Tethys Himalaya synclinorium" passes gradually to the north in a large dome of greenschist to eclogitic metamorphic rocks. As with the HHCS, these metamorphic rocks represent the metamorphic equivalent of the sediments forming the base of the Tethys Himalaya. The "Precambrian Phe Formation" is also here intruded by several Ordovician (c. 480 Ma[19]) granites.

Lamayuru and Markha Units (LMU)

The Lamayuru and Markha Units are formed by

Late Permian to Eocene
.

Exhumation of Metamorphic Rocks

The metamorphic rocks of the Himalaya can be very useful in deciphering and coming up with models of tectonic relationships. According to Kohn (2014), the exhumation of metamorphic rocks can be explained by the Main Himalayan Thrust. [20] Although the mechanism of emplacing higher grade metamorphic rocks on top of lower grade metamorphic rocks still strongly debated, Kohn believes that it is due to long periods of transportation of higher grade metamorphic rocks on the Main Himalayan Thrust. Essentially, the longer the higher grade rocks were spatially interacting with the thrust, the farther they were transported.

The exhumation of eclogite and granulite rocks can be explained by several different models. The first model includes slab tear where the lower plate tore off into the mantle leading to high amounts of rebound. The second model states that the rocks got to a certain point in subduction and then were forced back up through the channel they came down due to a space problem. The third model states that the thick continental crust of India further exasperated the space problem and caused the corner flow of those rocks back up the channel. The fourth model includes the rocks being transported along the Main Himalayan Thrust.

Indus Suture Zone (ISZ) (or Yarlung-Tsangpo Suture Zone) tectonic plate

See also: Lhasa terrane and Qiangtang terrane

ISZ, also called

Karakoram-Lhasa Block
) to the north. This suture zone is formed by:

Seismic Activity

The modern day rate of convergence between the Indian and Eurasian plates is measured to be approximately 17 mm/yr. [21]This convergence is accommodated through seismic activity in active fault zones. As a result, the Himalayan range is one of the most seismically active regions in the world. This region has experienced many high magnitude earthquakes in the last 100 years, including the 1905 Kangra Earthquake, 1975 Kinnaur Earthquake, 1991 Uttarkashi Earthquake, and the 1999 Chamoli Earthquake, all of which were recorded at magnitudes equal or greater than Mw 6.6.

A recent study (Parija et al, 2021) sought to quantify the Coulomb Stress Transfer in the Western Himalayas. Coulomb stress transfer is used to quantify how earthquakes release stress, identifying areas that are put under increased stress and those that have been unloaded. This study and those like it are important in understanding the current state of fault zones in the region, as well as their potential for rupture in the future. [21]

See also

Localized geology and geomorphology topics for various parts of the Himalaya are discussed on other pages:

Notes

  1. ^ A more modern paleogeographic reconstruction of the Early Permian can be found at "Paleotethys". Université de Lausanne. Archived from the original on 8 June 2011..
  2. ^ A more modern paleogeographic reconstruction of the Permian-Triassic boundary, see "Neotethys". Université de Lausanne. Archived from the original on 19 January 2011..
  3. ^ The fourfold division of Himalayan units has been used since the work of Blanford & Medlicott (1879) and Heim & Gansser (1939).

References

Citations

  1. ^ Stampfli 2000.
  2. ^ Stampfli et al. 2001.
  3. ^ Stampfli & Borel 2002.
  4. ^ Burbank et al. 1996.
  5. ^ DiPietro & Pogue 2004.
  6. ^ Dèzes 1999.
  7. ^ Ding, Kapp & Wan 2005.
  8. ^ Klootwijk et al. 1992.
  9. ^ Achache, Courtillot & Xiu 1984.
  10. ^ Patriat & Achache 1984.
  11. ^ Besse et al. 1984.
  12. ^ Besse & Courtillot 1988.
  13. ^ Klootwijk, Conaghan & Powell 1985.
  14. ^ Bingham & Klootwijk 1980.
  15. ^ Le Pichon, Fournier & Jolivet 1992.
  16. S2CID 232084060
    .
  17. ^ Frank, Gansser & Trommsdorff 1977.
  18. ^ Steck et al. 1993a.
  19. ^ Girard & Bussy 1998.
  20. doi:10.1146/annurev-earth-060313-055005
    .
  21. ^
    ISSN 0278-7407
    .

Sources

External links
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