• Himalayan mountain range and Tibetan plateau formed due to collision between Indian Plate and Eurasian Plate
• Collision began between 40 and 50 million years ago and continues today
• Both continental landmasses have about the same rock density, so one plate could not be subducted under the other
• Pressure of the impinging plates could only be relieved by thrusting skyward
• Collision zone was contorted and jagged Himalayan peaks were formed

Himalayas Formation

• The Himalayas are the youngest mountain chain in the world
• They emerged from a geosyncline called the Tethys Sea and uplift took place in different phases
• During Permian Period (250 million years ago), there was a supercontinent known as Pangaea
• Pangaea had two parts: Laurasia (present-day North America and Eurasia) and Gondwanaland (present-day South America, Africa, South India, Australia, and Antarctica)
• The Tethys Sea was a long, narrow, and shallow sea between Laurasia and Gondwanaland
• Many rivers flowed into the Tethys Sea, depositing sediments on its floor
• These sediments were subjected to powerful compression due to northward movement of the Indian Plate
• This compression resulted in the folding of sediments
• The summit of Mount Everest is made of marine limestone from the ancient Tethys Sea

• Sediments deposited on the floor of the Tethys Sea were folded due to the northward movement of the Indian Plate.
• Once the Indian Plate started plunging below the Eurasian Plate, these sediments were further folded and raised, forming the Himalayas.
• India is moving northwards at the rate of about five cm per year and crashing into the rest of Asia.
• Tibetan plateau was formed due to upthrusting of the southern block of the Eurasian Plate.
• The Indo-Gangetic plain was formed due to the consolidation of alluvium brought down by the rivers flowing from the Himalayas.
• The curved shape of the Himalayas convex to the south is attributed to the maximum push offered at two ends of the Indian Peninsula during its northward drift.

Phases of Himalayas formation

• The Himalayas do not comprise a single range but a series of at least three ranges running more or less parallel to one another. Therefore, the Himalayas are supposed to have emerged out of the Himalayan Geosyncline, i.e. the Tethys Sea in Six different phases following one after the other.
• 6 phases involved in the formation of Himalayas:
  • Phase 1 – 100 million years ago
  • Phase 2 – 71 million years ago
  • Phase 3 – The Drass volcanic arc
  • Phase 4 – Greater Himalayas were raised
  • Phase 5 – Rise of lesser Himalayas
  • Phase 6 – Rise of the Shiwalik ranges
• 
• Phase 1 (100 million years ago):
  • Indian plate was located between 10⁰ S – 40⁰ S over the reunion hotspot during the Cretaceous Period.
  • Movement of the plate attained its mass velocity as it was closer to the equator (14cm/yr).
• Phase 2 (71 million years ago):
  • Himalayan Orogenesis began around 71 million years ago.
  • The plate with the Gondwana continental piece drifted towards Northeast and collided with Eurasia.
  • The line of collision between the Tibetan Plateau and the Indian Plate is called the Indus-Tsangpo Suture Zone (ITSZ).
  • Murree Foredeep and Shiwalik foredeep were formed.
• Phase 3 (Oligocene):
  • The Drass volcanic area was formed, and in the Tethys crust, a series of volcanic eruptions took place.
  • The plate started anti-clock rotation and Drass became the Pivotal Axis.
• Phase 4 (30-35 million years ago):
  • Greater Himalayas were raised due to compressional thrust in the sediments of Murree foredeep.
  • The compressional thrust line is known as the Main Central Thrust (MCT).
• Phase 5 (15-20 million years ago):
  • Lesser Himalayas were lifted due to further movement in the plate during the Miocene.
  • MCT separates greater and lesser Himalayas, and the compressional thrust line along which the lesser Himalayas were lifted is called the Boundary Thrust/Fault (MBT of MBF) line.
• Phase 6:
  • Rise of the Shiwalik ranges due to partial feeding of the Shiwalik foredeep along the Himalayan Frontal Fault (HFF).
  • Shiwalik ranges represent partially folded sedimentary range.
• The Himalayan relief and structure can be studied based on the tecto-geological history.

Relief and Structure of Himalayas

• Tibetan Plateau:
  • Not a part of Himalayas, but formed due to Himalayan Orogeny
  • Indus-Tsangpo Suture Zone: compressional fault line extending for 3200 km from Indus gorge to Tsangpo gorge where rocks are crushed, pulverized, and mostly Paleozoic and ancient rocks are found
  • River Indus and Tsangpo flow through the reverse faulted line of discontinuity (Suture)
• Tethyan Himalayas:
  • Average height is 4000m
  • Compressed with Greater Himalayas, absence of longitudinal valley manifests high compressional forces
  • Submarine, sedimentary, metamorphic rocks, representing initial upliftment in Tethyan Geosyn or Murree foredeep
• Greater Himalayas:
  • Average height is 6000m
  • Extends from Mt. Namcha Barwa to Nanga Parbat for 2500km
  • Mightiest and most majestic mountain range, boasting hundreds of peaks rising above 7000m
  • Composed of metamorphic and sedimentary rocks
  • Core has Batholith representing intrusion of Magma (Granitic Magma)
• Main Central Thrust (MCT):
  • Zone of Tectonic Thrust and longitudinal axis along which Greater Himalayas were lifted
  • Tectonic compressional valley with fractured and pulverized rocks forming a synclinal basin b/w Greater Himalayas and Lesser Himalayas
• Lesser Himalayas:
  • Average height is 3800m, length is about 2400kms
  • Almost parallel to Greater Himalayas but segmented into several parallel and transverse ranges
  • Central part in Nepal is called the Mahabharatha range and eastern part is called Dafla hills, Mishmi hills, Miri hills, Abor
• Main Boundary Fault (MBF):
  • Longitudinal axis along with second major thrust in Himalayas acted
  • Not as deep as MCT
  • Compressional fault line with valleys being wide and rivers forming lakes in valleys where Lakestrene sediments were deposited
• Shiwaliks:
  • Average height is 800-1200m
  • Partially folded and formed of river sediments deposited in foredeep
  • Represent Hogback topography and 300m contour line demarcates boundary with Gangetic Plain
• Himalayan Frontal Fault (HFF):
  • Marks boundary between Himalayan ranges and Gangetic basin
  • Wide-angle thrust line where last compressional force in Himalayas Orogeny has taken place.