DEFINITION OF HIMALAYA MOUNTAINS

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The world’s tallest mountains, as

well as those exhibiting the greatest vertical relief over short

distances, form the Himalaya range that developed in the

continent-continent collision zone between India and Asia.

The range extends for more than 1,800 miles (3,000 km) from

the Karakorum near Kabul (Afghanistan), past Lhasa in Tibet,

to Arunachal Pradesh in the remote Assam province of India.

Ten of the world’s 14 peaks that rise to more than 26,000 feet

(8,000 m) are located in the Himalayas, including Mount

Everest, 29,035 feet (8,850 m); Nanga Parbat, 26,650 feet

(8,123 m); and Namche Barwa, 25,440 feet (7,754 m). The

rivers that drain the Himalayas feature some with the highest

sediment outputs in the world, including the Indus, Ganges,

and Brahmaputra. The Indo-Gangetic plain on the southern

side of the Himalayas represents a foreland basin filled by sediments

eroded from the mountains and deposited on Precambrian

and Gondwanan rocks of peninsular India. The northern

margin of the Himalayas is marked by the world’s highest

and largest uplifted plateau, the Tibetan plateau.

The Himalayas is one of the youngest mountain ranges

in the world but has a long and complicated history. This history

is best understood in the context of five main structural

and tectonic units within the ranges. The Subhimalaya

includes the Neogene Siwalik molasse, bounded on the south

by the Main Frontal Thrust that places the Siwalik molasse

over the Indo-Gangetic plain. The Lower Subhimalaya is

thrust over the Subhimalaya along the Main Boundary

Thrust, and it consists mainly of deformed thrust sheets

derived from the northern margin of the Indian shield. The

High Himalaya is a large area of crystalline basement rocks,

thrust over the Subhimalaya along the Main Central Thrust.

Further north, the High Himalaya sedimentary series or

Tibetan Himalaya consists of sedimentary rocks deposited on

the crystalline basement of the High Himalaya. Finally, the

Indus-Tsangpo suture represents the suture between the

Himalaya and the Tibetan Plateau to the north.

Sedimentary rocks in the Himalayas record events on the

Indian subcontinent, including a thick Cambrian-Ordovician

through Late Carboniferous/Early Permian Gondwanan

sequence, followed by rocks deposited during rifting and subsidence

events on the margins of the Tethys and Neotethys

Oceans. Collision of India with Asia was in progress by the

Early Eocene. This collision exposed the diverse rocks in the

Himalayas, revealing a rich geologic history that extends back

to the Precambrian, where shield rocks of the Aravalli Delhi

cratons are intruded by 500-million-year-old granites. Subduction

of Tethyan oceanic crust along the southern margin of

Tibet formed an Andean-style arc represented by the Transhimalaya

batholith that extends west into the Kohistan island

arc sequence, in a manner similar to the Alaskan range-Aleutians

of western North America. The obduction of ophiolites

and high-pressure (blueschist facies) metamorphism dated to

have occurred around 100 million years ago is believed to be

related to this subduction. Thrust stacks began stacking up on

the Indian subcontinent, and by the Miocene, deep attempted

intracrustal subduction of the Indian plate beneath Tibet

along the Main Central Thrust formed high-grade metamorphism

and generated a suite of granitic rocks in the Himalayas.

After 15–10 million years ago, movements were transferred to

the south to the Main Frontal Thrust, which is still active.

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