Geology of the Himalayan Mountains
The collision between the Indian subcontinent and Eurasian
continent, which started in Paleogene time and continues today,
produced the Himalayas and Tibetan Plateau, spectacular modern
examples of the effects of plate tectonics. Tibetan Plateau
itself is a collage of microplates or continental fragments that
were successively added to the Eurasian plate during the
Paleozoic and Mesozoic eras. Paleomagnetic analysis indicates
that these older microplates were in the southern hemisphere
during the Paleozoic era. Each older fragment, like the larger
Indian Plate, made a long northward journey as the intervening
ocean was subducted, and was accreted to the Eurasian continent.
The resulting sutures are marked by scattered occurrences of
ophiolite, ocean floor material that was caught up between the
crustal blocks during accretion. The collision that produced the
Himalayas was only the latest, albeit climactic, episode in this
long series of collisions.
The Indian plate continues to move northward relative to Asia
about 5 cm per year. Given the great magnitudes of the blocks of
the Earth's crust involved this is a remarkable rate, about twice
the speed at which your fingernails grow. This movement caused
the rocks along the leading edge of the Indian plate to deform
and fracture. Immense slices of Indian crust were pushed
southward and stacked up to produce the Himalayan orogen
(figure). Several distinct sequences of rock are recognized in
the Himalayas, separated from one another and rocks of the Indian
plate by northward-dipping fault zones. It is these zones that
accommodated the huge amounts of crustal shortening as the rock
sequences were slid one over another. From north to south, these
rock packages and fault zones are the Tibetan Sedimentary
Sequence (blue), South Tibetan Detachment Zone (Detachment
Fault), Greater Himalayan Sequence (red and pink), Main Central
Thrust (MCT) and Maha Bharat Thrust (MT), Lesser Himalayan
Sequence (light blue), Main Boundary Thrust (MBT) and Main
Frontal Thrust (MFT), and Gangetic Basin of the Indian
subcontinent (yellow).
NASA's on-line geomorphology book describes the
Himalayas and Tibetan Plateau and the
associated geology of Afghanistan, Pakistan, western India.
Project INDEPTH, which has done seismic profiling across Tibet
and the Himalaya, has an interesting, but technical, web page
with some great pictures at: http://www.geo.cornell.edu/geology/indepth/indepth.html.
Readings
- Allegre et al., 1984, Structure and evolution of the
Himalaya-Tibet orogenic belt: Nature, v. 307, p. 17-22.
- Bilham et al.., 1997, GPS measurements of present-day
convergence across the Nepal Himalaya: Nature, v. 386, p.
61-64.
- Le Fort, 1996, Evolution of the Himalaya, in The Tectonics
of Asia, Yin & Harrison (eds.), p. 95-109.
- Molnar, 1986, The geologic history and structure of the
Himalaya: American Scientist, v. 74, p. 144-154.
- Searle et al., 1987, The closing of Tethys and the
tectonics of the Himalaya: Geological Society of America
Bulletin, v. 98, p. 678-701.
- Searle & Treloar, 1993, Himalayaan tectonics - an
introduction: in Himalayan Tectonics, Treloar & Searle
(eds.), p. 1-7.
- Schelling, 1992, The tectonostratigraphy and structure of the
Eastern Nepal Himalaya: Tectonics, v. 11, p. 925-943.
- Tapponnier et al., 1986, On the mechanics of the
collision between India and Asia, in Collision Tectonics,
Coward & Ries (eds.): Geological Society Special Publication, no.
19, p. 115-157.
