Himalayas: an active orogen
History of plate motions:
- India broke free of Antarctica before 70 Ma
(part of continued break-up of Gondwana Land) and moved toward
Asia at rate of 10 cm/yr.
- collision initiated some 42 Ma (plus or minus
several Ma), but some debate.
- rate of motion after collision circa 5 cm/yr
which gives 2000 km of crustal overlap to look for
- diagram to right showing the northward movement of India, and collision with the Asian Continent. Image source: http://pubs.usgs.gov/gip/dynamic/himalaya.html .
Simple map view of the orogen. Note the distinctive curvature of the Himalayas, and the two distinct ends. Image source: http://pubs.usgs.gov/gip/dynamic/understanding.html .
Map from USGS showing earthquake distribution in the region.
USGS showing the epicenter of an earthquake in China. Much of the topography depicted here is actively tectonically generated topography. Along with the earthquake map above, they remind the viewer that the deformation associated with the India-Asia collision penetrates deeply into the Asian continent. Image source: http://neic.usgs.gov/neis/last_event/world_china.html .
Some major components of the Himalayas:
- MFT: main frontal thrust, changes with time,
deforms Quaternary river terraces .
- MBT: Plio-Pleistocene in age with Triassic
Megalodon linestone in hanging wall, stacked passive margin sediments.
- MCT: main central thrust: large basement-involved
thrust with > 100 km displacement and folded by underlying
thrusts. Overlying basement are Cambrian thorugh Eocene passive
margin sediments. Thrust intruded by granites.
- mid Miocene to Pleistocene (15 Ma-0 ma) molasse
- derived from the foreland fold-thrust belt
(i.e. the Himalayas).
- involved in thrusts, which in places ride
out over their own sedimentary debris.
- well known terrestrial vertebrate fauna
- Indus-Tsangbo suture zone: 2 belts of ophiolites
with 70 Ma old attached sediments. Glaucophane schists also indicate
previous subduction zone metamorphism.
- 15-30 Ma leucogranites (anatectic melts).
- Kohistan arc found on W side, K batholiths
and metamorphism, crustal cross section exposed, built on Lhasa
- Lhasa block: an accreted terrane with K age
- Bangong-Nujiang suture: ophiolite thrust
onto Middle-Upper Jurassic clastics,but overlain by Middle K
volcanic redbed unit -> Lower Cretaceous suturing of Lhasa
- Tien Shan Range in NE Tibet - flat lying
marine strata from 200-40 Ma.
Visible earth field trip to Himalayas:
There is about 300 km of shortening in the
Himalayas plus 600-700 km to form the crustal thickness observed
within the Tibetan plateau. Different workers have somewhat different
numbers, but all result in a deficit considering how much plate
motion has occurred between the two continents since initial collision. How was the rest accomodated?
- on W side: dextral Herat fault and sinistral
Quetta-Chaman fault bound one edge.
- on E side: many more strike-slip faults,
more complex, treated as a free boundary to the east, Altyn Tagh
sinistral fault and red River dextral fault bount one wedge.
- more information on escape tectonics.
pile it on and it will fail.
materials for Plate Tectonics, GEOL 3700, University of Nebraska
at Omaha. Instructor: H. D. Maher Jr., copyright. This material
may be used for non-profit educational purposes with appropriate
attribution of authorship. Otherwise please contact author.