Download The Alpine-Himalayan Orogeny

SPAZIO CLIL
The Alpine-Himalayan Orogeny
EUROPE
T
ASIA
Alps
Caucasus
Tien Shan
Atlas
AFRICA
Tibetan plateau
Zagros
Himalayas
Saudi
Arabia
China
India
Figure 1 The Alpine-Himalayan belt showing the chains of high mountains built by the ongoing
collision of the African, Arabian, and Indian plates with the Eurasian Plate.
60 Ma
Indian plate
Eurasian plate
(Tibet)
The Indian Plate moved
northward, carrying the Indian
subcontinent. As it subducted
under the Eurasian Plate, an
accretionary wedge accumulated
from the sediments and oceanic
crust scraped off the descending
plate. Rising magma from the
descending plate thickened the
Eurasian Plate crust.
Oceanic crust
Continental
crust
30÷50 Ma
The Indian subcontinent collided
with Tibet, but India was too
buoyant to be subducted into the
mantle, so India broke along the
Main Central Thrust fault.
Main central
trust
Eroded uplifted material
20÷30 Ma
As the collision continued, the
motion was taken up along the
thrust fault, and a slice of Indian
crust and shelf sediments stacked
onto the oncoming subcontinent.
10÷20 Ma
A second thrust fault, the Main
Boundary Fault, developed,
stacking a second slice of crust
onto India and lifting the first
slice. Thus two overthrust
slices make up the bulk of the
Himalayas.
Main boundary
fault
Main central
trust
Ganges plain
Himalayas
Tibetan Plateau
Main boundary
fault
Main central
trust
Figure 2 Cross sections showing the sequence of events that have caused the Himalayan orogeny,
simplified and vertically exaggerated. (After P. Molnar, The Structure of Mountain Ranges. Scientific American, July 1986, p. 70)
Fantini, Monesi, Piazzini - Elementi
o see orogeny in action, we look to the great
chains of high mountains that stretch from
Europe through the Middle East and across Asia,
known collectively as the Alpine-Himalayan belt
(see figure 1) The breakup of the ancient supercontinent Pangaea sent the continental crust of Africa, Arabia, and India northward, causing the ancient Tethys Ocean to close as its lithosphere was
subducted beneath Eurasia. These former pieces
of Gond­wana­land (the southern part of Pangaea)
collided with Eurasia in a complex se­
quence,
beginning in the western part of Eurasia during
the Creta­ceous period and continuing eastward
through the Cenozoic, raising the Alps in central Europe, the Caucasus and Zagros mountains
in the Middle East, and the Himalayas and other
high mountain chains across central Asia.
The Himalayas, the world’s highest mountains,
are the most spectacular result of this modern
episode of continent-continent collision. About 50
million years ago, the Indian subcontinent, riding
on the subducting Indian Plate, first encountered
the island arcs and continental volcanic belts that
then bounded the Eurasian Plate (see figure 2). As
the landmasses of India and Eurasia merged, the
Tethys Ocean disappeared through subduction.
Pieces of the oceanic crust were trapped along the
suture zone between the converging continents
and can be seen today as ophiolites along the Indus
and Tsangpo river valleys that separate the high
Himalayas from Tibet. The collision slowed India’s
advance, but the plate continued to drive northward. So far, India has penetrated over 2 000 km
into Eurasia, causing the largest and most intense
orogeny of the Cenozoic era.
The Himalayas were formed from overthrust
slices of the old northern portion of India, stacked
one atop the other. This process took up some of
the compression. Horizontal compression also
thickened the crust north of India, causing uplift of
the huge Tibetan Plateau, which now has a crustal
thickness of 60 to 70 km (almost twice the thickness
of normal continental crust) and stands nearly 5 km
above sea level. These and other zones of compression account for perhaps half of India’s penetration
into Eurasia. The other half has been accommodated by pushing China and Mongolia eastward,
out of India’s way, like toothpaste squeezed from
a tube. The mountains, plateaus, faults, and great
earthquakes of Asia, thousands of kilometers from
the Indian-Eurasian suture, are thus affected by
the Himal­ayan orogeny, which continues as India
ploughs into Asia at a rate of 40 to 50 mm/year.
di Scienze della Terra • Italo Bovolenta editore - 2013
1