Plate Tectonics

The collision between the Eurasian and African plates has resulted in the

formation of the Alps; this is a relatively subdued continental collision and

the Alpine orogenic zone is relatively narrow. One model for this collision

is illustrated in the cross section given in Figure 1–45. A flake of the upper

continental crust of the Eurasian plate has overridden the continental crust

of the African plate. The forces associated with the southward dipping sub-

duction of the Eurasian plate has driven the upper Eurasian crust several

hundred kilometers over the African crust. The lower Eurasian crust has

been delaminated and is being subducted into the mantle with the Eurasian

lithosphere. The underlying African crust is exposed through the overly-

ing upper Eurasian crust at several points in the Alps. The splitting of the

Eurasian crust at a depth of about 15 km requires an intracrustal decolle-

ment. This type of splitting is often observed in the geological record and is

attributed to a soft crustal rheology at intermediate crustal depths.

The continental collision between the Eurasian and the Indian plates has

resulted in a much broader orogenic zone that extends throughout much of

China. This orogenic zone is illustrated in Figure 1–46. The collision has

resulted in the formation of the Himalayas, the highest and most extensive

mountain range in the world. A satellite photograph of the Himalayas look-

ing to the northwest is given in Figure 1–47. Imbedded within the Himalayas

is the Indus suture, the actual boundary between the Indian plate and the

Eurasian plate. The Tibetan plateau is a broad region of elevated topogra-

phy with extensive faulting but very little recent volcanism. Faulting extends

throughout much of China. A substantial fraction of the largest historical

earthquakes has occurred on these faults and in many cases the death toll

has been very high. Reports claimed that there were 655,000 deaths during

the Tangshan earthquake of July 28, 1976. The largest reported death toll in

an earthquake was the 800,000 deaths attributed to the Shensi earthquake

on January 23, 1556.

It is necessary to explain why this orogenic zone is so broad and why the

orogeny is principally on the Eurasian plate with relatively little deformation

on the Indian plate. One explanation for the asymmetric deformation is that

the Eurasian lithosphere in Tibet and China was thin and weak prior to the

collision. This area may have resembled the present western United States

which has a weak and easily deformable lithosphere. A simplified model for

this continental collision is given in Figure 1–48. The Indian continental

crust and lithosphere have been thrust beneath the Eurasian crust across

the entire width of the Tibetan plateau.