spreading centers in the oceans and plate destruction at convergent plate boundaries. The cordilleran process that characterizes convergence between oceanic and continental plates involves the rifting of fragments hundreds of kilometers long from the continent and the collision and lateral motion of objects of comparable size. At times in earth history, plate motion has inevitably led to collisions between objects of continental dimensions.

The huge Alpine-Himalayan mountain chain represents the site of an ongoing continental collision between Eurasia and three fragments of the former continent of Gondwanaland: Africa, Arabia, and India. An earlier large-scale continental collision involved Gondwanaland itself, which collided with Laurasia 300-million-years ago to form the short-lived supercontinent of Pangea. Comparable collisions may be vaguely discerned in the older rock record—for example, in the original assembly of Gondwanaland about 600-million-years ago.

In the Mediterranean area the Alpine-Himalayan chain shows some of the complexities of continental collisions. Continental fragments collided tens of millions of years ago to form the Alps, the best studied of all mountain ranges. Crustal rocks were locally carried to depths of 100 km during this mountain-forming process, and they have been intensely deformed during several episodes. Widespread collapse of the elevated mountains is evident, a process also encountered at present in western North America. Collapse of mountain ranges by extension may be as general and as significant a part of their history as their construction by shortening. The kinds of integrated geological and geophysical approaches applied in western North America suggest that the mountain belts of the Alps and the Mediterranean region are uncoupled from the underlying mantle and are moving independently. Lateral movement, perhaps in response to the collision of the Arabian continental protuberance, is accommodated by the earthquake-generating faults in Turkey. This lateral motion may play a major part in the collision that is closing the Mediterranean. For example, over the past 25-million-years west-to-east movement of most of the Italian peninsula has pushed up the Apennine and Dalmatian mountains and, in its wake, has opened a small basin now filled by the Tyrrhenian Sea.

Perhaps the most distinctive feature of the collision of Arabia with Asia, in the sector of the Alpine-Himalayan belt immediately east of the Mediterranean, is the role it has played in forming the enormous accumulation of oil and gas in Saudi Arabia, the Gulf states, Kuwait, Iran, Iraq, Syria, and Turkey. This remarkable resource accumulation has depended on the association of the collision along with a number of other special circumstances; these are considered in Chapter 4, Resources of the Solid-Earth. As a first approximation, oil generated on the old continental margin that was the northern edge of Arabia appears to have been driven up-slope at the collision by shortening in front of the collisional Zagros Mountains.

The collision between India and Asia represents the most advanced stage of continental collision on Earth at this time. Phenomena associated with this collision include elevation of the highest mountains, the Himalaya; formation of the world's largest sedimentary body, the Bengal fan; and generation of violent earthquakes, including the most deadly earthquake in recent decades at Tangshan, near Beijing, China. The collisional zone in Pakistan and India has yielded far less oil and gas accumulations than that in Arabia. This is, at least in part, because the Indian continental margin formed at high latitudes where the accumulation of organic material was less abundant.

The study of continental convergence between Eurasia and fragments of Gondwana serves to unite scientists as well as science. Researchers from India, Nepal, Pakistan, the Commonwealth of Independent States, China, and the countries of Southeast Asia, in cooperation with each other and with solid-earth scientists from Europe, Australasia, and North America, are suggesting solutions to questions concerning the ongoing collision. They conclude that India is dipping under Asia, but how fast and by how much are unknown. Recent investigations considering earthquake mechanisms and Landsat imagery propose that China is bulging eastward into the Pacific to escape the squeeze of the collision. And innovative interpretations speculate that the Tibetan Plateau, unable to escape the squeeze, has reached a limit of thickness and is now beginning to collapse.

The Himalayan/Tibetan area stands 5,000 meters above sea level over an area of more than 1,000,000 km2, which is as large as the lower-standing mountainous area of western North America. Researchers propose that this mass of continental crust has become detached from its underlying lithospheric mantle, which has foundered into the convecting mantle layer. If recycling of continental crustal material and its roots is indeed happening on such a grand scale beneath the Himalaya, the Alps, and perhaps the American cordillera, current ideas about continental crustal growth and processes of recycling should be reexamined.



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