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As ocean basins shrink by subduction, portions of the ridge system may be subducted. Once the ridge is subducted, growth of the ocean basin by sea‐floor spreading ceases, the ocean basin continues to shrink, and the continents, microcontinents or arcs on either side are brought closer together as subduction proceeds. Eventually they converge to produce a continental collision.

When a continental collision (Dewey and Bird 1970) occurs, subduction eventually ceases. This occurs because most continental lithosphere is too buoyant to be subducted to great depths for prolonged periods of time. Small amounts may be subducted in this tectonic setting, likely because the density contrasts between the leading edges of the two plates are small. The continental lithosphere involved in the collision may be part of a continent, a microcontinent or a volcanic‐magmatic arc complex. Typically the collision occurs over millions to tens of millions of years as irregularly shaped ocean basins close at different times. As convergence continues, the margins of both continental plates are compressed and shortened horizontally and thickened vertically in a manner roughly analogous to what happens to two vehicles in a head‐on collision. However, in the case of continents colliding at a convergent plate boundary, the convergence occurs over millions of years. The result is a severe horizontal shortening and vertical thickening which results in the progressive uplift of a mountain belt and/or extensive elevated plateau that mark the closing of an ancient ocean basin (Figure 1.13).

Long mountain belts formed along convergent plate boundaries are called orogenic belts. The increasing weight of the thickening orogenic belt causes the adjacent continental lithosphere to bend downward to produce foreland basins adjacent to the orogenic belt. Large amounts of detrital sediments derived from the erosion of the mountain belts are deposited in such basins. In addition, increasing temperatures and pressures within the thickening orogenic belt cause regional metamorphism of the rocks within it. If the temperatures become high enough, partial melting may occur to produce melts in the deepest parts of orogenic belts which rise to produce a variety of felsic igneous rocks.

Figure 1.13 (a) Ocean basin shrinks by subduction, as continents on two plates converge. (b) Continental collision produces a larger continent from two continents joined by a suture zone. Horizontal shortening and vertical thickening are accommodated by folds and thrust faults in the resulting orogenic belt.

A striking example of a modern orogenic belt is the Himalayan Mountain range formed by the collision of India with Eurasia over the past 40 Ma. The continued convergence of the Indian micro‐continent with Asia has resulted in shortening and regional uplift of the Himalayan Mountain Belt along a series of major thrust faults and produced the Tibetan Plateau. Limestones near the summit of Mt. Everest, Earth's highest mountain, were formed on the floor of the Tethys Ocean that once separated India and Asia. They were then thrust to an elevation of nearly 9 km as that ocean was closed and the Himalayan Mountain Belt formed by continental collision. The collision has produced tectonic indentation of Asia, resulting in mountain ranges that wrap around India (Figure 1.14). The Ganges River in northern India flows approximately west‐east in a trough that represents a modern foreland basin.

Continental collision inevitably produces a larger continent. It is now recognized that supercontinents such as Pangea and Rodinia were formed as the result of collisional tectonics. Collisional tectonics only requires converging plates whose leading edges are composed of lithosphere that is too buoyant to be easily subducted. In fact all the major continents display evidence of being composed of a collage of terranes that were accreted by collisional events at various times in their histories.