Exam 10: Deformation, Mountain Building, and Earthapos;s Crust

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These accretions are called terranes and are fragments of seamounts, island arcs, and small pieces of continents that were carried on oceanic plates that collided with continental plates, thus adding them to the continental margins where mountain building was occurring.

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The Himalayas, the best example of an orogeny along a continental--continental plate boundary, began forming when India collided with Asia. Before that time, India was far south of Asia. As the Indian plate moved northward, a subduction zone formed along the southern margin of Asia where oceanic lithosphere was consumed, magma was generated, and large granitic plutons were emplaced. As a result, two continental plates were compressed and were sutured, together. Thus, the Himalayas are now within a continent rather than along a continental margin. Because continental lithosphere is not dense enough to be subducted, the leading margin of India was thrust beneath Asia, causing crustal thickening, thrusting, and uplift. Sedimentary rocks that had been deposited in the sea south of Asia were thrust northward, and two major thrust faults carried rocks of Asian origin onto the Indian plate. Rocks deposited in the shallow seas along India's northern margin now form the higher parts of the Himalayas.