The mountain-building episode that created the Himalayas began roughly 50 million years ago, when India began to collide with Asia.
Prior to the breakup of Pangaea, India was located between Africa and Antarctica in the Southern Hemisphere. As Pangaea fragmented, India moved rapidly, geologically speaking, a few thousand kilometers in a northward direction.
The subduction zone that facilitated India’s northward migration was near the southern margin of Asia (Figure 1A). Continued subduction along Asia’s margin created an Andean-type plate margin containing a welldeveloped continental volcanic arc and an accretionary wedge.
India’s northern margin, on the other hand, was a passive continental margin consisting of a thick platform of shallow-water sediments and sedimentary rocks.
Geologists have determined that one or perhaps more small crustal fragments were positioned on the subducting plate somewhere between India and Asia. During the closing of the intervening ocean basin, a small crustal fragment, which now forms southern Tibet, reached the trench.
This event was followed by the docking of India to Eurasia. The tectonic forces involved in the collision of India with Asia were immense, causing the more deformable materials located on the seaward edges of these landmasses to become highly folded and faulted (Figure 1B).
The shortening and thickening of the crust elevated great quantities of crustal material, thereby generating the spectacular Himalaya Mountains (Figure 2). As a result, tropical marine limestones that formed along the continental shelf now lie at the summit of Mount Everest.
In addition to uplift, crustal shortening caused rocks at the “bottom of the pile” to become deeply buried—an environment where these rocks experienced elevated temperatures and pressures (see Figure 1B).
Partial melting within the deepest and most-deformed region of the developing mountain belt produced magmas that intruded the overlying rocks. These environments generate the metamorphic and igneous cores of collisional mountains.
The formation of the Himalayas was followed by a period of uplift that raised the Tibetan Plateau. Seismic evidence suggests that a portion of the Indian subcontinent was thrust beneath Tibet—a distance of perhaps 400 kilometers (250 miles).
If this occurred, the added crustal thickness would account for the lofty landscape of southern Tibet, which has an average elevation higher than Mount Whitney, the highest point in the contiguous United States.
The collision with Asia slowed but did not stop the northward migration of India, which has since penetrated at least 2000 kilometers (1200 miles) into the mainland of Asia. Crustal shortening accommodated some of this motion. Much of the remaining penetration into Asia caused lateral displacement of large blocks of the Asian crust by a mechanism described as continental escape.
As shown in Figure 3, when India continued its northward trek, parts of Asia were “squeezed” eastward, out of the collision zone. These displaced crustal blocks include much of Southeast Asia (the region between India and China) and sections of China.