W. Hamilton used these consequences of plate tectonics to explain modern examples of mountain building, and J. Dewey and J. Bird used them to account for the geologic structures observed in ancient mountain belts (76).

Much of the early work on convergent plate boundaries interpreted mountain building in terms of two-dimensional models that consider deformations only in the vertical planes perpendicular to the strikes of the convergent zones. During a protracted continent-continent collision, however, crustal material is eventually squeezed sideways out of the collision zone along lateral systems of strike-slip faults. The best modern example is the Tethyian orogenic belt, which extends for 10,000 kilometers across the southern margin of Eurasia. At the eastern end of this belt, the convergence of the Indian subcontinent with Asia has uplifted the Himalaya, raised the great plateau of Tibet, re-elevated the Tien Shan Mountains to heights in excess of 5 kilometers, and caused deformations up to 2000 kilometers north of the Himalayan front. Earthquakes within these continental deformation zones have been frequent and dangerous.

In a series of studies, P. Molnar and P. Tapponnier explained the orientation of the major faults in southern Asia, their displacements, and the timing of key tectonic events as a consequence of the collision of the Indian continent with Asia (77). They investigated the active faulting in central Asia using photographs from the Earth Resources Technology Satellite, magnetic lineations on the ocean floor, and teleseismically determined focal mechanisms of recent earthquakes. By combining these remote-sensing observations with the plate-tectonic information, they demonstrated that strike-slip faulting has played a dominant role in the mature phase of the Himalayan collision (78).

The more diffuse nature of continental seismicity and deformation was consistent with the notion that the continental lithosphere is some-how weaker than the oceanic lithosphere, but a detailed picture required a better understanding of the mechanical properties of rocks. When subjected to differential compression at moderate temperatures and pressures, most rocks fail by brittle fracture according to the Coulomb criterion (Equation 2.1). Extensive laboratory experiments on carbonates and silicates showed that for all modes of brittle failure, the coefficient of friction µ usually lies in the range 0.6 to 0.8, with only a weak dependence on the rock type, pressure, temperature, and properties of the fault surface. This behavior has come to be known as Byerlee’s law (79), and it implies that the frictional strength of continental and oceanic lithospheres should be about the same, at least at shallow depths.

Rocks deform by ductile flow, not brittle failure, when the temperature and pressure get high enough, however, and the onset of this ductility depends on composition. Investigations of ductile flow began in 1911 with Theodore von Kármán’s triaxial tests on jacketed samples of marble.

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