boundary layer that can act as a mechanical stress guide, capable of transmitting forces for thousands of kilometers from one boundary to another (69). The essential elements of the subduction process were brought together in a 1968 paper by seismologists Brian Isacks, Jack Oliver, and Lynn Sykes (70). In addition to obtaining improved data on earthquake locations and focal mechanisms, they delineated a dipping slab of mantle material with distinctively high seismic velocity and low attenuation, which coincided with the Wadati-Benioff planes of deep seismicity (71). They found that they could account for their results, as well as most of the other data on plate tectonics, in terms of three mechanical layers, which J. Barrell and R.A. Daly had postulated earlier in the century to explain the vertical motions associated with isostatic compensation. A cold, strong lithosphere was generated by seafloor spreading at the ridge axis and subsequent conductive cooling of the oceanic crust and upper mantle, attaining a thickness of about 100 kilometers. It slid over and eventually subducted back into a hot, weak asthenosphere. Earthquakes of the Wadati-Benioff zones were generated primarily by stresses internal to the descending slab of oceanic lithosphere when it encountered a stronger, interior mesosphere at a depth of about 700 kilometers.

Deformation of the Continents

Plate tectonics was astounding in its simplicity and the economy with which it explained so many previously disparate geological observations. In the late 1960s and 1970s, geological data were reappraised in the light of the “new global tectonics,” leading to some important extensions of the basic plate theory. However, a major problem was the obvious contrast in mechanical behavior of the oceanic and continental lithospheres. Geophysical surveys in the ocean basins revealed much narrower plate boundaries than observed on land. The volcanic rifts of active crust formation along the mid-ocean ridges were found to be only a few kilometers wide, for example, whereas volcanic activity in continental rifts could be mapped over tens to hundreds of kilometers. Similar differences were observed for transform faults; in the oceans, the active slip is confined to very narrow zones, in marked contrast to the broad belts of continental strike-slip tectonics, which often involve many distributed, interdependent fault systems. For example, only about two-thirds of the relative motion between the Pacific and North American plates turned out to be accommodated along the infamous San Andreas fault; the remainder is taken up on subsidiary faults and by oblique extension in the Basin and Range Province (see Section 3.2).

In 1970, Tanya Atwater (72) explained the geological evolution of western North America over the last 30 million years as the consequence



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