An extensive review of geologic and tectonic features of western North America suggests that the interaction of oceanic plates with the continent follows a broad cyclical pattern. In a typical cycle, periods of rapid subduction (7–15 cm/yr), andesitic volcanism, and trench-normal contraction are followed by a shift to trench-normal extension, the onset of voluminous silicic volcanism, formation of large calderas, and the creation of major batholiths. Extension becomes pervasive in metamorphic core complexes, and there is a shift to fundamentally basaltic volcanism, formation of flood basalts, widespread rifting, rotation of terranes, and extensive circulation of fluids throughout the plate margin. Strike-slip faulting becomes widespread with the creation of new tectonostratigraphic terranes. A new subduction zone forms and the cycle repeats. Each cycle is 50–80 m.y. long; cycles since the Triassic have ended and begun at approximately 225, 152, 92, 44, and 15 Ma. The youngest two cycles are diachronous, one from Oregon to Alaska, the other from central Mexico to California. The transitions from one cycle to the next cycle are characterized by rapid and pervasive changes termed, in this chapter, “major chaotic tectonic events.” These events appear to be related to the necking or breaking apart of the formerly subducted slab at shallow depth, the resulting delamination of the plate margin, and the onset of a new subduction cycle. These are times of the most rapid apparent and true polar wander of the North American plate, when the plate appears most free to move relative to surrounding plates and relative to the mantle below the asthenosphere. In western North America, magmatism and tectonics during the Jurassic period are quite similar to magmatism and tectonics since mid-Cretaceous time except strike-slip faulting shifted in sense from left lateral to right lateral.

Abstract Two active transform faults are identified on land in Iceland. This observation leads to a new interpretation of the tectonics of Iceland that is generally consistent with the available geologic, geomorphic, and geophysical data. This new interpretation provides a framework that can be used to relate detailed geologic and geophysical studies in Iceland to worldwide processes at the crests of mid-ocean ridges. Nearly one-half of Iceland seems to have formed during a period of very slow spreading between about 9 and 20 m.y. B.P. The center of spreading within Iceland apparently shifted from western to eastern Iceland around 7 or 8 m.y. B.P. Iceland, the largest land mass on the mid-kxean ridge system, may have resulted from a change in the stress pattern on a broad fracture zone, allowing large volumes of lava to be erupted while there was little regional spreading.