During the late 1970s, the study of rifted margins was changed fundamentally by the recognition that simple models of thinning of continental lithosphere could account for many features of the subsidence observed on rifted or passive continental margins (McKenzie, 1978). The 1980s have produced at least three major new foci of research in the study of such margins and continental rifting in general. These new directions have emerged partly as a result of new data acquisition, primarily deep penetration seismic reflection profiling and complementary long offset seismic reflection-refraction experiments on rifted margins worldwide, and partly as a result of the application of land geologic observations of continental rifts to the study of margins.
The first of these new directions was the recognition of the importance of low-angle normal faulting in continental rifting. This avenue of research has changed our expectations of the large-scale patterns of rifting and rifted margin formation. We had come to expect that most rifts should be symmetric and that, upon plate reconstruction, conjugate rifted margins would also show symmetry at the time of initiation of sea-floor spreading. A number of investigators (Wernicke, 1981Wernicke, 1985; Lister and others, 1986) observed asymmetry in the shallow structures in continental rifts, and extended this idea to the whole lithosphere scale. The new model, usually referred to as the simple shear or delamination model, concentrates shearing along narrow zones at low angle to the surface and through the continental lithosphere. Most of the strain during continental extension takes place within this
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This volume presents syntheses in twelve chapters of the tectonic evolution of continent-ocean transitions of North America (Canada-Mexico-U.S.A.) since the Precambrian. The syntheses are interpretations based on the 19 continent-ocean transects across North American margins published by GSA as part of its Decade of North American Geology (DNAG) series. The transitional region is the part of North America between the craton, little deformed in Phanerozoic time, and the modern ocean basins. The region developed heterogeneously within plate boundary zones that led to sequences of passive, collisional, and active margins that differ place to place. Nine chapters address individual segments of the transitional region, two consider active and passive margin tectonics topically, and one treats the evolution of Phanerozoic transitions of North America as a whole.