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This chapter presents a synthesis of the structure and Phanerozoic tectonic evolution of continent-ocean transitions around nearly the entire North American continent. Its objectives are to compare and contrast the modern transitions and Phanerozoic histories of specific margins of North America discussed in other chapters of this book and in the Transect displays (see Foreword) and to present ideas on processes in the evolution of continent-ocean transitions.

A word about the focus on the continent’s margins and their place in the global scheme of tectonics may be warranted. The modern transitions between the North American continent and adjoining oceanic basins are where plate tectonics has most recently affected the continent aside from rigid translation. At active margins, where oceanic lithosphere underrides or slides along the continent, the development of the margin is ongoing and may have been continuous over a long time. In contrast, at passive margins where the continent has split and sea-floor spreading has occurred, the continent-ocean transition was developed in a discrete event and was discontinuous in time. At passive margins, the most recent North American transitions range from Holocene to mid-Mesozoic. At collisional margins, where the North American continent is or was on the underriding plate, the development of the margin was also discrete and discontinuous. As with passive margins, an existing collisional margin is generally the product of an ancient event.

During the development of all margin types, the motions between the North American plate and adjacent plates have commonly been broadly distributed and nonuniform

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