The evolution of the continental margins around each continent is of fundamental importance in estimating longitudinal separations of the continents. In the simplest plate tectonic cycle, a continent splits and separates into two or more continents, each of which eventually collides to form a continent similar to the original continent. In more-complex cases, a continent may split into several fragments, some or all of which might collide with continents different from the one they originally separated from. The age at which two continents separate can be estimated relatively precisely by applying the lithospheric stretching model (McKenzie 1978) to the stratigraphic sequences formed on each margin, even in orogenic belts (Wooler et al. 1992). In the absence of quantitative analyses, the time of separation may be difficult to estimate. Extensional faulting that preceded the formation of ocean floor and the separation of two continents may span some tens of millions of years, as in the present East African rift. The succeeding thermal phase, during which the margin subsides and the postrift passive margin sequence accumulates, continues until collision takes place. Flexure of the margin prior to actual collision gives rise to a characteristic time-subsidence signature.
Figure 2.5 Summary of methods used to make figures 2.4a-e. Black areas are large continents oriented by paleomagnetic data (Lau-rentia, Baltica, Siberia, and Gondwana, to which smaller fragments have been attached using visual, tectonic, and faunal data). Gray areas are large continents and their attached
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