Most of the current views on the evolution of passive margins have roots in ideas that were developed before 1930 in the context of continental drift and geosynclinal theory. These ideas include the concept of an Atlantic type of margin formed by rifting and continental drift; the presence of a thick sedimentary deposit beneath the continental shelves; and subsidence in response to such mechanisms as crustal thinning, igneous underplating, thermal contraction, flexure, and sediment loading. As large amounts of new surface and subsurface data were acquired from modern passive margins after World War II, owing to significant advances in technology for geological and geophysical exploration of the ocean basins, these early ideas were strengthened and modified. With the development of plate-tectonic theory, the origin of passive margins as rifted trailing edges of continents became widely accepted, and significant changes in thinking involved the role of passive margins and their implications for large horizontal displacements in the evolution of geosynclines. Within the past decade, a large number of geophysical models have been developed for passive margins that focus once again on the problem of the mechanisms of vertical movements of the Earth’s crust. At the same time, new developments in the acquisition and processing of data from ocean basins, especially deep-reflection data, have resulted in major new concepts about the deep structure of passive margins, including recognition of the importance of underplating and plutonic activity in the thinned rifted crust and the unexpected degree of faulting and formation of horizontal reflectors in the lower continental crust and subcrustal lithosphere.

Abstract Modeling of early Paleozoic passive margins in the Cordilleran and Appalachian orogens indicates that factors controlling growth of early Paleozoic passive-margin carbonate platforms were thermally controlled subsidence, time-dependent flexure of the lithosphere, and at least two orders of eustatic sea-level changes. Initiation of the carbonate platforms in Middle Cambrian time followed a marked reduction in supply of Lower Cambrian coarse siliciclastic material to the passive margins. Two-dimensional modeling of palinspastically restored cross sections implies that the reduction in relief of onshore sediment sources resulted mainly from increased time-dependent flexural rigidity and extension of the area of subsidence into the craton. Continued increase in rigidity and bending of the craton edge, combined with a long-term eustatic sea-level rise, further reduced the supply of siliciclastic material to the carbonate platforms, resulting in a progressive cratonward shift of the siliciclastic shoreline and cratonward expansion of the carbonate platforms. Additional evidence of eustatic controls on growth of the platforms is obtained from one-dimensional analyses of post-rift subsidence of the platforms. The effects of sediment loading and lithification are removed from cumulative subsidence curves, producing reduced cumulative curves, designated R1 curves. The first-order form of the R1 curves is exponential, matching closely the form of theoretical curves calculated from cooling plate models for passive margins. After subtracting best-fit model cooling curves from the R1 curves, the residual curves, designated R2 curves, contain evidence of two orders of "events" superimposed on the thermally controlled subsidence of the margins. One event is the long-term rise and fall of sea level observed in the two-dimensional modeling. The long-term event coincides temporally with the Sauk transgression-regression on the craton. The other consists of repeating short-term sea-level changes with wave lengths of 2 to 6 Ma. The short-term sea-level events have similar timing in the southern Canadian Rockies, in the Great Basin, and in the Virginia-Tennessee Appalachians, suggesting a eustatic control. These inferred eustatic events appear to have exerted a major influence on the lithologic framework of the carbonate platforms. The long-term eustatic fall in Late Cambrian and Ordovician time augmented the reduction in rate of net subsidence of the platforms resulting from decay of the thermal anomaly. The much slower subsidence probably was the principal cause of the marked expansion in Late Cambrian and Ordovician time of carbonate shoal facies within the platforms. The short-term eustatic events produced distinct cycles composed of fine-grained shaley material in their lower halves and coarser grained shoal facies in their upper halves. Apparently, each short-term sea-level rise reduced the rate of carbonate production sufficiently to allow widespread deposition of subtidal facies with large amounts of interbedded siliciclastic mud. During each short-term fall, rates of carbonate production increased and led to expansion of shoal facies across the platforms.