Further Reading

Prothero, Donald R., and Robert H. Dott. Evolution of the

Earth. 6th ed. Boston, Mass.: McGraw-Hill, 2002. Rogers, J. J. W., and M. Santosh. Continents and Supercontinents. Oxford: Oxford University Press, 2004. Windley, Brian F. The Evolving Continents. 3rd ed. Chichester, England: John Wiley & Sons, 1995.

passive margin Continental margins that are attached to the adjacent oceanic crust and do not have a plate boundary along the margin are known as passive or trailing margins. Most parts of the east coasts of North and South America, the west coasts of Europe and Africa, and most of the coastlines of India, Antarctica, and Australia are passive margins, many of which are characterized by thick accumula tions of marine carbonates, shales, and sandstones. Conditions for the formation, accumulation, and preservation of hydrocarbons are met along many passive margins, so contemporaneous and ancient passive margin sequences are the focus of intense petroleum exploration.

Trailing or passive margins typically develop from a continental rift and first form an immature passive margin, with the Red Sea being the main example present on the planet at this time. Rifting along the Red Sea began in earnest by 30 million years ago, separating Arabia from Africa. The Red Sea is characterized by uplifted rift shoulders that slope generally away from the interior of the sea, but have narrow down-dropped coastal plains where steep mountain fronts are drained by wadis (dry stream beds) with alluvial fans that form a typically narrow coastal plain. These have formed over stretched continental crust, forming many rotated fault blocks and grabens, intruded by mafic dike swarms that in some places feed extensive young volcanic fields, especially in Saudi Arabia. The center of the Red Sea, which is located in tropical to subtropical latitudes, has developed thick carbonate platforms along the stretched continental crust. As rifting and associated stretching of the continental crust proceeded, areas that were once above sea level subsided below sea level, but different parts of the Red Sea basin reached this point at different times. Together with global rises and falls of sea level, this led to episodic spilling of salty seawater into restricted basins which would then evaporate, leaving thick deposits of salt behind. As rifting continued these salts became buried beneath the carbonates, shales, and sandstones, but when salt gets buried deeply it rises buoyantly, forming salt domes that pierce overlying sediments. The movement of the salt forms broad open folds that in some places forms exceptionally good petroleum traps. Parts of the passive margins along the Red Sea are several kilometers thick and currently exhibit some of the world's best coral reefs. The center of the Red Sea is marked by steep slopes off the carbonate platforms, leading to the embryonic spreading center that is present only in southern parts of the sea. Abundant volcanism, hot black smoker vents, and active metalliferous and brine mineralization on the seafloor characterize this spreading center.

As spreading continues on passive margins, the embryonic or Red Sea stage gradually evolves into a young oceanic or mature passive margin stage where the topographic relief on the margins decreases and the ocean-to-passive-margin transition becomes very flat, forming wide coastal plains such as those along the east coast of North America. This transition is an important point in the evolution of passive margins, as it marks the change from rifting and heating of

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