Full Divergent Margin
The East continent has now drifted off the eastern side of the cross section, and only the West continent and the new ocean basin with its rifting center (mid oceanic ridge) remain. Heat rising to the surface from the convection cells remains concentrated at the rifting site in the center of the new ocean basin, so as the ocean basin widens the newly formed continental margin (now called a divergent continental margin [DCM], or a passive continental margin because it is geologically passive) moves away from the heat source, and cools. Cool crust is denser than warm crust and as the DCM cools it sinks, rapidly at first, but ever more slowly with time (a process called thermal decay). Thus, in about 5-10 million years the horsts which once were 3-5 kilometers above sea level sink below the waves. Ultimately it will take about 110 million years for the DCM to cool completely and stabilize, at which point it will be about 14 kilometers below sea level (Stage E).
A significant amount (wedge) of sediment is deposited on the DCM, expanding and thickening from a feather edge on the continent side toward the ocean basin. These sediments are derived from the eroding continent in the case of clastics, and by chemical and biological activity in the case of carbonates. It consists mostly of shallow-water marine deposits because subsidence and deposition occur at approximately the same rate.
When next to a stable Craton, the wedge of sedimentary rocks is dominated by mature sandstone, limestones, and dolomites, but if the continent is undergoing tectonic activity, many kinds of less mature sedimentary rocks may be introduced, such as along the east coast of North America today where sublithic sandstones and shales are common. The Virginia coastal region today is a modern DCM, at this point stabilized since the rifting which opened the Atlantic ocean occurred nearly 250 million years ago.
Contributed by Lynn Fichter
Thursday, August 21, 2014