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The process of ocean basin formation begins with a great surge of mafic volcanic activity along one side of the axial rift. Axial rifts do not usually split in two, down the middle, but separate along one side or the other (detailed drawing). In this model the activity is on the east (right) side of the rift and so the axial rift will remain with the western continent. At first the magma is injected as a large number of basaltic dikes into the now thinned and stretched granitic continental crust. So many dikes form in fact that it is finally hard to decide looking at them what the original rocks were, granite invaded by basalt, or basalt invaded by granite. This mixture of continental granite and injected basalt is called transition crust (principally because the speed of seismic (earthquake) waves traveling through it is transitional between the slower granite and the faster basalt.)
The mafic volcanic activity is concentrated at the rifting site, but is not confined there. Feeder dikes cut through the crust at many places, sometimes hundreds of miles to the sides of the axial rift. This magma may emplace as sills or laccoliths, or may surge to the surface to form fissure volcanos and lava flows.
As the volcanic activity continues, the two pieces of the original continent begin to drift apart and the gap between them fills with mafic igneous rock. Surge after surge of magma rises from the convection cells in the mantle into the continuously spreading gap as the continents move farther and farther apart. Within a few million years the two continents can be separated by thousands of kilometers.
Because all this new igneous rock is mafic and ultramafic in composition (basalt/gabbro near the surface and dunite/peridotite at depth), and high in density, it "floats" about 5 km below sea level. These layers of rock that form the oceanic lithosphere are the ophiolite suite.
The final result is that beginning with only one tectonic plate in Stage A, rifting has created a new divergent plate boundary and two plates, one on the west (containing Westcontinent) and one on the east (containing Eastcontinent).
As the new ocean basin begins to form the edge of the continent cools and subsides below sea level. And as the continental edge subsides below sea level the sea begins to transgress, or migrate across, the edge of Westcontinent (and Eastcontinent too). This is the beginning of deposition of Divergent Continental Margin (DCM) sediments, which will become much more prominent in the next few stages (see Early Divergent Margin). But initially, as the sea begins its transgression a layer of quartz sandstone is laid down as a beach deposit by the transgressing sea. Off shore from it is shallow shelf deposition. Its composition may be dominantly shale if there is a clastic source on the continent. But if the continent is stable, as Westcontinent is, and the climate is warm, then carbonate (limestone) deposition will dominate.
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