Divergent plate boundaries in continents

Rifts are elongate depressions formed where the entire thickness of the lithosphere has ruptured in extension. In these places the continents are beginning to break apart as immature divergent boundaries, and if successful, may form new ocean basins. The general geomorphic feature that initially forms is known as a rift valley. Rift valleys have steep, fault-bounded sides, with rift shoulders that typically tilt slightly away from the rift valley floor. Drainage systems tend to be short, internal systems, with streams forming on the steep sides of the rift, flowing along the rift axis, and draining into deep, narrow lakes within the rift. If the rift is in an arid

Brittle-ductile transition

Brittle-ductile transition

Brittle Ductile Transition

Crust-mantle boundary

Dikes

Upwelling asthenosphere

Lithosphere-asthenosphere boundary

Crust-mantle boundary

Dikes

Upwelling asthenosphere

Lithosphere-asthenosphere boundary

Brittle-ductile transition

Half-graben complex

Crust-mantle boundary

Detachment fault

Upwelling asthenosphere

Lithosphere-asthenosphere boundary

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Modes of extension in rifts. (A) Shows pure shear model, in which the lithosphere extends symmetrically and asthenosphere rises to fill the space vacated by the extending lithosphere. (B) Shows simple shear or asymmetric rifting, where a shallow-dipping detachment fault penetrates the thickness of the lithosphere, and asthenosphere rises asymmetrically on the side of the rift where the fault enters the asthenosphere. Faulting patterns are also asymmetric, with different styles on either side of the rift.

Figure showing simplified three-stage evolution of divergent margins. The young rift valley stage like that in the East African rift system has steep rift shoulders and basaltic and rhyolitic volcanoes. The young ocean stage, similar to the modern Red Sea, has seafloor spreading and steep rift shoulders. Mature ocean stage is like the modern Atlantic Ocean, with thick passive margin sequences developed on continental edges around a wide ocean basin.

Basalt eruption

Rhyolite dome

'.Continental;^

Mantle

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Mantle

Continents Divergent
Mid-ocean ridge
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environment, such as much of East Africa, the drainage may have no outlet and the water will evaporate before it can reach the sea. This process leaves distinctive deposits of salts and other minerals that form by being left behind during evaporation of seawater (evaporites), one of the hallmark deposits of continental rift settings. other types of deposits in rifts include lake sediments in rift centers, and conglomerates (cemented gravels) derived from rocks exposed along the rift shoulders. These sediments may be interleaved with volcanic rocks, typically alkaline (having abundant sodium, Na, and other alkali elements) and bimodal in silica content (i.e., basalts and rhyolites).

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