Rift basins are elongate depressions in the Earth’s surface
where the entire thickness of the lithosphere has ruptured
in extension. They are typically bounded by normal
faults along their long sides and display rapid lateral variation
in sedimentary facies and thicknesses. Rock types
deposited in the rift basins include marginal conglomerate,
fanglomerate, and alluvial fans, grading basinward into sandstone,
shale, and lake evaporite deposits. Volcanic rocks may
be intercalated with the sedimentary deposits of rifts and in
many cases include a bimodal suite of basalts and rhyolites,
some with alkaline chemical characteristics.
Many rifts in continents are associated with incipient
breaking apart of the continent to form an oceanic basin.
These types of rift system typically form three arms that
develop over domed areas above upwelling mantle material,
such as is observed in East Africa. Two of the three arms may
link with other three-pronged rift systems developed over
adjacent domes, forming a linked elongate rift system that
then spreads to form an ocean basin. This type of development
leaves behind some failed rift arms that will come to
reside on the margins of young oceans when the successful
rift arms begin to spread. These failed rift arms then become
sites of increased sedimentation and subsidence, and they also
tend to be low-lying areas and form the tectonic setting
where many of the world’s major rivers flow (for example,
the Nile, Amazon, Mississippi). Other rifts form at high
angles to collisional mountain belts, and still others form in
regions of widespread continental extension such as the Basin
and Range Province of the southwestern United States.
See also DIVERGENT OR EXTENSIONAL BOUNDARIES.
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