Wedge-shaped sedimentary basins that form
on the continent-ward side of fold-thrust belts, filling the topographic
depression created by the weight of the mountain belt.
Most foreland basins have asymmetric, broadly wedge-shaped
profiles with the deeper side toward the mountain range, and a
flexural bulge developed about 90 miles (150 km) from the
deformation front. The Indo-Gangetic plain on the south side
of the Himalayan Mountains is an example of an active foreland
basin, whereas some ancient examples include the Cretaceous
Canadian Rockies Alberta foreland basin, the Cenozoic
Flysch basins of the Alps, and the Ordovician and Devonian
clastic wedges in the Appalachian foreland basins. Foreland
basins are characterized by asymmetric subsidence, with
greater amounts near the thrust front. Typical amounts of
sudsidence fall in the range of about 0.6 miles (1 km) every
2–5 million years.
Deformation such as folding, thrust faulting, and repetition
of stratigraphic units may affect foreland basins near the
transition to the mountain front. These types of foreland
basins appear to have formed largely by the flexure of the
lithosphere under the weight of the mountain range, with the
space created by the flexure filled in by sediments eroded
from the uplifted mountains. Sedimentary facies typically
grade from fluvial/alluvial systems near the mountains to
shallow marine clastic environments farther away from the
mountains, with typical deposition of flysch sequences by turbidity
currents. These deposits may be succeeded laterally by
distal black shales, then shallow water carbonates over a
cross-strike distance of several hundred kilometers. There is
also often a progressive zonation of structural features across
the foreland basin, with contractional deformation (folds and
faults) affecting the region near the mountain front, and normal
faulting affecting the area on the flexural bulge a few
tens to hundreds of kilometers from the deformation front.
Sedimentary facies and structural zones all may migrate
toward the continent in collisional foreland basins.
A second variety of foreland basins is found on the continent-
ward side of non-collisional mountain belts such as the
Andes, and these are sometimes referred to as retroarc foreland
basins. They differ from the collisional foreland basins
described above in that the mountain ranges are not advancing
on the foreland, and the basin subsidence is a response to
the weight of the mountains, added primarily by magmatism.
Another variety of foreland basins is known as extensional
foreland basins and include features such as impactogens
and aulacogens, which are extensional basins that form
at high angles to the mountain front. Impactogens form during
the convergence, whereas aulacogens are reactivated rifts
that formed during earlier ocean opening. Many of these
basins have earlier structural histories, including formation as
a rift at a high angle to an ocean margin. These rifts are naturally
oriented at high angles to the mountain ranges when the
oceans close and become sites of enhanced subsidence, sedimentation,
and locally additional extension. The Rhine
graben in front of the Alpine collision of Europe is a wellknown
example of an aulacogen.
See also CONVERGENT PLATE MARGIN PROCESSES; PLATE
TECTONICS.
foreshock See EARTHQUAKES
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