Senin, 20 Juni 2011

DEFINITION OF FORELAND BASIN

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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