Foreland Basins

Foreland basins are wedge-shaped sedimentary basins that form on the continentward 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 pro files with the deeper side located toward the mountain range, and a flexural bulge developed about 90 miles (150 km) from the foothills of the mountains where the deformation front is located. The indo-Gangetic plain on the south side of the Himalaya 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 ordovi-cian 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 to 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 by 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 miles (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 basin is found on the continentward side of noncollisional 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, known as extensional foreland basins, include features such as impactogens and aulacogens, which are extensional basins that form at wide 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 wide 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 well-known example of an aulacogen.

Continue reading here: Rifts

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