A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure. The width and depth of the foreland basin is determined by the flexural rigidity of the underlying lithosphere, and the characteristics of the mountain belt. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that thin away from the mountain belt. Foreland basins represent an endmember basin type, the other being rift basins. Space for sediments (accommodation space) is provided by loading and downflexure to form foreland basins, in contrast to rift basins, where accommodation space is generated by lithospheric extension. Types of foreland basin Foreland basins can be divided into two categories: Peripheral (Pro) foreland basins, which occur on the plate that is subducted or underthrust during plate collision (i.e. the outer arc of the orogen) Examples include the North Alpine Foreland Basin of Europe, or the Ganges Basin of Asia Retroarc (Retro) foreland basins, which occur on the plate that overrides during plate convergence or collision (i.e. situated behind the magmatic arc that is linked with the subduction of oceanic lithosphere) Examples include the Andean basins, or Late Mesozoic to Cenozoic Rocky Mountain Basins of North America Foreland basin system DeCelles & Giles (1996) provide a thorough definition of the foreland basin system. Foreland basin systems comprise three characteristic properties: An elongate region of potential sediment accommodation that forms on continental crust between a contractional orogenic belt and the adjacent craton, mainly in response to geodynamic processes related to subduction and the resulting peripheral or retroarc fold-thrust belt; It consists of four discrete depozones, referred to as the wedge-top, foredeep, forebulge and back-bulge depozones (depositional zones) – which of these depozones a sediment particle occupies depends on its location at the time of deposition, rather than its ultimate geometric relationship with the thrust belt; The longitudinal dimension of the foreland basin system is roughly equal to the length of the fold-thrust belt, and does not include sediment that spills into remnant ocean basins or continental rifts (impactogens). Foreland basin systems: depozones The wedge-top sits on top of the moving thrust sheets and contains all the sediments charging from the active tectonic thrust