Foreland Basin Systems Revisited: Variations in Response to Tectonic Settings

Abstract

The four-part districting scheme (wedge-top, foredeep, forebulge, and backbulge depozones) applies to many foreland basin systems worldwide, but significant variations occur in the stratigraphic record. These variations depend on tectonic setting and the nature of the associated fold-thrust belt. Continued growth of the foldthrust belt by horizontal shortening requires foreland lithosphere to migrate toward the fold-thrust belt. The flexural wave set up by the topographic load may migrate � 1000km sideways through the foreland lithosphere, a distance that is comparable to the flexural wavelength. This extreme lateral mobility results in the vertical stacking of foreland basin depozones in the stratigraphic record. The standard stratigraphic succession consists of a several km-thick upward coarsening sequence, marked in its lower part by a zone of intense stratigraphic condensation or a major disconformity (owing to passage of the forebulge), and in its upper part by coarsegrained proximal facies with growth structures (the wedge-top depozone). Foredeep deposits always reside between the forebulge disconformity/condensation zone and wedge-top deposits, and backbulge deposits may be present in the lowermost part of the succession. Wedge-top deposits are vulnerable to erosion because of their high structural elevation, and preservation of backbulge and forebulge deposits depends in part on tectonic setting. Three main types of fold-thrust belt are recognized: retroarc, collisional (or peripheral), and those associated with retreating collisional subduction zones. Retroarc foreland basin systems (such as the modern Andean) are susceptible to far-field dynamic loading transmitted to the foreland lithosphere by viscous coupling between the subducting oceanic slab and the mantle wedge. This longwavelength subsidence adds to subsidence caused by the topographic flexural wave, allowing for preservation of well-developed forebulge and backbulge depozones. The absence of dynamic subsidence in collisional (peripheral) foreland basin systems (such as the modern Himalayan) renders forebulge and backbulge regions vulnerable to erosion and non-preservation. Retreating collisional foreland basin systems (such as those in the Mediterranean region) are often associated with large subducted slab loads, which produce narrow but very thick accumulations in the foredeep and wedge-top depozones. These foreland basin systems are characterized by very thick foredeep and wedge-top deposits, well beyond what would be expected from topographic loading alone. Changing lithospheric stiffness in collisional settings mayaffectpreservationofthebackbulgeandforebulgedepozones.Ifthesedistalforeland basin deposits are not preserved, roughly half the history of the orogenic event (as archived in the stratigraphic record) may be lost. Many foreland stratigraphic successions provide sufficient information to estimate thevelocityofmigrationoftheflexuralwavethroughtheforeland,whichmayinturnbe decomposed into propagation and shortening velocities in the thrust belt. Foreland basin subsidence curves may be inverted to produce an idealized flexural profile, from