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Models of Extensional Basins
Geometric, Thermal and Isostatic Consequences of Detachments in Continental Lithosphere Extension and Basin Formation
Many intracratonic basins exhibit a complicated history of both multiple extensional and compressional events, with the consequence that basins form on basins (i.e. successor basins). Seismic reflection imaging has confirmed that many extensional basins form on the collapsed hanging wall of normally reactivated low angle faults that sole into an intra-crustal or base-crustal detachment. The detachments may be an inherited collisional structure, such as the basal decollement of the European Variscides, or theologically produced by a compositional layering of the lithosphere. Although fault reactivation implies a simple shear process, existing models for crustal extension are based on extension by pure shear. We therefore investigate the importance of detachments in controlling extensional sedimentary basin formation using a coupled simple shear/pure shear model of continental extension.
The models assume that lithosphere extension is achieved by simple shear along low angle faults in the upper lithosphere, giving way with depth to pure shear below a level of horizontal detachment. Basin formation consists of three distinct but related components: 1) the geometric response of the lithosphere giving rise to crustal thinning by simple and pure shear; 2) the perturbation of the lithosphere temperature field during extension and its subsequent re-equilibration; and 3) the isostatic response of the lithosphere during the rift and post-rift stages of basin formation. The main consequence of the use of detachments during lithosphere extension is to allow the lateral separation of the rift and thermal subsidence basins. Although the total amount of lithosphere extension, detachment depth, and the relative position of the pure shear deformation with respect to the simple shear are major factors in controlling the basin geometry, fault shape is relatively unimportant.
The commonly assumed Airy isostatic condition during rifting leads to: 1) unrealistic deformation of the low angle fault and Moho, and locking of the fault zone; and 2) exaggerated thermal uplifts within the flanks of the rift sub-basin. As a consequence, significant lithosphere flexural rigidity must exist during the rifting phase of sedimentary basin formation, as well as during the post-rift thermal subsidence phase. By analogy with basin formation, the generation of Moho topography should also consist of the same two processes - a geometric-mechanical, and a restoring isostatic process. In general, assuming significant lithosphere flexural rigidity during rifting, isostatically produced Moho topography is minor relative to geometrically (mechanically) produced topography.
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