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Palinspastic Reconstruction and Forward Modelling |
Department of Geology, The University of Alabama, Tuscaloosa, AL 35487-0338, USA
The simultaneous area balance of multiple horizons in a graben system formed above a detachment is the basis for the lost-area section construction and validation technique. Area balance requires that the area displaced below the original regional level of each bed in a graben (the lost area) be balanced by the displaced area at the boundary of the system, given by the product of the displacement times the depth to detachment. For multiple horizons, this relationship is a straight line on a plot of lost area versus depth. The slope of the line is the displacement and the depth intercept is the depth of the detachment. In general, beds within the graben system undergo both visible and homogeneous layer-parallel extension. The homogeneous component can be calculated from the width of the graben system, the lost area, and the depth to detachment. Conversely, the detachment depth may be calculated from the total layer-parallel extension and the lost area. The technique is illustrated by application to the Rhine graben. The Rhine graben is inferred to have formed with its detachment at the base of a normal thickness (31 km) of crust and to have been uplifted. Crustal necking with a detachment at 15 km depth is a viable alternative but requires three times the layer-parallel extension of 6.3% measured at the top of the Permian and older basement. The large difference between the layer-parallel extension (6.3%) and the crustal extension (29%) is in close agreement with an area-balanced full-graben model having boundaries that slope at 60° toward the centrer of the graben.
