Abstract
The style of deformation of rocks from basin-infilling sequences in positively inverted natural basins was discussed upon the results of laboratory experiments carried out in sandboxes with sand packs laid down in the space between two wooden blocks. The space simulated stages of crustal extension leading to (1) a half graben due to extension above a listric extensional detachment, with the blocks simulating the footwall and hanging wall, or (2) a graben, with the blocks simulating the external margins that drifted apart above a horizontal detachment. Combinations of two different angles were used to simulate the dip of curved normal faults along the internal face of the wooden blocks. Backstops in the half graben had a convex up internal face. Backstops in the graben had a concave up internal face. Shortening as partitioned in forward and backward movements within the sand packs, and the kinematics of contraction was largely influenced by the convex or concave internal faces. A buttress effect characterized by rotation of the sand pack close to the footwall was stronger for footwall with steeper-dipping internal faces. The results were compared to other physical experiments and applied to an inverted basin found in nature.
Highlights
The geometry and tectonic features of inverted basins have been described since the beginning of the century
The progressive extension in experiments with convex-up backstop and dip angles of 60◦ resulted in a sequence of structures that compares well with the typical sequence observed in physical experiments of extension (McClay 1990)
Our experiments demonstrated that the backstop geometry exerts a strong control on the normal detachment reactivation
Summary
The geometry and tectonic features of inverted basins have been described since the beginning of the century. The formal definition of ‘inversion’ was only presented in 1981 by Glennie and Boegner (1981) According to these authors, ‘inversion’ represents the reversal of the regional stress regime, from extension to contraction, or the opposite. Inversion in orogenic belts involves deformation of continental margin prisms and is normally characterized by thin- and thick-skinned thrust sheets and nappes. This inversion style produces major crustal shortening and was discussed by several authors dealing with foreland fold-thrust belts such as the Appalachians, the Rocky Mountains, the Alps and the Himalayas (respectively in Mitra 1988, de Graciansky et al 1989, Hayward and Graham 1989, Jadoon et al 1994)
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