Mechanics of basin inversion from worldwide examples

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Abstract
Inversion is dependent on pre-existing basin configuration in the initial subsidence, usually extensional phase, and the resolution of compressional forces in the later shortening phase. External horizontal, rather than isostatic vertical forces are required for inversion because many deep sediment-filled basins around the world have never been inverted, but should have been, if isostatic rebound were the driving mechanism. Inversion can range in scale from basins to sub-basins to selected structures within basins. Typically, rift basins can be later inverted. Mainly by reactivation of older normal faults, inversion selects rift basins where, in pure shear, weakening because of necking or thinning of lithosphere has occurred; and where, in simple shear, mechanical detachment surfaces are available for subsequent movement. Some pre-existing lows or sags can apparently be inverted in the absence of reactivated normal faults, as in the southern Altiplano of Bolivia and offshore Sabah, Borneo. Basins can be inverted by dominantly strike-slip with some convergent component, e.g. offshore northeast Brazil, and by almost direct compression, Atlas Mountains, Morocco. A prime difference is that convergent strike-slip can reactivate relatively high-angle normal faults as reverse faults, whereas these faults tend to lock when subjected to more direct compression which then creates lower angle contractional faults. Usually, inversion is caused by a combination of compression and strike-slip (transpression) because the azimuth of maximum principal compressional stress to the direction of original basin trend vectors into an oblique-slip component. This is illustrated in northern Argentina where compression derived from Andean deformation is resolved obliquely (about 45°) against an older rift basin in the foreland nearby. Interestingly, for this area at least, the 45° angle which affords an equal contribution of strike slip and compression results only in reactivation of older normal faults and does not create younger contractional features. Apparently compressive forces can be transmitted backward from the lead edge of an underthrusted foreland plate to invert rather remote regions and basins that are carried on that plate; this may be a significant part of the mechanism for basin inversion in northwest Europe and the southern North Sea as that foreland region underthrusted along the Alpine system.