Balancing sections through inverted basins

Abstract

Abstract Basin inversion and the reactivation of earlier normal faults is now recognized as being widespread in many mountain belts. By ignoring the effects of basin inversion, serious errors in section construction and structural and tectonic interpretations can be made. Folds produced during thick-skinned basement-involved faulting are generally very different from those produced during thin-skinned thrust tectonics and hence different models must be used for section construction and restoration. In particular, block rotations are important. Rotation about a horizontal axis may lead to shortening of the graben or half-graben, expulsion of material from out of the graben and/or reactivation of the original normal fault. The resultant hanging wall folds are characterized by long gently dipping backlimbs and short hooked forelimbs. Various models for fold development are derived and compared for different inversion geometries and kinematics. Examples are taken from the North Sea. Many inverted faults involve components of strike-slip or oblique-slip movement, and hence simple 2D methods of section balancing cannot be applied. Basin inversion may have involved rotation of the fault blocks about vertical axes and/or lateral expulsion of material from the basin. Examples are described from South Wales, Syria and the French Alps. Vertical axis rotation appears a common mechanism for intraplate deformation, where the regional shear couple is applied at an angle to earlier fault blocks or crustal lineaments. The analysis of paleomagnetic data together with kinematic data enable the rotation to be determined and amounts of shortening strain estimated.