Strike-slip Basins and Inversion: a possible model for the Southern North Sea Gas Areas

Abstract

Summary This paper examines the structural style of strike-slip zones and of the associated inverted basins and presents a tectonic model for their evolution as part of a linked system. The structure of strike-slip zones may be modelled geometrically by turning an extensional or compressional dip-slip system on its side. The duplex geometries (Dahlstrom 1969a) become braided wrench zones and the transfer components (Gibbs 1984) become linking ‘flats’ with dip-slip displacements between steep strike-slip belts. The stratigraphic implications of this model are not discussed in detail. The model developed in this paper suggests that in an area such as the Southern North Sea Gas Area, the pattern of inversions at a regional scale can be related to major crustally linked systems of steep faults which ride at different levels on flat detachments. Where the detachments step up, or down the over-lying block or leaf will invert. At a more local scale individual structures may invert by progressive rotation of individual faults or fault bounded blocks. Simple shear geometries of both extension and shortening above a relatively simple basement shear zone can be modelled by reference to the strain ellipse. The rotation of strained elements gives rise to the classic ‘flower structure’ seen in cross-section. The Gas Area Basin is a particularly good area to study the effects of rotational simple shear because of the low strength salt decoupling cover from basement and for studying the long range regional inversions across crustal shear zones and the postulated detachments on long regional seismic lines. The data presented in this paper are intended to illustrate the variety of structural styles produced by progressive evolution of structure on a linked system of strike-slip and detachment faults.