Sandbox experiments of inverted listric and planar fault systems

Abstract

The kinematic and geometric evolution of basement-controlled inversion structures has been analyzed using a plane strain sandbox apparatus. The results of two representative scaled analogue models comprising rigid listric and planar detachment faults that both have cut-off angles of 60° to the horizontal, are described in detail. The hangingwall above these detachment geometries was constructed from alternate layers of sand and mica, in order to simulate an anisotropic sedimentary sequence. In both experiments, initial shortening at the onset of inversion was accommodated by bulk strain. This was followed by reactivation of the basal detachment together with the main basin controlling planar and listric faults which propagate upwards through the post-rift sequence at approximately the same angle as the upper part of the detachment fault (i.e. 60°). Reactivation of relatively steep, intra-basinal extensional faults within the cover was restricted to nucleation of reserve faults and thrusts in the region of the tips during approximately the first 10 to 15% contraction. This resulted in gently dipping thrusts that propagated upwards through the post-rift sequence to produce “harpoon” shaped structures. Later in the inversion phase, after approximately 15% contraction, backthrusts cut through and displaced the original antithetic extensional graben faults, producing downward tapering wedge-shaped structures at the margins of the basin. Although deformation in these experiments was limited to purely dip-slip displacement, the nature of the original extensional faults and their reactivated tip regions resulted in cross-sectional geometries that have a marked resemblance to strike-slip induced flower structures. Uplift was asymmetric and centred above the upper part of the main detachment above the listric fault but was more symmetrical and centred above the soling-out point of the detachment in the case of the planar fault geometry. The results of these experiments show distinct similarities to structures seen on seismic sections form the southern North Sea and other inverted sedimentary basins.