Inversion of the Broad Fourteens Basin (offshore Netherlands), a small-scale model investigation

Abstract

The southern North Sea exhibits spectacular examples of graben inversion which have been extensively documented by oil industry exploration. The Broad Fourteens Basin, which is located at the southern limit of the Southern Permian Salt Basin, displays numerous structural patterns of positive inversion, which are related to the inversion, which are related to the presence and thickness of Zechstein salts. In the northern part of the Broad Fourteens Basin, the Zechstein salt is rather thick (> 500 m), allowing decollement of the Mesozoic series during both extension and compression. Salt is missing to the southern part of this graben, and both Mesozoic series and Palaeozoic ‘basement’ display similar patterns of deformatiqn. The mechanical significance of inversion patterns was investigated through laboratory experiments on brittleductile models. Models were built with sand and silicone putty that represent, respectively, the frictional behaviour of the Mesozoic cover as well as the Palaeozoic basement, and the viscous behaviour of a decollement layer, mainly composed of Zechstein salts, which is intercalated between them. The models were scaled to fit natural configurations observed in the Broad Fourteens Basin. All small-scale experiments were carried out in two steps: (1) graben formation with synkinematic sedimentation, and (2) compression at various angles of obliquity to the graben. Experimental results show that the structures generated by or reactivated during inversion are strongly dependent on: (a) the strength of the decollement layer, which is a function of the layer thickness and of the displacement velocity applied at the model boundaries; and (b) the strength of the sedimentary overburden, which is a function only of its thickness (for a given lithology). Reactivation of normal faults occurs when the angle between the compression and the direction of the normal faults is smaller than 45. The ductile layer, which can be a source for diapirs during extension, plays during compression an important rô1e in the thrusting of the graben fill onto the adjacent platforms.