ABSTRACT: The geometry and kinematics of southern North Sea inversion structures

Elders, Christopher F¹, Kenneth R McClay¹, Graham Baker², Mario Moreno³, Zeina Al-Mussallam¹, James Healing¹, and Fernando Sanchez-Ferrer⁴
(1) Royal Holloway, University of London, Surrey, United Kingdom
(2) Arco British Limited, Guildford, United Kingdom
(3) Sipetrol (UK) Ltd, Woking, United Kingdom
(4) Norske Shell, Tananger, Norway

Late Cretaceous to Tertiary compression caused widespread inversion of Mesozoic extensional structures throughout the southern North Sea. However, a substantial thicknesses of Upper Permian Zechstein salt covers much of the basin and has decoupled both extensional and inversion structures in the pre-salt and post-salt sequences such that it is seldom possible to match fault displacements in these mechanically separate layers.

The style and distribution of inversion is largely controlled by the thickness of the salt layer. Where it is thick inversion is characterised by diapirism and the tightening of graben in the supra-salt layer. Where the salt is thinner it acts as a glide plane for reactivated basin margin faults. Where the salt is absent inverted extensional domino faults in the pre-salt sequence propagate directly into post rift sediments, forming spectacular growth anticlines above their tips.

The development of growth strata in the syn-inversion Upper Cretaceous Chalk allows accurate determination of the timing of inversion and its variation throughout the southern North Sea. Although thick salt tends to dampen the stratigraphic signature of the inversion, in areas where it is thin or absent, it can be shown that inversion proceeded in a series of pulses during the Late Cretaceous and Early Tertiary. Furthermore, where salt is absent 3D seismic data shows that differently oriented extensional faults were inverted at different times, implying a rotation of stresses between the main inversion events, with inactive extensional faults acting to transfer displacement between overlapping inversion structures.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia