Origin and timing of layer-bound radial faulting around North Sea salt stocks: New insights into the evolving stress state around rising diapirs

Abstract

3D seismic data are used to establish the origin and timing of layer-bound arrays of radial faults within a 1500-m-thick sequence of polygonally faulted, Eocene to middle Miocene claystones penetrated by two adjacent salt stocks in the UK Central Graben. Polygonal faults are organised into six laterally extensive tiers that formed during sediment compaction and dewatering during early burial. Dip and amplitude attributes of faulted horizons within the centre of the tiers shows that regional polygonal fault patterns change to radial patterns around the two salt stocks which have radii of c. 1.5 and 2.5 km respectively. Radial faults are confined to the same (120–450 m thick) tiers as those of polygonal faults, indicating that they are also layer bound. The radial-fault pattern in each tier is distinctive and commonly shows a range of fault spacings (154–356 m), maximum throws (7–40 ms), and C-shaped displacement–distribution profiles similar to those of laterally equivalent polygonal faults. These layer-bound radial faults are interpreted as a form of polygonal faults in which strikes have been strongly aligned by a local stress perturbation of salt stocks. Timing constraints indicate that layer-bound arrays of polygonal and radial faults formed in discrete phases during and at the close of sedimentation of each tier. The six tiers formed during the early Eocene, late Eocene, early Oligocene, late Oligocene, early Miocene and middle Miocene. The distance of the polygonal–radial-fault transition from the salt stocks vary from tier to tier but are typically within 800–4000 m of the salt-sediment contact. The position of the transition boundary is used to reconstruct the lateral limit of the salt/sediment-boundary-parallel stress or hoop stress field during key stages in the evolution of the Pierce salt stocks. The position of the radial–polygonal fault transition could be caused by two possible mechanisms: (1) limit of circumferential stretching caused by arching during diapiric rise or (2) lateral limit of hoop stresses produced by radial loading of the overburden from a pressurised salt stock.