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1 Chevron UK Ltd, 2 Portman Street, London, W1H 0AN, UK
2 Department of Geology, Royal Holloway and Bedford New College, University of London, Egham, Surrey, TW20 0EX, UK
The Palaeogene sequence of the Outer Moray Firth in the UK sector of the North Sea consists of a series of stacked submarine-fans and associated shelf deltaic deposits. The sequence was derived from the elevated Shetland Platform to the northwest. The clastic material was transported southeastwards along the axis of the Witch Ground Graben towards the basin low in the Central Graben.
Towards the top of the Palaeogene submarine-fan slope deposits, there is a disturbed sequence, Oligocene in age, which is interpreted to have been deformed by Miocene age extensional faulting. A semi-regional study of Quadrants 15, 16, 21 and 22 has revealed that the faults, which cut the Eocene to Lower Miocene section, terminate at the Middle Miocene unconformity. Fault activity coincided with a Middle Miocene tilting event indicated by the onlap of Upper Miocene sequences higher on the shelf slope. This tilting event acted as a trigger for extensional fault movement.
Detailed study of the extensional fault geometries illustrates the complex nature of the deformation. The dominant fault trend is northeast-southwest, commonly with the downthrown side towards the northwest. The faults do not exhibit a marked listric geometry nor do they have a common detachment horizon. A common feature is the decrease in observed brittle deformation with depth.
Analogue sandbox modelling illustrates the development of extensional faults associated with the gravitational collapse of a tilted sequence. Deformation within the sandbox models is dominated by non-rigid block rotation.
The Miocene-aged faulting in the Outer Moray Firth is interpreted to be a result of the gravitational collapse of the Palaeogene slope sequence. Fault dip towards the shelf was probably controlled by shear stresses within the deforming sequence and a downslope resistance to deformation. A non-rigid block rotation model is proposed as a possible mechanism for the deformation. A basal detachment is not a requirement of this model.
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