The tectonic history of the East Irish Sea Basin with reference to the Morecambe Fields

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Abstract
Regional seismic lines from the East Irish Sea area and a 2D radial seismic survey across the Morecambe Fields have been used to assess fault pattern evolution and the history of tilting in the East Irish Sea Basin. Extension and fault-controlled sedimentation began in Permian times and continued throughout the Permo-Triassic. Later inversion and erosion has removed the post-Triassic cover in the area. The main extension direction during basin evolution was ENE–WSW to NE–SW. Under this extension, NE–SW-trending basin-bounding faults in the north and south were dominated by strike-slip and oblique displacements. Offset of the pre-Permian basement suggests the amount of post-Carboniferous net extension varies from 10–16%. The basin can be divided into two discrete structural domains which controlled the geological evolution of the East Irish Sea Basin. Basin evolution in the northern part of the basin was controlled by easterly dipping faults which induced tilting towards the southwest or west. The southeastern part of the basin is dominated by two faults which dip to the west or southwest, inducing tilting towards the east. The complex structural and tilting history of the Morecambe Fields arises from their position at the boundary of these two domains. Detailed analysis of this area illustrates how the propagation of the Keys Fault (from the north) and the Crosh Vusta Fault (from the south) competed to dominate tilting history. The faults which now bound the Morecambe Fields show only minor influence on Permo-Triassic sedimentation patterns, implying that the main displacement on these faults post-dated the deposition of the Sherwood Sandstone. The development of these boundary faults is thought to be related to the accommodation of arching between the Keys Fault and Tynwald fault system. The differing tilting histories of the North and South Morecambe Fields are indicated by the differing orientations of a previously shared palaeo-hydrocarbon–water contact. This is now preserved as the top platy-illite surface and is interpreted as arising from the development of a tilt-transfer zone during the propagation and linking of the northerly and southerly propagating fault arrays. Analysis of the tilting history of the Morecambe area has allowed the top platy-illite surface to be mapped outside well control.