The role of sill intrusion in delimiting the lateral propagation of giant dyke swarms: emplacement of the Dogger Sill Complex, Southern North Sea

This study addresses the question of which factors control the lateral arrest of giant dykes. We present new seismic mapping of a large sill complex associated with the distal portion of the Mull Dyke Swarm in the Southern North Sea, which we name the Dogger Sill Complex. Fifteen key calibration wells show that dolerite sills ranging from 20 to 98 m thick intrude into potash-rich units of the Z3 cycle of the Zechstein Group. We mapped six main sills intruded within 5 km of three of the longest dykes belonging to the Mull Dyke Swarm, suggesting that these dykes fed the sills. The sills have a combined area of 429 km 2 and a median gross volume of c. 22.7 km 3 . Their concordant intrusion into the potash-rich Z3K unit implies that the physical properties of this unit exerted a primary control on sill emplacement. Intrusion of the sill complex is argued to have impacted the propagation of the dykes and contributed to their arrest only 50 km beyond the limit of the sills via the loss of the driving magma pressure. The intrusion of a large sill complex so close to the terminus of a giant dyke swarm may be a more widely developed mechanism for lateral dyke arrest than currently appreciated.