Geophysical evidence for ultrawide dykes of the late Carboniferous quartz-dolerite swarm of northern Britain

Abstract

A compilation of the late Carboniferous quartz-dolerite dyke swarm of northern Britain has been made using existing geological maps, augmented by new ground and airborne magnetic surveys. Aeromagnetic data in the North Sea show that dyke anomalies can be traced eastwards on an arcuate trend for up to 200 km from the UK coast, as far as the western margin of the Central Graben. Individual dykes, which are generally up to 30m wide onshore, attain widths of well over 1 km offshore. These are at least as wide as any known dykes. The swarm is probably as extensive and voluminous as the well-known and better-exposed Palaeocene tholeiitic swarm of NW Britain.

Emplacement mechanism of the Great Whin and Midland Valley dolerite sills

“…It seems probable that flood basalts were extruded in association with intrusion of the sills, if only because it is unlikely that the head of magma would still have been at its maximum value when magma flow had ceased and hydrostatic equilibrium was established. Extrusion would have occurred if the head of magma had been about 100 m greater than its equilibrium value at the end of the intrusive episode, or if the local topographic relief had been around 100 m above the lowest elevation of the ground surface in the area of the dyke swarm (Smythe 1994). Circumstantial evidence in support of the latter suggestion is provided by the presence of tholeiitic basalts with consistent ages in the Central Graben of the North Sea .…”

“…The system of dykes associated with the Great Whin and Midland Valley sills extends 200 km eastwards from the UK coast to the western margin of the Central Graben in the North Sea (Smythe 1994). Approximately 100 km further to the east, within the Central Graben, several wells have penetrated volcanic sequences within the Upper Carboniferous-Lower Permian Lower Rotliegend Group.…”

“…First, it is likely that the hydrostatic head of magma would have been slightly greater during the intrusive episode than it was at the end, when the magma source had become exhausted and hydrostatic equilibrium was established. Second, although it was assumed in the simple analysis above that the topographic surface was flat prior to intrusion, the system of dykes associated with the Great Whin and Midland Valley sills extends at least 200 km east of the present-day coastline (Smythe 1994), reaching the western margin of the Central Graben. If the contemporary ground surface had been around 100 m lower to the east, it could have been covered in flood basalts whose upper surface coincided with the level of hydrostatic head of the magma at the end of the intrusive episode.…”

A new stratigraphic trap for CO2 in the UK North Sea: Appraisal using legacy information

“…These isolated outcrops demonstrate that the proven offshore occurrences in the Central North do not mark the western limit of the Permo-Carboniferous volcanic province and suggest that the regional distribution of volcanic rocks is probably controlled by major Carboniferous rift structures. Onshore in Northern England and Southern Scotland, associated intrusive magmatism is widespread in the form of the Whin Sill, the Midland Valley Sill, and their related suite of basic dykes (Smith, 1992;Smythe, 1994).…”

Carboniferous and Permian magmatism in Scotland

“…Extending over 300 km from Barra (Outer Hebrides) and Kintyre in the west to the east coast of Scotland, it constitutes one of the major Phanerozoic dyke swarms of NW Europe. Offshore to the east, the swarm may continue at least as far as the Central Graben of the North Sea (Smythe 1994). Locally, some dykes are deflected to a NE trend along the Highland Boundary Fault.…”