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Emplacement mechanism of the Great Whin and Midland Valley dolerite sills

Goulty, N.R.

Authors

N.R. Goulty



Abstract

The Great Whin and Midland Valley dolerite sill complexes appear to have been emplaced laterally from the walls of feeder dykes. The overall thickness of each sill increases with depth, as would be expected if the magma finally reached a state of hydrostatic equilibrium. Variations in the thickness of the sills with estimated intrusion depth imply that the head of magma was about 100 m below the contemporary ground surface in the areas of the present-day outcrops at the end of the intrusive episodes. Before hydrostatic equilibrium was established, the magma pressure would probably have been somewhat greater, so it is likely that intrusion of the sills was accompanied by the extrusion of flood basalts. Step-and-stair transgressions of the bedding are commonly found within the sills, mostly stepping downwards in the direction of bedding dip. The reason for this directionality is that the weight of sediments floating on an intruding sill has a downdip component that applies a tensile stress to intersecting fractures below the sill when magma is moving downdip, and to intersecting fractures above the sill when magma is moving updip.

Citation

Goulty, N. (2005). Emplacement mechanism of the Great Whin and Midland Valley dolerite sills. Journal of the Geological Society, 162(6), 1047-1056. https://doi.org/10.1144/0016-764904-141

Journal Article Type Article
Publication Date 2005-11
Deposit Date Jan 13, 2009
Journal Journal of the Geological Society
Print ISSN 0016-7649
Publisher The Geological Society
Peer Reviewed Peer Reviewed
Volume 162
Issue 6
Pages 1047-1056
DOI https://doi.org/10.1144/0016-764904-141