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Determining magma flow in sills, dykes and laccoliths and their implications for sill emplacement mechanisms

Abstract

Three-dimensional seismic data from the Faeroe-Shetland Basin provides detailed information on the relationships between sills, dykes, laccoliths and contemporaneous volcanic activity. The data shows that sills are predominantly concave upwards, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Such morphologies are only partially modified by pre-existing faults. Sills can be sourced from dykes or the steep climbing portions of deeper sills. Both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to volcanic fissures, other sills and laccoliths, the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggests that the presence of lithological contrasts, particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level.

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Acknowledgements

Thanks to Conoco-Philips for support of this project through the release of seismic data and permission to publish the results. Thanks also to my colleagues in Birmingham for their help and advice during this project. The constructive reviews from Richard Davies and Thierry Menand are also acknowledged.

Author information

Correspondence to Ken Thomson.

Additional information

Ken Thomson–deceased, April 2007

Editorial responsibility: M. Ripepe

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Thomson, K. Determining magma flow in sills, dykes and laccoliths and their implications for sill emplacement mechanisms. Bull Volcanol 70, 183–201 (2007). https://doi.org/10.1007/s00445-007-0131-8

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Keywords

  • Sills
  • Dykes
  • Laccoliths
  • Magma flow patterns
  • Sill emplacement models
  • Fissure eruptions
  • Submarine volcanism
  • Three-dimensional seismic
  • Faroe-Shetland Basin
  • Volcanic continental margins