Shallow magmatic plumbing systems and edifices of monogenetic volcanic fields

Magmatic plumbing systems of monogenetic volcanic fields comprise an interconnected network of sills, dykes and inclined sheets; however, the morphology, distribution and emplacement mechanisms of these shallow plumbing systems are often overlooked due to the lack of observable field data. Using seismic data provides an opportunity to understand these systems and integrate seismic-scale features with macro-scale observations from the field.

Using a seismic dataset from the Faroe-Shetland Basin, NW Scotland, the Ben Nevis Monogenetic Volcanic Field (BNVF), this research provides an insight into how monogenetic volcanic fields are fed, and how the distribution of edifices can be primarily influenced by the structure of the substrate.

By comparing the seismic data with field observations from three separate field studies, this research focuses in on the shallow, ductile zone (<1 km) between the feeding intrusion(s) and the volcanic edifice, in order to determine the processes that occur during emplacement at a sub-seismic scale. This study provides a comprehensive assessment of the features of very shallow plumbing systems (<200 m) within a variety of water-saturated settings, including: passive and dynamic peperite (blocky and fluidal); R-T structures; clastogenic dykes; gas cavities; slurry cavities; protopillows and subsurface pillows; volcaniclastic injectites; and magma lobes and fingers.

The range of features displayed within the four case studies demonstrates that a variety of processes affect the emplacement of magma in the ductile zone. These processes include: (1) exploiting preferential horizons by host-rock fluidisation; (2) exploiting boundaries in the host-rock by the rotation of principal stresses; (3) explosive magmatic degassing; (4) cooling-contraction fragmentation; (5) explosions caused by rapid vapourisation of porewater; (6) a variety of molten fuel-coolant interactions (non-explosive to highly explosive); and (7) density contrasts.

Monogenetic volcanic fields and their plumbing systems have been identified in reflective seismic data in exploratory offshore sedimentary basins (e.g. Southern Australia Margin and the NE Atlantic Margin). This research presents unprecedented quantitative analysis of the thermal influence of diatremes using a range of techniques (vitrinite reflectance analysis, Raman spectroscopy and fluid inclusion techniques) to produce local isothermal models of the host-rock surrounding diatremes. This aims to provide a better understanding of the overall thermal effect of monogenetic plumbing systems on reservoir porosity and permeability, and source maturity.