HR: 0800h
AN: V11A-0371    [Abstracts]
TI: The Significance of Fingers and Lobes in Emplacement of Sill Complexes: Insights From Field Data
AU: * Schofield, N
EM: nxs582@bham.ac.uk
AF: School of Geography, Earth and Enviromental Science, University of Birmingham, Edgbaston, Birmingham, B5 7PQ, United Kingdom
AU: Thomson, K
EM: K.Thomson@bham.ac.uk
AF: School of Geography, Earth and Enviromental Science, University of Birmingham, Edgbaston, Birmingham, B5 7PQ, United Kingdom
AU: Stevenson, C
EM: c.t.stevenson@bham.ac.uk
AF: School of Geography, Earth and Enviromental Science, University of Birmingham, Edgbaston, Birmingham, B5 7PQ, United Kingdom
AU: Hutton, D
EM: d.h.hutton@bham.ac.uk
AF: School of Geography, Earth and Enviromental Science, University of Birmingham, Edgbaston, Birmingham, B5 7PQ, United Kingdom
AB: The understanding of the emplacement of sill complexes has developed greatly of the last decade from insights gained from 3D seismic data. Particularly, the identification of structures, such as units and lobes in the order of 100s m - km scale, which make up individual sills (e.g. Thomson & Hutton, 2004, Bull Volcanol, 66, 364–375). However the formation and significance of these structures in terms of the emplacement of sill complexes is only partially understood. We present field data from planar-segmented sills on the Isle of Skye, Scotland and saucer- shaped sills in the Karoo Basin, S Africa. In both of these areas constituent lobes can be confirmed but we have also observed smaller scale finger-like structures in the order of 10s – 100s m, which appear to make up these lobes. In planar-segmented sills (Skye) fingers are aligned along the axis of the lobes and have a NNW trend, consistent with a structurally controlled emplacement direction. In saucer-shaped sills (Karoo) with no obvious structural control, fingers are oriented radially about lobes, radiating outwards from the centre of the saucer. Fingers initially propagate separately, thickening vertically and laterally and coalescing to form lobes. Once the fingers have coalesced, the sill continues to thicken vertically. Finger structures clearly have a fundamental role in the initial propagation of sills. Detailed examination of fingers of the Golden Valley Sill in the Karoo Basin shows that there is a regular and predictable spacing between the crests and troughs between individual fingers. This spacing is consistent within individual lobes but varies from lobe to lobe. This implies that sills are constructed in a stepwise fashion and that models which assume one discrete intrusion event may be an oversimplification. The regular spacing of fingers also leads us to suspect that their wavelength may provide information about the propagation of the sill. It is proposed that the wavelength of the fingers of an individual lobe may be a function of the velocity and viscosity of the magma. According to our theory, shorter wavelength (narrow) fingers are produced by relatively low viscosity and higher velocity magmas, indicating that the sill front propagated relatively quickly. Longer wavelength (thicker) fingers result from higher viscosity, lower velocity magma, indicating that the sill propagated relatively slowly.
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8169 Sedimentary basin processes
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting