HR: 0800h
AN: V11A-0360    [Abstracts]
TI: Magnetic fabric of saucer-shaped sills in the Karoo Large Igneous Province
AU: * Polteau, S
EM: polteau@fys.uio.no
AF: Physics of Geological Processes - PGP, University of Oslo POBOX1048 - Blindern, Oslo, 0316, Norway
AU: Ferre, E C
EM: eferre@geo.siu.edu
AF: Department of Geology, Southern Illinois University, Carbondale, IL 62901-4324, United States
AU: Planke, S
EM: planke@vbpr.no
AF: VBPR, Forskningparken, Gaustadalléen 21, Oslo, 0349, Norway
AU: Neumann, E
EM: e.r.neumann@geo.uio.no
AF: Physics of Geological Processes - PGP, University of Oslo POBOX1048 - Blindern, Oslo, 0316, Norway
AU: Chevallier, L
EM: lchevallier@geoscience.org.za
AF: Council for Geoscience, PB BOX 572, Bellville, 7535, South Africa
AB: Magmatic sill intrusions commonly exhibit a saucer geometry in undeformed sedimentary basins and volcanic rifted margins. Current emplacement models are based on the analysis of the intrusion geometry and their spatial relationships with potential feeders, not on the knowledge of the magma flow geometry. The Karoo Basin of South Africa hosts hundreds of saucer-shaped sills. Amongst these, the Golden Valley Sill is well-exposed and displays the connections with adjacent and nested saucers. A combination of detailed fieldwork observations and the anisotropy of magnetic susceptibility measurements were used to identify strain markers that can be interpreted in terms of magma flow directions. A total of 113 localities (6 specimens/site), mostly including opposite sill margins, have been sampled for anisotropy of magnetic susceptibility (AMS) analyses. The magnetic properties were defined by measuring hysteresis cycles and K-T curves on 34 and 19 specimens, respectively. The majority of the localities display well-defined magnetic foliations that consistently dip outward from the centre of the Golden Valley Sill. This orientation of the magnetic foliation most likely represents inflation/deflation cycles of the intruding sill that interacts with non-static enclosing walls. In addition, four magma channels were identified and display an imbrication of the magnetic foliation that indicates an outward magma flow direction. In conclusion, the observed magma flow geometries derived from macroscopic flow indicators and the AMS data correlate well and are used to constrain an emplacement model for the Golden Valley Sill Complex. Finally, the emplacement model of sill complexes repeats the cycle -injection of magma - formation of a saucer-shaped sill - pressure build up - fracturation and pressure drop - channeling of magma - injection of (new batch of) magma - formation of a new saucer-shaped sill- until the magma supply stops.
UR: http://folk.uio.no/polteau
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting