HR: 0800h
AN: V21D-0645    [Abstracts]
TI: Emplacement of Saucer-Shaped Sills and Long Dykes: Constrains from Detailed Field Work and AMS Analyses in the Karoo Basin, South Africa.
AU: * Polteau, S
EM: polteau@fys.uio.no
AF: Physics of Geological Processes, PO-BOX 1048 - Blindern, Oslo, 0316 Norway
AU: Planke, S
EM: planke@vbpr.no
AF: VBPR, Forskningsparken, Gaustadalleen 21, Oslo, 0349 Norway
AU: Neumann, E
EM: e.r.neumann@geo.uio.no
AF: Physics of Geological Processes, PO-BOX 1048 - Blindern, Oslo, 0316 Norway
AU: Galerne, C
EM: chrisgal@fys.uio.no
AF: Physics of Geological Processes, PO-BOX 1048 - Blindern, Oslo, 0316 Norway
AU: Malthe-Sorenssen, A
EM: malthe@fys.uio.no
AF: Physics of Geological Processes, PO-BOX 1048 - Blindern, Oslo, 0316 Norway
AU: Podladtchikov, Y
EM: y.y.podladchikov@fys.uio.no
AF: Physics of Geological Processes, PO-BOX 1048 - Blindern, Oslo, 0316 Norway
AU: Svensen, H
EM: svensen@fys.uio.no
AF: Physics of Geological Processes, PO-BOX 1048 - Blindern, Oslo, 0316 Norway
AB: The plumbing system of large igneous provinces (LIPs) consists largely of sheet-like intrusions that are vertical (dykes) and sub-horizontal (sills). Detailed seismic mapping in the NE Atlantic and field work in the Karoo basin, South Africa, show that the magma tends to develop into interconnected 3D networks of saucer-shaped sills in undeformed basin provinces. Minor dykes, up to 1 m thick and 200 m long, may originate from the sills whereas major dykes are not involved in the formation of the saucer-shaped sill complexes. We have recently initiated a major integrated field, laboratory and theoretical study of magma emplacement processes in sedimentary basins. The remarkably well-exposed Golden Valley Sill Complex and two long (> 100 km) dyke systems in the central Karoo basin have been selected for detailed field, geochemistry, and anisotropy of magnetic susceptibility (AMS) studies. The Golden Valley Sill Complex contains four individual saucers, each having a horizontal inner sill, transgressive inclined sheets and sub-horizontal outer sills. Macroscopic magma flow indicators include ropy flow structures and tubes. The upper sill surfaces commonly exhibit large-scale undulations. A total of 115 localities have been analyzed for low field AMS to define the ellipsoid of anisotropy. The AMS and field data are interpreted in terms of syn-emplacement (magma flow directions) and post-emplacement (thermal contraction, shearing) processes. The flow indicators support an emplacement model where the saucers are fed from the center and transgress around the edges of the sub-horizontal inner sill. Localized shearing of the magnetic fabric and the undulations suggest a late phase of inward magma flow during cooling and contraction of the saucer. A total of 24 localities from two sets of dykes have been analyzed for AMS. The first set consists of NW-SE trending dykes, between 20 to 100 km long and 15-20 m wide, forming prominent topographic highs. The second set, the ``Gap Dykes'', consists of two parallel E-W trending dykes, each more than 300 km long and 100 m wide, having negative or no topographic expressions. Magma flow indicators are rare, and consist primarily of the upward bending of sedimentary strata in contact with the dykes suggesting an upward magma flow. AMS measurements on both dyke systems reveal vertical magnetic lineations, suggesting that the magma flowed vertically upward during emplacement of the dykes.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005