HR: 15:45h
AN: GP73A-08    [Abstracts]
TI: Modes of Basaltic Magma Flow and Propagation Through the Crust as Inferred From the Foum Zguid Dyke and its Host Sedimentary Rocks (Southern Morocco)
AU: * Silva, P F
EM: pmfsilva@fc.ul.pt
AF: IDL/CGUL, Edif. C8, Lab. 8.1.70, Campo Grande, Lisboa, 1749-016, Portugal
AU: * Silva, P F
EM: pmfsilva@fc.ul.pt
AF: ISEL/DEC, Rua Conselheiro Emídio Navarro, 1, Lisboa, 1959-007, Portugal
AU: Marques, F O
EM: fomarques@fc.ul.pt
AF: IDL/CGUL, Edif. C8, Lab. 8.1.70, Campo Grande, Lisboa, 1749-016, Portugal
AU: Henry, B
EM: henry@ipgp.jussieu.fr
AF: Paleomagnetism, IPGP and CNRS, 4 Av. de Neptune, Saint-Maur cedex, 94107, France
AU: Hirt, A M
EM: hirt@mag.ig.erdw.ethz.ch
AF: Laboratory of Natural Magnetism, Institute of Geophysics, ETH-Zürich, Zurich, CH-8092, Switzerland
AU: Font, E
EM: font_eric@hotmail.com
AF: IDL/CGUL, Edif. C8, Lab. 8.1.70, Campo Grande, Lisboa, 1749-016, Portugal
AU: Madureira, P
EM: pedro@uevora.pt
AF: Univ. Évora, D. Geociências and C. Geofísica de Évora, R. Romão Ramalho, 59, Évora, 7000-671, Portugal
AU: Lourenço, N
EM: nlourenco@emepc-portugal.org
AF: Centro de Investigação Marinha e Ambiental, Universidade do Algarve, Portugal
AU: Miranda, M
EM: jmmiranda@fc.ul.pt
AF: IDL/CGUL, Edif. C8, Lab. 8.1.70, Campo Grande, Lisboa, 1749-016, Portugal
AB: Dykes are the main feeders of mantle-derived magmas that cross the lithosphere and eventually reach the surface. Therefore, the study of flow in dykes is crucial in our understanding of how magmas migrate from the mantle source to the Earth's surface, through the crust, which we can access and study. This work focuses on magma flow and propagation of the Foum Zguid dyke (NE-SW, sub-vertical) through the upper crust, and how the sedimentary host rocks reacted to the intrusion mechanically and thermally. The dyke intruded an extensive fracture that shows no significant displacement parallel to the fracture surface. The petrofabric of igneous and host sedimentary rocks was determined by measurement of anisotropy of magnetic susceptibility (AMS). These results were complemented by detailed rock magnetic, structural and microscopic (petrography and SEM/EDS) studies. Important variations of the bulk magnetic properties and of the magnetic fabric for the igneous and sedimentary rocks are strictly related to the distance of the samples to the dyke wall. From AMS data of the igneous body it is possible to infer a dominant sub-vertical flow near the margins that changes to one steeper oblique towards NE for domains in the dyke middle. AMS fabric closest to the dyke wall likely records: i) mechanical processes associated with the first stages of opening and propagation of the dyke tip, or ii) later mechanical processes related to thickening of the dyke, with the chilled margins working as host- rocks. The forceful intrusion of a thick dyke induces deformation in the host sedimentary rock in two typical ways: (i) by homogeneous flattening, or (ii) by folding of sedimentary strata. AMS records such mechanical effects on the host rock, with a gradual transition from a bedding-parallel magnetic foliation away from the dyke to a dyke-parallel magnetic foliation close to the contacts (newly formed hematite crystallized under forceful injection). AMS evolution in the host sediment is accompanied by a significant change in the bulk magnetic properties, indicating important chemical and thermal effects related to the intrusive process.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1540 Rock and mineral magnetism
DE: 8414 Eruption mechanisms and flow emplacement
DE: 9305 Africa
DE: 9611 Jurassic
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2009 Joint Assembly