HR: 0830h
AN: GP11D-0288    [PDF]
TI: Magnetic versus Crystallographic Fabrics in Basaltic Lava Flows
AU: Bascou, J
EM: bascou@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique, CNRS and ISTEEM, Universit\'{e} Montpellier 2, cc049,Pl. E. Bataillon, Montpellier, 34095 France
AU: Camps, P C
EM: camps@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique, CNRS and ISTEEM, Universit\'{e} Montpellier 2, cc049,Pl. E. Bataillon, Montpellier, 34095 France
AU: * Plenier, G
EM: plenier@cerege.fr
AF: CEREGE, Universit\'{e} Aix Marseille III, BP 80, Europ\^{o}le m\'{e}diterran\'{e}en de l'Arbois, Aix en Provence, 13545 France
AU: Dautria, J
EM: dautria@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique, CNRS and ISTEEM, Universit\'{e} Montpellier 2, cc049,Pl. E. Bataillon, Montpellier, 34095 France
AB: Indicators of flow direction and sense in lava flows are often difficult to observe. To overcome this difficulty, anisotropy of magnetic susceptibility (AMS) can be used. However, a major issue is to know how consistently the axes of magnetic susceptibility (K1, K2 and K3) correlate with the flow direction.\\ We carried out a systematically sampling of the base, the middle part and the top of lava flow for which the flowing directions are well known. These quaternary flows, located in the area of P\'{e}zenas (southern France), are pluri - kilometric long, between 2 and 10 m thick, rather narrow ($<$ 500 m) and characterized by a weak slope ($<$10$^\circ$). Oriented cores offer an opportunity to investigate the relationships between flow direction, principal susceptibilities and crystallographic preferred orientations (shape and lattice orientations) of rock-forming minerals.\\ In thin-section, the opaque grains observed in reflected light are abundant (about 5 percent) and the largest in size (15-20 $\mu$m) are sub-automorphous. Microsonde analyses and thermomagnetic curves measured in the range 80 - 900 K indicate that titano magnetite ($x = 0.6$) is the dominant oxide mineral. FORC diagrams reveal that the magnetic grain sizes are both PSD-MD and PSD-SD assemblages. AMS measurements from 180 specimens reveal a tight clustering of the K3 axes close to the vertical and a weaker degree of clustering of K1, K2 axes. A significant enhancing of the magnetic fabric is observed after demagnetization by thermal treatment. Lattice Preferred Orientation (LPO) measurements of titano magnetite and plagioclase were performed using the electron backscattered diffraction (EBSD) technique. The highest fabric strength is observed from plagioclase measurements and the LPO of this mineral are correlated to the flow dynamic. For the samples collected from near the flow base a good correlation is observed between the AMS ellipsoid axes and the LPO of plagioclase: the K1 axis is close to the maximum concentration of the [001] axes, the K2 axis is close to the maximum concentration of [100] axes and the K3 axis is close to the preferential orientation of the (010) poles. The angle between the K3 axes and the vertical plane define a significant flow related imbrication of the magnetic foliation that can be used to infer the flow direction. Determination of relationships between the AMS axes and the flow related LPO of the plagioclase may represent a valuable constraints to interpret the ASM signature in lava flows.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting