Magnetic anisotropy produced by magma flow: theoretical model and experimental data from Ferrar dolerite sills (Antarctica)

Abstract

S U M M A R YVolcanic rocks forming sills, dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement. We model a sill as a steadystate flow of a Bingham fluid, driven by a pressure gradient in a horizontal conduit. The magma velocity as a function of depth is calculated from the motion and constitutive equations. Vorticity and strain rate are determined for a reference system moving with the fluid. The angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain. Magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations. It is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction, and that the magnetic foliation plane changes periodically from horizontal to vertical. The results are compared with the magnetic fabric of Ferrar dolerite sills (Victoria Land, East Antarctica) derived from low-field susceptibility measurements.

Anisotropy of Magnetic Susceptibility (Ams) in Volcanic Formations: Theory and Preliminary Results From Recent Volcanics of Broader Aegean.

“…1b) (Khan 1962, Ellwood 1978. These relationships have been also observed in laboratory materials in which the magma flow was simulated (Wing-Fatt & Stacey 1966, Dragoni et al 1997). …”

“…the flow plane and/or direc tion, of these rocks has been demonstrated in volcanic formations throughout the world (e.g. Dragoni et al 1997, Le Pennée et al 1998, Smith 1998, Varga et al 1998, Archanjo et al 2000, Canon-Tapia & Pinkerton 2000. The method favours great acceptance because: it exhibits high sensitivity, is non time-consuming and can be applied to a variety of formations -lava flows, pyroclastic deposits, ash and tuff flows, ignimbrites, sills and dykes.…”

Rock magnetism and magnetic anisotropy in folded sills and basaltic flows: A case study of volcanics from the Taimyr Peninsula, Northern Russia

“…in Figure 5). In this case, magnetic mineralogy has confirmed the possibility of SD behavior which always leads to the axis inversion during AMS measurement [3,18,26,29] .…”

Lateral Magma Flow in Mafic Sill‐complexes

“…7A, 7B; Galerne et al, 2008;Leat, 2008;Polteau et al, 2008b;Airoldi et al, 2012;Muirhead et al, 2012Muirhead et al, , 2014Svensen et al, 2012;Scheiber-Enslin et al, 2014). In the Ferrar magmatic province specifically, the role of sills in facilitating extensive lateral magma flow is supported by: (1) observed sill-sill feeding relationships (Muirhead et al, , 2014; (2) decreasing Mg# and MgO contents along the length of the Ferrar magmatic province that are consistent with fractional crystallization during lateral magma flow as far as 4100 km from the Weddell Sea region (Elliot et al, 1999;Elliot and Fleming, 2000;Leat, 2008); and (3) the consistent orientation of bridge and/or broken bridge structure long axes and magnetic lineations within sills and interconnected sheets across Antarctica (e.g., in the Theron Mountains, Antarctica) (Dragoni et al, 1997;Hutton, 2009;Airoldi et al, 2012). Overall, these data imply that sills facilitated magma transport lengthwise across the East Antarctic margin within Beacon Supergroup rocks (Storey and Kyle, 1997;Leat, 2008).…”