Anisotropy of magnetic susceptibility: Sedimentological, igneous, and structural‐tectonic applications

Abstract

As was noted by Graham [1954], anisotropy of magnetic susceptibility (AMS) data has many applications to the study of geological processes. This type of anisotropy expresses directional variation in the magnetization induced in a rock, most notably in its iron oxides and especially in its magnetite and hematite. The AMS is commonly expressed by an ellipsoid, of which the principal axes are K1, K2, and K3, from greatest to least. This ellipsoid is generally interpreted in terms of the distribution and shape of grains of magnetic minerals in a rock although the ellipsoid determined depends also on magnetocrystalline anisotropy and on factors of instrumental technique [Ellwood et al., in press, 1987], The challenge for the geoscientist in analyzing AMS data is first to identify the origin of the AMS geometry, then second to relate that geometry to the natural processes which produced the inferred or measured distribution of magnetic materials.

Anisotropy of magnetic susceptibility data bearing on the transport direction of mid-tertiary regional ignimbrites, Candelaria Hills area, West-Central Nevada

“…Ideally, magnetic fabric data determined from numerous locations distributed regionally across an area in a single ignimbrite sheet can be used to triangulate on a point of intersection that would lie near or in the source area of the eruption (i.e. a specific caldera ;Ellwood 1982;Macdonald and Ellwood 1987;Palmer and MacDonald 1999). Our data do not allow for the identification of unique intersection points, but do nonetheless reveal the likely transport direction.…”

Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)

“…Moreover, magnetic fabric studies are an important complement to paleomagnetic studies, both as a means of determining magnetic mineralogy (since it studies the paramagnetic versus ferromagnetic contribution) and as a way to determine possible deformation-induced and compaction deflections in natural remanent magnetization (e.g., Fuller, 1963;Kligfield et al, 1983;Cogné et al, 1986;Hirt et al, 1986;Lowrie et al, 1986;Mothersill and Borradaile, 1989;Vetter et al, 1989;Jackson and Tauxe, 1991). Anisotropy of magnetic susceptibility has shown great potential first as a means of determining rock-and mineral-orientation fabrics (see reviews by Hrouda, 1982;MacDonald and Ellwood, 1987;Borradaile, 1988;Rochette et al, 1992;Borradaile and Henry, 1997), but also as a method of determining the kinematic history in deformed samples where conventional strain methods cannot be applied (e.g. Owens, 1974;Rathore, 1979;Hrouda, 1987).…”

Ellipsoids (v1.0): 3-D magnetic modelling of ellipsoidal bodies

“…The function f ( r) was first presented by Dirichlet (1839) to describe the gravitational potential produced by homogeneous ellipsoids. Later, several authors also deduced and used this function for describing the magnetic and gravitational fields produced by triaxial, prolate and oblate ellipsoids (Maxwell, 1873;Thomson and Tait, 1879;DuBois, 1896;Peirce, 1902;Webster, 1904;Kellogg, 1929;Stoner, 1945;Osborn, 1945;Peake and Davy, 1953;Macmillan, 1958;Chang, 1961;Lowes, 1974;Clark et al, 1986;Tejedor et al, 1995;Stratton, 2007).…”

“…If the principal susceptibilities are different from each other, we say that the body has an anisotropy of magnetic susceptibility (AMS). The AMS is generally associated with the preferred orientation of the grains of magnetic minerals forming the rock (Fuller, 1963;Uyeda et al, 1963;Janák, 1972;Hrouda, 1982;Thompson and Oldfield, 1986;MacDonald and Ellwood, 1987;Rochette et al, 1992;Dunlop and Özdemir, 1997;Tauxe, 2003). For the particular case in which the principal directions coincide with the ellipsoid axes, the matrix U is equal to the matrix V (Eq.…”