Unraveling magnetic fabrics

Abstract

The anisotropy of magnetic susceptibility has been proven to be an excellent indicator for mineral fabrics and therefore deformation in a rock or sediment. Low-field anisotropy is relatively rapid to measure so that a sufficient number of samples can be measured to obtain a good statistical representation of the magnetic fabric. The physical properties of individual minerals that contribute to the observed magnetic fabric include bulk susceptibility and intrinsic anisotropy of the mineral phase, its volume concentration, and its degree of alignment. Several techniques have been developed to separate magnetic subfabrics arising from magnetization types, i.e., ferrimagnetism, antiferromagnetism, paramagnetism, and diamagnetism. Susceptibility anisotropy can be measured in low or high fields and at different temperatures in order to isolate a particular subfabric. Measuring the anisotropy of a remanent magnetization can also isolate ferrimagnetic fabrics. A series of case studies are presented that exemplify the value of isolating magnetic subfabrics in a geological context. It is particularly useful in rocks that carry a paramagnetic and diamagnetic subfabric of similar magnitude, such that they negate one another. Further examples are provided for purely paramagnetic subfabrics and cases where a ferrimagnetic subfabric is also identified.

Evidence for the Permo-Triassic transtensional rifting in the Iberian Range (NE Spain) according to magnetic fabrics results

“…AMS studies require a careful determination of the magnetic phases present in the studied rocks and their influence on fabric orientation (Borradaile, 1988;Hirt and Almqvist, 2012;Lanza and Meloni, 2006;Tarling and Hrouda, 1993). In order to address these issues, we need to characterize the present magnetic mineralogies and their relative contribution to the bulk AMS by applying different rock-magnetic techniques.…”

Paramagnetic metamorphic mineral assemblages controlling AMS in low-grade deformed metasediments and the implications with respect to the use of AMS as a strain marker

“…Martín-Hernández & Ferré 2007;Oliva-Urcia et al 2009;Hirt & Almqvist 2012) and by comparing magnetic methods with numerical models based on independent analyses of the mineral fabric (e.g. Siegesmund et al 1995;Martín-Hernández et al 2005).…”

Distinctive diamagnetic fabrics in dolostones evolved at fault cores, the Dead Sea Transform

“…In most deformed environments, the directions of the AMS axes show a fair to good correlation with the directions of the principal (finite) strain axes (e.g., Borradaile, 1987Borradaile, , 1988Borradaile, , 1991Tarling and Hrouda, 1993;Borradaile and Henry, 1997;Par es et al, 1999;Latta and Anastasio, 2007;Soto et al, 2007;Mamtani and Sengupta, 2009;Hrouda et al, 2009;Burmeister et al, 2009;Hirt and Almqvist, 2012). No reliable correlation has been established between the magnitudes of the AMS axes and those of strain magnitudes (e.g., Par es and Van der Pluijm, 2004;Latta and Anastasio, 2007;Borradaile and Jackson, 2010) mainly because these relationships are complex, and may be governed by the percentage of the Fe and Mn-minerals in the rock (Rochette, 1988;Borradaile and Henry, 1997;Schmidt et al, 2006).…”