Strain and magnetic fabric in the Santa Catalina and Pinaleno Mountains Metamorphic Core Complex Mylonite Zones, Arizona

Abstract

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference (ΔKi=[Ki−K¯]/K¯) and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains.

Plastic behavior of magnetite and high strains obtained from magnetic fabrics in the Parry Sound shear zone, Ontario Grenville Province

“…Empirical correlations are usually powerlaw relationships of the form (k,) = (V'?$, where ki is a factor derived from the magnitude of the AMS axes and fl represents the principal strain (stretches). Although empirical correlations require no a priori assumptions about the behavior of the AMS carriers, either passive-line or rigid-body rotation are again invoked to explain such correlations in mylonites (Goldstein 1980, Rathore et al 1983, Goldstein & Brown 1988, Ruf et al 1988.…”

“…Because mylonites in such shear zones often lack conventional strain markers and have textures that are difficult to quantify with optical methods, magnetic anisotropy has been utilized in many fabric studies of shear zones. Finite strain values (Rathore et al 1983, Goldstein & Brown 1988, Ruf et al 1988 and finite strain geometries (Goldstein 1980) have been determined from magnetic susceptibility ellipsoids measured by the anisotropy of magnetic susceptibility (AMS). These correlations between AMS and strain assume rigid-particle behavior for the mineral(s) that dominate the AMS.…”

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

“…In deformed rocks, the potential of susceptibility anisotropy as a strain gauge has received a lot of attention (e.g., Owens, 1974;Singh et al, 1975;Hrouda, 1976;Wood et al, 1976;Kligfield et al, 1977;Goldstein, 1980;Rathore et al, 1983;Henry and Daly, 1983;Borradaile, 1987;Cogné and Perroud, 1988;Hirt et al, 1988;Ruf et al, 1988;Pearce and Fueten, 1989;Kodama and Sun, 1990;Housen et al, 1995). The main reasons for this application are: (1) the relative ease by which the AMS ellipsoid can be established; (2) the sensitivity of the method to weakly developed rock fabrics; and (3) that essentially all rocks are magnetically anisotropic and therefore magnetic anisotropy offers a nearly universally applicable method to analyze deformation state of rocks, as opposed to the much smaller population of samples that may contain other strain indicators.…”

Evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the Main Central Thrust Zone (Garhwal Himalaya, India): Inferences from finite strain and AMS analyses

“…These above observations suggest that the development of AMS fabric is influenced by the passive rotation of magnetite grains as a function of strain. In the contrast, development of AMS fabric by passive rotation of magnetite grains is limited in part by the degree to which elongated grains can be aligned and as approaches a maximum degree, the degree of AMS also approaches a limit (Ruf et al, 1988). In the present case, the AMS fabric develops rapidly at low finite strain magnitude (Es) and as strain increases the P 0 values increases with the change in shape of AMS ellipsoid.…”