Graham J. Borradaile

Tectonic applications of magnetic susceptibility and its anisotropy

Abstract Anisotropy of low field magnetic susceptibility (AMS) is a versatile petrofabric tool. For magnetite, AMS primarily defines grain-shape anisotropy; for other minerals, AMS expresses crystallographic control on magnetic properties. Thus, we may infer the orientation-distribution of a dominant mineral from the AMS of a rock. AMS principal directions can record current directions from sediment, flow-directions from magma, finite-strain directions from tectonized rocks and stress-directions from low-strain, low-temperature, neotectonic environments. AMS measurements may reveal some aspects of the strain-path, where carefully selected. For example, we may compare different parts of a heterogeneously strained domain, different minerals in a homogeneously strained site, AMS with schistosity/mineral lineation, and AMS with remanence-anisotropy. Such measurements isolate the orientation-distributions of different minerals, adding a temporal scale to the kinematic sequence. Normally, we can interpret the principal directions of AMS distributions as a physically significant direction, such as a current direction, magmatic flow or finite-strain axis. However, calibrating the AMS ellipsoid shape against the magnitude of the controlling physical process is very difficult. Primarily, this is because the shape of the AMS ellipsoid combines contributions from several minerals whose individual AMS ellipsoids are of different shape. Thus, small variations in the proportions of minerals change the shape of the rocks AMS ellipsoid, even if the alignment process were of constant intensity. In deformed rocks, AMS is more strain-sensitive than calcite twinning or the alignment of calcite or quartz c-axes. Not all AMS fabrics relate to crystallographic or grain alignment. First, displacement fabrics generate AMS where an isotropic matrix of high susceptibility displaces unevenly spaced objects of low susceptibility and suitable scales. Second, AMS location fabrics occur where sub-isometric magnetite grains are close enough, in certain directions, for their demagnetizing fields to interact. This accounts for the AMS of many magnetite-dominated signals where there is no aligned magnetite. Third, the AMS of single-domain magnetite is inverse to shape so that such grains may oppose the AMS contribution of parallel minerals. Finally, transitional sedimentary-tectonic or magmatic-tectonic fabrics yield smeared, temporal sequences of AMS principal directions that cannot be immediately attributed to a single alignment process. These transitional AMS ellipsoids mix primary and secondary AMS components, making it difficult to characterize either component. However, such fabric combinations may permit us to recognize the sense of shearing in flow processes.

Anisotropy of magnetic susceptibility (AMS): Magnetic petrofabrics of deformed rocks

Abstract For 40 years magnetic anisotropy has provided successful geological interpretations of magnetic ellipsoid orientations; in contrast the interpretation of anisotropy magnitudes is far more convoluted. This is due to complexities at various levels within rocks, including different physical magnetic responses of different minerals, grain-scale magnetic anisotropy, the anisotropy of interacting ensembles, the mineralogical constitution of rocks and the processes and mechanisms that align minerals in nature. The chief factors determining the magnetic fabrics of tectonized rocks include: mineral-physics properties, crystal symmetry, mineral-abundances, tectonic symmetry and crystal orientation-distribution, strain or stress, kinematic history and certain tectono-metamorphic processes (e.g. diffusion, crystal plasticity, dynamic recrystallization, particulate flow, neomineralization). AMS ultimately provides an integrated record of some combination of these factors. Subfabrics due to distinct processes or events may be expressed in different mineral and/or grain-size fractions, and are superposed in the conventionally observed AMS. Their discrimination may be achieved by various laboratory techniques such as magnetization and torque measurements in weak and strong applied fields, anisotropy of ARM and IRM, gyroremanence, Rayleigh magnetization, chemical leaching. However, under limited circumstances, statistical approaches such as differential analysis, tensor standardization, symmetry of confidence regions for the principal axes may partly isolate different subfabric orientations.

Correlation of strain with anisotropy of magnetic susceptibility (AMS)

Existing correlations between strain and anisotropy of low-field magnetic susceptibility (AMS) have been re-assessed using a single parameter to express both anisotropies. TheP′ parameter (Hrouda, 1982) shows potential as a powerful single expression of the intensity of strain and of AMS. Previous correlations are improved by use of this parameter. Cautious optimism is justified for correlations between strain and susceptibility in a certain strain window between a lower limit (excluding the incomplete overprint of predeformation anisotropy) and an upper limit (excluding the effects of saturation anisotropy). For successful correlations the influence of stress-controlled recrystallisation should be minimal and the mineralogical sources of susceptibility must predate deformation.

Anisotropy of magnetic susceptibility: rock composition versus strain

Abstract The shape of the susceptibility ellipsoid for a metamorphic tectonite with a strong crystallographic preferred orientation of silicates is strongly influenced by the anisotropy of the most abundant magnetic silicate in the absence of magnetite. Where traces (

Particulate flow of rock and the formation of cleavage

Abstract Sliding on grain boundaries produces particulate flow during rock deformation. Intragranular deformation and particulate flow may be uncoupled (independent particulate flow), completely coupled (dependent particulate flow) or partially coupled (controlled particulate flow). In independent particulate flow grains are not deformed. In dependent particulate flow the grain sliding is limited and dependent on the incompatible deformation of grains. In controlled particulate flow, grain sliding is encouraged by other factors (e.g. pore-fluid pressure) but the rate of grain sliding is controlled by the deformation of grains. Cataclastic flow and superplasticity are two well known types of behaviour that involve intragranular deformation mechanisms and one or more types of particulate flow. The degree of coupling between intragranular deformation and particulate flow may change during the course of natural deformation. For example, pore-fluid pressure may decrease, causing a transition from controlled to dependent particulate flow. The early part of such a history would be dominated by particulate flow and intragranular deformation would predominate later. This explains bulk strains that exceed the strains of individual grains, and it explains cleavages (formed by the deformation of grains) which transect coeval folds and show no simple relationship with the bulk strain ellipsoid. It also indicates that strain data derived from grain shapes or intragranular features may not truly reflect bulk strain.

Magnetic anisotropy of some phyllosilicates

Magnetic susceptibility, anisotropy of susceptibility and hysteresis of single microcrystals of chlorite, biotite, phlogopite, muscovite, zinnwaldite and fuchsite were measured in low and high magnetic fields with an alternating gradient force magnetometer (Micromag). Their properties are sufficient to account for the low field susceptibility (AMS) of most micaceous rocks. Nearly all samples show some ferromagnetic contribution at low fields due to inclusions of pseudosingle domain and multidomain magnetite. The paramagnetic contribution isolated at high fields usually exceeds the ferromagnetic contribution. The paramagnetic susceptibility is intrinsic to the silicate lattice and agrees with values predicted from chemical composition within the limits of error. The minimum susceptibility is nearly parallel to c, another axis is parallel to b and the third susceptibility (usually the maximum) is close to a. The paramagnetic susceptibility has a disk-shaped magnitude ellipsoid with strong anisotropy (P′ < 2). The ferromagnetic contributions at low fields have more variably shaped ellipsoids with greater eccentricity (P′ < 5). The silicate lattice does not constrain their orientation. Our technique cannot determine the principal axes of the ferromagnetic component. However, its principal values usually correspond with the paramagnetic principal susceptibilities in order of magnitude. Thus, the combined paramagnetic-ferromagnetic anisotropy recognised in routine studies of AMS should faithfully represent the petrofabric of most micaceous rocks. Nevertheless, nearly 10% of our samples have incompatible anisotropy ellipsoids for the silicate host and magnetite inclusions. These yield a net inverse AMS that does not correctly represent the orientation of the silicate lattice. Therefore, some caution is necessary in petrofabric-AMS studies of micaceous rocks.

Anisotropy of magnetic susceptibility of some metamorphic minerals

Abstract The anisotropy of susceptibility of metamorphic rocks can be due to paramagnetic rock-forming silicates such as amphiboles, chlorites and micas. It is not always necessary to invoke fabrics of separate grains of iron oxide to explain the anisotropy. Minimum estimates of lattice anisotropies of typical samples of silicates have maximum-to-minimum ratios of 1.1–1.7. Since the magnetic anisotropies of most metamorphic rocks are less than this, these minerals can control the anisotropy of susceptibility because their preferred crystallographic orientations are usually very strong in comparison with the preferred dimensional orientation of magnetite and because they are more abundant than magnetite.

Atlas of deformational and metamorphic rock fabrics

I. Introduction.- Glossary of Cleavage Terms.- II. Processes Contributing to Development of Cleavage.- Possible Geometrical Changes.- Possible Material Processes.- Extensions and Examples.- III. Possible Links Between Observables and Processes.- The Structure of the Problem.- Instances Where a Process Can Be Inferred.- Commentary.- Appendix I: A System for Drawing Conclusions from Observables.- IV. The Plates.- Continuous Cleavage.- Section 1 Continuous Cleavage Formed by Coarse, Aligned Grains.- Section 2 Fine Continuous Cleavage in Rocks Composed Largely of Phyllosilicates.- Section 3 Continuous Cleavage in Rocks Composed Largely ofNon-Phyllosilicate Minerals.- Spaced Cleavage.- Section 4 Crenulation Cleavage with Gradational Boundaries.- Section 5 Zonal Crenulation Cleavage with Discrete Boundaries.- Section 6 Crenulations Bounded by Cracks.- Section 7 Disjunctive Cleavage Defined by Simple Cracks.- Section 8 Disjunctive Cleavage Defined by Wiggly Cracks or Seams.- Section 9 Cleavage Defined by Anastomosing Seams.- Section 10 Cleavage Defined by Planar Seams.- Section 11 Cleavage Defined by Wispy Seams.- Section 12 Cleavage Defined by Flame-like Seams.- Section 13 Cleavage Defined by Seams Differentiated Without Dissolution or Disaggregation Aspects.- Section 14 Cleavage Defined by Differentiation on Grounds of Texture or Geometry but not Composition.- Other Topics.- Section 15 Nonplanar Differentiation and Blastesis.- Section 16 Cleavage: Indications of Genesis and Strain.- Section 17 Cleavage and Polyphase Deformation.- Section 18 Cleavage Refraction and Cleavage-fold Relationships.- References.

Transected folds: A study illustrated with examples from Canada and Scotland

Transected folds have contemporaneous cleavage that is not parallel to their axial surface, but cuts through the axial surface and both limbs with the same sense. Such folds can develop with perfectly synchronous formation of folds and cleavage. This may occur in rocks undergoing a bulk coaxial strain history or in an approximately coaxial phase of a more complex strain history in which the strain axes do rotate relative to the deforming rocks. In a noncoaxial strain history, transected folds may also arise by a different mechanism, in which cleavage formation is slightly delayed relative to fold formation. This may be achieved where the grain-shaping (cleavage-forming), intragranular deformation mechanisms are initially suppressed because rock flow is more easily accommodated by intergranular movement. The temporary local volume increases required by this “grain-boundary sliding” may occur during de-watering. Transected folds may readily occur and have already been described in many areas. Therefore, field mapping techniques assuming a special geometrical relationship between folds and cleavage should be used only where the degree, or absence, of transection of folds by their contemporaneous cleavage has been established.

Experimental Deformation of Synthetic Magnetite-Bearing Calcite Sandstones' Effects on Remanence, Bulk Magnetic Properties, and Magnetic Anisotropy

We have quantified effects of experimental deformation on the magnetic properties of a set of synthetic “calcite sandstone” samples containing magnetite. The deformation was carried out in a microcomputer-controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10−5 s−1. Most samples were deformed under dry conditions, but a few were deformed with a pore fluid present; the samples deformed under dry conditions required substantially higher differential stresses. Macroscopically ductile shortening strains of up to 25% produced the following irreversible changes in magnetic properties: (1) increased bulk coercivity, remanence coercivity, and mean anhysteretic remanence susceptibility; (2) decreased mean low-field susceptibility; (3) decreases in the component of remanence parallel to shortening; (4) smaller decreases for most samples in the component normal to shortening, resulting in a net “rotation” of the remanence away from the shortening axis; (5) larger decreases in the normal component in a few samples, resulting in a net “rotation” of the remanence towards the shortening axis; (6) increased magnetic anisotropy; and (7) increased “deformation” of initial magnetic ellipsoids. A comparison of data for samples deformed under dry and wet conditions (higher and lower differential stresses, respectively) indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain (i.e., rotation and displacement of particles), whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress.