Magnetic characterization using a three-dimensional hysteresis projection, illustrated with a study of limestones

International audienceLimestones provide an important source of palaeomagnetic information despite their low content of submicroscopic remanence-bearing minerals. The chief sources of these minerals are thought to be clastic volcanic magnetite and titanomagnetite, and organic magnetite, the latter mostly from bacterial sources. Chemically remagnetized limestones carry magnetite or pyrrhotite. Three hysteresis properties prove useful in identifying and characterizing these mineralogical influences on limestones: the ratio of zero-field maximum remanence to saturation remanence (M r /M s) in an applied field, coercivity of remanence (B cr) and coercivity (B c). To a lesser extent K f /M s may be useful, where K f is the ferrimagnetic susceptibility. Traditionally, these have been plotted on a combination of 2-D graphs that of necessity only preserve two variables (Day et al. 1977; Wasilewski 1973). However, we found that magnetic discrimination and characterization of the limestones was much easier on a three-axis hysteresis projection that preserves the values of B cr , B c and M r /M s as independent variables. Using logarithmic scales, the regression surfaces through the data become almost planar and distinguish pelagic, shallow marine, shelf and remagnetized limestones on the basis of the slope and intercept of the associated regression surface. Clearly, there are sensitive sedi-mentological, geochemical or organic influences that dictate the magnetic mineralogy through sedimentary environment. Moreover, the 3-D plot of hysteresis criteria affords easy recognition of remagnetized limestones and may permit the rejection of material unsuitable for palaeomagnetic study. The 3-D hysteresis projection may be useful for the characterization of other rocks and magnetic material

Thermal Enhancement of Magnetic Fabrics in High Grade Gneisses

International audienceThe orientation-distributions of AMS and AARM ellipsoids are non-coaxial in the Kapuskasing Zone granulites. AARM is due to the preferred orientation of magnetite which mostly postdates the silicate fabric. The AMS orientation-distribution combines susceptibility anisotropies of magnetite and the silicates. Whereas heating does not increase the bulk susceptibility, it does improve the definition of the orientation-distributions of both AMS and AARM. However, the orientation-distribution of AARM is most enhanced and therefore post-heated samples emphasize the magnetite subfabric. Thus, heating does not clarify the interpretation of the silicate fabric in this study. However, it does change the samples' AMS fabric in such a way as to reveal the magnetite subfabric which can only otherwise be detected by more tedious AARM measurements

Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties, and magnetic anisotropy

NSF EAR 88-04820 and NSERC A686

Structures Related to the Emplacement of Shallow-Level Intrusions

A systematic view of the vast nomenclature used to describe the structures of shallow-level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions

Unraveling the geometry of the New England oroclines (eastern Australia): Constraints from magnetic fabrics

The southern New England Orogen (NEO) in eastern Australia is characterized by tight curvatures(oroclines), but the exact geometry of the oroclines and their kinematic evolution are controversial. Here we present new data on the anisotropy ofmagnetic susceptibility (AMS), which provide a petrofabric proxy for the finite strain associated with the oroclines.We focus on a series of preoroclinal Devonian-Carboniferous fore-arc basin rocks, which are aligned parallel to the oroclinal structure, and by examining structural domains, we test whether or not the magnetic fabric is consistent with the strain axes. AMS data show a first-order consistency with the shape of the oroclines, characterized, in most of structural domains, by subparallelism between magnetic lineations, “structural axis” and bedding. With the exception of the Gresford and west Hastings domains, our results are relatively consistent with the existence of the Manning and Nambucca (Hastings) Oroclines. Reconstruction of magnetic lineations to a prerotation (i.e., pre–late Carboniferous) stage, considering available paleomagnetic results, yields a consistent and rather rectilinear NE-SW predeformation fore-arc basin. This supports the validity of AMS as a strain proxy in complex orogens, such as the NEO. In the Hastings Block, magnetic lineations are suborthogonal to bedding, possibly indicating a different deformational historywith respect to the rest of the NEO