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Sedimentary rocks of the Itararé Group, deposited during the Late Paleozoic Ice Age in the Paraná Basin of South America, were collected throughout the state of São Paulo, Brazil, for an anisotropy of magnetic susceptibility (AMS) and rock‐magnetic study. A recent paleomagnetic study conducted on the same samples had determined that these rocks were largely remagnetized during the Cretaceous; however, rock‐magnetic experiments demonstrate that the AMS is dominantly carried by paramagnetic minerals and therefore is unaffected by the secondary magnetic overprints. AMS data are analyzed in terms of their shape and orientation, and according to the relationship between the q‐value (magnetic lineation/foliation) and the imbrication angle (β) of the minimum susceptibility axes with respect to bedding (q–β diagram). Using multiple lines of evidence, we demonstrate that AMS records primary sedimentary fabrics that reflect the depositional environments and paleocurrent conditions in which these rocks were deposited. The magnetic fabrics consistently record a SE‐NW paleocurrent orientation, with dominant direction of transport to the NW throughout the entire state of São Paulo, in agreement with ice flow and sediment transport directions reported from limited numbers of sites possessing sedimentary structures and ice‐kinematic indicators. Key Points: A rock magnetic and anisotropy of magnetic susceptibility study was conducted on the subglacial Itararé Group sedimentary rocks of BrazilRock‐magnetic evidence reveals a complex magnetic mineralogy and allows determining a dominantly paramagnetic origin for the AMSAMS analyses determine a dominant sedimentary transport direction to the NW and allow determining the depositional regimes for these rocks [ABSTRACT FROM AUTHOR]

Basal ice of glaciers and ice sheets frequently contains a well-developed stratification of distinct, semi-continuous, alternating layers of debris-poor and debris-rich ice. Here, the nature and distribution of shear within stratified basal ice are assessed through the anisotropy of magnetic susceptibility (AMS) of samples collected from Matanuska Glacier, Alaska. Generally, the AMS reveals consistent moderate-to-strong fabrics reflecting simple shear in the direction of ice flow; however, AMS is also dependent upon debris content and morphology. While sample anisotropy is statistically similar throughout the sampled section, debris-rich basal ice composed of semi-continuous mm-scale layers (the stratified facies) possesses well-defined triaxial to oblate fabrics reflecting shear in the direction of ice flow, whereas debris-poor ice containing mm-scale star-shaped silt aggregates (the suspended facies) possesses nearly isotropic fabrics. Thus, deformation within the stratified basal ice appears concentrated in debris-rich layers, likely the result of decreased crystal size and greater availability of unfrozen water associated with high debris content. These results suggest that variations in debris-content over small spatial scales influence ice rheology and deformation in the basal zone. [ABSTRACT FROM AUTHOR]

Anisotropy of isothermal remanent magnetization (AIRM) is useful for describing the fabrics of high-coercivity grains, or alternatively, the fabrics of all remanence-carrying grains in rocks with weak remanence. Comparisons between AIRM and other measures of magnetic fabric allow for description of mineral-specific or grain-size-dependent fabrics, and their relation to one another. Additionally, when the natural remanence of a rock is carried by high-coercivity minerals, it is essential to isolate the anisotropy of this grain fraction to correct paleodirectional and paleointensity data. AIRMs have been measured using a wide range of applied fields, from a few  mT to several T. It has been shown that the degree and shape of AIRM can vary with the strength of the applied field, for example, due to the contribution of separate grain subpopulations or due to field-dependent properties. To improve our understanding of these processes, we systematically investigate the variation of AIRM and the anisotropy of partial isothermal remanence (ApIRM) with applied field for a variety of rocks with different magnetic mineralogies. We also test the additivity of A(p)IRMs and provide a definition of their error limits. While A(p)IRM principal directions can be similar for a range of applied field strengths on the same specimen, the degree and shape of anisotropy often show systematic changes with the field over which the (p)IRM was applied. Also, the data uncertainty varies with field window; typically, larger windows lead to better-defined principal directions. Therefore, the choice of an appropriate field window is crucial for successful anisotropy corrections in paleomagnetic studies. Due to relatively large deviations between AIRMs calculated by tensor addition and directly measured AIRMs, we recommend that the desired A(p)IRM be measured directly for anisotropy corrections. [ABSTRACT FROM AUTHOR]

Magnetic anisotropy measurements are becoming increasingly common to many studies within the different disciplines of geology, involving sedimentary, igneous and metamorphic rocks. A plethora of techniques exists for measuring magnetic anisotropy of rocks. Some are rapid and non-destructive while others are more labor-intensive or may result in alteration of the magnetic minerals. All, however, have the potential of revealing a wealth of information when measured and interpreted correctly. In broad terms, anisotropy techniques subdivide into measurements of susceptibility, remanence and torque; here we consider the first two of these. Anisotropy of magnetic susceptibility (AMS) is by far the most utilized, and measures composite fabrics. Magnetic susceptibilities in high fields and low temperatures, however, are being increasingly used to isolate the paramagnetic contribution to the fabrics. When distinguishing between fabrics carried by different ferromagnetic phases, or to separate these from the diamagnetic and paramagnetic contributions to the fabric, then remanence anisotropy techniques become necessary. Anisotropies of thermal remanence (ATRMs), of anhysteretic remanence (AARM) and of isothermal remanence (AIRM) are the most common examples. Remanence anisotropy may be measured over the full spectrum of magnetic coercivities or over a targeted range (e.g. partial or ApARM). Moreover, anisotropies may be calculated using only the resolved field-parallel component of the vector, in which case a minimum of six different orientations is necessary to obtain a complete symmetric tensor, or using the three components (full vector) of the measured magnetic vectors (e.g. AvARM), in which case three orthogonal applied magnetizations are the minimum requirement. In this study we utilize a variety of magnetic remanence room temperature techniques to measure remanence anisotropy of selected coarse and finer grained gneiss-granulitic specimens with well-pronounced fabrics. Results are compared to room temperature AMS and are interpreted in terms of the applicability of instrumentation/technique to specific rock-magnetic properties. [ABSTRACT FROM AUTHOR]

Magnetic fabrics provide information on the preferred alignment of minerals in rocks. The preferred orientation of ferromagnetic grains specifically can be described by various kinds of remanence anisotropy and partial remanence anisotropy, or isolated from high-field anisotropy of magnetic susceptibility (HF-AMS) measurements. This contribution focuses on comparing results from anisotropy of (partial) anhysteretic remanent magnetization (A(p)ARM), anisotropy of (partial) isothermal remanence (A(p)IRM), anisotropy of (partial) thermal remanence (A(p)TRM), and isolated ferromagnetic sub-fabrics from HF-AMS. Experiments were performed on a collection of igneous, metamorphic and sedimentary rocks. A(p)ARMs and A(p)IRMs can be strongly coercivity-dependent, both in terms of principal directions and in the degree and shape of the anisotropy, indicating different fabrics of different grain sizes. Additionally, remanence anisotropy and the ferromagnetic fabrics from HF-AMS can be largely different, indicating that the large MD grains dominating AMS have different fabrics than those of the smaller remanence-carrying grains. These results indicate that a given sample may display a range of magnetic anisotropy that depend on the experimental parameters and the grains targeted in any specific experiment. Hence, measuring ferromagnetic fabrics with a variety of methods provides a more detailed insight into preferred orientations of sub-populations of ferromagnetic grains. Researchers can use these results to determine the ferromagnetic anisotropy most relevant to their specific applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Magnetic fabric in the central Tarom Basin records NE-SW layer parallel shortening. • Magnetic fabric in the SW margin of the basin is a primary sedimentary fabric. • NE-SW orogen-perpendicular syndepositional extension occurs in the SW margin. • Normal faults in the SW margin were induced by gravity instabilities. • Middle-late Miocene extension is not regionally pervasive. The upper plate of the Arabia-Eurasia collision zone experienced orogen-perpendicular to orogen-parallel extension from 25–22 to 10–9 Ma. Although such an extension occurred during widespread collisional deformation, it is not clear if it is a local feature or if represents a major phase of upper plate extension. In this study we combine anisotropy of magnetic susceptibility (AMS) with fault kinematic analysis and sedimentologic data from 16.2- to 7.6-My-old deposits of the Upper Red Formation of the intermontane Tarom Basin (NW Iran). These strata present syndepositional, normal faults and offer the possibility to gain new insights into the spatial extent of such a Miocene extension. AMS data from the central and northern sectors of the basin document a tectonic fabric with a magnetic lineation parallel to the strike of the orogen, suggesting a compressional tectonic overprint. Conversely, the southern margin of the basin presents a purely sedimentary magnetic fabric despite a ~NE–SW orogen-perpendicular extension. This suggests that basin formation was not driven by extensional tectonics. Rather, the normal faults are gravity instabilities induced as also documented by coeval landslide deposits. This allows concluding that the orogen-perpendicular extension observed in few sectors of the collision zone is not regionally pervasive and hence it is not controlled by large-scale processes. Combined, our results indicate that if orogen-parallel extension associated with tectonic denudation and metamorphic core complex development occurred in certain sectors of the collision zone (Takab complex), it must have ended before 19–16 Ma, when widespread upper plate contractional deformation started. [ABSTRACT FROM AUTHOR]