Abstract
The use of the Anisotropy of Magnetic Susceptibility (AMS) has become a rather common practice in Earth Sciences since the pioneer note by Graham (1954). The versatility of the technique, and the rapidness in obtaining and processing AMS data largely improved in the past thirty years, and has generated a wealth of literature, notably on mudrock fabrics. The assessment of the current trends in magnetic fabric studies reveals that AMS has one of its largest potential in sedimentary rocks from structural settings where the ductile component of deformation is cryptic or hindered by the brittle component. Abundant evidence provided by AMS data reveal that deformation extents beyond the deformation or cleavage front in contractional settings, including fold-and-thrust belts and active accretionary prisms, configuring magnetic fabrics as a standard method for fabric quantification in deformed sedimentary rocks.
Highlights
Numerous rock-magnetic studies where paramagnetic and ferromagnetic susceptibilities have been quantified revealed that typically the former dominates in mudstones (e.g., Martín-Hernández and Hirt, 2001 and references therein), the anisotropy of magnetic susceptibility (AMS) is dominated by the paramagnetic component, and most by the shape anisotropy of clay minerals very fine magnetic particles attached to the clay fabric might contribute (Kodama and Sun, 1992)
It has been demonstrated that magnetic axes in biotite crystals conform to the density distributions of mineral lattice planes obtained by x-ray goniometry (Richter et al, 1993; Schmidt et al, 2009) (Figure 2). These results reveal that densities from x-ray for chlorite and mica are perfectly reflected by the distribution of the minimum susceptibility axes
MAGNETIC ANISOTROPY AND WEAK DEFORMATION Since the seminal paper by Graham (1966) where he pointed out that AMS in sediments indicate “the final shape distortions,” many scholars have exploited the property in order to retrieve the strain imprint in sedimentary rocks, in the weak deformation realm
Summary
Numerous rock-magnetic studies where paramagnetic and ferromagnetic susceptibilities have been quantified revealed that typically the former dominates in mudstones (e.g., Martín-Hernández and Hirt, 2001 and references therein), the AMS is dominated by the paramagnetic component, and most by the shape anisotropy of clay minerals very fine magnetic particles attached to the clay fabric might contribute (Kodama and Sun, 1992). ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) The low field magnetic susceptibility of a rock (the ratio of magnetization to the applied field or K = M/H) is given by the total contribution of its bulk mineralogy, including paramagnetic (e.g., phyllosilicates, iron-bearing feldspars), diamagnetic (e.g., quartz, calcite) and ferromagnetic (sensu lato; e.g., magnetite, goethite, hematite) grains.
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