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Metamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in central Sweden, an old exhumed orogenic thrust zone that is characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about their origin in relation to composition and structural influences on measurements at different seismic scales. Here, we present a new integrative study of cross-scale seismic anisotropy analyses combining mineralogical composition, microstructural analyses, and seismic laboratory experiments from the COSC-1 borehole, which sampled a 2.5-km-deep section of metamorphic rocks deformed in an orogenic root now preserved in the Lower Seve Nappe. While there is strong crystallographic preferred orientation in most samples in general, variations in anisotropy depend mostly on bulk mineral composition and dominant core lithology as shown by a strong correlation between these. This relationship enables to identify three distinct seismic anisotropy facies providing a continuous anisotropy profile along the borehole. Moreover, comparison of laboratory seismic measurements and electron-backscatter diffraction data reveals a strong scale dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross-validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy over seismic scales., Journal of Geophysical Research: Solid Earth, 126 (5), ISSN:2169-9313, ISSN:0148-0227, ISSN:2169-9356

Given shale's potential to serve as both a caprock for geological carbon sequestration and as the host for nuclear waste disposal, this study focuses on the structural characterization around a 1.5–3 m thick fault within the Opalinus Clay host rock and petrophysical characterization of both the host rock and fault core. Seven boreholes were drilled at the Mont Terri Rock laboratory in late 2018 to constrain the orientation of the so-called ‘Main Fault’ and several fracture families within the fault core using core and image logs. Contrary to typical fault models, matrix permeability and seismic velocity measurements show similarities in the host rock and within the Main Fault. In the clay-rich samples, measurements performed in laboratory at varying confining pressures indicate p-wave velocities range from 2.60 to 2.95 km/s perpendicular to foliation and 3.38–3.58 km/s parallel to foliation at near in-situ confining pressures, which increase with similar anisotropy at higher confining pressures (200 MPa). Since the permeability of the host rock and fault zone is very low (10−19–10−21 m2) flow is expected to prevail along fractures, where critical stress fault analysis suggests that potential hydraulically conductive fractures are actually conjugate to the Main Fault trend.

Recent drilling of the first Collisional Orogeny in the Scandinavian Caledonides scientific borehole (COSC-1) near Are, Sweden permitted a laboratory investigation of seismic anisotropy on high met ...

Mineral alignment can provide valuable information on a rock's geological history. Many structural, tectonic, or geodynamic studies rely on determining the preferred orientation of rock-forming minerals, either by direct texture determination or using anisotropy measurements as proxies. Robust interpretations of anisotropy require detailed understanding of its relationship with texture. This study investigates the texture, magnetic and seismic anisotropies of ultramafic rocks from the Ivrea-Verbano Zone (IVZ), Northern Italy, with a special focus on the interplay between anisotropy contributions carried by different minerals. Texture was obtained from electron backscatter diffraction, magnetic anisotropy from low- and high-field anisotropy of susceptibility measurements, and seismic anisotropy from P- and S-wave velocities along the three principal directions of the macroscopic fabric, using 22 mm diameter cores. Texture-based models of magnetic anisotropy agree well with corresponding measurements. Larger variability is observed for seismic anisotropy. Possible explanations are (1) velocities were measured in three directions, insufficient to determine the full elasticity tensor, and (2) seismic anisotropy depends on grain boundaries, pores, and cracks in addition to crystallographic preferred orientation. The contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively, leading to a larger overall anisotropy, or negatively, resulting in a weaker anisotropy. Seismic anisotropy is mostly controlled by olivine, whereas magnetic anisotropy can be dominated by olivine, pyroxene, or hornblende. This study illustrates that, although the relationship between texture and anisotropy can be complex, it is possible to quantitatively predict magnetic, and to a lesser degree seismic, anisotropy from texture data. Even though both magnetic and seismic anisotropies are common proxies for texture, they are not coaxial, and no simple relationship between the degree of magnetic and seismic anisotropy was found. These results underline the importance of understanding different minerals' contributions to anisotropy in upper mantle rocks.

Geophysical Research Letters, 41 (20), ISSN:0094-8276, ISSN:1944-8007

The magnetic fabric of a rock, defined by the anisotropy of magnetic susceptibility (AMS), is often used as a tectonic indicator. In order to establish a quantitative relationship between AMS and mineral texture, it is important to understand the single crystal intrinsic AMS of each mineral that contributes to the AMS of the rock. The AMS and crystallographic preferred orientation (CPO) of amphiboles, olivine and pyroxenes has been analyzed in a series of amphibolites, peridotites and pyroxenites that do show preferred mineral alignment. The CPO of each mineral phase was determined based on electron backscatter diffraction (EBSD). Whole- rock AMS was computed based on the CPO and single crystal AMS of the respective minerals. A comparison between measured and modelled magnetic anisotropy shows that the directions of the principal susceptibility axes agree well in amphibolite and peridotite. Pyroxenite is a good example for competing AMS fabrics in polyphase rocks.