Pressure, temperature and lithological dependence of seismic and magnetic susceptibility anisotropy in amphibolites and gneisses from the central Scandinavian Caledonides

Mohsen Bazargan,Hem Bahadur Motra,Bjarne Almqvist,Sandra Piazolo,Christoph Hieronymus

doi:10.1016/j.tecto.2021.229113

Mohsen Bazargan, Hem Bahadur Motra + Show 3 more

Open Access

https://doi.org/10.1016/j.tecto.2021.229113

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Abstract

As a petrofabric indicator, anisotropy of magnetic susceptibility (AMS) can potentially be used to infer seismic properties of rocks, and in particular seismic anisotropy. To evaluate the link between AMS and seismic anisotropy we present laboratory measurements of elastic wave velocities and anisotropy of magnetic susceptibility (AMS) for eight samples from the deep drilling investigation forming a part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project. The samples consist of a representative suite of mid crustal, deformed rock types, namely felsic and biotite-rich gneisses, and amphibolites (mafic gneisses). Compressional (P) and shear (S) waves were measured at confining pressures from room pressure to 600 MPa and temperature from room condition to 600 °C. Seismic anisotropy changes with increasing temperature and pressure, where the effect of pressure is more significant than temperature. Increasing pressure, considering the range of samples, results in an increase in mean wave speed values from 4.52 to 7.86 km/s for P waves and from 2.75 to 4.09 km/s for S waves. Biotite gneiss and amphibolite exhibit the highest anisotropy with P wave anisotropy (AVp) in the ranges of ~9% to ~20%, and maximum S- wave anisotropy exceeds 10%. In contrast, Felsic gneisses are significantly less anisotropic, with AVp of <7% and AVs of <6%. Up to 20% anisotropy may be generated by microcracks at 600 MPa and 600 °C, which is likely originating from thermal expansion of anisotropic minerals. An agreement is found between AMS and seismic anisotropy, although this is only a case if mean magnetic susceptibility (kmean) ranges between ~1 × 10−5 to ~1 × 10−3 [SI]. Such kmean values are common in rocks dominated by paramagnetic matrix minerals. Based on our results we propose that such samples are the most likely to be useful for the prediction of seismic anisotropy based on their AMS.

Highlights

Laboratory measurements of seismic anisotropy provide petrophysical signatures that allow insight into the structure of the middle and lower crust
All felsic gneiss samples show similar ultrasonic wave speeds and AVp, where the latter ranges from 4-6 % at the highest applied pressure (600 MPa) and temperature (600 C) (Tables 2 and 3)
The experimental data show that this hysteresis effect is largest along the Z-axis (Fig. 5), which is the axis that is normal to the orientation of the foliation plane, suggesting that this plane exhibits the largest amount of interaction with microcracks; i.e., microcracks tend to orient parallel to the foliation plane

Summary

Introduction

Laboratory measurements of seismic anisotropy provide petrophysical signatures that allow insight into the structure of the middle and lower crust. Journal Pre-proof observed seismic anisotropy in the middle and lower crust can be explained mainly by CPO of mica (e.g., biotite and muscovite) and amphibole, based on microstructure-based calculations. These two groups of minerals are among the most anisotropic in the crustal mineral inventory and tend to develop strong CPO’s in ductile deformed rocks (Fountain et al, 1976; Burlini and Fountain, 1993; Cholach and Schmitt, 2006). Almqvist and Mainprice (2017) provide a comprehensive overview of common minerals in the continental crust and their microstructures affect seismic anisotropy

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