Elastic anisotropy of Tambo gneiss from Promontogno, Switzerland: a comparison of crystal orientation and microstructure-based modeling and experimental measurements

Abstract

Felsic and mafic gneisses constitute large proportions of the upper and lower continental crust. Gneisses often display high anisotropy of elastic properties associated with preferred orientations of sheet silicates. Here we study the elastic anisotropy of a sample of Tambo gneiss from Promontogno in the Central Alps. We apply optical microscopy, time-of-flight neutron diffraction, neutron and X-ray tomography to quantify mineral composition and microstructures and use them to construct self-consistent models of elastic properties. They are compared to results of ultrasonic measurements on a cube sample in a multi-anvil apparatus and on a spherical sample in an apparatus that can measure velocities in multiple directions. Both methods provide similar results. It is shown that models of microstructure-derived elastic properties provide a good match with ultrasonic experiment results at pressures above 100 MPa. At a pressure of 0.1 MPa the correspondence between the model and the experiment is worse. This may be caused by an oversimplification of the model with respect to microfractures or uncertainties in the experimental determination of S-wave velocities and elastic tensor inversion. The study provides a basis to determine anisotropic elastic properties of rocks either by ultrasonic experiments or quantitative models based on microstructures. This information can then be used for interpretation of seismic data of the crust.

Intrinsic Elastic Anisotropy of Westerly Granite Observed by Ultrasound Measurements, Microstructural Investigations, and Neutron Diffraction

“…Preferred orientation of alkali feldspar has similar symmetry, but the texture itself is weaker. This is similar to plagioclase and alkali feldspar textures observed in orthogneiss (Vasin et al., 2017). Biotite shows typical preferred orientation of cleavage planes close to a (001) fiber, but with a large random texture component.…”

“…and alkali feldspar textures observed in orthogneiss (Vasin et al, 2017). Biotite shows typical preferred orientation of cleavage planes close to a (001) fiber, but with a large random texture component.…”

Preferred Orientation of Quartz in Metamorphic Rocks from the Bergell Alps

“…Also muscovite inhibits grain boundary motion during dynamic recrystallization. The texture of Triassic quartzites is similar to that documented in underlying muscovite gneiss from the Tambo nappe that transformed from old granite, with also very strong muscovite texture (10.0 mrd) and a much weaker quartz texture (1.7 mrd) [112].…”

Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

“…For a recent multimethod approach, see Vasin et al . [], which combines neutron and X‐ray imaging, effective medium models, and laboratory measurements.…”

“…For spherical measurement setups [i.e., Pros et al, 1998Pros et al, , 2003, there is still only a limited range in applied pressure and no temperature. For a recent multimethod approach, see Vasin et al [2017], which combines neutron and X-ray imaging, effective medium models, and laboratory measurements.…”