Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

Abstract

Rocks in the continental crust are long lived and have the potential to record a wide span of tectonic history in rock fabric. Mapping rock fabric in situ at depth requires the application of seismic methods. Below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations influenced by deformation processes. Interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor. We compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements, including electron backscatter diffraction and ultrasound, on crustal rocks. The majority of samples developed fabric at pressures corresponding to middle to lower crustal condition. Tensor symmetry is a function of mineral modal composition, with mica‐rich samples trending toward hexagonal symmetry, amphibole‐rich samples trending toward an increased orthorhombic symmetry component, and quartz‐feldspar‐rich samples showing a larger component of lower symmetries. Seventy‐seven percent of samples have a best fit hexagonal tensor with slow‐axis symmetry, as opposed to mantle deformation fabric that usually has fast‐axis symmetry. The best fit hexagonal approximation for crustal tensors is not elliptical but deviates systematically from elliptical symmetry with increasing anisotropy, an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations. We present empirical linear relationships between anisotropy and ellipticity for crustal rocks. The maximum out‐of‐plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for nonelliptical symmetry. The elliptical assumption results in a bias of up to 1.4 times true anisotropy.

Dynamics of the lithosphere and upper mantle in light of seismic anisotropy

“…There are possible other contribution to anisotropy besides LPO due to past and present mantle flow, such as preserved shape preferred fabrics or LPO within the crust [e.g. Godfrey et al, 2000;Brownlee et al, 2017], or the effects of partial melt [e.g. Blackman et al, 1996;Holtzman and Kendall, 2010;Hansen et al, 2016a].…”

“…Vectorial tomography exploits the fact that individual minerals such as olivine show certain characteristics which link different elastic parameters, allowing reduction in the parameter space that has to be explored by a seismological inversion [Montagner and Anderson, 1989;Plomerová et al, 1996;Xie et al, 2015]. Similar relationships between parameters such as P and S wave anisotropy, for example, can be established for upper mantle LPO assemblages [Becker et al, 2006a or crustal rocks [Brownlee et al, 2017], and the resulting scaling relationships can improve inversion robustness [Panning and Nolet, 2008;Chevrot and Monteiller, 2009;Xie et al, 2015;Mondal and Long, 2019].…”

Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

“…• More work still needs to be done to include other parameters influencing seismic anisotropies, intrinsic and extrinsic, to build a database of natural rocks properties [5,55] , across the crustal profile from shallow to lower crust [6,39,54]. Funding: Neutron experiments were funded by ILL (www.ill.eu)…”

“…In the last two decades, a great effort has been made to quantify the seismic response of natural aggregates, chiefly by using the 2Dimensional approach of the EBSD (Electron Back Scattered Diffraction) to reconstruct the Orientation Distribution Function (e.g. [5][6][7]). Carbonatic rocks mainly occur at the surface and at shallow levels, as sedimentary cover, but can be found with the crust, as marbles.…”

Telewavesim: Python software for teleseismic body wave modeling

“…The integration of obspy (Krischer et al, 2015) Stream objects allows flexibility in manipulating and visualzing the resulting seismograms. The software contains a wide range of elastic stiffness definitions extracted from the literature (Brownlee et al, 2017) to accurately represent seismic anisotropy due to various mineralogies and rock types. The software also accurately models acoustic reverberations from an overlying column of water, effectively simulating ocean-bottom seismograph station recordings.The software will be useful in a variety of teleseismic receiver-based studies, such as P or S receiver functions and shear-wave splitting from core-refracted teleseismic shear waves (i.e.…”