Seismic velocity anisotropy in mica-rich rocks: an inclusion model

Abstract

SUMMAR YWe calculated seismic wave velocity anisotropy caused by the preferred orientation of mica minerals by using the differential effective medium method (DEM). Spheroidal biotite crystals with their c-axes coinciding with the symmetry axis of the spheroid are embedded in an isotropic matrix up to a volume ratio of 30 per cent. All crystals are aligned with their c-axes parallel to the symmetry axis of the effective homogeneous medium, which shows transverse isotropy. The effect of crystal shape on anisotropy was studied by changing the aspect ratio (the ratio between the minor axis and the major axis of the spheroid) from 0.01 (flat spheroid) to 1 (sphere). The S-velocity anisotropy becomes large as the crystal shape becomes flat, whereas the P-velocity anisotropy shows only small changes with changes of the crystal shape. In particular, biotite generates large S-wave anisotropy, and the anisotropy becomes stronger as the aspect ratio of the biotite crystal becomes smaller. When the volume ratio of the mica mineral is large, the P-wave phase velocity surface shows considerable deviation from the ellipse, and the SV-wave phase velocity surface forms a large bulge and crosses the SH-wave phase velocity surface (singularity) in the plane including the symmetry axis. These results show an interesting contrast when compared with the effect of crack or pore shape on seismic velocity anisotropy: crack (or pore) shape affects the P velocity more than the S velocity. We also calculated Thomsen's anisotropic parameters, e, c and d as functions of the crystal aspect ratio and the mica volume ratio.

Crustal Radial Anisotropy of the Iran Plateau Inferred From Ambient Noise Tomography

“…Other mechanisms like coherent distributions of gaps and aligned cracks can lead to radial anisotropy in the upper crust and shallow structures, usually termed as shape preferred orientation. Preferential orientation of the mica lattices is usually known as the main factor of anisotropy in the middle crust (Nishizawa & Yoshino, 2001; Shapiro et al., 2004) and amphiboles may play the main role for anisotropy in the lower crust (Barberini et al., 2007). Most minerals in the upper mantle (Olivine, Orthopyroxene, and, Clinopyroxene) are intrinsically anisotropic.…”

Crustal Deformations of the Central North China Craton Constrained by Radial Anisotropy

“…Located in this transition zone, the FWR zone may be of inherited weakness and thus is more susceptible to the regional extension processes during the late Mesozoic to the Cenozoic (Zhang et al, 2003). Generally, the continental rifting would promote horizontal alignment of anisotropy minerals, such as micas (Lynner et al, 2018;Mainprice & Nicolas, 1989;Nishizawa & Yoshino, 2001;Shapiro et al, 2004;Xie et al, 2013), through ductile deformation in the middleto-lower crust beneath the brittle-ductile transition boundary. In the case of horizontal alignment of micas, the formed sheets exhibit pronounced positive radial anisotropy with the symmetry axes of the aligned anisotropy minerals oriented vertically (Xie et al, 2013).…”

Midcrustal Deformation in the Central Andes Constrained by Radial Anisotropy

“…While volumetrically small in the crust, micas are the most important anisotropic minerals (e.g., Lloyd et al, 2009;Tatham et al, 2008). Quartz and other volumetrically dominant crustal minerals are isotropic to weakly anisotropic or do not deform in such a way as to produce strong radial anisotropy (Lloyd et al, 2009;Mahan, 2006;Nishizawa & Yoshino, 2001;Ward et al, 2012). When deformed, micas tend to align such that the seismically slow axis orients orthogonally to the deformation direction.…”