Seismic velocity anisotropy of phyllosilicate-rich rocks: characteristics inferred from experimental and crack-model studies of biotite-rich schist

Abstract

S U M M A R YSeismic velocity anisotropy of biotite schist (30 per cent-mode biotite) was measured under confining pressures up to 150 MPa. The rock shows weak orthotropy which was altered from transverse isotropy (TI) generated by biotite-preferred orientation. The orthotropy was caused by microfolding in the rock. The velocity increase under confining pressure indicates that most crack planes are aligned parallel to the cleavage planes (silicate sheet) of the oriented biotite minerals. The anisotropy of the rock is basically TI due to both the aligned biotite minerals and cracks, which have a common symmetry axis. We found that other sheet silicate-rich rocks have a similar anisotropy with the biotite schist, in which the TI-type anisotropy is characterized by the slow P-and S-wave velocities along the symmetry axis. This is caused by the preferred orientation of sheet silicate minerals and the extremely slow P-and S-wave velocities along the axis perpendicular to the silicate sheet compared to the directions along the silicate sheet. When rock contains a large percentage of highly oriented sheet silicates, the fast and slow shear waves exchange their polarities at some off-symmetry axis directions, indicating that the qSwave (quasi-S wave) velocity exceeds the SH-wave velocity. The phase velocity distribution of qS wave shows an asymmetry with respect to the angle from the symmetry axis, which is characterized by a bulge in this distribution located near the symmetric axis. This is inherent to most sheet silicate minerals. When crack density of aligned cracks increases, the P-wave velocity along the symmetry axis decreases considerably. The qS-wave phase velocity in the off-axis directions also decreases, in accordance with the decrease of the P velocity along the symmetry axis. The asymmetry of the qS-wave phase velocity distribution increases as the P-wave velocity decreases along the symmetry axis. This relationship can be well understood by means of Berryman's extended Thomsen approach.

High Vp/Vs ratio: Saturated cracks or anisotropy effects?

“…Ultrasonic measurements on rocks containing anisotropic fabrics, both mineral and crack induced, are more common however. For example, Nishizawa and Kanagawa [2010] have recently observed “apparent” Vp/Vs ratio as high as 2.4, for waves propagating parallel to the crack and mineral fabrics in biotite rich micaschists, under dry and elevated pressure (150 MPa) conditions. Similarly, Sarout and Guéguen [2008] observed Vp/Vsv ratios of 2.35 in dry consolidated clays for wave propagating parallel to the fabric, under 200 MPa isotropic stress.…”

Influence of mineralogy on granite decay induced by temperature increase: Experimental observations and stress simulation

“…7a) used for modeling the maximum principal stress distribution upon heating at 400°C for two extreme cases: the first one in which quartz thermo-physical properties were assigned to all crystals (Fig. 7b) and the second one in which The single-crystal elastic constants, C ij , in GPa (Nishizawa and Kanagawa, 2010) and the crystalline coefficients of thermal expansion, α ij , in 10 −6 K −1 for biotite (hexagonal symmetry) (Hall et al, 2008). microcline thermo-physical properties were assigned to all crystals (Fig.…”

Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

“…Some of the major contributors of seismic anisotropy may arise from (1) weaknesses in the crust (Nishizawa & Kanagawa, ), (2) melt lenses (Kendall, ), and/or fossil anisotropy (e.g., Silver & Chan, ) among other sources, and (3) the alignment of olivine minerals in the upper mantle governed by dislocation creep (Karato & Wu, ). Anisotropy in the crust can be from shallow sources due to the alignment of sheet silicates (e.g., Nishizawa & Kanagawa, ), faults, cracks, and microcracks in the upper 10–15 km (e.g., Kaneshima et al, ). Fossil anisotropy is crystallographic fabric preserved in the crust and/or lithospheric mantle imprinted by past or present‐day orogenic processes (e.g., Tian & Santosh, ; Silver & Chan, ).…”