Magnitude and symmetry of seismic anisotropy in schists and geophysical implications

English Abstract:

We have calibrated the magnitude and symmetry of seismic anisotropy in various schists by laboratory measurements at pressures up to 600 MPa and analyses of EBSD fabrics in order to evaluate whether seismic properties of the schists can be reasonably well approximated by a transverse isotropic (TI) symmetry. The average bulk anisotropy values for chlorite schists, mica schists, phyllites and sillimanite-mica schists at 600 MPa are 12.0±4.1%, 12.8±5.6%, 12.8±9.0% and 18.4±4.4%, respectively. These schists show much higher V_p anisotropy (13.1±6.1%) than other categories of metamorphic rocks such as granitic gneisses, felsic mylonites, granulites and peridotites. V_p anisotropic pattern of the schists depends mainly on the interference effects between mica and quartz. Basal slip of mica causes V_p(X)≈V_p(Y)>>V_p(Z) while prism <a> slip of quartz increases V_p(Y) but decreases V_p(X). Accordingly the quartz-mica schists display V_p(Y)>V_p(X)>V_p(Z). However, schists containing considerable volume fractions of quartz deformed by prism <c> slip show V_p(X)>V_p(Y)>V_p(Z). Mica schists containing amphibole, kyanite and sillimanite are of orthorhombic symmetry due to the needle minerals whose fast c-axes are aligned preferentially parallel to the lineation. The average polarization direction of fast split SKS and SKKS waves (φ) between the Ailao Shan-Red River fault zone and the Nabang Fault zone in west Yunnan is 110±19°, which is approximately parallel to the absolute plate motion (APM) of the region but at high angles with the strikes of the crustal fault zones. The average delay time is 1.58±0.44 s. However, Pms phases, which are generated at the Moho and their wave path are confined within the crust only, display their fast polarization directions parallel essentially to the strikes of the fault zones and splitting times of up to 0.45 s with typical values of 0.30 s. Hence, S-wave anisotropy in the west Yunnan is consistent with a model of two anisotropic layers: the upper layer related to frozen fabrics of mica-rich and amphibole-bearing rocks within the crust, which deformed during folding and strike-slip shear, whereas the lower layer related to present-day APM-driven flow in the lithospheric mantle and asthenosphere. The data indicate that mechanical decoupling occurs between the crust and mantle of the region.