Magnitude and symmetry of seismic anisotropy in mica- and amphibole-bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau Ji, Shaocheng; Shao, Tongbin; Michibayashi, Katsuyoshi; ...

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Ji, Shaocheng ; Shao, Tongbin ; Michibayashi, Katsuyoshi ; Oya, Shoma ; Satsukawa, Takako ; Wang, Qian ; Zhao, Weihua ; Salisbury, Matthew H ; Macquarie University. Department of Earth and Planetary Sciences
We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica- or amphibole-bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually assumed in the interpretation of seismic data. The average bulk V[p] anisotropy at 600MPa for the chlorite schists, mica schists, phyllites, sillimanite-mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show V[p] anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite-mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite, and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation, forming orthorhombic anisotropy with V[p](X)>V[p](Y)>V[p](Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica- or amphibole-bearing schists and mylonites are approximately transversely isotropic in terms of S wave velocities and splitting although their P wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan Plateau. The N-S to NW-SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica- and amphibole-bearing rocks deformed by predominantly compressional folding and subordinate strike-slip shear. These blocks have been rotated clockwise 70-90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia. 27 page(s)
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