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. Issue 9 (21st September 2015)

• Record Type: Journal Article
• Authors: Ji, Shaocheng; Shao, Tongbin; Michibayashi, Katsuyoshi; Oya, Shoma; Satsukawa, Takako; Wang, Qian; Zhao, Weihua; Salisbury, Matthew H.
• Is Part Of: Journal of geophysical research. Volume 120:Issue 9(2015:Sep.)
• Page Start: 6404
• Page End: 6430
• Publication Date: 2015-09-21
• Subjects: mica schist -- seismic wave velocities -- anisotropy -- shear wave splitting -- crystallographic preferred orientation -- Tibetan Plateau; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2015JB012209
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: 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 Vp anisotropy at 600 MPa 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 Vp 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 Vp ( X ) > Vp ( Y ) > Vp ( 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. Key Points: Departure of schists from transverse isotropy by amphibole, sillimanite, and quartz Decoupling between crust and upper mantle beneath the southeast Tibetan Plateau No southeastward extrusion for the Sibumasu and Indochina blocks since 45 Ma