Abstract
Muscovite is a major constituent mineral in the continental crust that exhibits very strong seismic anisotropy. Muscovite alignment in rocks can significantly affect the magnitude and symmetry of seismic anisotropy. In this study, deformation microstructures of muscovite-quartz phyllites from the Geumseongri Formation in Gunsan, Korea, were studied to investigate the relationship between muscovite and chlorite fabrics in strongly deformed rocks and the seismic anisotropy observed in the continental crust. The [001] axes of muscovite and chlorite were strongly aligned subnormal to the foliation, while the [100] and [010] axes were aligned subparallel to the foliation. The distribution of quartz c-axes indicates activation of the basal<a>, rhomb<a> and prism<a> slip systems. For albite, most samples showed (001) or (010) poles aligned subnormal to the foliation. The calculated seismic anisotropies based on the lattice preferred orientation and modal compositions were in the range of 9.0â21.7% for the P-wave anisotropy and 9.6â24.2% for the maximum S-wave anisotropy. Our results indicate that the modal composition and alignment of muscovite and chlorite significantly affect the magnitude and symmetry of seismic anisotropy. It was found that the coexistence of muscovite and chlorite contributes to seismic anisotropy constructively when their [001] axes are aligned in the same direction.
Highlights
Seismic anisotropy originating in the interior of the earth provides important information for understanding tectonic processes, deep earth structures and geodynamics [1â8]
We present data on deformation microstructures and lattice preferred orientation (LPO) of minerals revealed by Electron backscatter diffraction (EBSD), and the seismic properties of strongly deformed phyllite collected from the Geumseongri Formation in Gunsan, Korea, to understand the deformation mechanism of minerals and the causes of seismic anisotropy in the middle crust of highly deformed tectonic boundaries
The deformation microstructures and seismic anisotropies of greenschist-facies phyllites from the Geumseongri Formation in Gunsan, Korea, were studied to understand the relationship between LPOs, deformation conditions, mineral assemblages, and seismic patterns of deformed rocks in the middle crust
Summary
Seismic anisotropy originating in the interior of the earth provides important information for understanding tectonic processes, deep earth structures and geodynamics [1â8]. The fast S-wave polarization direction in continental crust is parallel to the major tectonic boundary formed by compressive regime [4,11,13]. These seismic patterns are usually attributed to the fault or fluid-filled cracks [14â16] and the orientation of anisotropic fabrics and structures [2,12,17]. Many studies using the receiver function technique [9,18], acoustic wave velocity measurements in laboratory settings [15,16,19], and fabric analysis on naturally [5,19â23] and experimentally [24,25] deformed rock samples have suggested that the layering and lattice preferred orientation (LPO) of anisotropic minerals is one of the important factors controlling the seismic anisotropy in the middle crust below the depth of microcrack closure (P â 150â250 MPa [15,16,18,26]). Phyllosilicate minerals show very strong anisotropy and it has been suggested that these minerals play an important role in affecting seismic anisotropies observed in various tectonic settings [5,17,22,30â34]
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