HR: 16:32h
AN: V14A-03    [Abstracts]
TI: The Origin of Seismic Reflectivity in Ultra High-Pressure Metamorphic Terrain: the Case From Chinese Continental Scientific Drilling
AU: * Jin, Z
EM: zmjin@cug.edu.cn
AF: Department of Earth Sciences, China University of Geosciences, Wuhan, 430074 China
AU: Ou, X
EM: xgou@gig.ac.cn
AF: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 China
AU: Xu, H
AF: Graduate school, China University of Geosciences, Wuhan, 430074 China
AU: Jin, S
AF: Department of Earth Sciences, China University of Geosciences, Wuhan, 430074 China
AU: Wu, Y
AF: Graduate school, China University of Geosciences, Wuhan, 430074 China
AB: The elastic wave velocities of 700 samples from main hole of Chinese Continental Scientific Drilling (CCSD) have been performed in the conditions at room temperature and pressure. The continuous profile of seismic velocities (Vp and Vs) from 100m to 2000m was constructed, which provides an important petrophysical data for interpretation of geophysical model. Fresh eclogite yields the biggest compressional wave velocity (7.86km/s), and gneiss has smaller velocity (5.53-5.71km/s). Seismic velocity of eclogite significantly decreases with increase of retrogressive metamorphism. Overall average Vp velocity of main hole of 2000m rocks is 6.2km/s, which is correspondent to conclusion with high velocity zone (6.2-6.3km/s) of the upper crust in Dabie-Sulu orogenic belt inferred from geophysical methods. Most rocks at the main hole have pronounced seismic anisotropy. The velocity of Vp and Vs with water-saturated rock increases by a factor of 19% and 10%, respectively. Conversely, anisotropy of Vp and Vs with water-saturated rocks reduces 3%-4%. Reflective coefficient (Rc) of different lithological boundaries is the main factor producing seismic reflection. The lithological boundaries between rutile-bearing eclogite and gneiss have the biggest reflective coefficient (0.24-0.31). The boundaries between fresh eclogite and retrogressive eclogite is characterized by strong reflection coefficient (Rc=0.27). Reflection coefficient of paragneiss contacting with fresh eclogite is 0.24. The margin between paragneiss and orthgneiss has the lowest seismic coefficient (Rc=0.02) if the rocks did not experience strong ductile shearing. The mylonitized gneiss in ductile shear zone is a good seismic reflector. P-wave anisotropy of gneiss increase with strengthening mylonitization. The measurements of elastic wave velocity in this study provide an important constraint on the origin of seismic reflection of Ultrahigh-pressure metamorphic terrain in Chinese Continental Scientific Drilling.
DE: 1744 Tectonophysics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting