HR: 0830h
AN: S11C-0292    [PDF]
TI: Field Observations of Crustal Seismic Anisotropy: Implications for Mapping Tectonic Structure in Metamorphic Terranes
AU: * Christensen, N I
EM: chris@geology.wisc.edu
AF: Univ. Wisconsin, Dept. Geology, U. Wisc., Madison, WI 53706 United States
AU: Okaya, D
EM: okaya@usc.edu
AF: Univ. So. California, Dept. Earth Sciences, USC, Los Angeles, CA 900890740 United States
AU: Meltzer, A
EM: ameltzer@lehigh.edu
AF: Lehigh Univ., Dept. Earth Sci., 31 Williams Dr., Bethlehem, PA 18015 United States
AU: Brocher, T
EM: brocher@usgs.gov
AF: USGS, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Holbrook, W S
EM: steveh@uwyo.edu
AF: U. Wyoming, Dept. Geology & Geophysics, Laramie, WY 82071 United States
AB: The study of seismic anisotropy within continental tectonic provinces provides earth scientists with a powerful tool for measuring and quantifying deformation within the crust. Preferred mineral alignment observed in metamorphic terranes produced by recrystallization during metamorphism is associated with planar structures such as slaty cleavage, schistosity, and gneissic layering. These structures are often pervasive for tens to hundreds of kilometers and produce significant compressional wave seismic anisotropy as well as shear wave splitting. Observations of crustal anisotropy within (1) slates of the chlorite subzone of the Haast schist terrane of South Island, New Zealand, (2) lower greenschist facies phyllites and metagraywackes of the Valdez Group Chugach terrane in southern Alaska, (3) amphibolite facies mica schists within the Yukon-Tanana terrane in the eastern Alaska range and (4) amphibolite facies quartzofeldspathic gneisses, approaching granulite grade, within the Nanga Parbat-Haramosh massif demonstrate that crustal anisotropy is not limited to rocks of any particular metamorphic grade and thus can be present at all crustal levels. Two refraction lines at approximately right angles shown up to 10% compressional wave anisotropy in relatively low grade metapelites of the Haast schist terrane. Fast velocities parallel the strike of the upturned slaty cleavage. Measured field velocities in the Chugach terrane, obtained from observed first arrival travel times, demonstrate significant compressional wave anisotropy (~9%) with fastest directions oriented approximately east-west and parallel to foliations observed in outcrops. Within the Alaskan Yukon-Tanana terrane variations in seismic velocities of the first arrivals correlate with field observations of regional dips of foliated schists. A northward shallowing of foliation dips produces an observed northward increasing seismic velocity. The core of the Nanga-Parbat massif forms a large-scale antiformal structure with an axial orientation of N10 degrees E with near vertical lineations. Observations of local seismicity show shear wave splitting which originates within the high-grade granitic and metasedimentary gneisses of the massif. Laboratory velocity measurements on rocks collected from surface exposures within these four regions are consistent with the magnitudes and directions of the observed anisotropies. We conclude that future field investigations designed specifically to study crustal seismic anisotropy, combined with laboratory measurements, will provide valuable information on the structure, magnitude and extent of crustal tectonic deformation.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7203 Body wave propagation
DE: 7260 Theory and modeling
DE: 8025 Mesoscopic fabrics
DE: 8110 Continental tectonics--general (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting