CRUSTAL STRUCTURAL GEOLOGY OBSERVED BY SEISMIC ANISOTROPY

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.

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.