HR: 0830h
AN: S11C-0317    [PDF]
TI: Crustal Seismic Velocity Anisotropy Beneath the Ruby Mountains Metamorphic Core Complex from 3-Component Wide-Angle Seismic Data
AU: * Johnson, R A
EM: johnson@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th St., Tucson, AZ 85721 United States
AU: Satarugsa, P
EM: peangta@kku.ac.th
AF: Khon Kaen University, Department of Geotechnology Faculty of Technology, Khon Kaen, 40002 Thailand
AB: Metamorphic core complexes, such as the Ruby Mountains-East Humboldt Range core complex in NE Nevada, expose rocks deformed at deep upper to middle crustal levels during extreme crustal extension. The northern Ruby Mountains expose ductilely deformed mid-Tertiary metamorphic rocks that have been tectonically exhumed from depths of ~13-14 km. However, the southern Ruby Mountains expose unmetamorphosed shallow-level sedimentary rocks. Analysis of Moho reflections from seismic data along the eastern flank of the Ruby Mountains shows that crustal thickness varies between 30.5 and 33.5 km, with no apparent correlation between amounts of extension, exhumation, or local topographic relief, implying that large-scale flow of ductile crustal rocks may have been important in the development of crustal structure beneath the Ruby Mountains. Ductile flow in rocks can impart significant seismic velocity anisotropy due to alignment of minerals. However, the calculated velocity anisotropies for all crustal levels beneath the Ruby Mountains are significantly lower than are determined from laboratory measurements on representative metamorphic rocks from the area. Analyses of shear-wave-splitting results from 3-component normal-incidence to wide-angle reflection/refraction data from the Ruby Mountains indicate that upper crustal metamorphic rocks, including areas containing mylonitic fabrics, are characterized by velocity anisotropies of 0.6-2.5%. Orientations of fast shear waves near the surface and in the upper crust are approximately parallel to the regional maximum horizontal stress in the Nevada part of the Basin and Range Province, suggesting that the bulk anisotropy in the upper crust may be controlled by stress-induced fractures. From middle and lower crustal levels beneath the Ruby Mountains, shear-wave splitting was not identified in the 3-component seismic data. Lack of evidence in the data for shear-wave splitting in the lower crust suggests that velocity anisotropy is less than ~3% in an area that has experienced intense extensional deformation and probable crustal flow.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting