Implications on the nature of extended crust from multicomponent seismic studies of the Ruby Mountains metamorphic core complex, northeastern Nevada

Fundamental problems about the nature of extended crust include the role of plastic flow and mid-crustal detachment zones during extension, the possibility of fluids in the crust and magma ponded at the Moho, the amount of underplating, and the similarity of core complex crust to Basin and Range crust. In 1992 and 1993 the University of Wyoming conducted a multicomponent wide-angle seismic experiment in the Ruby Mountains of the Basin and Range to address these problems related to the nature of extended crust. The northern Ruby Mountains expose upper- and mid-crustal rocks of an extensive metamorphic core complex, whereas the southern Ruby Mountains consist of low-grade miogeoclinal rocks. The wide-angle reflection/refraction profile transected the southern and northern Ruby Mountains with a 95 km long N-S profile located adjacent to the Ruby Mountains uplift. Recording systems spaced in 100 m intervals or less recorded P- and S-wave energy from 18 explosive sources at offsets between 0 and 130 km.; Traveltime inversion of the wide-angle reflection and refraction data reveals that the upper crustal P- and S-wave velocities increase with depth and laterally from south to north. Middle and lower crustal layers have laterally and vertically constant average velocities and dip 5-10{dollar}spcirc{dollar} to the south. The relatively high seismic velocities in the lower crust of the Ruby Mountains indicate that fluid-filled pores, if they exist, do not reduce seismic velocities significantly. Therefore, the lower crustal porosity is probably much smaller than 1-2 vol. %. Magnetotelluric data from the Ruby Mountains indicate that the electrical resistivity is about 20 {dollar}Omega{dollar} m at depths between about 22 and 37 km. Brine contents of 0.1-0.2 vol. % are compatible with this mid to lower crustal resistivity. The velocity profile is consistent with a maximum of 6-7 km of mafic underplating in the southern Ruby Mountains and a maximum of 12-15 km of underplating in the northern Ruby Mountains. The upper to middle crust in the Ruby Mountains is seismically anisotropic probably due to superposed Tertiary plastic flow patterns. Doming and inflation of a mid-crustal layer in the seismic model is compatible with Tertiary flow of material from the southern to the northern Ruby Mountains that compensated about 2-5 km of core complex exhumation. Several high frequency reflections (25-50 Hz) are interpreted as originating from mid-crustal shear zones that dip 5-22{dollar}spcirc{dollar} to the north. Precritical Moho reflections (0-70 km offset, 10-11 s normal two-way traveltime) are weaker and less continuous than mid-crustal reflections (7-9 s normal two-way traveltime). Strong and continuous Moho reflections occur at postcritical ({dollar}>{dollar}80 km) offsets only. Modeling shows that the Moho depth increases from about 32.5 km in the southern Ruby Mountains to about 34.5 km in the northern Ruby Mountains. CDP data from a 12 km long E-W profile indicates that the depth of the Moho varies not only in N-S direction, but also increases east of the northern Ruby Mountains. (Abstract shortened by UMI.)...