Two‐dimensional travel time inversion for the crustal P and S wave velocity structure of the Ruby Mountains metamorphic core complex, NE Nevada

Abstract

Problems about the nature of extended crust in the Basin and Range include the role of plastic flow during extension, the possibility of fluids in the crust, and the amount of mantle‐derived material added to the crust. To address these problems, the University of Wyoming conducted a multicomponent wide‐angle seismic experiment in the Ruby Mountains of the Basin and Range. The northern Ruby Mountains expose upper and middle crustal rocks of a metamorphic core complex, whereas the southern Ruby Mountains consist of low‐grade miogeoclinal rocks. The wide‐angle experiment consisted of a 95 km long N‐S profile that extended from the southern to the northern Ruby Mountains. Travel time inversion of the wide‐angle reflection data reveals relatively high seismic velocities in the midcrust to lower crust which suggest that fluid‐filled pores, if they exist, do not reduce the seismic velocities significantly. Therefore the midcrustal to lower crustal porosity is probably much smaller than 1–2 vol %. The average P wave velocity for the crust is 6.4–6.5 km/s in the north and 6.1–6.2 km/s in the south. This difference suggests that larger amounts of mantle‐derived material were added during the Tertiary to the core complex crust of the northern Ruby Mountains than to the “normal” Basin and Range crust of the southern Ruby Mountains. The velocity profile is consistent with a maximum of 6–7 km of mantle‐derived material in the south and a maximum of 12–15 km of mantle‐derived material in the north. Thus volumetric mafic intrusions probably accommodated a maximum of 7 km of core complex exhumation. Shear wave splitting indicates that the upper to middle crust is seismically anisotropic probably due to superposed Tertiary plastic flow patterns. Doming and inflation of a midcrustal layer in the seismic model are compatible with Tertiary flow of material from the south to the north that compensated about 2–5 km of core complex exhumation. Precritical Moho reflections (0–70 km offset, 10–11 s normal two‐way travel time) are weaker and less continuous than midcrustal reflections (7–9 s normal two‐way travel time). Strong and continuous Moho reflections occur at postcritical (>80 km) offsets only. Modeling shows that the Moho depth increases from about 32.5 km in the south to about 34.5 km in the north.