| Summary: | Seismic studies in the area of the Ruby Mountains metamorphic core complex and adjacent basins of northeast Nevada provide new evidence for Cenozoic tectonic evolution of the Ruby Mountains. Results from interpretation of industry seismic data show that (1) asymmetric basins flanking the Ruby Mountains were created by normal faults beginning in the late Eocene-early Oligocene; (2) the metamorphic core complex detachment fault system was cut by the normal fault system; and (3) total subsidences of Huntington and Lamoille basins, and Ruby basins are ∼4.5 and ∼5.0 km. Analysis of crustal-scale 3-component normal-incidence to wide-angle seismic data shows that (1) the crust along the eastern flank of the Ruby Mountains can be divided into three layers corresponding to the upper, middle and lower crust; (2) upper crustal rocks likely consist of metaquartzite, schist, granite gneiss, and granite-granodiorite with P-wave velocities (Vp) of 5.80-6.25 km/s, S-wave velocities (Vs) of 3.20-3.72 km/s, Poisson's ratios (sigma) of 0.22-0.25, and anisotropy of 0.6-2.5%; (3) possible middle crustal rocks are paragranulite, felsic granulite, felsic amphibolite gneiss, granite-granodiorite, and mica-quartz schist with Vp of 6.35-6.45 km/s, Vs of 3.70-3.75 km/s, and σ of 0.24; (4) lower crustal rocks most likely consist of granulite- rather than amphibolite-facies rocks with Vp of 6.60-6.80 km/s, Vs of 3.85-3.92 km/s, σ of 0.24-0.25, and anisotropy of less than 3%; (4) depth to the Moho varies irregularly between 30.5 and 33.5. Interpretation of these results suggests that (1) Cenozoic extension of the Ruby Mountains and adjacent basins began by late Eocene-early Oligocene; (2) depth to Moho does not reflect local surface relief on the eastern flank of the Ruby Mountains and adjacent basin; (3) fluid-filled fractures and mafic large-scale underplating are unlikely in the lower crust; (4) the present seismic velocities of highly extended core complex crust and normally extended Basin and Range crust are similar; and (5) orientations of fast shear waves near the surface and in the upper crust are parallel to sub-parallel to the regional maximum horizontal compressive stress in the Nevada part of the Basin and Range province.
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