Constraints on crustal composition beneath a metamorphic core complex: results from 3-component wide-angle seismic data along the eastern flank of the Ruby Mountains, Nevada

Abstract

Metamorphic core complexes expose rocks deformed at deep upper to middle crustal levels during extreme crustal extension. However, mechanisms of crustal extension and exhumation of core complexes remain to be fully understood. Detailed study of crustal velocity structure and inferences about the composition of the crust beneath core complexes (and nearby) provide useful constraints on core-complex evolution. P- and S-wave velocity structures determined from seismic experiments along the eastern flank of the Ruby Mountains metamorphic core complex, Nevada, show that the crust can be divided into three main layers corresponding to the upper, middle and lower crust. We interpreted crustal composition by integrating results of P-wave velocities ( V p), S-wave velocities ( V s), Poisson's ratios ( σ), seismic anisotropy, and reflection character with published geologic maps of the area. Near-surface estimates of V p, V s, σ, and anisotropy of 1.90–4.8 km/s, 1.01–2.75 km/s, 0.25–0.33, and 0.6–2.5%, respectively, are consistent with surface exposures of unconsolidated to consolidated sedimentary rocks, limestone, dolomite, siltstone, sandstone, porous sandstone, conglomerate, and weathered granite. Results from analysis of reflection responses, V p, V s, σ, and anisotropy also indicate that: (1) upper crustal rocks most likely consist of metaquartzite, schist, granite gneiss, and granite–granodiorite with V p of 5.80–6.25 km/s, V s of 3.20–3.72 km/s, σ of 0.22–0.25, and anisotropy of 0.6–2.5%; (2) possible middle crustal rocks are paragranulite, felsic granulite, felsic amphibolite gneiss, granite–granodiorite, and mica–quartz schist with V p of 6.35–6.45 km/s, V s of 3.70–3.75 km/s, and σ of 0.24; and (3) lower crustal rocks most likely consist of granulite- rather than amphibolite-facies rocks with V p of 6.60–6.80 km/s, V s of 3.85–3.92 km/s, σ of 0.24–0.25, and anisotropy of <3%. Our principal conclusions are: (1) significant addition of gabbroic rocks (underplating) is unlikely in the lower crust; (2) lower crustal rocks were stretched into sub-horizontal geometries with aligned minerals during extension, creating seismic lamellae in the lower crust; (3) present-day seismic velocities of highly extended core complex crust and normally extended Basin and Range crust are similar; and (4) orientations of fast shear waves near the surface and in the upper crust are sub-parallel to the regional maximum horizontal compressive stress in the Nevada part of the Basin and Range province.