The crustal structure of the Northwestern Basin and Range Province, Nevada, from wide‐angle seismic data

Abstract

I present a model of lithospheric P‐wave velocity structure in the northwestern Basin and Range province, Nevada, based on wide‐angle seismic data collected in 1986 under the auspices of the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL). The lithosphere in the Basin and Range consists of (1) 2‐ to 3‐km‐thick low‐velocity (2.5–4.0 km/s) sedimentary basins and high‐velocity (>4.5 km/s) ranges overlying a basement of velocity 5.5–6.1 km/s, (2) a mid‐crust (6.1–6.2 km/s) which consists of rocks of probable granitic to granodioritic composition in greenschist to amphibolite facies and which includes two local low‐velocity zones (5.5–5.7 km/s), (3) a laterally varying lower crust which consists of rocks of probable gabbroic composition in upper amphibolite to granulite fades and which comprises layers of 6.6 and 7.0–7.2 km/s average velocity, (4) a complex crust‐mantle transition best modeled as a velocity gradient zone 1–2 km thick, and (5) an upper mantle with velocities of 7.7–7.9 km/s which includes a local low‐velocity zone (7.5 km/s). Crustal thickness in the region varies laterally from 28 to 37 km, significantly thicker than several previous estimates. The upper mantle velocities and velocity gradients differ slightly on the two perpendicular profiles, suggesting possible azimuthal anisotropy. A wedge‐shaped, eastward‐thickening lower crust of high seismic velocity (7.0–7.2 km/s) on the eastern pan of the survey may represent the westward edge of “rift‐stage” Précambrien continental crust or, alternatively, mafic material underplated and intruded into the crust during Cenozoic extension. Comparison of the velocity model with coincident seismic reflection data shows that the reflection and refraction Mohos are coincident across the entire profile, thus indicating that deep reflections (4–10 sec two‐way travel time) seen on COCORP and PASSCAL‐piggyback reflection profiles originate entirely within the crust. The top of the reflective zone, however, lies consistently above the mid‐crustal increase in average velocity, indicating that the reflectivity is not restricted to the mafic part of the crust; on PASSCAL piggyback profiles, in fact, the middle (silicic) crust appears more reflective than the lower (mafic) crust This can be explained as the result of either mafic, sill‐like intrusions or ductile strain fabrics. Both of these processes are likely to have accompanied Cenozoic extension in the region.