The Basin and Range province of the western United States is characterized by active east-west extension at strain rates of ~1 cm yr-1, high regional heat flow of ~90 mW m-2, and widespread late Cenozoic bimodal rhyolitic-basaltic magmatism. The 1986 Program for Array Seismic Studies of the Continental Lithosphere Basin and Range seismic experiment imaged a portion of northwestern Nevada to determine the crustal structure and to assess reported difference between refraction versus reflection determinations of Moho depth and how the crustal composition and structure has been influenced by volcanic and extension mechanisms. Our interpretation of the refraction/wide-angle reflection data suggests that the crust is fairly uniform in thickness and varies by less than 5 km over the 280 km east-west profile and 3 km over its 220 km north-south length. We characterize the velocity structure by five layers: (1) and upper most curst, composed of sedimentary rocks and basement that has an average velocity of 5.7 km s-1, (2) a middle crust that extends to a depth of 18--22 with an average velocity of 6.1 km s-1, (3) a 10--12 km thick lower crust with an average velocity of 6.6 km s-1, (4) a 2--5 km thick transitional crust-mantle boundary defined by a 7./6 km s-1 velocity, and (5) an upper mantle with an average Pn velocity of 7.9--8.0 kms s-1. These data show a uniformily thick crust of 28 km¿5 km beneath northwestern Nevada and a lack of evidence for significant low-velocity crustal layers. In general, the entire crustal column exhibits uniform positive velocity gradients of 0.02--0.04 s-1. The midcrustal discontinuity is interpreted as a small velocity increase of 0.2 km s-1 at ~18 km depth.

The absence of a significant midcrustal reflection and the presence of large velocity gradients throughout the crust arises either from (1) a systematic increase in mafic composition with depth, or (2) possibly high lower crustal pore pressure that acts to decrease the velocity contrast across the midcrustal boundary. Our interpreted seismic velocity structure is consistent with a broad range of bulk compositions for the crust. Upper crustal velocities are consistent with bulk compositions that range from metasediments and granites at shallow depth to diorites at midcrustal levels, while lower crustal velocities suggest bulk compositions that range from diorites and amphibolities, at midcrustal depths, to mafic granulites and gabbros at the base of the crust. The uniform crustal thickness of 30--34 km in northwestern Nevada is close to the crustal average for the Basin and range province but is thin when compared to surrounding regions, such as the Sierra Nevada and Colorado Plateau where crustal thickness is greater than 40 km. Similarily northwestern Nevada Pn velocities of 7.9--8.0 km s-1 are near the province-wide average of 7.9 km s-1. Temperature-corrected Pn velocities, from a 90 mW m-2 geotherm for the Basin and Range to 60 mW m-2 geotherm for the Basin and Range to 60 mW m-2 for the stable interior of the United States, are within one standard deviation of the continental average of 8.05 km s-1 and suggest a uniform upper mantle composition across the Basin and Range. In addition to these observations, the homogeneity of the velocity structure beneath the western Basin and Range argues for a youthful Moho and crust that has been reworked by province-wide late Cenozoic extension, episodic magmatism, and underplating. ¿American Geophysical Union 1990