ELSEVIER Tectonophysics243 (1995) 255-276 TECTONOPHYSICS The bright Moho reflection in the 1986 Nevada PASSCAL seismic experiment Ramon Carbonell *'1, Scott B. Smithson Department of Geology and Geophysics, Unicersity of Wyoming, P.O. Box 3006, Laramie, WY82071, USA Received 14 May 1993; revised version accepted 7 July 1994 Abstract Detailed study of the reflected phases from the Moho, identified in the seismic reflection recordings of the 1986 Nevada PASSCAL lithospheric seismic experiment, place important constraints on the fine structure and geological nature of the Mohorovicic discontinuity. Two different spectral ratio techniques are used to estimate the reflectivity of the Moho. We evaluate the ratio of the reflected PmP phase to the incident source amplitude. We also calculated the spectral ratio of the PmP phase with a reference reflection. Reflectivity estimates, calculated using both spectral ratio techniques, are very consistent. High reflectivity within the range 0.28-0.39 was obtained. The analysis of the spectral characteristics suggests a layered structure for the Moho with an average thickness between 85 and 250 m. These results require a layered Moho structure with high impedance contrasts. Amplitude modeling using a reflectivity algorithm demonstrates that constructive interference from a 1.0-1.5-km-thick layered structure produces amplitudes and waveforms comparable to the data. To achieve such high reflectivities, 20-25% of the Moho consists of layers with velocities below 6.5 km/s. The remaining layers have velocities between 7.7 and 8.1 km/s. Geologically, the low velocities are consistent with velocities characteristic of basaltic magmas (partial melts). The high velocities are characteristic of mantle material. The partial melts may represent an expression of present-day underplating. I. Introduction The Basin and Range has been the objective of intensive geologic and geophysics investiga- tions for the last decade and has become an important testing ground for the ideas about ex- tension of continental lithosphere. A large num- * Corresponding author. 1Present address: Institute of Earth Sciences "Jaume Almera", Consejo superior de lnvestigaciones Cientificas, Marti i Franques s/n, 08028 Barcelona, Spain. ber of seismic refraction and reflection surveys has targeted the nature and geometry of the Cenozoic extension and the nature and evolution of the Moho in this province. One of the most remarkable and repetitive features observed on the deep seismic surveys acquired in the Basin and Range province is a subhorizontal, high-am- plitude and laterally persistent zone of Moho reflections (Klemperer et al., 1986; Hauser et al., 1987; Jarchow and Thompson, 1989). An abnor- mally high-amplitude near-vertical incidence Moho reflection characterizes the 1986 Nevada PASSCAL (Program for Array Seismic Studies of 0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0040-1951(94)00142-1