Comment on “Crustal structure and composition of the southern Foothills Metamorphic Belt, Sierra Nevada, California, from seismic data” by Kate C. Miller and Walter D. Mooney

Abstract

We appreciate the comments of Murdock [this issue] on our recent study [Miller and Mooney, 1994] of the southern Foothills Metamorphic Belt of the Sierra Nevada, California. These comments are of particular interest in that they cite evidence for seismic anisotropy in the upper crustal metamorphic rocks of our study area [Murdock, 1980]. Such anisotropy can be expected based on laboratory measurements of rock cores [e.g., Christensen, 1966], but field measurements of seismic anisotropy have only recently been reported [Brocher and Christensen, 1990; Rabbel and Mooney, 1995]. Thus the 1972 study of Murdock [1980] is one of the first field measurements of anisotropy in metamorphic rocks. Murdock [1980] reports results from an analysis of records of seven shots recorded at approximately 30 stations laid out along three Wansects within a study area that overlaps our own in the southern Foohills Metamorphic Belt. The primary result of Murdock [1980] is that near-surface velocities range from 1.7 to 6.0 km s -1 and that he basement refractor (Pg) has a velocity of 6.3 km s-1. We acknowledge that we omitted the work of Murdock [ 1980] from our references but find his results to be consistent with our own. We present a model which contains a 5.24 km s -1 layer from the surface to4 km depth. This velocity is constrained primarily by shots recorded at receivers lying in arrays perpendicular to the strike of the Foothills Metamorphic Belt. Since a number of these stations are in the Great Valley and the Sierran batholith, our shallow model reflects a somewhat wider range of geology. We have already pointed out hat he depth to the top of the 6.3 km s-1 is poorly constrained. Murdock's report of velocities as low as 6.0 km-1 due to anisotropy in the lower layer may also help explain the lower-velocity estimate found in our analysis. Our knowledge of the crustal structure of the Sierra Nevada is limited and has been controversial for more than 30 years. However, we disagree with Murdock's statement that showing east dip on the Moho beneath the foothills is unwarranted. Figure 8 of Oppenheimer and Eaton [1984] clearly shows east dip from 30 to approximately 39 km along the cross section drawn in Figure 12 of Miller and Mooney [1994]. If anything we have underestimated the amount of dip across this region. Furthermore, results from a recent (1993) seismic refraction profile across the southern Sierra Nevada, 100 km south of this study area (S. Ruppert et al., Crustal structure and thickness of the southern Sierra Nevada from seismic refraction profiles, submitted to Geophys. Res. Lett., 1995; M. M. Fleidner et al., Three-dimensional crustal structure of the southern Sierra