JOURNAL OF GEOPHYSICAL RESEARCH,VOL. 87, NO. B4, PAGES 2619-2633, APRIL 10, 1982 CRUSTAL STRUCTURE OF THE EASTERN SNAKE RIVER PLAIN DETERMINED FROM RAY TRACE MODELING OF SEISMIC REFRACTION DATA M. A. Sparlin 1 and L. W. Braile Department of Geosciences, Purdue University, West Lafayette, Indiana 47907 R. B. Smith Department of Geology and Geophysics, University of Utah Salt Lake City, Utah 84112 Abstract. Ray trace travel time modeling of the seismic refraction record sections for a profile from near Soda Springs, Idaho, to near McKay, Idaho, was used to derive a crustal model across the eastern Snake River Plain (ESRP). The derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the ESRP. The inter- pretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the ESRP when compared with the upper •rust beneath the adjoining Northern Rocky Mountain and Basin and Range provinces. The most pro- minent features of the crustal structure inferred by the ray trace modeling are as follows: (1) The northwest margin of the ESRP was modeled as a fault structure, downthrown on the SRP side (SE) wi•h an offset of greater than 4 km. The southeast margin, conversely, appears to be downwarped with possible minor faulting. Paleozoi c sedimentary rocks are present beneath the Cenozoic volcanics of the ESRP possibly to and beyond 40 km from the southeastern margin of the ESRP near Blackfoot, Idaho. (2) The modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the ESRP. (3) An inter- mediate 6.5 km/s layer occurring beneath the ESRP, interpreted from a refraction profile located along the axis of the ESRP, was found to be localized within the ESRP margins. This layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region. (4) A density model of the crust across the ESRP was prepared with the densities selected using the interpreted seismic velocities as a constraint. The gravity field calculated from this model resulted in a good match to observed gravity data over the eastern Snake River Plain. Introduction During the 1978 Yellowstone-Snake River Plain (Y-SRP) seismic profiling experiment, a partially reversed seismic refraction profile was recorded along a northwest-southeast line across the axis of the eastern Snake River Plain (ESRP). Four shotpoints located along this profile resulted 1Now at Arco Oil and Gas Company, Dallas, Texas 75221. Copyright 1982 by the American Geophysical Union. Paper number 1B1333. 0148-0227 / 8 2/00lB-1333 $05.00 in refracted and reflected arrival phases, which have been interpreted in order to infer the crustal structure beneath the ESRP and the tran- sition in crustal models between the ESRP and adjacent provinces. The objective of this paper is to describe ray trace travel time modeling applied to the seismic data recorded along this northwest-southeast line. In addition, the anom- alous gravity field of the derived crustal model is compared with observed gravity data across the ESRP in order to confirm and further con- strain the crustal model. The details of the 1978 Y-SRP seismic profiling experiment and shot- point and seismograph information are presented by Smith et al. [this issue] and Braile et al. [this issue]. Data Acquisition and Presentation Seismograph stations and shot locations for the eastern SRP phase of the Y-SRP seismic experiment are shown in Figure 1. Four shot- points were recorded along the axis of the ESRP; SP5, SP8, SP4 and SP2; and four shotpoints along a profile approximately perpendicular to the ESRP axis; SP7, SP8, Gay Mine, and Conda Mine. Shotpoint 8 was recorded both transverse (NW- SE) and parallel (NE-SW) to the axis of the ESRP. Shotpoint locations, origin times, sizes, and other relevant information for these sources are given by Braile et al. [this issue]. The 196-km seismic refraction line oriented roughly perpendicular to the axis of the ESRP and inter- secting both the northwest and southeast bound- aries of the SRPnear Arco and Blackfoot, idaho, respectively, has been interpreted using ray trace modeling. This refraction profile will be termed the Conda-SP7 line or the 'crossline.' The interpretation procedure and the derived crustal model along this profile are the principal topics of this paper. In addition, a 47-km- long reversed profile from SP8 to SP4 near the center of the ESRP (Figure 1) was•interpreted. This profile is particularly relevant to the Conda-SP7 interpretation because the two profiles intersect near the axis of the ESRP. As described in detail by Smith et al. [this issue] and Braile et al.. [this issue], seismic data were recorded along the Conda-SP7 pro fil '•_ utilizing portable seismographs with timing referenced to crystal clocks and radio signal WWV or the radio broadcast time standard WWVB. Seismograms recorded on FM tape or analog records (240 mm/min or 480 •u/min fast rotation smoked- paper drum recordings) were subsequently digi- tized for plotting as reduced-time record sec- 2619