Crustal structure of the Eastern Snake River Plain determined from ray trace modeling of seismic refraction data

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 interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the ESRP when compared with the upper crust beneath the adjoining Northern Rocky Mountain and Basin and Range provinces. The most prominent 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) with an offset of greater than 4 km. The southeast margin, conversely, appears to be downwarped with possible minor faulting. Paleozoic 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 intermediate 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.