MANTLE REFLECTIONS BENEATH THE MONTANA GREAT-PLAINS ON CONSORTIUM FOR CONTINENTAL REFLECTION PROFILING SEISMIC-REFLECTION DATA

A systematic search for reflections from the upper mantle has been carried out on 2000 km of Consortium for Continental Reflection Profiling seismic reflection data from 14 different sites across the United States. The general observation from these data is the striking contrast between the generally pervasive reflectivity of the crust and the relative lack of coherent reflectivity in the upper mantle, a contrast that cannot always be attributed to lack of signal penetration. In order to image more of the upper mantle section on the seismic data, seismic field files were increased from 16 to 28 s two-way travel time by extended correlation processing. While the results reveal an upper mantle generally devoid of laterally continuous coherent energy, pronounced local reflections, called here the Lonesome Lake Complex, are observed at upper mantle travel times beneath the Montana Plains. Reflections at mantle travel times do not ensure mantle reflectors, as other mechanisms operating at shallower levels in the crust may be responsible for the observed reflections. However, multiple reflections and converted waves from the crust are unlikely explanations for the Lonesome Lake Complex. Although energy from outside the plane of profile cannot be ruled out, circumstantial evidence suggests that these reflections are likely due to intramantle heterogeneity. The Lonesome Lake Complex joins examples of upper mantle reflectivity observed on other deep seismic data worldwide in establishing the mantle as an productive target for future deep seismic surveys.