Date i ftBSTBACT A partially reversed seismic refraction profile utilizing mine blasts as sources was recorded across southern British Columbia from Sparwood to the Highland Valley. The westwardly directed profile consisted of 32 short period seismograms covering 440 km, while the reversed line extended 330 km with 41 seismograms. The FM field tapes were digitized, filtered, and combined into vertical component record sections. Instrument and shot amplitude corrections were applied in order to

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... isplay the true relative amplitude variation along the profile. As well as geometrical technigues, least sguares delay-time methods and disc ray theory synthetic seismograms were used to interpret the data. Where the profile crosses the Eocky Mountain Trench and the Okanagan Valley, the seismic data show arrival time and amplitude anomalies in the approximately 6.0 km/s upper crustal refraction arrivals. An intermediate crustal layer of velocity 7.05 km/s, depth 29 km, and thickness 9 km is defined by second arrival data recorded 100 to 200 km west of the Kaiser shot point. There is poorer evidence, based on data between 80 and 180 km from the Highland Valley shot point, for a layer with a velocity of 7.35 km/s, a depth of 15 to 24 km, and a thickness of 6 to 10 km. The M-i i discontinuity generally dips to the east from an approximate depth of 33 km near the Highland Valley, to about 42 km just to the west of the Kaiser shot point. The upper mantle velocity is 7.8 km/s between the Okanagan Valley and Kootenay Lake. Delay time interpretations suggest that a 165 km wavelength anomaly in average crustal velocity and/or depth to the mantle occurs in the Eastern Metamorphic Belt. . The anomaly is probably associated with a deep transition zone between the Arrow Lakes and Kootenay Lake. The results of the present study are applicable primarily to the eastern Interior Plateaux and the Eastern Metamorphic Belt south of 51°N. The guidance and assistance of my supervisor. Dr.R.M. Clowes, has been greatly appreciated. Dr. R.M. Ellis who read the manuscript and provided field assistance also deserves thanks. The technical preparation and companionable field work of Chris West, and the technical clarifications by Ecb Meldrum were much appreciated. Messrs. Special appreciation is due my wife Yasmin, whc keypunched the drafts and encouraged a healthy academic perspective. This project was supported by the National Research Council cf Canada, the Department of Energy, Mines and Resources of Canada, and the British Columbia Department Mines and Fatroleum Resources. 2 little corroborating seismic data. The cooperation of the mining companies in obtaining shot statistics offered a relatively inexpensive opportunity to utilize the exceptionally large shots in a seismic investigation of a poorly understood region, 2. Regional Geology. The narrow region of southern Eritish Columbia within which the profile is located cresses several geological zones (Figure 1). At the eastern end, the Kaiser shot point is in the Rocky Mountains. This area was once a stable platform of Hudsonian shield-type recks overlain by miogeoclinal sedimentary seguences. In several pulses of activity, the first beginning in the Mesozoic, and the most recent ending in the Eocene, a series of low angle thrust sheets have been stacked progressively eastward. There is general agreement that, in the easternmost Front Ranges, a decollement separates the largely passive crystalline basement from the overthrust sediments. However, considerable debate has arisen over the degree of basement involvement in the thrusting in the Main and Western Ranges of the Rocky Mountains. The lateral shortening of the sedimentary section is partially controlled by basement deformation, and the degree of shortening will constrain the minimum westerly distance of the Prcterozoic continental shelf-slope boundary from the present western margin of the