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An unreversed seismic refraction profile has been recorded in the southern Rocky Mountain Trench from 50°N to 53°N. Using blasts from two open-pit coal mines, 44 recordings were obtained over a distance of 540 km. These were combined into a record section in which instrument and shot variations were included to show amplitude variations along the profile. Interpretation involved Weichert-Herglotz integration of p -delta curves to obtain a velocity-depth structure and the calculation of synthetic seismograms for comparison with the record section. Refractors with apparent P -wave velocities of 6.5 to 6.6 km/sec and 8.22±0.04 km/sec are interpreted as the surface of the Precambrian basement and the Mohorovicic discontinuity, respectively. A prominent travel-time delay associated with the 6.5 km/sec branch is interpreted in two possible ways. One explanation is the existence of a crustal low-velocity zone beginning 3 km beneath the basement, depth of 6.5 km, and having a depth extent of 9 to 15 km with associated velocities of 5.5 to 6.1 km/sec, respectively. The second interpretation proposes a high-angle crustal fault near Radium. The resultant model has an up-fault structure with depth to basement of 6.5 km and depth to the M-discontinuity of 51 km and a down-fault structure with corresponding values of 12.1 and 58 km. On the basis of gravity and magnetic trends, the fault strikes northeasterly. In either interpretation, a velocity gradient is present in the lower crustal section and the thickness of the crust is in excess of 50 km. Analysis of larger amplitude arrivals shortly after the P n phase is consistent with the interpretation of a low-velocity zone, 8 km beneath the M-discontinuity and approximately 7 km thick.

A seismic refraction survey along the southern Rocky Mountain Trench, Canada