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A seismic refraction survey along the southern Rocky Mountain Trench Bennett, Geoffrey Taylor

Abstract

Deducing the structure and origin of the Rocky Mountain Trench has proven to be a difficult problem in the past. To understand this feature more fully and to obtain information about the entire crustal section, 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, forty-four useful recordings were obtained over a distance of 540 km. Three components of short period ground motion were recorded by tape recording systems; the vertical component was also recorded by elements of the Mica array. Careful attention to amplitude scale factors results in the formation of a record section in which the energy pattern varies uniformly along the profile. A geometric ray theory interpretation involving Weichert-Herglotz integration of p-delta curves is used to obtain a velocity-depth structure. Approximate synthetic seismograms are then calculated using modified ray theory. Refractors with apparent P-wave velocities of 6.5 - 6.6 km/s and 8.22 ± 0.04 km/s are interpreted as the surface of the Precambrian basement and the Moho discontinuity, respectively. A velocity gradient is present in the lower crustal section. The depth to basement beneath the western Rocky Mountains at 50°30'N is calculated to be 6.5 ± 1 km. Near Radium, a significant anomaly in the seismic data is best interpreted as a northeasterly-trending normal fault with a downthrow of 5.6 ± 1 km to the northwest. The directions are inferred from gravity and magnetic trends in the region. Alternatively, the anomaly could represent a disappearance of the basement surface west of the east wall of the Trench. An anomalously thick crustal section is inferred from the data. A preferred model gives a depth of 51 ± 2 km southeast of Radium and 58 ± 2 km to the northwest. Study of a converted phase leads to the conclusion that there may be a discontinuity on the Moho surface beneath the Trench near 52°N. Analysis of arrivals shortly after the Pn phase is consistent with the interpretation of a low velocity zone, approximately 7 km thick, 8 km beneath the Moho.

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