Gravity evidence against a high-angle fault crossing the Rocky Mountain Trench near Radium, British Columbia

One of three explanations of a prominent time delay in the 6.5 km/s branch of a recent seismic refraction survey in the Rocky Mountain Trench suggested a high-angle crustal fault crossing the trench near Radium, British Columbia. If the density contrast between basement and cover rocks is 0.1 g/cm3, a gravity anomaly of approximately 18 mGal should be observed. To test the fault hypothesis, a gravity survey has been carried out in and adjacent to the trench in the Radium area. The resultant data are not consistent with the proposed fault model. The principal feature of the data is a pronounced low, which coincides with the trench throughout the survey area. The low is due to Cenozoic fill and interpretation by two-dimensional modelling indicates the thickness of fill is about 550 m to the north and 420 m to the south of Radium. As a result of this survey, the two alternative hypotheses to explain the seismic data must be reconsidered. These are (1) the existence of a crustal low velocity zone, and (2) a major deformation of the basement and overlying rocks due to the trench being an ancient zone of weakness, which coincides with the western limit of the continental Precambrian craton. As reflections from the top of a low velocity zone are not observed, the second alternative is preferred.