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UBC Theses and Dissertations

Gravity and seismic studies in the southern Rocky Mountain trench Spence, George D.

Abstract

as one of three explanations of a prominenttine delay in the 6.5 km/s branch of their seismic refraction survey in the Rocky Mountain Trench, Eennett et al (1975) suggested a high-angle crustal fault crossing the trench near Radium. 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 modeling indicates the thickness of fill is about 550 m to the north and 420 m to the south of Radium. An analysis has also been performed of the shear-wave data recorded during the seismic survey of Bennett et al (1975). Although the quality of the S save data is poor, they show consistent behavior with the P save data. There is weak evidence suggesting a basement refractor velocity of 3.5 km/s and a Moho refractor velocity of 4.2-4.5 km/s. The corresponding Poisscn's ratios are 0.30 and 0.28-0.32. To determine maximum and minimum depth limits to the Hcho allowed by the seismic data, an extremal analysis was performed on both the P and S wave record sections. From the P wave data, the limits on crustal thickness beneath the Rocky Mountain Trench are 52 km and 60 km; from the S wave data, the limits are 47 km and 59 km. Is a result of these additional studies, the tao alternative hypotheses of Bennett et al (1975) 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 the low velocity zone are not observed by Bennett et al (1975), the second alternative is preferred.

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