Deep Geology from Seismic Reflection Studies in Canada

In 1964, the Department of Physics at the University of Alberta initiated a programme to explore the feasibility of recording reflected seismic energy from boundaries within the lower crust and from the Mohorovičić discontinuity. This was the first attempt by a North American institute to systematically develop and apply industry-type seismic reflection techniques for investigations of the deep crust. For the first study a modification of the expanding spread technique was applied to an area of southern Alberta, where the general crustal layering was known from the results of reversed seismic refraction profiling. Kanasewich and Cumming (1965) reported on the success of this study by providing convincing evidence that reflections from the deep crust had been recorded. In particular, a strong lower crustal reflector at a two-way travel time of 11.4 sec was phase correlated over a distance of about 13 km, enabling the compilation of a T2-X2 plot and the determination of an average vertical velocity and depth to the reflector. Within experimental error, the 34 km depth to the reflector (later named the Riel discontinuity) matched the results from an earlier seismic refraction survey along the same line (Cumming and Kanasewich 1966). Encouraged by this initial success the group at the University of Alberta expanded its programme and by the late 1960s to the mid-1970s seismologists at three other institutes (University of Saskatchewan, University of Manitoba and the Earth Physics Branch of the Federal Department of Energy, Mines and Resources) were shooting short seismic reflection profiles across various parts of western Canada. It was during this period, in 1973, that Mair and Lyons (1976) recorded the first strong evidence that Vibroseis was a viabie source for probing the deep crust. They recorded a relatively continuous reflection from the Mohorovičić discontinuity along a 20 km line in the Cordilleran Orogen. In 1975, in an effort to consolidate funding and pool the limited human resources and recording/processing capabilities available in Canada, crustal seismologists from across the country joined together to form the Consortium for Crustal Reconnaissance Using Seismic Techniques (COCRUST). Since its inception most major crustal seismic surveys on the Canadian landmass have been conducted under the COCRUST umbrella. Unfortunately, due to a continued low level of funding,COCRUST has yet to capitalise fully on the earlier successes in Canadian reflection seismology. To date, most of the COCRUST funding has been directed to regional refraction surveys; accompanying short reflection profiles have been obtained only at critical locations. In this paper we review briefly, in chronological order, some of the important results of the pre-COCRUST era and then we present the conc1usions obtained from two COCRUST sponsored seismie reflection surveys and a recent government/industry sponsored intermediate depth reflection survey. Finally, we describe briefly the latest Canadian initiatives (LITHOPROBE) for mapping the third dimension of geology during the 1980s and beyond.