Summary. During the summer of 1982 the Canadian Consortium for Crustal Reconnaissance using Seismic Techniques (COCRUST) conducted a major long‐range seismic refraction and wide‐angle reflection experiment across the Grenville province of the Canadian Shield. Three seismic lines each approximately 300 km in length were located (i) along the Ottawa–Bonnechere graben, (ii) perpendicular to the graben and (iii) perpendicular to the Grenville Front. Geological evidence indicates that the graben may have originated from a failed arm of the St Lawrence rift system, and the Grenville Front marks the boundary between the Grenville province and the much older Superior province. Other geological features of the Grenville province that were traversed by the profiles were the Central Metasedimentary belt, and the Central Gneiss belt. Analysis of the data involved conventional travel‐time procedures coupled with the use of synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures. Results from the survey indicate a variation in near surface seismic velocity from 5.8 to 6.4 km s−1 with the highest values occurring in the Central Metasedimentary belt just north of Marmora, Ontario. Near Mont Laurier, Quebec, a zone of low velocity near‐surface material was found which is probably related to a nearby gravity low. Upper crustal velocity gradients differed from one profile to another but there was little evidence for any significant intermediate velocity discontinuity such as the Conrad. A study of wide angle reflected waves from the Mohorovickić discontinuity (Moho) showed that all the major tectonic features in the region have an expression at depth. The Moho is a very well defined sharp discontinuity beneath the Gneiss belt and there is strong evidence for a significant thickening of the crust by at least 5 km in the vicinity of the Grenville Front. The boundary between the Central Metasedimentary and Gneiss belts is characterized by a 2 to 3 km fault‐like step in the Moho with the thinner part being under the Gneiss belt. The Moho is very disturbed and poorly defined along major portions of the Ottawa graben and in some localities there is evidence for a rise in high velocity material at its base. This gives added support to the theory that the graben is similar to major rift structures on other continents.

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