Tectonic framework and relative ages of structures within the Ottawa-Bonnechere graben.

Abstract

The Ottawa-Bonnechere graben forms a 55 km wide topographic low extending from near Montreal through Ottawa. It is part of the St. Lawrence rift system, which also includes the Saguenay graben in Quebec. Cambrian to Ordovician carbonate and fine clastic sedimentary rocks of the St. Lawrence Platform overlie Grenvillian basement rocks and, in the Ottawa-Hull area, are cut by several generations of brittle faulting. Relative ages of faults and associated structures are determined or inferred from field studies of key outcrops. Three periods of faulting are defined, and the orientations of the paleostress fields associated with each period are modelled using fault and fault surface lineations orientation data. The oldest generation of fault structures, referred to as D$\sb1$, formed in response to a stress field in which the greatest principal horizontal stress was oriented northwest. The second and third periods of faulting (D$\sb2$, D$\sb3$) occurred when the greatest principal horizontal stress was oriented west-northwest and southwest, respectively. Both D$\sb2$ and D$\sb3$ involved the reactivation of the existing faults and the development of new faults. (Abstract shortened by UMI.)