Evolution of faulting and paleo-stress field within the Ottawa graben, Canada
Abstract We relate families of faults in the Ottawa graben to periods in the tectonic evolution of eastern North America. The Ottawa graben is a seismically active structure that coincides with a 55 km wide topographic depression extending from near Montreal through Ottawa. It is part of the St. Lawrence rift system that also includes the seismically active Saguenay graben. The Cambrian to Ordovician sedimentary rocks that overlie Grenvillian basement rocks in the Ottawa area are cut by several generations of brittle faults. Relative ages of faults and associated structures are determined, or inferred, from field studies of key outcrops. Three periods of faulting are defined. The orientations of the paleo-stress fields associated with each period are modelled using fault and fault surface lineations orientation data. The oldest generation of faults (D 1 ) formed in response to a stress field in which the greatest principal stress ( σ 1 ) was horizontal and oriented NW. D 2 and D 3 occurred when σ 1 was oriented WNW and SW, respectively. Hence the paleo-stress field underwent a counterclockwise rotation from D 1 to D 3 . The D 2 and D 3 periods involved the reactivation of faults and the development of new faults. D 1 structures are kinematically congruent with the compressional direction associated with the closing of the Iapetus Ocean during Paleozoic time. Emplacement of Cretaceous carbonatite dykes coincided with D 2 , which occurred during the Mesozoic when the stress regime was associated with the opening of the Atlantic Ocean. The relative timing of these structures supports both Paleozoic and Mesozoic ages of mineralization events within the Ottawa graben. D 3 structures are consistent with the post-Cretaceous stress field in eastern North America. Recent seismicity within the Ottawa graben may involve reactivations of faults belonging to the families identified in this paper.