The Ottawa graben is a Neoproterozoic intracratonic rift in northeastern North America that was reactivated throughout the Phanerozoic and persists as a modern seismically active zone of lithospheric weakness with extant topography. U-Pb geochronology of detrital zircon grains, paleoflow directions, and stratigraphic data from the Potsdam Group provide evidence of early Paleozoic rifting and reactivation of the passive-margin Ottawa graben. Early to middle Cambrian rifting (ca. 515−505 Ma) coincided with the opening of fault-bounded subbasins that were filled with arkose derived from local rift shoulders consisting mainly of ca. 1176 Ma Grenville sources from the Frontenac terrane. Additionally, the local presence of ca. 1442 Ma zircon grains suggests extensive opening of the Ottawa graben along the modern Ottawa River Valley, which provided a localized conduit for transport of sediment sourced from parts of the Central Gneiss belt, ∼350−400 km to the west. Early to middle Cambrian rifting of the Ottawa graben coincided with evidence of more widespread rifting across the Laurentian margin, suggesting that the Laurentian passive-margin rift-to-drift transition did not occur until the middle Cambrian. Later, late middle to early late Cambrian reactivation of the Ottawa graben resulted in topographic inversion marked by uplift of the adjacent Adirondack Dome, and radial dispersal of sediment with introduction of ca. 1060−1000 Ma detrital zircon grains throughout the Ottawa graben. A second episode of reactivation and topographic inversion occurred during the earliest Ordovician, marked by subsidence of the Adirondack Dome and uplift of parts of the northern Ottawa graben. This resulted in southeastward drainage and the reintroduction of ca. 1442 Ma zircon grains via reworking of older Potsdam strata and/or direct sourcing from parts of the Central Gneiss belt to the west. These two enigmatic, passive-margin early Paleozoic reactivations are correlated with events along the Laurentian margin, for example, early Furongian shelf erosion, especially near the Saguenay graben, and they are linked to perturbations in the intraplate stress field driven by plate-boundary forces originating from the peri-Laurentian and Iapetan regions.

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