Ottawa-Bonnechere Graben

The Ottawa-Bonnechere Graben (also known as the Ottawa Graben) is a seismically active structure that coincides with a 55 km (34 mi) wide topographic depression extending from near Montréal through Ottawa. It is part of the St. Lawrence rift system that also includes the seismically active Saguenay graben. This rift valley was formed when the Earth's crust moved downward about a kilometre between two major fault zones known as the Mattawa and Petawawa faults. These ancient faults are still active and occasionally release stress in the form of earthquakes, such as the 1935 Timiskaming earthquake and the 2000 Kipawa earthquake. The length of the graben is about 700 km (435 mi).

The Ottawa-Bonnechere Graben runs from the Montreal area on the east to near Sudbury and Lake Nipissing on the west. On the east, it joins the Saint Lawrence rift system, a half-graben which extends more than 1000 km along the Saint Lawrence River valley and links the Ottawa and Saguenay Graben.

The 200 km (124 mi) segment of the Ottawa-Bonnechere Graben west of Ottawa was the first to be recognized as a graben. Since then, it has been traced west to Lake Nipissing, and northwestwards from the confluence of the Mattawa and Ottawa Rivers up the valley of the latter stream to Lake Timiskaming and the Montreal River valley. This latter branch is the Timiskaming Graben. At the rifts' western termini, the main faults split into divergent smaller faults. The graben has been interpreted as a Late Proterozoic to Early Paleozoic failed arm of the Iapetus Ocean, the precursor to the Atlantic Ocean. The main Ottawa-Bonnechere Graben is associated with collapse of the regional carbonate platform and formation of deep water shale basins by ~452 mya (million years ago); similar events formed the Temiskaming Graben ~449-451 mya. These graben were reactivated during the breakup of supercontinent Pangaea some 150 mya.

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