Failed Rift ArmsA continental rift is described as the zone of crustal deformation that occurs when

Failed Rift ArmsA continental rift is described as the zone of crustal deformation that occurs when the continental lithosphere is subject to extensional deformation (stretching). Often, continental rifting occurs because of a mantle plume rising from deep in the mantle and presses on the Earth’s crust from below. The pressure from the mantle plume puts tremendous stress on the crust and causes it to thin bow upwards. When sufficient stress is put on the crust it ‘cracks’, and forms a series of three rift arms at approximately 120 degrees to each other. This is known as a triple junction. A modern-day example of an active triple junction is the East African Rift system. In the early stages of continental rifting, grabens are formed. A graben is an elongated, downthrown fault block which is bounded on either side by normal faults. As rifting progresses, and the lithosphere continues to stretch, an ocean basin can be formed. An example of this is the Atlantic Ocean, which started approximately 130 mya as a rift in the supercontinent Pangaea, and is still spreading from the Mid-Atlantic Ridge. While it is possible that all three rift arms of a triple junction remain active, usually one arm becomes inactive while in the earlier stages of rifting and the other two rift arms form the main rift system. These failed rift arms are known as aulocogens.Old failed rift arms are often preserved within the continent. The Ottawa-Bonnechere graben is an example of an Paleozoic (542 – 251 mya) failed rift arm that is part of the larger St. Lawrence rift system, located in eastern Canada. This rift system formed during an extensional event leading to the formation of the Iapetus Ocean, a precursor to the Atlantic Ocean. The NE-SW trending graben stretches approximately 700 km, from Montreal through to the Ottawa Valley. The 55 km wide graben is bounded by two primary normal faults, known as the Petawawa fault and the Mattawa fault. Many smaller faults are also present, and the rocks show evidence for multiple generations of faulting. In terms of geology, the Ottawa-Bonnechere graben is comprised of Cambrian to Ordovician sedimentary rocks overlying predominantly metasedimentary and metavolcanic basement rocks. Continental rifting in the Ottawa-Bonnechere area ceased while the rift was still in the early graben stage. It is interesting to think how different the surface of the Earth could look if rifting had been allowed to continue. Would Ottawa and Montreal be separated by an ocean?The formation of the faults associated with the graben structure during the early stages of rifting are preserved as planes of weakness within the rocks. These old faults are likely to become reactivated if tectonic stress acts on the region. This potential for fault reactivation is of particular interest to researchers and governments determining the seismic risk of an area. The Ottawa-Bonnechere graben has been reactivated several times during the Appalachian Orogen and currently during the Cordilleran Orogeny on the west coast of North America. Today minor earthquakes are common in the area around the Ottawa-Bonnechere graben. The vast majority are too small to be felt, but larger earthquakes also occur such as the 2000 Timiskaming earthquake which measured at magnitude 5.2.CDSourceshttp://bit.ly/2qJvpJg (paywall)http://bit.ly/2rkHo2xhttp://bit.ly/2rkHlEchttp://bit.ly/2snDMveImage Sourceshttp://bit.ly/2rNRHOahttp://bit.ly/2sysDa4 -- source link

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