Great Falls Tectonic Zone
Major intracontinental shear zone between the Hearne craton and Wyoming craton / From Wikipedia, the free encyclopedia
The Great Falls Tectonic Zone is a major intracontinental shear zone between the Hearne craton and Wyoming craton basement rock of the Archean Eon which form part of the North American continent.[1][2][3][4][5] The zone is an area about 100 miles (150 km) wide extending from the southwestern Idaho-Montana border across Montana to the northwestern Montana-Saskatchewan-North Dakota border.[4][6] It is named for the Great Falls of the Missouri River, a major geologic feature of the area. The central and western portions of the zone are believed to be about 1.1 to 3.3 billion years old.[7] The central part of the zone lacks Archean rock, however, leading at least one group of scientists to speculate that it was formed very late in the Paleoproterozoic Era.[7]
The Great Falls tectonic zone has been periodically active since the Proterozoic, and possibly as late as the Holocene.[6] Little of the zone is visible due to Phanerozoic cover, the exception being the Little Belt Mountains.[2] However, it is believed that the tectonic zone controlled the geologic development of nearby basins and subbasins.[8]
The Great Falls tectonic zone was first identified in 1985.[4] Geologists originally believed the zone was part of the Wyoming craton, but now conclude that it is distinct from it.[2] There is continuing controversy over whether the region is a shear zone or suture,[1][9] and the role the zone played in the formation of the North American continent.[2] At one time, both the Great Falls Tectonic Zone and the Vulcan structure were both considered sutures, but debate remains open on the point.[2] At least one group of geologists has concluded the zone represents the closure of an ocean basin.[2] The zone lacks gravity anomalies or electromagnetic signatures which would allow scientists to conclude that it was generated by subduction.[4] There is significant evidence that the zone has been periodically remineralized since Precambrian times.[10] Square Butte, Shaw Butte, Crown Butte and the other structures of the Adel Mountains Volcanic Field lie astride the tectonic zone near the city of Great Falls.[11]