The Algoman orogeny, known as the Kenoran orogeny in Canada, was an episode of mountain-building (orogeny) during the Late Archean Eon that involved repeated episodes of continental collisions, compressions and subductions. The Superior province and the Minnesota River Valley terrane collided about 2,700 to 2,500 million years ago. The collision folded the Earth's crust and produced enough heat and pressure to metamorphose the rock. Blocks were added to the Superior province along a boundary that stretches from present-day eastern South Dakota into the Lake Huron area. The Algoman orogeny brought the Archean Eon to a close, about ; it lasted less than 100 million years and marks a major change in the development of the Earth's crust.
The Canadian Shield contains belts of metavolcanic and metasedimentary rocks formed by the action of metamorphism on volcanic and sedimentary rock. The areas between individual belts consist of granites or granitic gneisses that form fault zones. These two types of belts can be seen in the Wabigoon, Quetico and Wawa subprovinces; the Wabigoon and Wawa are of volcanic origin and the Quetico is of sedimentary origin. These three subprovinces lie linearly in southwestern- to northeastern-oriented belts about wide on the southern portion of the Superior Province.
The Slave province and portions of the Nain province were also affected. Between about 2,000 and these combined with the Sask and Wyoming cratons to form the first supercontinent, the Kenorland supercontinent.
Overview Through most of the Archean Eon, the Earth had a heat production at least twice that of the present. The timing of initiation of plate tectonics is still debated, but if modern-day tectonics were operative in the Archean, higher heat fluxes might have caused tectonic processes to be more active. As a result, plates and continents may have been smaller. No broad blocks as old as 3 Ga are found in Precambrian shields. Toward the end of the Archean, however, some of these blocks or terranes came together to form larger blocks welded together by greenstone belts.
Two such terranes that now form part of the Canadian Shield collided about . These were the Superior province and the large Minnesota River Valley terrane, the former composed mainly of granite and the latter of gneiss. This led to the mountain-building episode known as the Algoman orogeny in the U. S. (named for Algoma, Kewaunee County, Wisconsin), and the Kenoran orogeny in Canada. Its duration is estimated at 50 to 100 million years. The current boundary between these terranes is known as the Great Lakes tectonic zone (GLTZ). This zone is wide and extends in a line roughly 1,200 kilometers long from the middle of South Dakota, east through the middle of the Upper Peninsula of Michigan, into the Sudbury, Ontario region. The region remains slightly active today. Rifting in the GLTZ began about at the end of the Algoman orogeny.
The orogeny affected adjacent regions of northern Minnesota and Ontario in the Superior province as well as the Slave and the eastern part of the Nain province, a far wider region of influence than in subsequent orogenies. It is the earliest datable orogeny in North America and brought the Archean Eon to a close. The end of the Archean Eon marks a major change in the development of the Earth's crust: the crust was essentially formed and achieved thicknesses of about under the continents.
Tectonics The collision between terranes folded the Earth's crust, and produced enough heat and pressure to metamorphose then-existing rock. Repeated continental collisions, compression along a north–south axis, and subduction resulted in the uprising of the Algoman Mountains. This was followed by intrusions of granite plutons and batholithic domes within the gneisses about ; two examples are the Sacred Heart granite of southwestern Minnesota and the Watersmeet Domes metagabbros (metamorphosed gabbros) that straddle the border of Wisconsin and Michigan's Upper Peninsula. After the intrusions solidified, new stresses on the greenstone belt caused movement horizontally along several faults and moved huge blocks of the crust vertically relative to adjacent blocks. This combination of folding, intrusion and faulting built mountain ranges throughout northern Minnesota, northern Wisconsin, Michigan's Upper Peninsula and southernmost Ontario. Igneous and high-grade metamorphic rocks are associated with the orogeny.
By extrapolating the now-eroded and tilted beds upward, geologists have determined that these mountains were several kilometers high. Similar projections of the tilted beds downward, coupled with geophysical measurements on the greenstone belts in Canada, suggest the metavolcanic and metasedimentary rocks of the belts project downward at least a few kilometers.
Greenstone
The action of metamorphism on the border between granite and gneiss bodies produces a succession of metamorphosed volcanic and sedimentary rocks called greenstone belts. Most Archean volcanic rocks are concentrated within greenstone belts; the green color comes from minerals, such as chlorite, epidote and actinolite that formed during metamorphism. After metamorphism occurred, these rocks were folded and faulted into a system of mountains by the Algoman orogeny.
The volcanic beds are thick. About the greenstone belt was subjected to new stresses that caused movement along several faults. Faulting on both small and large scales is typical of greenstone belt deformation. These faults show both vertical and horizontal movement relative to adjacent blocks. Large-scale faults typically occur along the margins of the greenstone belts where they are in contact with enclosed granitic rocks. Vertical movement may be thousands of meters and horizontal movements of many kilometers occur along some fault zones.
Some time before , masses of magma intruded under and within the igneous and sedimentary rocks, heating and pressing the rocks to metamorphose them into hard greenish greenstones. They began with fissure eruptions of basalt, continued with intermediate and felsic rocks erupted from volcanic centers and ended with deposition of sediments from the erosion of the volcanic pile. The rising magma was extruded under a shallow ancient sea where it cooled to form pillowed greenstones. Some of Minnesota's pillows probably cooled at depths as great as and contain no gas cavities or vesicles.
Most greenstone belts, with all of their components, have been folded into troughlike synclines; the original basaltic rock, which was on the bottom, occurs on the outer margins of the trough. The overlying, younger rock units – rhyolites and greywackes – occur closer to the center of the syncline. The rocks are so intensely folded that most have been tilted nearly 90°, with the tops of layers on one side of the synclinal belt facing those on the other side; the rock sequences are in effect lying on their sides. The folding can be so complex that a single layer may be exposed at the surface many times by subsequent erosion.
Volcanic activity
As the greenstone belts were forming, volcanoes ejected tephra into the air which settled as sediments to become compacted into the greywackes and mudstones of the Knife Lake and Lake Vermilion formations. Greywackes are poorly sorted mixtures of clay, mica and quartz that may be derived from the decomposition of pyroclastic debris; the presence of this debris suggests that some explosive volcanic activity had occurred in the area earlier. The volcanism took place on the surface and the other deformations took place at various depths. Numerous earthquakes accompanied the volcanism and faulting.
Superior province
The Superior province forms the core of both the North American continent and the Canadian shield, and has a thickness of at least . Its granites date from 2,700 to 2,500 million years ago. It was formed by the welding together of many small terranes, the ages of which decrease away from the nucleus. This progression is illustrated by the age of the Wabigoon, Quetico and Wawa subprovinces, discussed in their individual sections. Later terranes docked on the periphery of continental masses with geosynclines developing between the fused nuclei and oceanic crust. In general the Superior province consists of east–west-trending belts of predominately volcanic rocks alternating with belts of sedimentary and gneissic rocks.
Due to down warping along elongate zones, each belt is essentially a large downfold or downfaulted block. The areas between individual belts are fault zones consisting of granite or granitic gneiss. Its western part contains a regional pattern of east–west-trending wide granitic greenstone and metasedimentary belts (subprovinces). Western Superior province's mantle has remained intact since the 2,700-million-year-ago accretion of the subprovinces.
Both folding and faulting can be seen in the Wabigoon, Quetico and Wawa subprovinces. These three subprovinces lie linearly in southwestern- to northeastern-oriented belts of about wide (see figure on right). The northernmost and widest province is the Wabigoon. It begins in north-central Minnesota and continues northeasterly into central Ontario; it is partially interrupted by the Southern province. Immediately to the south, the Quetico subprovince extends as far west in north-central Minnesota, and extends further to the northeast. It is completely interrupted by a narrow band of the 1,100- to 1,550-million-year-old Southern province to the northeast of Thunder Bay. The Wawa subprovince is the most southerly of the three; it begins in central Minnesota, continues northeast to Thunder Bay, Ontario, Canada, (where its southern border just skims north Thunder Bay) and then extends east beyond Lake Superior. The northern boundary continues in a roughly northeasterly heading, while the southern border dips south to follow the northeast shore of Lake Superior.
Fault zones
The three subprovinces are separated by steeply dipping shear zones caused by continued compression that occurred during the Algoman orogeny. These boundaries are major fault zones.
The boundary between the Wabigoon and Quetico subprovinces seems to have been also controlled by colliding plates and subsequent transpressions. This Rainy Lake – Seine River fault zone is a major northeast–southwest trending strike-slip fault zone; it trends N80°E to cut through the northwest part of Voyageurs National Park in Minnesota and extends westward to near International Falls, Minnesota and Fort Frances, Ontario. The fault has transported rocks in the greenstone belt a considerable distance from their origin. The greenstone belt is wide at the Seven Sisters Islands; to the west the greenstone interfingers with pods of anorthositic gabbro. Radiometric dating from the Rainy Lake area in Ontario show an age of about 2,700 million years old, which favors a moving tectonic plate model for the formation of the boundary.