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The Algoman Orogeny
The GLTZ
The Earth has been gradually collecting larger and larger volumes of lighter, low density rocks on its surface. This changed the continental crustal characteristics from small thin heavier belts of crust collected in the Archean, into lighter much larger plates of crust that interact in the modern type of tectonics. The first step in the process was the collection of the individual belts of the Superior province, along with the łglueing˛ effects of early granitic plutonism emplaced during and after the collection of the belts. The collision of the gneissic terrane onto the southern edge of the Superior province marks the next step in the slow change in tectonics. The significance was in the size of the terrane, it reinforces the idea of the increasing size of continental crust. This gneissic terrane originally extended several hundred kilometers east to west, making it more of a protocontinent than a Superior province belt.
The boundary that separates the two colliding bodies, the Superior province with the Wawa subprovince on its edge to the north, and a gneissic terrane to the south, is called the Great Lakes tectonic zone (GLTZ). The GLTZ is a fault zone of highly deformed rocks. Collision began along the GLTZ around 2.7 bya, and continued for many tens of millions of years, this is partly because the collision is interpreted to have happened at an angle. Suturing, the last stage of closure, started in western South Dakota and scizzored closed to the east. Some places show evidence of two episodes of closure, as if the terrane closed half way, stopped then later closed the rest of the way.
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Suturing of one continental block onto another usually takes place because a subduction zone exists underneath one of the blocks along an edge. This subduction zone takes in the oceanic crust that is connected to the other block. As time progresses there comes a point where all of the oceanic crust is consumed and the two blocks meet. When this happens, a number of scenarios can happen. The most common is for the block with the subduction zone underneath it to be thrust onto the other. Or looking at it from the other direction, the subducting oceanic crust pulls the attached continental block under the other for a short time. This continental block resists being pulled down primarily because the density of continental crust is less than the underlying mantle. The result is a thrusted crumpled up overlap many tens of kilometers wide producing not only a mountain range, but a shear zone that defines the boundary between the two bodies. This shear zone contains rocks that are deformed in a brittle fashion in the upper ten kilometers or so, then in the lower below ten kilometers rocks show ductile deformation. By the time the collision is complete, and all the magmatism has calmed down, it is a very complicated structure.
In the case of the GLTZ, the gneissic protocontinent was consuming oceanic crust as it feed in from the north. During the collision with the Superior province, the gneissic block was thrust up onto the Superior provinces edge. Today on the surface of the earth, we see the GLTZ as a ductile shear zone. This means that the upper 10 or more kilometers of rocks involved in the original collision have been eroded away. Reconstruction of the original depth of the rocks can be done using specific minerals that have known pressures and temperatures of development, that can be translated into depth into the crust. Pressure and temperature can be determined by evidence such as which minerals are forming around other minerals, which are fracturing brittely, and which ones are deforming ductiely. The true characteristics of these minerals are known from laboratory experiments. Description of the Gneissic terrane
The gneissic terrane itself is mainly a migmatitic felsic gneiss with some schist texture, with some smaller amounts of amphibolite schists, metagabbro and metasediments. This terrane started to develop around 3.5 bya, and was essentially a full microcontinent with its own protracted highly deformed history by the time it sutured into place. Most of the terrane has experienced the same temperatures of 650-750C, and pressures of 4.5-7.5 kbar which is considered to be granulite facies metamorphism. This terrane experienced distinct high grade metamorphic events around 3.05 bya, and 2.6 bya. The first most likely created during formation of the terrane, the second during suturing. Emplacement of the Scared Heart Granite follows the suturing event at 2.7 bya as a post tectonic intrusion. The granite cuts the gneisses and is relatively undeformed with only a minor foliation. Its a typical late tectonic pink medium grained granite that was intruded around 2.6 bya after the suturing of the gneissic terrane onto the Superior province. Similar intrusions farther east along the GLTZ show later dates reinforcing the scizzor model. There is also evidence for a much lower thermal event around the time of the Penokean Orogeny which will be discussed later. This later event also reactivated some of the faults associated with the GLTZ.
Rifting after collision
After suturing, the region fell tectonically quiet for a few hundred million years. The mountains that were built, eroded away turning into sediments that covered the area. Then beginning around 2.5-2.2 bya the gneissic block, which at this point was part of the continent at the edge, broke apart in a rifting sequence. A rifting sequence is when continental crust drifts over mantle currents that are flowing straight up and away as part of the convection cells flow pattern like in a spreading center for a midoceanic ridge. Mantle currents can also change developing a spreading center beneath a continent, producing the same result. As the crust positioned above such a situation responds to the currents, it thins out, producing extensional features such as horst and graben typified by the basin and range environment of the southwestern U.S. In this theoretical case, the crust at the spreading center may be elevated by the pressure from the upwelling mantle currents producing breaks or alterations in the sedimentation record due to this elevation, and emplacing dikes in to the stressed and fractured crust.
While the crust of the gneissic block was thinning out, the sea encroached in the opening basin, and sediments piled up, however, because of the poor exposures on this part of the sedimentation record its hard to determine to what degree the sedimentary record was altered. There are associated dikes, and the expected faulting characteristics that fit with this extensional model.
As an intresting side note, there is some evidence to suggest that the other half of the rifted southern edge is the Wyoming province.
The pattern of sedimentation from this rifting environment continues into the Penokean orogeny which is the next major tectonic event in the Great Lakes region. The sediments are part of the Banded Iron formations, and have such great significance in the change of Earth's atmosphere and oceans that I have chosen to discuss them in a section by themselves, even though professionally these sediments are now considered part of the Penokean orogeny due to its effect on them.
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