Great Lakes tectonic zone
Encyclopedia
During the Late Archean Eon the Algoman orogeny
added landmass to the Superior province
by volcanic activity and continental collision
along a boundary that stretches from present-day South Dakota, U.S., into the Sudbury, Ontario, Canada, Lake Huron region.
This crustal boundary is the Great Lakes tectonic zone. It is 1400 km (869.9 mi) long, and separates the older Archean gneissic terrane to the south from younger Late Archean greenstone
-granite terrane
to the north.
The zone is characterized by active compression during the Algoman orogeny (about ), a pulling-apart (extensional
) tectonics (2,450 to 2,100 million years ago), a second compression during the Penokean orogeny (1,900 to 1,850 million years ago), a second extension during Middle Proterozoic
time (1,600 million years ago) and minor reactivation during Phanerozoic
time (the past 500 million years).
Collision began along the Great Lakes tectonic zone (GLTZ) with the Algoman mountain-building event and continued for tens of millions of years. During the formation of the GLTZ, the gneissic Minnesota River Valley subprovince was thrust up onto the Superior province's edge as it consumed the Superior province's oceanic crust. Fragmentation of the Kenorland
supercontinent began and was completed by . The Wyoming province
is the continental landmass that is hypothesized to have rifted away from the southern Superior province portion of Kenorland, before moving rapidly west and docking with the Laurentia
supercontinent 1,850 to 1,715 million years ago. Sedimentation from the GLTZ-rifting environment continued into the Penokean orogeny, which is the next major tectonic
event in the Great Lakes region
. Several earthquakes have been documented in Minnesota, Michigan's Upper Peninsula and Sudbury in the last 120 years along the GLTZ.
that separates the more than 3,000-million-year-old Archean gneiss
ic terrane to the south – Minnesota River Valley subprovince – from the 2,700-million-year-old Late Archean greenstone-granite
terrane to the north – Wawa Subprovince of the Superior province. The GLTZ is 50 km (31.1 mi) wide.
The collision of the gneissic Minnesota River Valley (MFV) subprovince onto the southern edge of the Superior province was another process in the slow change in tectonics which marks the end of the Archean Eon. This gneissic terrane originally extended several hundred kilometers east to west, making it more of a protocontinent than a future Superior province belt. The boundary that separates the two colliding bodies is the Great Lakes tectonic zone; it is a fault zone of highly deformed rocks. Collision began along the GLTZ around and continued for tens of millions of years. The collision is interpreted to have happened obliquely at an angle, beginning in the west.
events, one and the other . The first was probably during formation of the terrane, the second was during suturing. The growth of the Superior province greenstone-granitic terranes ended with the suturing of the Minnesota River Valley gneiss terrane to the basalt
ic Wawa subprovince. Suturing, the last stage of closure, started in South Dakota and continued eastward.
During the formation of the GLTZ, the MRV protocontinent consumed the Superior province's oceanic crust as the subprovince came in from the south. Suturing of one continental block onto another usually occurs because a subduction
zone exists beneath one of the blocks. The subduction zone consumes the oceanic crust connected to the other block. After the oceanic crust is consumed, the two blocks meet and the subducting oceanic crust pulls the attached continental block under the other. During the collision with the Superior province, the MRV gneissic block was thrust up onto the Superior province's edge; this resulted in a thrusted crumpled fault tens of kilometers wide producing a mountain range, and a shear zone which defines the boundary between the two terranes. Tectonism along the zone began during the docking of the two terranes into a single continental mass, and culminated in the early Proterozoic, where deformation took place under low to intermediate pressures.
began around under a hotspot
near Sudbury and was completed by around . This is when the Wyoming province is hypothesized to have drifted away from the Superior province.
deformed the sequences in the Lake Superior region.
ping in the Marquette, Michigan, U.S., area provides information of the structure for the zone along a 10 km (6.2 mi) strike. The GLTZ was an active dextral strike-slip
zone south of Marquette, passing under the large Marquette anticline
. P.K. Sims and W.C. Day suggest that the kinematics
determined in the exposed GLTZ – which are consistent – are applicable to its entire length.
In the Marquette area, the GLTZ is a northwest-striking zone of metamorphic rock about 2 km (1.2 mi) wide that was crushed by the dynamics of tectonic movements. Shear zone boundaries are subparallel and strike N60°W; the foliation
in mylonite
within the GLTZ strikes N70°W and dips S75°W. A stretching lineation (line of tectonic transport) in the mylonite foliation plunges 42° in a S43°E direction. In the Sims-and-Day model, this last collision in the assembly of the Superior province resulted from northwest-directed tectonic transport of the Minnesota River Valley subprovince terrane against the terrane of the Superior province. The collision was oblique, resulting in dextral-thrust shear along the boundary.
and sill
s of diabase, quartz-feldspar fine-grained instrusive rocks (aplite
) and quartz-feldspar-mica coarse-grained intrusive rocks (pegmatite
) are common.
Most of the region's crystalline rock bodies of Late Archean age are part of the greenstone-granite terrane of northern Minnesota, northwestern Wisconsin and the western part of the Upper Peninsula of Michigan. Lithologies
of the rocks are usually gneissose and migmatitic
. Repeated metamorphism and deformation caused extensive recrystallization, intense foliation, shear zones and folding. There are east-northeast- to east-trending faults in the gneissic rocks south of the Great Lakes tectonic zone in Minnesota, south of the Midcontinent Rift System in Wisconsin and in the Upper Peninsula of Michigan.
age data indicates that there are four crystalline rock complexes 3,400 million years old in the Lake Superior region. The best-known units are the Morton Gneiss and the Montevideo Gneiss complexes, along the Minnesota River Valley in southwest Minnesota. Rocks exposed in the Minnesota River Valley include a complex of migmatitic granitic gneisses, schistose to gneissic amphibolite
, metagabbro and paragneisses. The complex of ancient gneisses is intruded by a younger, weakly deformed granite body, the Sacred Heart granite.
The 1,850-year-old Penokean magmatism in Wisconsin represents margin-type igneous activity terminated by collision. Some of the Penokean granites show iron enrichment similar to the magnetite
series, rather than the low-oxygen concentration of the magnetic titanium oxides. Penokean-age rocks in the northern Wisconsin and the Upper Peninsula of Michigan contain areas of low-pressure, low- to high-temperature metamorphism. The folding and metamorphism increased in intensity to the south and southeast, and produced the isolated gneissic 1,755-million-year-old Watersmeet Domes which straddle the border of Michigan and northeastern Wisconsin.
In the western portion of the Marquette syncline
, a second episode of GLTZ reactivation took place during the uplift of the post-Huronian 2,400- to 2,100-million-year-old granitic Southern Complex.
The Northern and Southern complexes of the Upper Peninsula are highly migmatized and intensely foliated, with the intensity of foliation increasing toward margins. The western part of the Southern Complex shows intricate phases of folding and foliaton. These Late Archean rocks form a roughly north-south belt lying south of Marquette extending to the Michigan-Wisconsin border.
, a differentiated sequence of intrusive volcanic rocks – norite
, gabbro and
granophyre
– overlain by breccias and metasedimenary rocks. The sublayer consists of a mass of basic to ultrabasic inclusions of varying size and frequency of occurrence. Sudbury gabbro
varies between a gabbro and a norite, depending upon the local silicates' ratios. The quartz biotite
gabbro is medium- to coarse-grained, the Climax quartz monzonite
is medium-grained.
In eastern Sudbury area the rock is highly crystalline hornblendic gneiss, which apparently dips at a rather low angle toward the southeast.
A paleostress analysis of the eastern exposures near Sudbury shows continuing dextral offset during the Penokean orogeny.
igneous rocks on each side of the rift margins. By the Superior and Wyoming provinces had completely separated. From about 2,100 to 1,865 million years ago the Wyoming craton drifted in a westward direction until it docked with the Superior province, northwest of its original position.
In the northern Black Hills of southwest South Dakota the 2,600- to 2,560-million-year-old Precambrian cystalline core, the Blue Draw Metagabbro, is a 1 km (0.621372736649807 mi) thick layered sill. The East Bull Lake intrusive suite, in the southern Superior province near Sudbury, Ontario, aligns spatially with the Blue Draw Metagabbro if the Superior and Wyoming cratons are restored to the Kenorland configuration proposed by Roscoe and Card (1993). These layered mafic intrusions are of similar thickness and identical age, and occur along a rifted belt.
Recent paleomagnetic and geochronological data from the central Wyoming craton support the hypothesis that the Huronian
(in southern Ontario) and Snowy Pass (in southeastern Wyoming) supergroups were adjacent to each other at and may have evolved as a single sedimentary rift basin between 2,450 and 2,100 million years ago. These Huronian and Snowy Pass sedimentary rocks are similar, each having 2,450- to 2,100-million-year-old epicratonic rifts succeeded by a 2,100- to 1,800-million-year-old passive sedimentary margins.
, with a magnitude of 4.6, and a felt area of 82000 square kilometre covering parts of four states.
Wisconsin has had no earthquakes along the GLTZ, Michigan's Upper Peninsula has had four earthquakes in the vicinity of the GLTZ – Negaunee, Newberry and two in Sault St. Marie – and the Sudbury area has had three earthquakes.
Algoman orogeny
During the Late Archaen Eon repeated episodes of continental collisions, compressions and subductions generated a mountain-building episode known as the Algoman orogeny; it is known as the Kenoran orogeny in Canada. The Superior province and the Minnesota River Valley microcontinent collided about...
added landmass to the Superior province
Superior craton
The Superior craton forms the core of the Canadian Shield at the heart of the North American continent. It extends from Quebec in the east to eastern Manitoba in the west...
by volcanic activity and continental collision
Continental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
along a boundary that stretches from present-day South Dakota, U.S., into the Sudbury, Ontario, Canada, Lake Huron region.
This crustal boundary is the Great Lakes tectonic zone. It is 1400 km (869.9 mi) long, and separates the older Archean gneissic terrane to the south from younger Late Archean greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
-granite terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
to the north.
The zone is characterized by active compression during the Algoman orogeny (about ), a pulling-apart (extensional
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
) tectonics (2,450 to 2,100 million years ago), a second compression during the Penokean orogeny (1,900 to 1,850 million years ago), a second extension during Middle Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
time (1,600 million years ago) and minor reactivation during Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
time (the past 500 million years).
Collision began along the Great Lakes tectonic zone (GLTZ) with the Algoman mountain-building event and continued for tens of millions of years. During the formation of the GLTZ, the gneissic Minnesota River Valley subprovince was thrust up onto the Superior province's edge as it consumed the Superior province's oceanic crust. Fragmentation of the Kenorland
Kenorland
Kenorland was one of the earliest supercontinents on Earth. It is believed to have formed during the Neoarchaean Era ~2.7 billion years ago by the accretion of Neoarchaean cratons and the formation of new continental crust...
supercontinent began and was completed by . The Wyoming province
Wyoming craton
The Wyoming craton is a craton located in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah...
is the continental landmass that is hypothesized to have rifted away from the southern Superior province portion of Kenorland, before moving rapidly west and docking with the Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
supercontinent 1,850 to 1,715 million years ago. Sedimentation from the GLTZ-rifting environment continued into the Penokean orogeny, which is the next major tectonic
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
event in the Great Lakes region
Great Lakes region (North America)
The Great Lakes region of North America, occasionally known as the Third Coast or the Fresh Coast , includes the eight U.S. states of Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania and Wisconsin as well as the Canadian province of Ontario...
. Several earthquakes have been documented in Minnesota, Michigan's Upper Peninsula and Sudbury in the last 120 years along the GLTZ.
Location
During the Late Archean Eon the Algoman orogeny – which occurred about – added landmass through volcanic activity and continental collision along a boundary that stretches from present-day South Dakota, U.S., into the Sudbury, Ontario, Canada, region. The farthest west into South Dakota is 99°W, which is about 55 km (34.2 mi) from the Minnesota – South Dakota border. This crustal boundary is the Great Lakes tectonic zone (GLTZ). It is a 1400 km (869.9 mi) long paleosutureSuture (geology)
In structural geology, a suture is a major fault zone through an orogen or mountain range. Sutures separate terranes, tectonic units that have different plate tectonic, metamorphic and paleogeographic histories...
that separates the more than 3,000-million-year-old Archean gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
ic terrane to the south – Minnesota River Valley subprovince – from the 2,700-million-year-old Late Archean greenstone-granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
terrane to the north – Wawa Subprovince of the Superior province. The GLTZ is 50 km (31.1 mi) wide.
Collision
Suturing
The MRV subprovince experienced two distinct high-grade metamorphicMetamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
events, one and the other . The first was probably during formation of the terrane, the second was during suturing. The growth of the Superior province greenstone-granitic terranes ended with the suturing of the Minnesota River Valley gneiss terrane to the basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic Wawa subprovince. Suturing, the last stage of closure, started in South Dakota and continued eastward.
During the formation of the GLTZ, the MRV protocontinent consumed the Superior province's oceanic crust as the subprovince came in from the south. Suturing of one continental block onto another usually occurs because a subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
zone exists beneath one of the blocks. The subduction zone consumes the oceanic crust connected to the other block. After the oceanic crust is consumed, the two blocks meet and the subducting oceanic crust pulls the attached continental block under the other. During the collision with the Superior province, the MRV gneissic block was thrust up onto the Superior province's edge; this resulted in a thrusted crumpled fault tens of kilometers wide producing a mountain range, and a shear zone which defines the boundary between the two terranes. Tectonism along the zone began during the docking of the two terranes into a single continental mass, and culminated in the early Proterozoic, where deformation took place under low to intermediate pressures.
Rifting
After suturing, the region was tectonically quiet for a few hundred million years. The Algoman Mountains had been built and then eroded into sediments that covered the area. Fragmentation of this Archean supercontinentSupercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
began around under a hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
near Sudbury and was completed by around . This is when the Wyoming province is hypothesized to have drifted away from the Superior province.
Cessation of rifting
The pattern of sedimentation from this rifting environment continued into the Penokean orogeny, which is the next major tectonic event in the Great Lakes region. During the Penokean orogeny (1,850 to 1,900 million years ago), compressionCompression (geology)
In geology the term compression refers to a set of stresses directed toward the center of a rock mass. Compressive strength refers to the maximum compressive stress that can be applied to a material before failure occurs. When the maximum compressive stress is in a horizontal orientation, thrust...
deformed the sequences in the Lake Superior region.
GLTZ in Marquette, Michigan, area
Recent geologic mapGeologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
ping in the Marquette, Michigan, U.S., area provides information of the structure for the zone along a 10 km (6.2 mi) strike. The GLTZ was an active dextral strike-slip
Strike-slip tectonics
Strike-slip tectonics is concerned with the structures formed by, and the tectonic processes associated with, zones of lateral displacement within the crust or lithosphere.-Deformation styles:-Riedel shear structures:...
zone south of Marquette, passing under the large Marquette anticline
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
. P.K. Sims and W.C. Day suggest that the kinematics
Kinematics
Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....
determined in the exposed GLTZ – which are consistent – are applicable to its entire length.
In the Marquette area, the GLTZ is a northwest-striking zone of metamorphic rock about 2 km (1.2 mi) wide that was crushed by the dynamics of tectonic movements. Shear zone boundaries are subparallel and strike N60°W; the foliation
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
in mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
within the GLTZ strikes N70°W and dips S75°W. A stretching lineation (line of tectonic transport) in the mylonite foliation plunges 42° in a S43°E direction. In the Sims-and-Day model, this last collision in the assembly of the Superior province resulted from northwest-directed tectonic transport of the Minnesota River Valley subprovince terrane against the terrane of the Superior province. The collision was oblique, resulting in dextral-thrust shear along the boundary.
Composition of rock
Early Archean rocks generally form elongate, domal or cicular bodies that are several kilometers thick. Late-stage dikesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
and sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
s of diabase, quartz-feldspar fine-grained instrusive rocks (aplite
Aplite
Aplite in petrology, the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens...
) and quartz-feldspar-mica coarse-grained intrusive rocks (pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
) are common.
Most of the region's crystalline rock bodies of Late Archean age are part of the greenstone-granite terrane of northern Minnesota, northwestern Wisconsin and the western part of the Upper Peninsula of Michigan. Lithologies
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
of the rocks are usually gneissose and migmatitic
Migmatite
Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...
. Repeated metamorphism and deformation caused extensive recrystallization, intense foliation, shear zones and folding. There are east-northeast- to east-trending faults in the gneissic rocks south of the Great Lakes tectonic zone in Minnesota, south of the Midcontinent Rift System in Wisconsin and in the Upper Peninsula of Michigan.
Minnesota
Crystalline rocks are more prominent in Minnesota, where they underlie 8882 square kilometre, than they are in either Wisconsin or Michigan's Upper Peninsula.Montevideo and Morton gneiss complexes
Recent radiometricRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
age data indicates that there are four crystalline rock complexes 3,400 million years old in the Lake Superior region. The best-known units are the Morton Gneiss and the Montevideo Gneiss complexes, along the Minnesota River Valley in southwest Minnesota. Rocks exposed in the Minnesota River Valley include a complex of migmatitic granitic gneisses, schistose to gneissic amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
, metagabbro and paragneisses. The complex of ancient gneisses is intruded by a younger, weakly deformed granite body, the Sacred Heart granite.
Sacred Heart granitic bodies
The Sacred Heart granitic bodies that occur along portions of the Minnesota River Valley are relatively unfractured and unfoliated, and may represent passive intrusions into folded metasedimentary rocks. It is a typical late-tectonic medium-grained pink granite that was intruded around , after the suturing of the MRV gneissic terrane onto the Superior province. Similar intrusions farther east along the GLTZ show later dates, reinforcing the theorized closure from west to east.Northern Wisconsin
Late Archean lithologies in northwestern Wisconsin and the Upper Peninsula of Michigan are similar to the Sacred Heart granite and consist of gneisses and migmatites.The 1,850-year-old Penokean magmatism in Wisconsin represents margin-type igneous activity terminated by collision. Some of the Penokean granites show iron enrichment similar to the magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
series, rather than the low-oxygen concentration of the magnetic titanium oxides. Penokean-age rocks in the northern Wisconsin and the Upper Peninsula of Michigan contain areas of low-pressure, low- to high-temperature metamorphism. The folding and metamorphism increased in intensity to the south and southeast, and produced the isolated gneissic 1,755-million-year-old Watersmeet Domes which straddle the border of Michigan and northeastern Wisconsin.
Michigan's Upper Peninsula
Compressive deformation during the Penokean orogeny reactivated the GLTZ, which followed deposition of the Marquette Range Supergroup sediments and resulted in a north-side up motion along steep brittle-ductile faults in the eastern, low-grade portion of the Marquette TroughTrough (geology)
In geology, a trough generally refers to a linear structural depression that extends laterally over a distance, while being less steep than a trench.A trough can be a narrow basin or a geologic rift....
In the western portion of the Marquette syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
, a second episode of GLTZ reactivation took place during the uplift of the post-Huronian 2,400- to 2,100-million-year-old granitic Southern Complex.
The Northern and Southern complexes of the Upper Peninsula are highly migmatized and intensely foliated, with the intensity of foliation increasing toward margins. The western part of the Southern Complex shows intricate phases of folding and foliaton. These Late Archean rocks form a roughly north-south belt lying south of Marquette extending to the Michigan-Wisconsin border.
Sudbury, Ontario
The Sudbury structure is located northwest of Sudbury at the erosional boundary between the Archean Superior province and the overlying sequence of early Proterozoic continental margin deposits.The original url used showed the entire article as a PDF. It doesn't seem to work when the url is typed into Google. The url=ftp:// geo.igemi.troisk.ru/archive/Geophysics/geo2000/geo65n06/geo6506r18901899.pdf The structure consists of the Sudbury Igneous ComplexSudbury Igneous Complex
The Sudbury Igneous Complex is a 1,844 million year-old impact melt sheet in Greater Sudbury, Northern Ontario, Canada. It is part of the Sudbury Basin impact structure, and is classified as a lopolith.-References:*...
, a differentiated sequence of intrusive volcanic rocks – norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
, gabbro and
granophyre
Granophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
– overlain by breccias and metasedimenary rocks. The sublayer consists of a mass of basic to ultrabasic inclusions of varying size and frequency of occurrence. Sudbury gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
varies between a gabbro and a norite, depending upon the local silicates' ratios. The quartz biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
gabbro is medium- to coarse-grained, the Climax quartz monzonite
Monzonite
Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals...
is medium-grained.
In eastern Sudbury area the rock is highly crystalline hornblendic gneiss, which apparently dips at a rather low angle toward the southeast.
A paleostress analysis of the eastern exposures near Sudbury shows continuing dextral offset during the Penokean orogeny.
General information
An episode of hotspot gabbro magmatism occurred at the eastern edge of the Wyoming craton, south of current-day Sudbury. Continental rifting is exhibited by emplacement of maficMafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
igneous rocks on each side of the rift margins. By the Superior and Wyoming provinces had completely separated. From about 2,100 to 1,865 million years ago the Wyoming craton drifted in a westward direction until it docked with the Superior province, northwest of its original position.
Before rifting
The final assembly of supercontinent Kenorland was finished by 2,600 to 2,550 million years ago; the southern Superior province – with the Minnesota River Valley subprovince attached – and the current-day southeastern border of the Wyoming province abutted each other from the Sudbury area westerly about 625 km (388.4 mi) to the Wisconsin-Michigan state line on Lake Superior.The distance measurements in the Wyoming province section are derived by the original author of this article with the use of three resources: 1. Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario|author1=Easton, R.M.|author2=Jobin-Bevans, L.S.|author3=James, R.S.|year=2004|publisher=Ontario Geological Survey|report=Open File Report 6135|page=4|url=http://mgmudrey.brinkster.net/Compressed/ILSG%2052%20pt%206%20East%20Bull%20Lake%20.cv.PDF |accessdate=April 4, 2010. Page 4 has a map with a scale given. 2. Using that scale with landmarks on the maps from page 8 of the Dahl reference and 3. Mapquest. The original author arrived at approximate values using the scales, maps and a ruler. The geographic locations (Sudbury, Duluth, Wisconsin, etc.) given with the measurements are present-day locations. The hotspot was 125 km (77.7 mi) south of the East Bull Lake suite, approximately under present-day Sudbury. The Blue Draw Metagabbros – in the Black Hills of South Dakota – were 625 km (388.4 mi) west of Sudbury and 150 km (93.2 mi) south of the western-most contact of the two provinces on the Wyoming province.During rifting
The 2,170-million-year-old intrusive events that affected the Superior and the Wyoming cratons indicate that the plume had moved 330 km (205.1 mi) west, centered in the opening between the Superior province and the rifting Wyoming province. The Wyoming province was rotating away, with the Blue Draw Metagabbro being the pivot point. Harlan's reconstruction of this pivot is shown to the right. At this time the two provinces are in contact at only one point north of the Blue Draw Metagabbro; that point of contact was 875 km (543.7 mi) from Sudbury and 95 km (59 mi) southwest of Duluth, Minnesota. The Blue Draw Metagabbro is now 935 km (581 mi) west of Sudbury and remains about 150 km (93.2 mi) south of the Superior-Wyoming provinces' junction.After complete separation
The 2,125- to 2,090-million-year-old mafic magmatic events affecting the Superior and Wyoming cratons show the hotspot having moved 500 km (310.7 mi) west from Sudbury, and the two provinces have rifted so that they are separated by 100 km (62.1 mi). That narrowest distance between the two cratons is 1150 km (714.6 mi) from Sudbury, in east-central South Dakota. The Blue Draw Metagabbro is now 950 km (590.3 mi) west of Sudbury and 200 km (124.3 mi) south of the Superior province's southern border.Before rifting
Swarms of mafic dikes and sills are typical of continental rifting and can be used to time supercontinent breakup. Intrusion of the 2,475- to 2,445-million-year-old Matachewan-Hearst Mafic Dike Swarm and the 2,490- to 2,475-million-year-old East Bull Lake suite of layered mafic intrusive rocks are interpreted as indicating early Paleoproterozoic, mantle-hotspot driven rifting centered near Sudbury, Ontario, during the onset of Kenorland breakup. Radiometric dating shows that the Wyoming province's Blue Draw Metagabbro was undergoing rifting at , the same time the emplacement of the 250 km (155.3 mi) long belt of mafic layered intrusions in the Sudbury region.In the northern Black Hills of southwest South Dakota the 2,600- to 2,560-million-year-old Precambrian cystalline core, the Blue Draw Metagabbro, is a 1 km (0.621372736649807 mi) thick layered sill. The East Bull Lake intrusive suite, in the southern Superior province near Sudbury, Ontario, aligns spatially with the Blue Draw Metagabbro if the Superior and Wyoming cratons are restored to the Kenorland configuration proposed by Roscoe and Card (1993). These layered mafic intrusions are of similar thickness and identical age, and occur along a rifted belt.
Recent paleomagnetic and geochronological data from the central Wyoming craton support the hypothesis that the Huronian
Huronian Supergroup
The Huronian Supergroup is a Proterozoic geologic feature of the Superior craton of the Canadian Shield in Ontario and Quebec. It extends from the city of Sault Ste. Marie in the west to the Ontario-Quebec border to the east....
(in southern Ontario) and Snowy Pass (in southeastern Wyoming) supergroups were adjacent to each other at and may have evolved as a single sedimentary rift basin between 2,450 and 2,100 million years ago. These Huronian and Snowy Pass sedimentary rocks are similar, each having 2,450- to 2,100-million-year-old epicratonic rifts succeeded by a 2,100- to 1,800-million-year-old passive sedimentary margins.
During rifting
Much of the southeastern Superior province was bisected by the more than 300000 square kilometre 2,172- to 2,167-million-year-old Biscotasing Diabase Swarm which trended northeast from Sudbury. In southcentral Wyoming province there is a 2,170 ± 8-million-year-old quartz diorite dike of Wind River Range.After complete separation
By , the Wyoming craton is thought to have completely separated from the southern Superior province, this is consistent with the occurrence of a 2,076- to 2,067-million-year-old hotspot centered just south of the Superior province and east of the MRV. The 2,125- to 2,101-million-year-old Marathon and 2,077- to 2,076-million-year-old Fort Frances dikes, both on the present-day Superior province north of the Great Lakes tectonic zone, are consistent with rifting during this time period.Earthquakes
Minnesota has been the most seismically active in the region of Minnesota, Wisconsin, Michigan's Upper Peninsula and southern Ontario. Several earthquakes have been documented in Minnesota in the last 120 years, with at least six in the GLTZ. The epicenters show a clear relationship to tectonic features of the state; four epicenters lie along the Great Lakes tectonic zone. Depths are estimated at 5 to 20 km (3.1 to 12.4 mi). The best-documented event occurred on July 9, 1975, near Morris, Minnesota1975 Morris earthquake
The 1975 Morris earthquake occurred in Morris, Minnesota, on July 9, 1975 at 14:54:15 UTC, or 9:54 a.m. The strongest instrumentally recorded rupture in the history of the state, it registered at magnitude 4.6 on the Richter scale. It was the first earthquake to be recorded on any seismic...
, with a magnitude of 4.6, and a felt area of 82000 square kilometre covering parts of four states.
Wisconsin has had no earthquakes along the GLTZ, Michigan's Upper Peninsula has had four earthquakes in the vicinity of the GLTZ – Negaunee, Newberry and two in Sault St. Marie – and the Sudbury area has had three earthquakes.