User:Bettymnz4/Penokean Orogeny

Name, age and location
The Penokean orogeny is a 1875- to 1825-million-year-old tectonic mountain-building event south of Lake Superior in the United States (U.S.). This major igneous activity is named for the Penokee Range in northeast Wisconsin; it is called the Huronian orogeny in Canada. It represents the initial phase of a major period of crustal growth of the North American craton, and is considered the end of the Middle Precambrian age.

This intruded magma formed the St. Cloud, Minnesota, U.S., granites, folded the rocks of the Cuyuna Range in east central Minnesota and further metamorphosed Minnesota's Morton-Montevideo gneisses. The tilted beds of the Thomson Formation southwest of Duluth, Minnesota, U.S., are part of the large folds formed during this time period.

Roots of the ancient line of the Penokean Mountains are a zone of deformed Archean and early Proterozoic rocks along the southern edge of the Superior province, extending from east-central Minnesota through northern Wisconsin and northern Michigan. Its northern boundary is the Niagara fault zone and its southern border is the Spirit Lake tectonic zone.

Synopsis
This orogeny folded and metamorphosed the Precambrian sedimentary rocks. The sedimentary rocks deposited in the northern part of the Animikie Basin, on the Mesabi and Gunflint iron ranges, were hardly affected. The beginning of the Penokean orogeny is recorded by the Lake Superior banded-iron formations, including those in the Marquette, Gogebic, Mesabi and Gunflint iron ranges. Banded-iron formations are alternating sedimentary strata of iron and chert. Animikie sediments include the iron formations, which were subsequently deformed by the collision of an island arc.

The region's iron-bearing strata were deposited throughout the progressive growth and destruction of a rifted continental margin, and was over by 1850 million years ago.

This orogeny was a major event in the formation of the North American continent and occurred during a worldwide period of mountain building and continent formation. The Penokean orogeny began with rifting of the southern edge of the Archean craton along an east–west line roughly parallel to the Great Lakes Tectonic Zone, the suture between the Minnesota River valley subprovince and the Superior province. The orogeny happened in two phases. First the island arc Pembine-Wausau terrane collided with the ancient North American craton along with volcanoes formingin its back-arc basin.

The Paleoproterozoic crust in the north-central U.S. represents intact juvenile terranes accreted to the rifted Superior craton. Progressive accretion of juvenile arc terranes from 1900 to 1600 million years ago. The newly defined Spirit Lake Tectonic Zone marks the southern limit of Archean and Penokean-interval rocks.

In comparison to the western U.S., little tectonism has occurred in the Great Lakes area in the last 1000 million years which provides a uniquely preserved record of the Precambrian evolution of the continental U.S. lithosphere.

A second crustal body collided into the island arc 1840 million years ago. The Penokean orogeny closed with some post-tectonic magmatism.

Niagara Fault Zone
The style and amount of deformation shows that tectonics had evolved and the crust was now behaving as modern crust does. Modern styles involve large rigid bodies of continental crust colliding and producing patterns of folds, faults and rock types that could have only have been produced with the collision of large bodies of crust. The most obvious is a semicontinuous major Niagara Fault Zone that runs from western Minnesota through Wisconsin into central Upper Michigan. This continuous fault zone indicates large sections of continental crust were involved. This is different from the small blocks of crust seen in the Superior province. The first and second collision deformed a large band of rocks along the line of the suture. The main Niagra fault zone plane dips shallowly to the south which shows the Pembine-Wausau terrane was thrust up on to the craton from the south. As the two opposing blocks collided, the Superior province was partly subducted under the Pembine-Wasuau terrane causing some of the sedimentary rocks of the passive margin to be folded, and compressed to the north up onto the craton along with rocks from the Pembine-Wasuau Terrane. The present-day Niagra fault zone is a zone of ductile deformation indicating it formed at considerable depth, near the roots of the colliding terranes. The overlying 10 km of rocks have been eroded off.



Pembine-Wausau terrane
Sedimentation styles of the passive margin changed as they came to a close; the sedimentary environment changed from deep-water shales from Archean rocks to coarser clastic rocks derived from a younger Proterozoic source. This change is interpreted to be from the island arc as it closed in on the passive margin from the south just before its collision with the passive margin. Sediments that were shedding off the island arc accumulated on top of the previously deposited passive margin sequences in a style called forestepping – where the younger sediments are deposited farther and farther inland on the stable craton as sediments are still being shed from the craton.

The Penokean orogeny began 1880 million years ago when an oceanic arc – Pembine–Wausau Terrane – collided with the southern margin of the Superior Province marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. The newly established subduction zone caused continued arc volcanism until 1850 million years ago when a fragment of Archean crust – now the basement of the Marshfield terrane – arrived at the subduction zone. The convergence of the Superior and Marshfield provinces resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine–Wausau island arc were thrust northward onto the Superior province causing subsidence of a foreland basin in which sedimentation began 1850 million years ago in the south – Baraga Group – and 1835 million years ago in the north – Rove and Virginia formations.

This island arc – the Pembine–Wausau terrane – was the first of the two collisions involved in the Penokean orogeny; it collided into the Superior province's passive margin 1850 million years ago. Geographically, the Pembine-Wausau terrane extends 75 km to the south of and 175 km east–west along the suture line between this terrane and the Superior province called the Niagra fault zone. The Pembine-Wausau terrane was produced by a southward subduction zone on top of oceanic crust, and is composed mainly of volcanic and plutonic rocks that are broken down into two intervals. The rocks in the northern part tend to be older, deposited around 1900 to 1860 million years ago. The rocks in the central and southern part have a range of compositions similar to back-arc basin volcanism, and are younger with ages of 1845 to 1835 million years ago.

The older 1900- to 1860-million-year-old interval consists of metamorphosed basalt, andesite and dacite flows. Early rocks in this sequence are typified by the tholeiitic basalts and andesites of the Quinnesec formation of 1,900 million years ago. Island arc rocks are solidified magmas derived from oceanic plate material that has been reheated during subduction under another oceanic plate. Oceanic plates are created and destroyed quickly in geological terms, so the rocks which comprise island arcs are not very old. Island arcs tend to be short-lived bodies that accrete onto a larger continental body because they sit above subduction zones which try to subduct a continent. The older portion of the Pembine-Wausau Terrane has these characteristics, it contains no Archean-derived igneous rocks and probably developed less than 75 million years before its suturing onto the Superior province.

The younger 1855- to 1835-million-year-old interval of rocks are deposited in the central and southern portions of the Pembine-Wausau terrane. Basic basalts and rhyolites were being deposited at the same time. These volcanic rocks have a wide range, but are generally higher in the quantity of silica ranging from andesite to rhyolite, and are moderately enriched in rare earth elements.

Spirit Lake tectonic zone
The northern border of the Spirt Lake tectonic zone extends from about 43&deg;N in northwestern Iowa northeasterly to about 44&deg;75' and 91&deg;W in mid-Wisconsin. Its southern border extends from about 40&deg; and 93&deg;W at the Iowa-Missouri state line northeasterly to 43&deg;25' and 88&deg;W at the Wisonsin-Michigan border.

In Precambrian Basement Beneath the Central Midcontinent United States as Interpreted from Potential Field Imagery, the Geological Society of America places the Spirit Lake trend as trending northeasterly from northeast Nebraska through the extreme southeastern tip of South Dakota, through northwestern Iowa and into Minnesota where it ends at the Midcontinent Rift.

Marshfield terrane
The second phase involved the microcontinent Marshfield terrane, which forms parts of Wisconsin and Illinois. The episode lasted about 10 million years.

In the southern fold-and-thrust belt, tectonic thickening resulted in high-grade metamorphism of the sediments 1830 million years ago}}. At this time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny.

Evidence for Penokean orogeny
Gneiss domes preserved throughout the 1870- to 1820-million-year-old Penokean orogenic belt are collisional features.

These rocks resemble modern island arc and back arc basins.

The Thomson Formation, near the center of the basin, folded into broad open folds a few kilometers across, elongated in an east-west direction; these beds dip to the north or to the south. Virtually all of the outcrops in the Thomson Formation consist of tilted beds that are on the flanks of large folds.

Accretion of these terranes to the Archean craton showed that plate tectonic styles had evolved further, and were now behaving in more of a modern Wilson Cycle. The Penokean orogeny assembled the last major crustal pieces in the Great Lakes region, which were subsequently modified by the Wolf River batholith and the Midcontinental Rift.

Two major phases of early Proterozoic Penokean folding are recognized in the Thomson Formation of east-central Minnesota. Effects of the first deformation are found in the southern two-thirds of the exposed Thomson Formation; the second deformation affected the entire region of outcrop. Evidence indicates that northward-directed nappes developed during the early phase of folding; this evidence includes lithologic differences between the areas of one and two deformations, the pervasive nature of a foliation in the area of its occurrence, facing directions of folds and the refraction pattern of the foliation in graded metagraywacke-slate beds. The data show that an area of several hundred square kilometers is on the upper limb or limbs of a large, northward-directed recumbent fold or folds (nappes) of the early phase of Penokean deformation.

consisted of four stages of deformation, three of which occurred during a prolonged period of regional metamorphism. The first two stages of deformation were possibly caused by gravity sliding northward off an ancestral Penokean range located in central Wisconsin. The deformation probably started while the sediments were still soft, and it produced a pervasive west-northwest–trending foliation in the middle Precambrian rocks.

In east-central Minnesota, it is marked by multiply deformed and highly metamorphosed supracrustal rocks of the early Proterozoic Denham and Thomson formations. Structural features similar to those in the supracrustal rocks also exist in the 2700-million-year-old McGrath gneiss. Intense deformation occurred in the footwall of the major thrust, which marked the boundary between downgoing and overriding plates during continental subduction. Sedimentary rocks of the Thomson Formation deposited on the footwall during loading caused by thrusting eventually became incorporated into the deformation zone. Early-formed structures related to footwall deformation are a dominantly well-developed foliation in the gneiss and isoclinal, recumbent folds with a bedding-subparallel foliation in the Denham and lower Thomson formations. Footwall deformation was followed by imbrication and accretion onto the hanging wall during uplift associated with continued compression and isostatic rebound. Later-formed structures associated with imbrication and deformation within the hanging wall consist of folding of the foliation and development of shear zones in the McGrath Gneiss and open-to-close, upright-to-overturned folds in the Denham and Thomson formations.

The volcanic rocks primarily represent new crustal material that had only a limited Archean input, probably through mixing of subducted sediments into the magma source area. The sedimentary rocks in the upper part of the Marquette Range Supergroup – graywackes of the upper Michigamme Formation. These graywackes probably are foredeep deposits derived from the volcanic rocks of the Wisconsin magmatic terrane to the south. The deposition of the upper Michigamme Formation dates the final convergence of the Wisconsin magmatic terrane with the continental margin.

The 1900- to 1700-million-year-old Penokean events involved major growth of new crust from the mantle.

In central Wisconsin the orogeny is dominated by the intrusion of variably deformed and metamorphosed tonalite-trondhjemite suites, in part hosted by Archean gneisses and migmatites. The magmatism is calc-alkaline and similar in composition to modern high-potassium orogenic andesites. An Archean component most likely was incorporated during emplacement.

Nd and Pb isotopes indicate significant direct involvement of older continental crust during the generation of the Penokean synorogenic intrusive suite in Wisconsin. Initial values for plutons show a clear trend of increasing Nd, from –6 to +4 southward from the Superior Province to the suture between the Pembine-Wausau and Marshfield terranes, indicating decreasing amounts of contamination by older crust. Synorogenic intrusive rocks acquired their isotopic ratios through interaction with Archean crust, rather than through contamination of a mantle source region by sediment subduction. Direct dating of the exposed basement in the Pembine-Wausau terrane indicates that Archean crust is 2607 ± 22 million years).

The preferred tectonic model for the Penokean orogeny involves back-arc rifting of the southern Superior province during a major period of Penokean orogenesis, followed by collision of the Marshfield terrane to the south, which was also intruded by Penokean-age magmas. In addition to the isotopic and geochronology data presented here, this model is supported by gravity data which indicate the presence of low-density material (probably Archean crust) south of the Niagara Fault Zone; this feature is interpreted to be a buried extension of the southern margin of the Superior Province beneath the Pembine-Wausau terrane.

Later plutons of tonalite and granodorite intruded the earlier rocks just before collision with the craton 1870 to 1855 million years ago. Volcanic rocks from andesite to rhyolite continued to be deposited onto the island arc. The younger volcanic rocks in this region become more differentiated, because of the absence of fresh magmas from the subducting oceanic plate, besides the normal differentiation of partially melted magmas.

The second collision involved a mature microcontinent, containing Archean gneisses and early Proterozoic plutonic rocks, into the southern edge of the Pembine-Wausau terrane. there is little exposure of a suture zone between the Pembine-Wausau terrane and the microcontinent. It is thought to most likely have been subducting to the north, due to the presence of later volcanic in the range of 1.845 to 1.835 bya.

The full extent of this microcontinent body is not known, but is suspected to reach into northern Illinois. This microcontinent is the Marshfield terrane and shows a long history of development. The complexity of this history can be demonstrated by an exposure near Stevens Point, Wisconsin, U.S. The first preserved rocks in this region are at least 3000-million-year-old Archean gneisses which are minor on the exposed surface, but play a larger role in interacting with magmas at greater depths that produced later intrusions. Later emplacement of amphibolite dikes – probably during the Penokean orogeny – cut the earlier rocks, and are followed by some minor diabase, apilite and pegmatites.

After active plate motions ceased in the Great Lakes region by 1840 million years ago a post-tectonic suite of granitoid rocks intruded the cooling crust. These rocks have a high degree of differentiation, because they have a high iron content and are very sodic. These rocks intruded the Pembine-Wausau terrane mostly, but minor other intrusions exist elsewhere. This closes the Penokean orogeny, and leaves the area as a stable craton with eroding mountains, and a sea far to the south.

The third and fourth stages of deformation were caused by uplift of rigid blocks of lower Precambrian basement rocks; this uplift produced prominent grabens such as the Marquette and Republic troughs. Metamorphism began very early in the deformational sequence, peaked during the third stage of deformation, and ended in a period of retrograde metamorphism.

Chert – silica oxide or quartz – is chemically precipitated in oceans and show an abundance of oxygen in the seas at the time of deposition which records the introduction of abundant free oxygen into our atmosphere.

Comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive Penokean orogen have been removed or overprinted by younger tectonic events.