Trans-Hudson orogeny

Trans-Hudson orogen (blue) surrounded by the Wyoming Hearne-Rae and Superior cratons (pink) that constitute the central core of the North American craton (Laurentia).

The North American craton, also called Laurentia.

File:Wyomig, Mojave, Yavapai, Mazatzal, Trans-Hudson.gif

Trans-Hudson orogen and the Wyoming, Superior and Hearne cratons

The Trans-Hudson orogeny, Trans-Hudsonian orogeny, or Trans-Hudson orogen (THO), (also referred to as the Trans-Hudsonian Suture Zone (THSZ) or Trans-Hudson suture), was the major orogenic event that formed the North American craton (also called Laurentia), and hence North America. The Trans-Hudson orogen, along with the Wyoming, Hearne-Rae and Superior cratons that it sutured together, is the cratonic core of North America, and contains a network of Paleoproterozoic orogenic belts that together comprise the largest Paleoproterozoic orogenic belt in the world. These orogenic belts include the margins of at least nine formerly independent microcontinents. These microcontinents were themselves sections of at least three major supercontinents, including Laurasia, Pangaea and Kenorland, and contain parts of some of the oldest cratonic continental crust on Earth. These old cratonic blocks, along with accreted island arc terranes and intraoceanic deposits from earlier Proterozoic and Mesozoic oceans and seaways, were sutured together in the Trans-Hudson oregon (THO). The event occurred 1.9 - 1.8 billion years ago and resulted in extensive folding and thrust faulting along with metamorphism and granitic intrusion.

Overview

The Trans-Hudson orogeny was the culminating event of the Paleoproterozoic Laurentian assembly, which occurred after the Wopmay orogeny (West of Hudson Bay, ca. 2.1-1.9 Ga.). The Trans-Hudson orogeny resulted from the collision of the Superior craton of eastern Canada with the Hearne craton in northern Saskatchewan and the Wyoming craton of the western United States, along with the Archaen microcontinent Sask craton trapped in the THO western interior. The northern edge of the THO is marked by an exposed continental collision zone on Baffin Island (Canada) that occurred between ca. 1.85-1.835 Ga. On Baffin Island, ca. 1.845 Ga. crustal thickening occurred that was associated with accretion of an intra-oceanic arc terrane and subsequent regional metamorphic activity characterized by the growth of retrograde, upper amphibolite-facies assemblages that define a clockwise, decompressive P–T path (pressure-temperature) path. Final mineral growth on Baffin Island is bracketed between 1.82-1.81 Ga. and supports a progressive change from plate-margin to intraplate processes within an evolving convergent orogeny. Collectively, these collisions resulted in a mountain range that rivaled the grandeur of the Himalayas. Similar to the Himalayas, the THO was alao a result of continent-continent collision along a suture zone. Only the roots of this mountain chain remain, but these can be seen in northeastern Saskatchewan and in the Black Hills of South Dakota. The Trans-Hudson orogenic belt extends from Baffin Island, across Hudson Bay west through Saskatchewan, and south through the western portions of the Dakotas and Nebraska and some parts of Montana.

The Trans-Hudson orogeny and the consequent upheaval of the continental crust in the middle Proterozoic eon caused the area around the Great Lakes to become a flattened plain, which in turn led to the creation of the intercontinental basin and the interior and central plains of the United States (the Great Plains are the westernmost portion of North America's Interior Plains, which extend east to the Appalachian Plateau).

The Black Hills of South Dakota is one of the few exposed portions of the Trans-Hudson orogenic belt. The nature and timing of this portion of the THO event in southern Laurentia is poorly understood compared to the exposed northern segments in Canada. The Black Hills offer the only surface exposure of the deformed and metamorphosed belt of Paleoproterozoic continental margin rocks in the collisional zone between the Archean Wyoming and Superior provinces. Based on geophysical evidence, this zone has been broadly interpreted to be the southern extension of the THO that was later truncated by the ~1.680 Ga. Central Plains orogen.

Sequence of events

Marine evidence indicates that the area initially opened to form an ocean called the Manikewan Ocean. Faulting, sedimentary and igneous rocks all indicate that divergence formed a rift valley that continued to spread until it resulted in a passive margin in which there was no tectonic activity. Shallow marine deposits formed on the continental shelves, and oceanic crust formed on the margins of the continental cratons as the divergence continued. Eventually the divergence stopped, then reversed direction, and collision occurred between continental land masses. During the Wopmay orogeny, subduction occurred as oceanic crust of the Slave craton was subducted beneath an eastward moving continental plate. Likewise, during the Trans-Hudson orogeny, rifting at first separated the Superior craton from the rest of the continent. Then the Superior craton reversed its direction and the ocean basin began to close. A subduction zone formed as the oceanic crust of the Superior craton was subducted beneath the Hearne and Wyoming craton with the Sask craton in the middle. Volcanic arcs developed as the cratons collided, eventually resulting in the THO mountain building (orogeny).

During the opening and then closure of the Manikewan Ocean, the following sequence of events occurred:

• 1) Deposition of passive margin sequences, beginning at ca. 2.16 Ga.
• 2) Ocean closure beginning at ca. 1.92 Ga in the western THO, leading to the formation of the La Ronge/Lynn Lake, the Flin Flon Domain/Glennie island arcs, and the Pelican Thrust located between the two volcanic arc complexes and the Archaen Sask craton.
• 3) Beginning at about 1.88 Ga, accretion of the La Ronge/Lynn Lake arcs to the Hearne craton margin and related deposition of an extensive molasse/foredeep sequence. This event was contemporaneous with intraoceanic accretion leading to the formation of the Flin Flon/Glennie accretionary complex.
• 4) Continental arc magmatism along the SE margin of the Churchill plate between 1.86 Ga and 1.85 Ga.
• 5) Cessation of continental arc magmatism at about 1.850 Ga, perhaps related to collision of the Flin Flon/Glennie complex (western THO) and Narsajuaq arc (Baffin segment), with the Churchill plate.
• 6) Opening of the Kissinew back-arc basin during the interval 1.85-1.84 Ga, with the Flin Flon complex now forming part of the active arc and Granville Lake Structural Zone forming part of the remnant arc.
• 7) Collision between the Sask Craton and Churchill Plate, beginning at ca. 1.84 Ga, leading to the inversion of the Kisseynew basin and deposition of molasse deposits.
• 8) Terminal collision involving the Superior craton, beginning at ca. 1.83 Ga and ongoing until approximately 1.77 Ga. (Corrigan, 2004).).
• 9) The final stages of continental collision where marked by 1.84-1.82 Ga. magma generation within the Kisseynew domain related to interaction between the Sask craton and the Flin Flon–Glennie complex, and a 1.82-1.79 Ga. collision of the Rae and Superior cratons. A decrease in subduction angle likely resulted from northward-directed subduction of more buoyant crust, with associated changes in thermal conditions leading to melting over a wide area. Ultimately subduction ceased, and the slab foundered into the mantle, potentially resulting in the generation of alkalic and potassic magmas within the Rae and Hearne cratons.

Geographic areas

Kisseynew back-arc basin

The Kisseynew back-arc basin (Kisseynew domain) is a sedimentary gneiss belt that is one of the most extensive tectonic segments of the Paleoproterozoic Trans-Hudson orogen in north-central Canada. It comprises turbidites (horizontally spreadout sediments deposited from a swiftly-moving downward-flowing current, such as from an existing continental slope) and continental sandstones, both of which are interlayered with volcanic rocks and intruded plutons. The dating of the encompanying detrital zircons and the dates of the intruded crosscutting plutons constrain the dates of the turbidite sedimentation to 1.855-1.841 Ga. This postdates the early deformation and metamorphism of the Trans-Hudson orogeny associated with arc-arc and arc-continent collisions. The turbidites are deposited in a back-arc basin behind a retreating subduction boundary. The Kisseynew domain and its southern flank are marked by 1.84-1.83 Ga magmatic rocks related to subduction of back-arc basin oceanic crust. Collapse of the Kisseynew "basin" by ductile fold-and-thrust deformation began during 1.84-1.83 Ga magmatism and continued through peak metamorphism at ca. 1.820-1.805 Ga. The extensive high-temperature/low-pressure metamorphism explains the thickening and thermal relaxation of the basin-fill rocks that are marked by a 1.84-1.83 Ga elevated geotherm magmatism.

Reed Lake Belt

The Reed Lake Belt is part of the internal zone of the Trans-Hudson Orogen and consists of Paleoproterozoic volcanic, plutonic and minor sedimentary rocks. The exposed portion of the belt is 250 km long by 75 km wide. Although it has an apparent easterly trend, this is an artifact of the belt's tectonic contact with gneissic metasedimentary, metavolcanic and plutonic rocks to the north (Kisseynew Domain) and the east-trending trace of Phanerozoic platformal cover rocks to the south. In reality the Reed Lake greenstone belt extends hundreds of kilometres to the south-southwest beneath a thin, geophysically transparent Phanerozoic cover. To the north the Reed Lake Belt is tectonically overthrust by younger metasedimentary rocks of the Kisseynew domain and by nappes of metavolcanic rocks of the same age. In fact, the entire Reed Lake-Snow Lake Domain is an imbricated thrust wedge carried on a lower detachment zone overridden by high grade Kisseynew gneisses.

The NATMAP Shield Margin Project and LITHOPROBE Trans-Hudson Orogen Transect have shown that the Reed Lake greenstone belt (and contained VMS deposits) is only one of three components in a northeast-dipping stack that was juxtaposed during the 1.84-1.80 Ga collisional deformations.

1) at the lowest structural level (exposed in the Pelican Window): metaplutonic rocks and paragneisses (3.20-2.40 Ga) of the "Sask craton". 2) at intermediate structural levels: Reed Lake Belt (now defined to include the Attitti Block and Paleoproterozoic rocks in the Hanson Lake Block) and Glennie Domain (together comprising the "Reed Lake-Glennie Complex." 3) at the highest structural levels: marine turbidites (Burntwood Group; 1.85-1.84 Ga) and partly coeval distal facies of alluvial-fluvial sandstones (Missi Group) in the Kisseynew Domain.

The Reed Lake Belt is usually described by two stratigraphic groups: the 1.92-1.87 Ga. Amisk Group volcanic-plutonic rocks, and the Missi Group continental sedimentary rocks. The Reed Lake Belt is a collage of distinct tectonostratigraphic assemblages composed of structurally juxtaposed volcanic and sedimentary rock that were emplaced in a variety of tectonic environments. The major 1.92-1.88 Ga components include significant juvenile arc and juvenile ocean-floor rocks, and minor ocean plateau/ocean island basalt. The juvenile arc assemblage comprises tholeiitic, calc-alkaline and lesser shoshonitic and boninitic rocks similar in major and trace element geochemistry to modern intraoceanic arcs. Ocean-floor basalt sequences are exclusively tholeiitic, and are geochemically similar to modern N- and E-type MORBs erupted in back-arc basins. Evolved arc assemblages and Archean crustal slices are present within the Reed Lake Belt as minor components. Collectively, these tectonostratigraphic assemblages were juxtaposed in an accretionary complex at Ca. 1.88-1.87 Ga. as a result of arc-arc collisions. The collage was a basement to a 1.87-1.83 Ga post-accretion arc magmatism that was expressed as voluminous caic-alkaline plutons and rarely preserved caic-alkaline to alkaline volcanic rocks. Unroofing of this accretionary collage, and the deposition of continental alluvial-fluvial sedimentary rocks (Missi Group) and marine turbidites (Burntwood Group) occurred Ca. 1.85-1.84 Ga, coeval with the waning stages of post-accretion arc magmatism. The sedimentary suites were imbricated with volcanic assemblages in the eastern Reed Lake Belt during 1.85-1.82 Ga juxtaposition of the supracrustal rocks along pre-peak metamorphic structures. Post ca. 1.83 Ga structures formed the present southwest-verging fold style at the northeastern end of the Reed Lake Belt. Emplacement of voluminous granitoid plutons and regional deformation related to the occurred during the Ca. 1.8 Ga Hudsonian Orogeny.

Superior Boundary Zone

The Superior Boundary Zone is a narrow, southeastern, ensialic foreland zone bordering Superior craton, comprising the Thompson Belt, Split Lake Block, and Fox River Belt.

Reindeer Zone

The Reindeer Zone is a 400 km wide collage of Paleoproterozoic (1.92-1.83 Ga) arc volcanic rocks, plutons, volcanogenic sediments, and younger molasse, divisible into several lithostructural domains. Most of the rocks here evolved in an oceanic to transitional, subduction-related arc setting, with increasing influence of Archean crustal components to the northwest. The zone overlies Archean basement exposed in structural window that are now recognized as the "Sask craton."

Wathaman-Chipewyan Batholith

Wathaman-Chipewyan Batholith is Andean-type continental-margin, magmatic arc emplaced 1.86-1.85 Ga.

Northwestern Hinterland Zone

The Northwestern Hinterland Zone is a complexly deformed region that includes the Peter Lake, Wollaston, and Seal River domains, and other parts of the Cree Lake Zone, now included in Hearne Province.

Economic geology

The Reed Lake Belt is one of the largest Proterozoic volcanic-hosted massive sulphide (VMS) districts in the world, containing 27 Cu-Zn- (Au) deposits from which more than 183 million tonnes of sulphide have been mined. Most of mined VMS deposits in the Reed Lake belt are associated with juvenile arc volcanic rocks providing a powerful focus for future explorations. Gold mineralization has been less studied but at Reed Lake has been shown to be associated with late brittle-ductile shear zones that follow peak tectonic and metamorphic activity within the Trans-Hudson Orogen. At Snow Lake, preliminary investigations suggest a long history of gold mineralization with at least some gold introduced prior to metamorphism.

See also

• Canadian Shield
• North American craton
• Wyoming craton
• Superior craton
• Hearne craton
• Rae craton

References

• Trans-Hudsonian Suture--Collision of Superior with Hearne/Wyoming Province
• John B. Brady, et al, eds. (2004) Precambrian Geology of the Tobacco Root Mountains, Montana (Special Papers (Geological Society of America), 377.) ISBN 0-8137-2377-9
• LITHOPROBE Seismic Processing Facility (LSPF). (1990). "Phase V Proposal - Executive Overview: Trans-Hudson Orogen Transect (THOT). [[1]]
• LITHOPROBE Seismic Processing Facility (LSPF). (1998). "Trans-Hudson Orogen Transect." [[2]]