The Trans-Hudson orogeny, Trans-Hudsonian orogeny, Trans-Hudson orogen (THO), or Trans-Hudson Orogen Transect (THOT), (also referred to as the Trans-Hudsonian Suture Zone (THSZ) or Trans-Hudson suture), was the major mountain building (orogeny) event that formed the Precambrian Canadian Shield, the North American craton (also called Laurentia), and the forging of the initial North American continent. It is the largest Paleoproterozoic orogenic belt in the world. It consists of a network of belts that were formed by Proterozoic crustal accretion and the collision of pre-existing Archean continents. The event occurred 2.0-1.8 billion years ago.
The Trans-Hudson orogen sutured together the Hearne-Rae, Superior, and Wyoming cratons to form the cratonic core of North America in a network of Paleoproterozoic orogenic belts. These orogenic belts include the margins of at least nine independent microcontinents that were themselves sections of at least three former major supercontinents, including Laurasia, Pangaea and Kenorland (ca. 2.7 Ga), 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 Orogen (THO) and resulted in extensive folding and thrust faulting along with metamorphism and hundreds of huge granitic intrusions.
The THO is a right-angled suture zone that extends eastward from Saskatchewan through collisional belts in the Churchill province, through northern Quebec, parts of Labrador and Baffin Island, and all the way to Greenland as the Rinkian belt and Nagssugtodidian Orogen. Westward it goes across Hudson Bay through Saskatchewan and then extends 90 degrees south through eastern Montana and the western Dakotas, downward through eastern Wyoming and western Nebraska, and is then cut off by the Cheyenne belt - the northern edge of the Yavapai province (see Trans-Hudson Orogen map and the THOT Transect map. To the south, the orogen contributed to the subsurface Phanerozoic strata in Montana and the Dakotas that created the Great Plains.
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, 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 occurred between 1.82-1.81 Ga. and occurred as 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 Trans-Hudson orogeny was also the 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 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 remaining exposed portions of the Trans-Hudson orogenic belt. It is a huge, elliptically domed area in northwestern South Dakota and northeastern Wyoming about 125 miles long and 65 miles wide where the THO uplift caused erosion to remove the overlying cover of the Proterozoic marine sedimentary rocks and expose the granite and metamorphic rocks that formed the core of the dome. The peaks of the Black Hills are 3,000 to 4,000 feet above the surrounding plains, while Harney Peak - the highest point in South Dakota - has an altitude of 7,242 feet above sea level. These central spires and peaks all are carved from granite and other igneous and metamorphic rocks that form the core of the uplift. 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:
- Deposition of passive margin sequences, beginning at ca. 2.16 Ga.
- 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.
- 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.
- Continental arc magmatism along the SE margin of the Churchill plate between 1.86 Ga and 1.85 Ga.
- 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.
- 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.
- 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.
- Terminal collision involving the Superior craton, beginning at ca. 1.83 Ga and ongoing until approximately 1.77 Ga. (Corrigan, 2004).).
- 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.
The Trans-Hudson Orogen is the only fully preserved Early Proterozoic orogenic belt in North America. It is a network of belts formed by Proterozoic crustal accretion and the collision of pre-existing Archean continents: the Hearne-Rae provinces to the northwest, the Superior Province to the southeast, and the Wyoming province to the southwest. In the northern Saskatchewan and Manitoba provinces, the 500-km-wide orogen includes four major lithotectonic zones:
- a northwest hinterland zone (divided into smaller domains);
- an Andean-type magmatic arc batholith;
- an internal zone of juvenile Proterozoic crust;
- a southeast foreland zone (with prominent, pervasive east-dipping seismic reflectors throughout the crust (not west-dipping as expected) where the orogen bounds the Superior Province).
Northwestern hinterland zone
The Northwestern hinterland zone is a complex tectonically 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.
The Reindeer zone to the north is a 500 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 these rocks 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.
The Wathaman-Chipewyan batholith is an Andean-type continental-margin, magmatic arc emplaced 1.86-1.85 Ga.
The Glennie domain is in the western segment of the orogen in the internal zone in Saskatchewan, and consists mostly of 2.4–2.5 Ga felsic plutonic rocks enveloped by the Nistowiak Thrust. The Nistowiak Thrust is a folded, 1– to 2-km-thick, upper amphibolite facies mylonite zone formed during emplacement of the Flin Flon–Glennie Complex across the ancient Saskatchewan continental area. It correlates to the Pelican Thrust, which envelops Archaean basement windows in the Hanson Lake Block 100 km to the east. There is an internal high strain zone within the overlying nappe pile called the Guncoat Thrust that is composed primarily of mylonitized porphyroclastic pelitic and psammitic migmatites. U-Pb geochronological results suggest 1.889–1.837 Ga calc-alkaline plutonism, thrust stacking, peak metamorphism and associated 1.837-1.809 Ga anatexis, isotopic closure of titanite at 1.790–1.772 Ga, and intrusion of late granitic rocks at 1.770–1.762 Ga. This is in agreement with ages from the Hanson Lake Block, and La Ronge, Kisseynew, and Flin‐Flon domains in Saskatchewan and Manitoba, and from the Ungava[disambiguation needed]‐Baffin portion of Trans-Hudson Orogen, suggesting broadly synchronous thermotectonic processes along a strike length of 2,000 km. This suggests that the Saskatchewan continental section rifted from the Superior and/or Hearne Provinces ca. 2.1 Ga and that the Maniwekan Ocean developed between the Rae-Hearne and Superior cratons as their plate motions opened and closed. The consequent crustal formations around parts of the arc-granitoid Glennie domain surface features are penetrated by Archean basement "windows". These Archaean basement "windows" is what identifies the previously unknown Archean microcontinent, now called the Sask craton.
Kisseynew back-arc basin
The Kisseynew back-arc basin (Kisseynew domain) is a metasedimentary 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 spread out 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. These turbidites are interpreted to have been deposited in a back-arc basin behind a retreating subduction boundary. 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 result was that the high-grade core of the Kisseynew domain was structurally emplaced over adjacent low-grade terranes. 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.
Flin Flon greenstone belt
The Flin Flon greenstone 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 Flin Flon greenstone belt extends hundreds of kilometres to the south-southwest beneath a thin, geophysically transparent Phanerozoic cover. To the north the Flin Flon greenstone 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 Flin Flon-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 Flin Flon 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: Flin Flon greenstone belt (now defined to include the Attitti block and Paleoproterozoic rocks in the Hanson Lake block) and Glennie Domain (together comprising the "Flin Flon-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 Flin Flon greenstone 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 Flin Flon greenstone 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 Flin Flon greenstone 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 calc-alkaline plutons and rarely preserved calc-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 Flin Flon greenstone 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 Flin Flon greenstone belt. Emplacement of voluminous granitoid plutons and regional deformation related to the occurred during the Ca. 1.8 Ga Hudsonian Orogeny.
Flin Flon domain
The Flin Flon domain is in the center of the Trans-Hudson Suture Zone and extends over the border of the Manitoba-Saskatchewan segment east and west. It is west of the Superior craton, south of the Kisseynew Domain, and east of the Glennie Domain.
The Sask craton is exposed in two structural windows in the western Flin Flon and Glennie domains to a depth of at least 25 km over an area of at least 100,000 km2. The presence of the Sask craton may have arrested the complete collision of the Superior and Hearne cratons and allowed the preservation of a significant amount of juvenile magmatic and sedimentary terranes in allochthons within the internides (Reindeer Zone). The sole of these juvenile terranes is the Pelican Thrust, which is seismically highly recognizable and is considered to be a well-defined orogen-scale discontinuity that facilitated underthrusting of the Archean Sask craton below Paleoproterozoic juvenile rocks from before 1.825 Ga until after 1.805 Ga. This led to thickening of the sedimentary rocks in the overlying Kisseynew Domain and the development of localized high temperature–low pressure peak-metamorphic conditions coeval with melting and advection of heat within this part of the orogen. The Sask craton was initially identified by seismic reflection along the Glennie and western Flin Flon domains and is now known to extend at depth northward and eastward well into Manitoba. Based on MT measurements, the Sask craton is resistive relative to the Paleoproterozoic volcano-sedimentary belts. Lower parts of the crust in the western Glennie Domain may be portions of the Sask craton that are significantly more complicated and comprise interleaved Archean and Paleoproterozoic rocks. The Sask craton includes rocks as old as 3.10 Ga, but it is dominated by a ca. 2.45 Ga magmatic event, and overprinted by metamorphism and deformation during the THO. The range in ages observed, together with Pb isotopic compositions, suggest that the Sask craton is not a fragment of the Superior or Hearne cratons. The Sask craton is associated with a crustal root that trends in a north-northeast direction of about 200 km. In addition, a lithosphere region may be spatially associated with the Sask craton lithosphere below the Fort à la Corne kimberlite. However, the mantle lithosphere preserved below the Sask craton may have been replaced by Superior craton mantle lithosphere during orogenesis.
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.
The Flin Flon greenstone belt is one of the largest Proterozoic volcanic-hosted massive sulfide (VMS) districts in the world, containing 27 Cu-Zn- (Au) deposits from which more than 183 million tonnes of ore have been mined. Most of mined VMS deposits in the Flin Flon 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.
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