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Geological map of the exposed Early Proterozoic Trans-Hudson Orogen between the Archean Superior craton to the southeast and Archean Hearne craton to the northwest.

Geologists wax enthusiastic about the Trans-Hudson Orogen because it is one of Earth's great examples of a preserved collisional belt. Perhaps, they cherish the orogen's very complexity, an outcome of a colourful tectonic history, represented by rift and strike-slip as well as compressional styles, juvenile oceanic crust to remnants of an old Archean craton, and just about everything else in between.
In Saskatchewan-Manitoba the Trans-Hudson orogenic belt is subdivided into four major tectonic zones which our slide shows. We see a southeastern foreland zone (yellow with TB and adjacent dark red), a large internal zone of juvenile Proterozoic crust (includes FFB, KD, HLB, GD and RD), an Andean-type magmatic arc batholith (red with WB), and a northwest hinterland zone (yellow with WD).

The southeastern foreland zone is the narrow Churchill or Boundary Zone, which includes the Thompson belt (in dark red and yellow). The zone forms a dogleg going from southwest to east along the edge of the Superior craton. Petrochemistry of volcanic rocks in this zone suggests that the volcanics and the associated metasediments (= sediments altered or metamorphosed by heat and pressure), formed in a rift (where the crust partially split).

The internal Reindeer Zone extends over to the other side of the THO (to the opposite fold belt marked in yellow). The Reindeer Zone is a 400-km-wide collage of Early Proterozic (1.9 to 1.8 Ga) island-arc volcanics, plutons (underground magma intrusions), volcanogenic (derived from eroded volcanoes) sediments, and younger molasse (= detritus from mountain ranges) derived from newly formed mountains. These Early Proterozoic island-arc terranes were severely deformed by the vice action of the collisions, and include imbricated sheets of rock formations thrust upon and into each other. Remember that between them the Superior and Hearne-Rae Provinces crunched the whole works together.

Geochronological analysis using isotope measurements has shown the Wathaman-Chipewyan batholith (red with WB) to have been emplaced at about 1,855 Ma. (A "batholith" is a large-volume magmatic intrusion into overlying rocks, and comprises many individual plutons.) It sits between the arc terrane of the Reindeer zone to the southeast and reworked continental rocks previously eroded from the Archean Hearne-Rae craton to the northwest.

The fourth zone is the hinterland of the sequence, the Wollaston Fold belt (yellow with WD). This zone also is highly deformed and metamorphosed, and complexly interfolded with Archean basement rocks.

Many disciplines of the earth sciences contribute to the research sorting out this complex overall picture of past tectonic events. Comparisons of this old orogen are made with, say, the western part of the modern Himalayas where comparable oceanic rocks are sandwiched between two converging continents (India northward against Eurasia). Let's cut a vertical slice through this map, crossing its tectonic trends.

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