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The Trans-Hudsonian Suture Zone or orogen is a large feature that extends from Hudson Bay, through Saskatchewan, through the western part of the Dakotas, and into Nebraska. It is the suture zone that formed when the Superior Province collided with the Hearne/Wyoming Province about 1.9 - 1.8 billion years ago. (See sharp versions of the simplified basement map of North America and a more detailed basement map. Both maps show Lithoprobe's study areas.)
Most of the Trans-Hudsonian Suture Zone is covered by younger sedimentary rocks. However, this zone it is readily visible on maps that show variations in the magnetic field of the earth (known as aeromagnetic anomaly maps). Rocks accreted to the margin of a continent have variable compositions and thus variable contents of magnetite. The resultant pattern on a map of aeromagnetic anomalies is a series of magnetic stripes parallel to the margin of the continent. The Trans-Hudsonian Zone truncates the magnetic patterns in the older terranes. See the low resolution aeromagnetic map of western Canada in Lithoprobe's Slide Set or the high resolution aeromag map in the Geological Atlas of the Western Canada Sedimentary Basin and the aeromagnetic map of Montana, complied by the U.S. Geological Survey.
The collision had been considered to be similar in some ways to the currently active collision between India and Asia. (See the section "Continental-continental convergence" in the USGS text.) Just as the Himalaya, the world's highest mountain range, formed as a result of the continent-continent collision, there was also a great mountain range that formed along this suture zone. Only the roots of the mountain chain remain, but these can be seen in northeastern Saskatchewan (and in the Black Hills of South Dakota). When Lithoprobe ran seismic lines across the Trans-Hudson suture in northeastern Saskatchewan and northwestern Manitoba, they made an astounding discovery. The suture zone had an antiformal core of Archean rocks with thrust sheets converging from both sides. (This is shown as the Glennie Domain on the map that shows the central Reindeen Zone.)
In Montana and North Dakota, some 700 kilometers to the south of the Lithoprobe THOT profile, COCORP ran seismic lines across the Trans-Hudson suture zone and found the same antiformal structure in the core. (Reference: see article by Baird et al, 1995; in COCORP publications .)
The Trans-Hudson orogeny had a profound effect on basement rocks, both in Montana and in Alberta. The older Archean rocks underwent retrograde metamorphism at about 1.8 billion years and many igneous rocks were emplaced at that time. Phlogopite rich veins were injected into the lithospheric mantle, providing the source for potassium-rich magmas which were subsequently generated during Eocene volcanism. As shown on the Deep Probe seismic refraction profile, both the Wyoming Province and the Medicine Hat Block have abnormally thick crust with the addition of 10 or more kilometers at the bottom of the crust of high temperature, dark-colored rocks called mafic granulites. Xenoliths from the lower crust which were brought up in Eocene magmas consistently give ages of about 1.8 billion years. The evidence to date strongly suggests that there was major crustal thickening by underplating of the Wyoming province and the Medicine Hat block with basaltic magmas generated by the Trans-Hudson orogeny.
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Plate Tectonics of Montana (Page 4 of 14)
