Under the flat-lying sedimentary rocks of Central and Eastern Montana are highly metamorphosed rocks that are known collectively as the basement complex. In Montana the basement complex is typically more than 2.5 billion years old and thus has an age designation called Archean. Basement rocks in the southeast half of Montana belong to the Wyoming Province and those in the northwest half, to the Hearne Province. The Great Falls Tectonic Zone (GFTZ) separates the two provinces. (See the Wyoming Province and the Hearne Province on the map of Precambrian basement in North America where the GFTZ is labeled "GF" but is partially hidden by the yellow line marking the southern boundary of the Alberta (AB) Lithoprobe research area. This is the same map from the Lithoprobe Project that was linked in the previous paragraph.)

The Wyoming Province is a microplate consisting of Archean-age rocks that occupies most of Wyoming and the southeastern half of Montana. Most of the Wyoming Province is covered by younger rocks. However, these old rocks are exposed in a series of uplifted mountain ranges; namely, the Little Belt, Beartooth, Highland, Ruby, and Tobacco Root Ranges in Montana and the Big Horn, Wind River, Laramie and Teton Ranges and the Sweetwater Uplift in Wyoming. You can see the location where these Archean-age (>2.5 billion years) rocks are exposed in Montana by using the Acrobat Reader to view Figure 1 in the online article by Steve Harlan and co-workers.

Because of good exposures in these high mountains rocks in the basement complex of the Wyoming Province are quite well known and many complexities of their long history have been unraveled. (See for example, general references for Wyoming geology and Don Blackstone's map of the Precambrian basement in Wyoming, unfortunately not available online.) There are some very old rocks in the Beartooth Mountains, including a gneiss that is 3.5 billion years old and a quartzite that contains zircons which are nearly 4 billion years old. (The age of the earth is 4.6 billion years.) Uplift and erosion brought Archean-age rocks in the Tobacco Root Mountains that had been buried to depths of 25 kilometers (8 kilobars of pressure) to the surface by Cambrian time (550 million years ago). David Mogk at Montana State University in Bozeman actively studies these old, old rocks in Montana and has a number of publications.

The Hearne Province is exposed in northeastern Saskatchewan, where it has received much study. The Introduction to Saskatchewan Geology website (maintained by Saskatchewan Energy and Mines) has options for viewers at the beginners level and those at an advanced level. [For details, see Introduction to the Precambrian Shield (including a generalized map), the Hearne Province, and a map showing individual domains in the Hearne Province.] Very highly metamorphosed rocks of Archean age in Northern Idaho suggest that the Hearne Province may extend that far west.

The continuation of the Hearne Province of basement complex from northeastern Saskatchewan to the southwest into Montana is covered by younger sedimentary rocks. Thus information concerning its nature has to be obtained from geophysical surveys, using gravity, aeromagnetic, and seismic techniques, and from core samples obtained from deep wells that have penetrated the basement complex. A cooperative study (called DEEPPROBE) between Canadian and U.S. universities has advanced the understanding of lithospheric structure on the east side of the Rocky Mountains, in general, and the Hearne and Wyoming Provinces, in particular, by making a seismic refraction profile from the Northwest Territories almost to the Mexican border. (The results are discussed in an article in GSA Today that can be downloaded in pdf format. Download the file to disk and rename the file by deleting bin and terminating with .pdf.) A new map of the basement complex in Montana and southern Alberta shows that the Medicine Hat Block in northwest Montana has behaved as a more or less rigid element within the otherwise highly deformed Hearne Province. Two features on this map, the Vulcan Low and Matzhiwin High, wrap around the north side of the Medicine Hat Block. The structures across these three zones were investigated as part of the Lithoprobe project with deep seismic reflection profiling in southern Alberta. Preliminary processing of seismic line 29 and line 25 suggests that Vulcan Low and Matzhiwin Low rocks underthrust the Medicine Hat Block. The DeepProbe seismic refraction profile also allows this interpretation for depths in the range of 25 - 40 km.

The southeastern boundary of the Medicine Hat Block is the Great Falls Tectonic Zone separates the Wyoming Province and the Hearne Province (i.e. the Medicine Hat Block). The Great Falls Tectonic Zone, which extends at the minimum from Challis, Idaho to the Bear's Paw Mountains in Montana, had been considered to be a suture zone in which a continent/continent type collision occurred between the Wyoming and Hearne Provinces during the Archean about 2.8 billion years ago. However, the latest study suggests that the Great Falls Tectonic Zone may have subsequently functioned more like a transform fault or continental shear zone than a suture zone. The lack of outcrops makes this a difficult problem to study. In any case, the two provinces were joined.

(Copyright by David Baker)

Plate Tectonics of Montana (Page 3 of 14)