Wyoming craton

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The Trans-Hudson Orogen (blue) is girded by Wyoming-, the Hearne- RAE and Superior - Kraton (purple). Together they form the heart of the North American craton (Laurentia).
The North American Crater, also known as Laurentia .

The Wyoming Craton (also Wyoming Province ) is the American craton (old continental mainland core) in the central western United States . It extends over Montana , Wyoming, and parts of northern Utah .

During the Trans-Hudson orogeny, the Wyoming craton was welded to the continental fragments, Superior Kraton and Hearne - Rae-Kraton, which had previously collided along the Trans-Hudson suture zone .

In the Wyoming craton, 3600-3000 million year old gneisses have been preserved in places , and isotope analysis shows that crustal parts of the same age were incorporated into younger plutons . This leads to the conclusion that the origins of the Wyoming craton go back to the Paleoarchean , it was igneously and tectonically overprinted in the Neoarchean and finally subjected to a stretching process in the Mesoproterozoic that led to continental rifting .

Geological overview

The Precambrian basement Wyoming consists of three major geological terranes :

The Colorado orogen collided with the Wyoming craton in the period between 1780 and 1750 mya . As a result of the collision with the Colorado ore and the Trans-Hudson ore, the archaic Wyoming craton experienced a very strong tectonic overprint on its southern and eastern edge.

The Wyoming craton consists mainly of two rock complexes - on the one hand, granitic plutons , which are 2800 to 2550 million years old and are associated with gneisses and migmatites , and on the other hand, supracrustal metavulcanites and metasediments, which only make up less than 10%. The granitoids usually contain potassium and come mainly from older (3100 to 2800 million years old) recycled gneisses. The relatively strong magnetic contrast between the granitoids and the gneisses enables the rock units to be mapped even in areas of terrain where the basement is covered by sediments. The magnetic data reveal an almost semicircular arrangement of the archaic units, open to the north.

Wyoming Province subdivisions

The Wyoming Province can be divided into three sub-provinces (from young to old):

  • the terrane in the south
  • the Beartooth-Bighorn igneous zone
  • the metasediment province of Montana.

The archaic rocks of the metasediment province of Montana and the Beartooth-Bighorn igneous zone are distinguished

  1. old age (rock age up to 3500 Ma , detritic zircon ages up to 4000 Ma and neodymium model ages exceeding 4000 Ma)
  2. Definitely enriched 207 Pb / 204Pb isotope ratio - this excludes an accumulation of exotic terranes in this crustal area
  3. relatively thick mafic lower crust (15-20 kilometers).

Already before 3000–2800 Ma they were cronated and thus an integral part of the Wyoming province. Further crust growth by means of continental arc magmatism took place between 2680 and 2500 Ma through the terranes attached to the southern edge of the craton. Towards the end of the Archaic era, all three sub-provinces in what is now the Wyoming Kraton were welded together.

Between 1900 and 1800 mya the now consolidated crustal area of ​​Wyoming was attached to Laurentia.

Paleoproterozoic mobile crustal belts (in turn 2400–1700 million years old) subsequently surrounded the Wyoming province on its western and southern edges and inevitably led to renewed tectonic movements accompanied by magmatic processes.

Chamberlain (2003) divides the Wyoming province into five sub-provinces due to a different interpretation of the processes in the late Archean:

  • the metasediment province of Montana
  • the Bighorn Sub-Province
  • the Sweetwater Sub-Province.

These three sub-provinces formed the archaic core. Such as:

  • the Sierra Madre-Medicine Bow crust block and
  • the Black Hills-Hartville crust block.

The last-mentioned crust blocks may have behaved allochthonously compared to the 3000 million year old craton.

With the analytical method “Deep Probe” it was found that in the area of ​​the Bighorn sub-province there is a thick lower crust, which was undercut by 2700 Ma in connection with mafic igneous activity. The Sweetwater sub-province is characterized by an east-west trending tectonic structure that was created in at least three temporally related tectonic thrusts (basin formation, narrowing phase and island arc magmatism ). This structure (including the 2620 million year old Oregon Trail structure) in turn mapped the location and orientation for the later Proterozoic rift valley events and the Laramic uplift processes.

The current lithosphere structure of the Wyoming province is the result of billions of years of cumulative crustal processes such as B. crust growth, tectonic transformations and contrasting lithosphere structure. If there has ever been a noticeable growth of the crust since 3000 Ma, it is probably due to a combination of island arc magmatism with the pushing of mafic lower crust.

Origin of Laurentia

The deposition process of the Selway Terran began on the west side of the Wyoming Craton as early as 1860 million years ago . From around 1840 mya there was a collision between the Hearne and the Superior Kraton during the Trans-Hudson orogeny in Canada . The first movements of the same orogeny then seized the Wyoming craton from 1810 mya in the northern and eastern sections (western north and south Dakota, southeastern Montana). The final union of the eastern Wyoming craton with Laurentia happened in the period 1780-1740 mya - on the southern edge of the growing continent, the accretion of a volcanic island arc, the Colorado ore, took place. The mountain-forming movements finally found their conclusion in a post-tectonic magmatic phase, which z. B. secreted the granite of Harney Peak in the Black Hills .

Colorado orrogen deposition

During the Paleoproterozoic around 1760 mya, the Colorado orogen crossed the Wyoming craton along the 500-kilometer-wide Cheyenne Belt , which was named after the city of Cheyenne in Wyoming and consists mainly of Proterozoic island arch rocks. As a result of this collision, the Wyoming Province itself was intensely deformed and metamorphosed within the resulting suture 75 km . The suture - a 5-kilometer-wide NW-vergenter mylonite belt , the south side of which has been raised 10 kilometers - runs through the Laramie Mountains and divides them into two very different halves. But the earlier collision on the eastern edge of the craton with the paleoproterozoic Trans-Hudson orogen was not without consequences - the archaic rocks in the exposed Hartville crust block and in the Laramie Mountains were also heavily tectonically stressed.

In the Laramie Mountains and in the neighboring Medicine Bow Mountains , the Colorado orogen is intruded by around 1,400 million year old mesoproterozoic anorthosites and syenites of the Laramie anorthosite complex, as well as granites (such as the ilmenite- bearing Sherman granite) . These intrusions not only penetrate the Cheyenne Belt, but also encroach on crystalline rocks in the Wyoming province. Overall, however, they are only the northern section of a very broad intrusive belt that runs through the entire Colorado orogen.

Laramian orogeny and Neogene fracture tectonics

The Wyoming Craton owes its spectacular mountain ranges to a compressive deformation phase that took place during the Laramian orogeny around 60 million years ago. The basement blocks consisting of Precambrian rocks were raised to high crustal levels during this deformation, the vertical offset of the basement can be up to 9250 meters! Subsequent erosion then created the current topography. The Sevier orogeny , which occurred a little earlier in western Wyoming, was of a tangential nature and had only produced relatively thin thrust faults - uninterrupted magnetic anomalies indicate that the basement in this region was little deformed and only slightly raised.

In the period Pliocene - Pleistocene the last major fractional tectonic phase occurred. a. the Teton Range tipped out on steep faults. The vertical offset on the east side of this chain is 7800 meters.

See also

Individual evidence

  1. ↑ David A. Foster, Paul A. Mueller, David W. Mogk, Joseph L. Wooden, James J. Vogi: Proterozoic Evolution of the Western Margin of the Wyoming Craton: Implications for the Tectonic and Magmatic Evolution of the Northern Rocky Mountains . In: Can. J. Earth Science . 43, 2006, pp. 1601-1619. doi : 10.1139 / E06-052 .
  2. ^ A b Early Archean to Mesoproterozoic evolution of the Wyoming Province: Archean origins to modern lithospheric architecture. Retrieved December 13, 2018 .
  3. ↑ a b c P.K. Sims, CA Finn, VL Rystrom: Preliminary Precambrian Basement Map Showing Geologic-Geophysical Domains, Wyoming . In: United States Geological Survey: USGS Open-File Report 01-199 (Ed.): Open-File Report . 2001.
  4. ^ PA Mueller, CD Frost: The Wyoming Province: a distinctive Archean craton in Laurentian North America . In: Canadian Journal of Earth Sciences . 43, No. 10, 2006, pp. 1391-1397. doi : 10.1139 / E06-075 .
  5. ↑ New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laure. In: pubs.geoscienceworld.org. September 1, 1999, accessed December 13, 2018 .