Great Falls Tectonic Zone
Encyclopedia
The Great Falls Tectonic Zone is a major intracontinental shear zone between the Hearne craton and Wyoming craton
, basement rock
of the Archean
Eon which form part of the North American continent
. The zone is an area about 100 miles (150 km) wide extending from the southwestern Idaho
-Montana
border across Montana to the northwestern Montana-Saskatchewan
-North Dakota
border. It is named for the Great Falls of the Missouri River
, a major geologic feature of the area. The central and western portions of the zone are believed to be about 1.1 to 3.3 billion years old. The central part of the zone lacks Archean rock, however, leading at least one group of scientists to speculate that it was formed very late in the Paleoproterozoic
Era.
The Great Falls tectonic zone has been periodically active since the Proterozoic, and possibly as late as the Holocene
. Little of the zone is visible due to Phanerozoic
cover, the exception being the Little Belt Mountains
. But it is believed that the tectonic zone controlled the geologic development of nearby basins and subbasins
.
The Great Falls tectonic zone was first identified in 1985. Geologists originally believed the zone was part of the Wyoming craton, but now conclude that it is distinct from it. There is continuing controversy over whether the region is a shear zone or suture
, and the role the zone played in the formation of the North American continent. At one time, both the Great Falls Tectonic Zone and the Vulcan structure
were both considered sutures, but debate remains open on the point. At least one group of geologists has concluded the zone represents the closure of an ocean basin. The zone lacks gravity anomalies
or electromagnetic signatures which would allow scientists to conclude that it was generated by subduction
. There is significant evidence that the zone has been periodically remineralized
since Precambrian
times. Square Butte, Shaw Butte, Crown Butte and the other structures of the Adel Mountains Volcanic Field
lie astride the tectonic zone near the city of Great Falls
.
Wyoming craton
The Wyoming craton is a craton located in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah...
, basement rock
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
of the Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
Eon which form part of the North American continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
. The zone is an area about 100 miles (150 km) wide extending from the southwestern Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
-Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
border across Montana to the northwestern Montana-Saskatchewan
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....
-North Dakota
North Dakota
North Dakota is a state located in the Midwestern region of the United States of America, along the Canadian border. The state is bordered by Canada to the north, Minnesota to the east, South Dakota to the south and Montana to the west. North Dakota is the 19th-largest state by area in the U.S....
border. It is named for the Great Falls of the Missouri River
Great Falls of the Missouri River
The Great Falls of the Missouri River are a series of waterfalls on the Missouri River in north-central Montana in the United States. The five falls, which are located in a area of the river, are:*Black Eagle Falls ;*Colter Falls ;*Rainbow Falls ;...
, a major geologic feature of the area. The central and western portions of the zone are believed to be about 1.1 to 3.3 billion years old. The central part of the zone lacks Archean rock, however, leading at least one group of scientists to speculate that it was formed very late in the Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
Era.
The Great Falls tectonic zone has been periodically active since the Proterozoic, and possibly as late as the Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
. Little of the zone is visible due to Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
cover, the exception being the Little Belt Mountains
Little Belt Mountains
The Little Belt Mountains are a section of the Rocky Mountains in the U.S. state of Montana. Situated mainly in the Lewis and Clark National Forest, the mountains are used for logging and recreation for the residents of Great Falls, Montana...
. But it is believed that the tectonic zone controlled the geologic development of nearby basins and subbasins
Depression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...
.
The Great Falls tectonic zone was first identified in 1985. Geologists originally believed the zone was part of the Wyoming craton, but now conclude that it is distinct from it. There is continuing controversy over whether the region is a shear zone or suture
Suture (geology)
In structural geology, a suture is a major fault zone through an orogen or mountain range. Sutures separate terranes, tectonic units that have different plate tectonic, metamorphic and paleogeographic histories...
, and the role the zone played in the formation of the North American continent. At one time, both the Great Falls Tectonic Zone and the Vulcan structure
Vulcan structure
The Vulcan structure is a long major convergent boundary between the Medicine Hat Block and the Loverna Block, Archean basement rock structures which lie between the Wyoming craton and Hearne craton on the western edge of the North American craton...
were both considered sutures, but debate remains open on the point. At least one group of geologists has concluded the zone represents the closure of an ocean basin. The zone lacks gravity anomalies
Gravity anomaly
A gravity anomaly is the difference between the observed acceleration of Earth's gravity and a value predicted from a model.-Geodesy and geophysics:...
or electromagnetic signatures which would allow scientists to conclude that it was generated by subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
. There is significant evidence that the zone has been periodically remineralized
Mineralization (geology)
In geology, mineralization is the hydrothermal deposition of economically important metals in the formation of ore bodies or "lodes".The first scientific studies of this process took place in Cornwall, United Kingdom by J.W.Henwood FRS and later by R.W...
since Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
times. Square Butte, Shaw Butte, Crown Butte and the other structures of the Adel Mountains Volcanic Field
Adel Mountains Volcanic Field
The Adel Mountains Volcanic Field is an ancient volcanic field of heavily eroded 75 million year old igneous rocks about 40 miles long and 20 miles wide in west-central Montana about 30 miles southwest of the city of Great Falls...
lie astride the tectonic zone near the city of Great Falls
Great Falls, Montana
Great Falls is a city in and the county seat of Cascade County, Montana, United States. The population was 58,505 at the 2010 census. It is the principal city of the Great Falls, Montana Metropolitan Statistical Area, which encompasses all of Cascade County...
.