Wide fold belts began their formation about 10 billion years ago in the late period of the Proterozoic era. They frame and separate the main ancient platforms that have a Precambrian basement. This structure spans a great width and extent - more than a thousand kilometers.
Scientific definition
Folded (mobile) belts are tectonic structures of the lithosphere that separate ancient platforms from each other. Mobile belts are characterized by high tectonic activity, the formation of sedimentary and magmatic accumulations. Their other name is geosynclinal belts.
Main mobile belts of the planet
There are five global fold belts:
- Pacific or Circum-Pacific. Frames the depression of the Pacific Ocean, uniting the plates of Australia, both Americas, Asia, Antarctica. Relatively the youngest belt, characterized by increased seismic and volcanic activity.
- Ural-Mongolian fold belt. Stretches from the Urals to the Pacific Ocean throughCentral Asia. Occupies a position within the continent. It is also called the Ural-Okhotsk.
- North Atlantic belt. It separates the North American and East European platforms. Divided by the Atlantic Ocean and occupies the eastern part of North America and the northwestern part of Europe.
- Arctic fold belt.
- Mediterranean - one of the main mobile belts. Starting in the Caribbean, like the North Atlantic, it is divided by the Atlantic and continues its advance through the southern and Mediterranean countries of Europe, Northwest Africa, Asia Minor and the Caucasus. By the name of the mountain systems included in it, it is known as the Alpine-Himalayan fold belt.
In addition to global geosynclines, there are two small mobile belts that completed their formation in the Baikal Proterozoic era. One of them captures Arabia and East Africa, the other - the west of Africa and the east of South America. Their contours are blurred and not well defined.
Formation history
The common thing in the history of these areas is that they were formed in places where ancient ocean basins used to be. This is confirmed by repeated exposures to the surface of relics of the oceanic lithosphere, or ophiolites. The inception and development of mobile belts is a long and difficult period. Since the late Proterozoic period, ocean basins were born, volcanic and non-volcanic arcs of islands arose, and continental plates collided with each other.
Main geologic althe processes of formation of rocks took place in the Baikal era of the end of the Precambrian period, the Caledonian era at the end of the Silurian, the Hercynian in the Paleozoic era, the Cimmerian at the end of the Jurassic period - the beginning of the Cretaceous, the Alpine era in the Oligocene period. All fold belts have experienced more than one complete cycle in their development from the origin of the ocean to completion.
Development stages
The development cycle includes several stages of development: inception, initial stage, maturity, the main stage - the creation of mountain ranges or orogeny. In the final stage of development, sprawling occurs, cutting off mountain peaks, and a decrease in seismic and volcanic activity. High peaks give way to a more relaxed platform mode.
The most important changes in the main fold belts of the Earth occur along the length of their location.
The history of the development of geosynclinal belts and areas from formation, rifting to the final and relict stage, was systematized and divided into 6 cycles by geographer Wilson. The scheme, which includes six main stages, is named after him - the "Wilson cycle".
Young and ancient fold belts
For the Arctic belt, development and transformation ended with the Cimmerian era. The North Atlantic completed its development in the Caledonian era, most of the Ural-Mongolian fold belt - in the Hercynian.
The Pacific and Mediterranean geosynclines are young mobile belts;present time. These structures are characterized by the presence of mountains with high and sharp peaks, mountain ranges along the folds of the terrain, significant fragmentation of the relief, and many seismically active areas.
Types of moving belts
The Pacific fold belt is the only one of all that belongs to the type of continental marginal structures. Its origin is associated with the subduction of the lithospheric plates of the oceanic crust under the continents. This process is not completed, therefore this belt is also called subduction belt.
The four other geosynclines belong to intercontinental belts that arose instead of secondary oceans, which were formed at the site of the destruction of the huge continent of Pangea. When there is a collision (collision) of the continents limiting the mobile belts, and the complete absorption of the oceanic crust, intercontinental structures stop their development. That is why they are called collisional.
Internal structure
Folded belts in their internal composition are a mosaic of fragments of a wide variety of rocks, continents and the seabed. The presence on the scale of this structure of blocks with a length of many kilometers, consisting of parts of Pangea or continental fragments of the ancient Precambrian crust, gives grounds for distinguishing individual folded massifs, mountain regions or entire continents. Such folded massifs, for example, are the mountain systems of the Urals, Tien Shan, and the Greater Caucasus. Sometimes a historical or relief feature serves as the basis for associationarrays into entire folded regions. Examples of such areas in the Alpine-Himalayan folded belt are the Carpatho-Balkan, in the Ural-Hunting - East Kazakhstan.
Boundary deflections
In the process of formation of tectonic folded structures on the border of platforms and mobile areas, advanced or foothill foredeeps (Predural, Ciscaucasian, Ciscarpathian marginal troughs) are formed. Deflections do not always coexist with movable belts. It happens that the mobile structure is directly stretched for many kilometers deep into the platform, an example of this is the Northern Apaches. Sometimes the absence of a foothill trough may be due to the fact that the foundation of the adjacent platform has a transverse uplift (Mineralovodskoe in the Caucasus). Depending on the method of connecting platforms with movable belts, two types of articulation are distinguished: along the forward deflections and along the seams or shields. The depressions are filled with a thickness of marine, lagoonal and continental rocks. Depending on the filling structure, certain minerals are formed in the foothill depressions:
- Marine continental terrigenous rocks.
- Coal-bearing layers (coal, sandstone, mudstone).
- Halogen formations (s alts).
- Barrier reefs (oil, gas, limestone).
Miogeosynclinal zones
Characterized by location along the edge of continental platforms. The crust of the platforms plunges in steps under the main complex of the outer zone. In terms of composition and topography, the outer zones are uniform. The sedimentary complex of the miogeosynclinal zone acquires a descending scaly structure, with separate overthrusts, in places reaching several kilometers. In addition to the main ones, there are separate thrusts of the opposite direction in the form of triangular folds. At depth, such folds are revealed by cut overthrusts. The complex of outer zones is usually torn from the base and moved up to tens of kilometers in the direction of the main platform. The structure of the miogeosynclinal zone is sandy-clayey, clayey-carbonate or marine rock deposits that form in the early stages of rock formations.
Eugeosynclinal zones
These are the inner zones of mountain-fold structures, which, unlike the outer zones, are characterized by sharp drops with maximum marks. The specificity of these zones is tectonic ophiolite covers, which can be located on the sedimentary rocks of the outer zones or directly on their basement in case of thrusting of tectonic plates. In addition to opheoliths, the inner zones are fragments of fore-arc, back-arc, and inter-arc depressions that have undergone metamorphosis under the influence of high temperatures and pressure. Elements of reef structures are not uncommon.
How do mountains form?
Mountain landscapes are directly related to folded belts. Mountain systems such as the Pamirs, the Himalayas, the Caucasus, which are part of the Mediterranean mobile belt, continue to form at the present time. Complex tectonic processes are accompanied in these areas by a number of seismic phenomena. The formation of mountains begins with the collision of platforms, as a result of which deflections of the earth's crust are formed. Magma emerging through tectonic faults forms volcanoes and lava outlets to the surface. Gradually, the troughs are filled with sea water, in which various organisms live and die, settling to the bottom and forming sedimentary rocks. The second stage begins when the rocks submerged by the deflection under the action of the buoyancy force begin to rise upward, forming mountain ranges and depressions. The processes of deflection and increase are very slow and take millions of years.
Young, relatively recently formed mountains are also called folded. They are composed of rocks crumpled into folds. Modern folded mountains are all the highest peaks of the planet. Massifs that have approached the stage of destruction, smoothing of peaks, have gentle slopes, are folded-blocky.
Mineral resources
Mobile structures are the main deposits of minerals. High seismic activity, magma ejections, high temperatures and pressure drops lead to the formation of rocks of igneous or metamorphic origin: iron, aluminum, copper, manganese ores. In geosynclines there are deposits of precious metals, combustible substances.
