Wide folded belts began to form about 10 billion years ago in the late period of the Proterozoic era. They frame and share the main ancient platforms that have a Precambrian foundation. This structure covers a large width and length - more than thousands of kilometers.
Scientific definition
Folded (mobile) belts are tectonic structures of the lithosphere that separate ancient platforms from each other. Mobile zones are characterized by high tectonic activity, the formation of sedimentary and magmatic clusters. Their other name is geosynclinal belts.
The main mobile belts of the planet
There are five global folding belts:
- Pacific or pacific. It frames the hollow of the Pacific Ocean, uniting the plates of Australia, both Americas, Asia, Antarctica. Relatively youngest belt, characterized by increased seismic and volcanic activity.
- Ural – Mongolian folded belt. It stretches from the Urals to the Pacific Ocean through Central Asia. It occupies a position within the continent. It is also called the Ural – Okhotsk.
- North Atlantic belt. Separates the platforms North American and Eastern European. It is divided by the Atlantic Ocean and occupies the eastern part of North America and the North-Western part of Europe.
- Arctic fold belt.
- The Mediterranean is one of the main mobile zones. Starting in the Caribbean, like the North Atlantic, it is divided by the Atlantic and continues to 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 folded 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 thoroughly defined.
Formation history
The common thing in the history of the emergence of these areas is that they were formed in places where ancient ocean basins were previously located. Confirmation of this is the repeated emergence on the surface of relics of the oceanic lithosphere, or ophiolites. The laying and development of mobile zones is a long and difficult period. Since the late Proterozoic period, ocean basins have arisen, volcanic and non-volcanic arcs of islands arose, and continental plates collided.
The main geological processes of rock formation took place in the Baikal era at the end of the Precambrian period, the Caledonian era at the end of the Silurian, Hercynian in the Paleozoic era, Cimmerian at the end of the Jurassic - early Cretaceous, Alpine era in the Oligocene period. All folded belts in their development have gone through more than one complete cycle from the occurrence of the ocean to its completion.
Developmental stages
The development cycle includes several stages of development: laying, initial stage, maturity, the main stage - the creation of mountain ranges or orogenesis. In the final stage of development, sprawl, cutting of mountain peaks, and a decrease in seismic and volcanic activity occur. High peaks give way to a calmer platform mode.
The most important changes in the main fold zones of the Earth occur along the length of their location.
The history of the development of geosynclinal belts and areas from formation, riftogenesis to the final and relict stage, was systematized and divided into 6 cycles by the geographer scientist 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 folded belt - in Hercynian.
The Pacific and Mediterranean geosynclines are young mobile zones, the development processes in them are still going on. These structures are characterized by the presence of mountains with high and sharp peaks, mountain ranges along the folds of the area, significant fragmentation of the relief, and many seismically active regions.
Types of Moving Belts
The Pacific fold belt is the only one that belongs to the type of marginal – continental structures. Its occurrence is associated with the pushing of lithospheric plates of the oceanic crust beneath the continents. This process is not completed, therefore this belt is also called subduction.
The four remaining geosynclines belong to the intercontinental zones that arose instead of the secondary oceans, which were formed at the site of the destruction of the vast continent of Pangea. When there is a collision (collision) of the continents bounding the mobile zones, and the complete absorption of the oceanic crust, intercontinental structures cease to develop. They are therefore 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 long for many kilometers, consisting of parts of Pangea or from the continental fragments of the ancient Precambrian crust, provides the basis for the identification of individual folded massifs, regions of mountains 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 combining arrays into entire folded areas. Examples of such regions in the Alpine-Himalayan folded belt are the Carpathian-Balkan, in the Ural-Okhotnichy - East Kazakhstan.
Edge deflections
In the process of formation of tectonic folded structures at the border of platforms and mobile areas, advanced or piedmont deflections are formed (Pre-Ural, Pre-Caucasus, Pre-Carpathian marginal depressions). Deflections are not always adjacent to moving belts. It happens that the mobile structure is directly stretched for many kilometers into the depths of the platform, as exemplified by the Northern Apaches. Sometimes the absence of foothill deflection may be due to the fact that the foundation of the adjacent platform has a transverse elevation (Mineralovodskoe in the Caucasus). Depending on the method of connecting the platforms to the moving belts, two types of articulation are distinguished: along the forward deflections and along the seams or panels. Depressions are filled with the thickness of marine, lagoon and continental rocks. Depending on the filling structure, certain minerals are formed in the foothill basins:
- Continental marine terrigenous rocks.
- Carboniferous layers (coal, sandstones, mudstones).
- Halogen formations (salts).
- Barrier reefs (oil, gas, limestone).
Myogeosynclinal zones
They are characterized by the location along the edge of the continental platforms. The platform bark is stepped into the main complex of the outer zone. In their composition and relief, the external zones are uniform. The sedimentary complex of the myogeosynclinal zone acquires a descending scaly structure, with individual thrusts, in places reaching several kilometers. In addition to the main ones, there are separate overthrusts of the oncoming direction in the form of triangular folds. At a depth, such folds are revealed by cut overthrusts. The complex of external zones is usually torn off the base and moved up to a dozen kilometers in the direction of the main platform. The structure of the myogeosynclinal zone is sandy-clay, clay-carbonate or marine deposits that form in the early stages of rock formations.
Evgeosynclinal zones
These are the inner zones of the mountain-folded structures, which, in contrast to 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 foundation in case of tectonic plate pushing. In addition to opheolites, the inner zones are fragments of pre-arc, back-arc, and inter-arc depressions that have experienced metamorphoses under the influence of high temperatures and pressure. Reef structures are not uncommon.
How do mountains arise?
Mountain landscapes are directly connected with folded belts. Mountain systems such as the Pamir, the Himalayas, the Caucasus, which are part of the Mediterranean mobile belt, continue their formation at the present time. Complex tectonic processes are accompanied in these areas by a number of seismic phenomena. The formation of mountains begins with a collision of platforms, as a result of which deflections of the earth's crust are formed. The magma emerging through tectonic faults forms volcanoes and lava outcrops 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 begin to rise upward under the action of the buoyancy force, forming mountain ranges and depressions. The processes of deflection and growth are very slow and take millions of years.
Young, relatively recently formed mountains are also called folded. They are composed of crumpled rocks. Modern folded mountains are all the highest peaks of the planet. Arrays that have reached the stage of destruction and smoothing of the peaks have gentle slopes and are folded – block.
