The lithosphere of our planet is mobile, subject to constant changes on the scale of geological time and has a complex structure. One of the tectonic structures of global importance are folded (geosynclinal) belts. More about this in this article.
The concept of a folded belt
Geosynclinal (folded or mobile) belt is a geotectonic unit characterized by magmatic, seismic and volcanic activity. As well as large-scale metamorphic processes and a certain set of folded structures with relatively high mobility. Geosynclinal belts are distinguished by the complex of their constituent formations, that is, the aggregates of rocks that arose in similar geodynamic settings.
The length of the belts reaches tens of thousands of kilometers in length. The width is in the order of hundreds or thousands of kilometers.
In the modern sense, folded belts are associated with activecontinental margins and collision zones of continental plates. Belts arise at the boundaries of lithospheric plates moving towards each other (such boundaries are called convergent).
Structure of moving belts
Belts are composed of folded (geosynclinal) areas - large formations that differ from adjacent areas in age and features of their evolution. The regions, in turn, are formed from similar in structure or origin fold systems of close age, such as the Baikalides, Caledonides, Hercynides, and others. So, the Ural Mountains are an example of the Hercynian fold system, the Himalayas are an example of the Alpine system.
Geosynclinal regions and systems within the belt are separated by many different tectonic structures. These are deep faults, microcontinents, fragments of continental and oceanic crust, igneous intrusions, island arcs or their remnants. Microcontinents are fragments of ancient Proterozoic continents and can be of considerable length - up to hundreds of kilometers.
The following zones are distinguished by the nature of mountain building processes in fold belts:
- forward (marginal) trough - the junction area of the platform and the folded area;
- outer zone of the peripheral geosynclinal system, formed through the processes of growth and accretion of various structural elements (for example, island arcs);
- inner zone of orogen, which is characterized by manifestations of metamorphism and intense transverse compressiondue to collision (collision) of continental blocks.
Earth's main mobile belts
Currently, there are five largest fold belts on the planet, differing in their development and age:
- The Pacific belt, bordering the Pacific Ocean along the edges of all continents in contact with this ocean. Sometimes, due to its gigantic length, it is divided into the West Pacific and East Pacific (Cordillera) belts. Despite this division, which reflects some structural differences, the Pacific geosynclinal belt is characterized by the common nature of the tectonic processes occurring in it.
- Alpine-Himalayan (Mediterranean) belt. It stretches from the Atlantic to Indonesia, where it contacts the western part of the Pacific belt. In the Tien Shan region, it practically merges with the Ural-Mongolian. The Alpine-Himalayan geosynclinal belt contains relics of the Tethys Ocean (Mediterranean, Black, Caspian Seas) and a number of microcontinents, such as Adria in Southern Europe or the Indosinian microcontinent in Southeast Asia.
- The Ural-Mongolian (Ural-Okhotsk) belt extends from Novaya Zemlya through the Ural fold system to the south and further east to Primorye, where it articulates with the Pacific belt. Its northern part in the area of the Barents Sea is in contact with the North Atlantic belt.
- The North Atlantic Fold Belt runs along the eastern edge of North America and further northwest and north Europe.
- Arcticthe belt covers the mainland along the Arctic Ocean from the Canadian Arctic Archipelago through Greenland to Taimyr.
Types of geosynclinal belts
Depending on the laying conditions, there are two main types of folded belts:
- Subduction (marginal continental). The formation of the belt is associated with the process of subsidence of plates bearing oceanic crust under the edges of plates, including island arcs or active continental margins. Now there is one fold belt of this type - the Pacific. In the eastern part of the belt, the subduction process proceeds with the subsidence of oceanic plates under the continental margin. At the same time, powerful folded systems (Cordillera, Andes) form along the edge of the mainland, and there are no volcanic arcs and marginal seas in the subduction zone. The Western Pacific part of the belt is characterized by other types of subduction due to the peculiarities of the structure of lithospheric plates.
- Collision (intercontinental). They are formed at the convergent boundaries of lithospheric plates as a result of the convergence and connection of the continental masses that make up these plates. The remaining four of the existing geosynclinal belts belong to this type. The bark during the collision process is intensively crushed with the formation of mountain ranges with a complex internal structure.
Evolution of fold belts
Let's consider the development of folded structures in the subduction zone. In gener althe processes of subsidence of one plate under another lead to the growth of the continental crust on the hanging (upper) edge of the subduction zone as a result of accretion due to the peeling and crushing of the sedimentary cover from the subducting plate. Subduction zones are characterized by powerful volcanic activity. Active volcanism manifests itself throughout the Pacific belt, forming the so-called Pacific Ring of Fire, and, along with accretion and other processes, takes part in mountain building.
The build-up of continental crust and the thrust of continental plates lead to a reduction in the ocean. In the geological past, there were oceans that "closed" due to convergent (counter) movement of plates. These are the famous Tethys, Iapetus, Paleoasian, Boreal oceans.
If both interacting plates contain continental blocks, when they collide, the fold belt enters a new stage of development, characterized by a complex of extremely complex processes involving various tectonic structures.
Collision leads to plate coalescence, as the continental plate cannot sink into the mantle due to the low density of most of its constituent rocks. At the same time, active tectonic processes in geosynclinal belts gradually fade, and plates can begin a new stage of their evolution (for example, rifting), often in another region.
History and present of mobile belts of the earth's crust
The formation of most of the existing fold belts is associated with the "closure" of the ancient oceans and the collision of the continents. Yes, UralThe Mongolian belt arose as a result of the disappearance of various parts of the Precambrian Paleoasian Ocean, such as the Ural, Turkestan, Mongolian-Okhotsk oceans. The North Atlantic belt was formed on the site of the Iapetus Ocean. During the collision of the ancient continents into the supercontinent Laurussia. The disappearance of the Boreal Ocean led to the emergence of the Arctic belt. In subsequent epochs, the North Atlantic and Arctic belts were dissected by the young Atlantic Ocean.
Pacific and Alpine-Himalayan are active modern geosynclinal belts. Both manifest themselves in Eurasia. Kamchatka, the Kuriles, Sakhalin, and the Japanese Islands are regions of the West Pacific mobile belt. As for the Alpine-Himalayan belt, almost all of it, with the exception of Northwest Africa (Maghrib) and part of the Caribbean region, is located on the territory of the Eurasian supercontinent.
The formation of the Alpine-Himalayan fold belt covers a long period. The laying of some of its sections began in the Late Proterozoic. But basically the belt is composed of areas of Mesozoic and Alpine folding. Seismic activity and the growth of mountain structures are manifested in all parts of the belt. In addition, in the Mediterranean, where there is still a remnant of the Tethys Ocean and subduction processes are underway, volcanic activity is observed. Thus, the formation of the belt is in full swing and far from complete.
