Adjacent Mobile Belts Research Articles

THE DISTINQUISHING FEATURES OF THE FAULTS IN THE PLATFORM COVER: RESULTS OF THE APPLICATION OF TECTONOPHYSICAL APPROACH TO THE STUDY OF THE TAMBEY HYDROCARBON DEPOSIT (YAMAL PENINSULAR)

The study was aimed to identify the features of the formation and regularities of manifestation of faults in the platform environment applying the tectonophysical approach to the study of the structure of the Tambey hydrocarbon deposit (northern Yamal), largest in the West Siberia. Such research is important in the oil and gas industry at the present stage of transition from the exploitation of declining unique and large deposits to exploration and exploitation of deposits of complex structure. The tectonophysical approach was applied consistently in three levels of research. Initial consideration was given to regular trends in the structure of the platform cover in the context of general tectonophysical ideas of disjunctive faults, their inner structure and formation features. Then, the identification of a network of large fault zones has been done at the regional level for the northern Yamal on the basis of the lineament analysis of the relief and optical modeling, three main stages of its formation have been reconstructed, and there have been identified the features of the state of stress, among other factors determining the Tambey deposit contours in three areas – western and northern Tambey and Tassyi. Finally, based on tectonophysical interpretation of 3D seismic attribute analysis data and elastoplastic modeling experiment results, for the northern Tambey area at the local level there were identified the faults zones, the features of their structures in rheologically stratified unit, and the paragenetic relationship with the regional-level structures. The study has shown that the structure of the sedimentary cover, whose formation is tectonically influenced by the adjacent mobile belts, is zone-block. It reflects the zone-block structure of the basement, though, in contrast, is not represented by narrow main-fault planes (1 st -order faults). The blocks in the cover contact along rather wide zones, the inner structure of which corresponds to the early stages of faulting and is represented by a dense network of the 2 nd -order fractures and faults. The fault zones are characterized by an inhomogeneous – segment – structure which is determined by an initially irregular development of deformations and complicated by rheological stratification of the sedimentary cover. Fault segments in relatively brittle rocks (sandstones) are composed of long faults whereas in more ductile (clayey) varieties these are wide parts of concentration of small faults and fractures. A style of the zone-block structure and the types of dynamic environments of its formation might be specific in different regions. The application of tectonophysical approach to the analysis of the geological-geophysical information, obtained for certain deposits, will make it possible to identify the structural conditions for hydrocarbon accumulation and migration in the sedimentary cover which is essential to choose an effective method of deposit exploitation.

The Bastar Craton of Central India: Tectonostratigraphic evolution and implications in global correlations

The Bastar Craton occupies a key position in the Peninsular India, being located between three cratonic nuclei of India, the Dharwar Craton in the southwest, the Singhbhum Craton in the northeast, and the Bundelkhand Craton in the northwest. The southeastern and the northern margins of the craton are bordered by the Proterozoic orogenic belts constituting the Eastern Ghats Mobile Belt and the Central Indian Tectonic Zone (the Satpura Mobile Belt), respectively. Analysis of the tectonostratigraphic evolution of the craton indicates the formation of the craton during Paleoarchean Era (~3582 Ma). The Archean evolution of the craton is marked by the development of granite-greenstone belts containing chemogenic sediments (Banded Iron Formations) and gold mineralisation . High-grade gneisses and granulites were developed during three orogenic episodes in the terrane (~2700 Ma Bailadila orogeny , ~2900 Ma Bengpal orogeny and ~3400 Ma Sukma orogeny). The Paleoproterozoic evolution of the craton is gleaned from the record of siliciclastic, volcaniclastics and chemogenic precipitates (carbonates, Banded Manganese Formation, and Banded Iron Formation) on the northern margin of the craton, which were deposited during the Great Oxidation Event (GOE; 2500–2300 Ma) and Paleoproterozoic glaciation (2400–2300 Ma) as well as immediately after the GOE (2050–1850 Ma). Effects of orogenic activities in the adjacent mobile belts are marked by the metamorphic and magmatic activities affecting the rocks at the margin of the craton. The Satpura orogenic event on the northern margin of the craton led to the amalgamation of the Bastar Craton with the Bundelkhand Craton at ~2250 and ~2100 Ma, forming the major part of the Peninsular India. Orogenic collapse (extension) developed basins transverse to the orogenic trend, where clastic, volcaniclastic and chemogenic sediments (Khairagarh and Chilpi Groups) were deposited. The southeastern margin of the craton has records of migmatisation and granite emplacement during the Eastern Ghats orogeny at ~1700 Ma. A number of sedimentary basins , aligned along a line parallel to the Eastern Ghats Mobile Belt, were developed during the orogenic collapse of the Eastern Ghats. Later orogenic activities along the Eastern Ghats were responsible for the development of depositional hiatus/unconformity within these sedimentary basins of the craton. Mafic dykes of different generations (2400 Ma, 2100 Ma and 1890 Ma) cutting across the Archean basement complex provide suitable markers for paleomagnetic reconstructions of the craton. The Archean and early Paleoproterozoic rock assemblages and mineralisation patterns in the craton show a broad similarity with the Western Australian Craton, indicating near-neighbourhood positions of the two ancient cratonic blocks. Paleomagnetic studies have shown the separation of these two blocks at ~2000 Ma, with the development of a paleo-ocean which closed subsequently during the amalgamation of the Bastar Craton with the East Antarctic Shield to form the Eastern Ghats Mobile Belt in three phases at ~1700–1600 Ma, ~1050–950 Ma and ~500 Ma, with extensional phase between 1450 and 1100 Ma. The Bastar Craton preserves important clues regarding the evolution of the ocean and atmospheric conditions in the early Earth. • The Bastar and Yilgarn Cratons were juxtaposed at ~2400 Ma. • Bundelkhand-Pilbara Cratons collided with Bastar-Yilgarn Cratons at ~2000 Ma. • Satpura-Singhbhum-Capricorn Orogen developed at ~2000 Ma. • No collision/accretion took place along the CITZ after ~1800 Ma. • New tectonic model for 1950–1000 Ma is proposed for Bastar-Yilgarn Cratons.

Edge-driven convection and thinning of craton lithosphere: Two-dimensional thermal-mechanical modeling

There are usually abrupt changes in lithospheric thickness at the boundaries between ancient cratons and adjacent young mobile belts. Lateral variations in temperature and density between the two can trigger small-scale mantle convection (edge-driven convection, EDC). Here, we use two-dimensional thermal-mechanical simulations to explore the EDC caused by the lithospheric step between a craton and a mobile belt, and its role in the thinning of the craton lithosphere. The results show that the impact of EDC on a craton depends on the properties of the craton lithosphere and on their contrasts with the adjacent mobile belt, given the same initial condition. When the craton lithospheric density is relatively large, a high-strength craton has strong ability to resist EDC, and craton lithospheric thinning is limited to the edge. In contrast, the ability of a low-strength craton to resist EDC is weak, and the craton lithosphere is gradually eroded by the downward flow, eventually leading to large-scale thinning of the cratonic lithosphere. When the craton lithospheric density is relatively small, regardless of the strength of the cratonic lithosphere, the craton can well withstand the impacts of EDC. In this case, upwelling flow drives cratonic lithosphere materials to the base of the mobile belt, and lithospheric thinning only occurs at the edge of the craton lithosphere. The Archean North China Craton (NCC) was surrounded by Phanerozoic mobile belts, and its lithospheric thinning first occurred on the northern and eastern margins, a process that lasted for a long time. We suggest that EDC has played an important role in lithospheric thinning of the NCC, in particular the initiation of lithospheric thinning, but we cannot rule out the additional contributions from other mechanisms.

Tectonophysical approach to the analysis of geological and geophysical data on gas-condensate deposits with the complex platform cover

The article presents the results of the tectonophysical approach to the analysis of stress fields and the structure of gas–condensate deposits with the complex platform cover. The discussed case is the Kovykta license area (LA) in Eastern Siberia, Russia. In the upper part of the cross section, the network of fault zones was identified from the relief lineaments and structural data. The dynamic conditions for faulting (compression, extension, and strike-slip) were reconstructed by the paragenetic analysis. The state of crustal stresses in the study area was studied by tectonophysical modeling using gelatin as an optically active material. The applied method was successful in distinguishing between the zones of faults in the platform cover, which differ in the degree of their activity in the specified stress fields. The lower part of the cross section in the NE segment of the Kovykta LA is considered as an example of the tectonophysical interpretation of the electrical and seismic survey data in order to identify the fault zones and reconstruct the corresponding stress fields. Based on the synthesis of the analyzed data, it is revealed that the deposits like the Kovykta gas condensate field (GCF) show the zone-block structure of the platform cover formed under the influence of several stress fields closely associated with the stages of tectogenesis in the adjacent mobile belts. The next objective is to enhance the tectonophysical approach in order to develop a hierarchical model of the GCF zone-block structure, which details need to be known for improving the prediction of sites with the complicated stress-strain state of rocks and mitigating the risks associated with drilling exploration and production wells.

Lithospheric structure of an Archean craton and adjacent mobile belt revealed from 2‐D and 3‐D inversion of magnetotelluric data: Example from southern Congo craton in northern Namibia

Archean cratons, and the stitching Proterozoic orogenic belts on their flanks, form an integral part of the Southern Africa tectonic landscape. Of these, virtually nothing is known of the position and thickness of the southern boundary of the composite Congo craton and the Neoproterozoic Pan‐African orogenic belt due to thick sedimentary cover. We present the first lithospheric‐scale geophysical study of that cryptic boundary and define its geometry at depth. Our results are derived from two‐dimensional (2‐D) and three‐dimensional (3‐D) inversion of magnetotelluric data acquired along four semiparallel profiles crossing the Kalahari craton across the Damara‐Ghanzi‐Chobe belts (DGC) and extending into the Congo craton. Two‐dimensional and three‐dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger DGC orogen to the older adjacent cratons. We find Damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent Congo craton. The Congo craton is characterized by very thick (to depths of 250 km) and resistive (i.e., cold) lithosphere. Resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during Pan‐African magmatism. Graphite‐bearing calcite marbles and sulfides are widespread in the Damara belt and account for the high crustal conductivity in the Central Zone. The resistivity models provide new constraints on the southern extent of the greater Congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south.

Spatiotemporal relationships of dike magmatism in the Kola region, the Fennoscandian Shield

A brief geological and petrographic characterization of the Early Precambrian dike complexes of the Kola region is given along with data on new estimates of dike age and analysis of their distribution over the entire Fennoscandian Shield. The emplacement of dikes in the Archean core of the shield continued after consolidation of the sialic crust 2.74–1.76 Ga ago. After the Svecofennian Orogeny, dikes continued to form in the west in the area of newly formed crust, while the amagmatic period began in the Archean domain. The intense formation of dikes in the Svecofennian domain lasted approximately for 1 Ga (1.8–0.84 Ga). The younger igneous rocks in the crustal domains of different age are less abundant and localized at their margins. A similar distribution of dikes is characteristic of other shields in different continents. This implies that the formation of the sialic crust in the shields is not completed by its consolidation and formation of the craton. For 1 Ga after completion of this process, the crust is underplated by mantle-derived magmas. This process is reflected at the Earth’s surface in the development of mantle-derived mafic and anorogenic granitoid magmatism. The process of crust formation is ended as the subcratonic lithosphere cools and the amagmatic period of the craton history is started. Beginning from this moment, the manifestations of cratonic magmatism were related either to the superposed tectonomagmatic reactivation of the cold craton under the effect of crust formation in the adjacent mobile belts or to the ascent of mantle plumes.

Early precambrian A-granitoids in the Aldan Shield and adjacent mobile belts: Sources and geodynamic environments

Systematized geological, geochronologic, geochemical, and Sm-Nd isotopic geochemical data obtained over the past decade on A-granitoids in the Aldan Shield and in adjacent mobile belts surrounding it in the south make it possible to identify the sources from which the rocks were derived and the geodynamic environments in which they were generated. The territory in question provides evidence of five episodes of Early Precambrian within-plate magmatism, including the derivation of A-granites: at 2.62, 2.40–2.52, 2.07, 1.87–1.88, and 1.70–1.74 Ga. Although all of the granitoids were derived within plates, the environments of their derivation were different: (i) postcollisional lithospheric extension at 2.64 and 1.87–1.88 Ga in an anorogenic environment and (ii) in relation to the activity of mantle plumes at 2.40–2.52, 2.07, 1.74–1.70 Ga. The postcollisional magmatism generated only potassic granitoids of the subalkaline type, whereas the anorogenic magmatic rocks comprise both subalkaline granitoids (of K series) and alkaline granites (of Na series), which are intensely fractionated and strongly enriched in incompatible elements. A-granitoids in the Aldan Shield and its surrounding folded structures were derived from mixed mantle-crustal sources. The sources of the subalkaline granitoids were dominated by the material of the continental lower crust, while the alkaline granitoids were derived from mantle sources. Thereby the mantle source material of the anorogenic granitoids consisted of an OIB-type component, and the postcollisional granitoids were derived from MORB and OIB sources.

Proterozoic platform sequences of Peninsular India: Implications towards basin evolution and supercontinent assembly

A group of less disturbed and unmetamorphosed ‘Proterozoic’ platformal sediment package lying over deformed and metamorphosed Archean to Paleoproterozoic basement in several parts of Peninsular India is informally termed as ‘Purana’ basins in Indian stratigraphy. The trend shift in the understanding of these basin fills in last two decades, often supported by international collaborations, has made significant advancements in respect to unconformity and ‘Sequence’ development, relationship with accretion and break-up of the supercontinents, initiation and evolution of the basins in different tectonic set ups, nature of sediment provenance, paleoclimate and paleo-weathering patterns and basin water chemistry. Geophysical backups encompassing the basins, occasionally extending up to the craton and adjacent mobile belts helped in refining the understanding on the basement structure and tectonic forcings. The present paper is an attempt towards the collation of the database generated in last two decades in order to present the status of current understanding on different ‘Purana’ basins with regard to interactions of physical, chemical and biological parameters of the Proterozoic Earth.

Provenance composition and features of geological evolution of the Late Vendian foreland basin of the Timan orogen

The provenance composition and geological evolution of different segments in the distal zone of the Late Vendian foreland basin of the Timan orogen were deciphered on the basis of sequence stratigraphic reconstructions and precision geochemical data on the Upper Vendian fine-grained terrigenous rocks from the Southeast White Sea region, Vychegda, Verkhnekama, and Shkapovo-Shikhan basins, and the Kvarkush-Kamennogorsk and Bashkirian meganticlinoriums. The Upper Vendian of the Southeast White Sea region is subdivided into four sequences: Agma, Solza, Zimnie Gory, and Erga. The tracing of sequence boundaries and lateral facies associations from the Southeast White Sea Region to the South Urals made it possible to identify the main stages of the evolution of sedimentary filling of the foreland basin: (I) shallow water epiplatformal basin, (II) isolated basin, (III) lowstand system tract, and (IV) progradation of delta platforms. Position of data points of the Upper Vendian shales, silty mudstones, and mudstones plotted in the diagrams Th-La, Ni-Cr, GdN/YbN-Eu/Eu*, Sc-Th/Sc, La/Sm-Sc/Th, and Co/Hf-Ce/Cr, together with their REE systematics and data on Nd model ages indicates that the Proterozoic, including Neoproterozoic rocks of the Timan thrust-folded belt served as the main source for fine aluminosiliciclastic material in the distal zones of the Timan foreland basin. Obtained results are consistent with the concept that the Vendian stage of the evolution of the Eastern European platform is related to the intense input of allochthonous material in its peripheral area owing to the activation of orogenic processes in the adjacent mobile belts and allow us to significantly specify the paleogeographical reconstructions.

Late orogenic, large-scale rotations in the Tien Shan and adjacent mobile belts in Kyrgyzstan and Kazakhstan

Most of Kazakhstan belongs to the central part of the Eurasian Paleozoic mobile belts for which previously proposed tectonic scenarios have been rather disparate. Of particular interest is the origin of strongly curved Middle and Late Paleozoic volcanic belts of island-arc and Andean-arc affinities that dominate the structure of Kazakhstan. We undertook a paleomagnetic study of Carboniferous to Upper Permian volcanics and sediments from several localities in the Ili River basin between the Tien Shan and the Junggar–Alatau ranges in southeast Kazakhstan. Our main goal was to investigate the Permian kinematic evolution of these belts, particularly in terms of rotations about vertical axes, in the hope of deciphering the dynamics that played a role during the latest Paleozoic deformation in this area. This deformation, in turn, can then be related to the amalgamation of this area with Baltica, Siberia, and Tarim in the expanding Eurasian supercontinent. Thermal demagnetization revealed that most Permian rocks retained a pretilting and likely primary component, which is of reversed polarity at three localities and normal at the fourth. In contrast, most Carboniferous rocks are dominated by postfolding reversed overprints of probably “mid-Permian” age, whereas presumably primary components are isolated from a few sites at two localities. Mean inclinations of primary components generally agree with coeval reference values extrapolated from Baltica, whereas declinations from primary as well as secondary components are deflected counterclockwise (ccw) by up to ∼ 90°. Such ccw rotated directions have previously also been observed in other Tien Shan sampling areas and in the adjacent Tarim Block to the south. However, two other areas in Kazakhstan show clockwise (cw) rotations of Permian magnetization directions. One area is located in the Kendyktas block about 300 km to the west of the Ili River valley, and the other is found in the Chingiz Range, to the north of Lake Balkhash and about 400 km to the north of the Ili River valley. The timing of the ccw as well as cw rotations is clearly later than the disappearance of any marine basins from northern Tarim, the Tien Shan and eastern Kazakhstan, so that the rotations cannot be attributed to island-arc or Andean-margin plate settings — instead we attribute the rotations to large-scale, east–west (present-day coordinates), sinistral wrenching in an intracontinental setting, related to convergence between Siberia and Baltica, as recently proposed by Natal'in and Şengör [Natal'in, B.A., and Şengör, A.M.C., 2005. Late Palaeozoic to Triassic evolution of the Turan and Scythian platforms: the pre-history of the palaeo-Tethyan closure, Tectonophysics, 404, 175–202.]. Our previous work in the Chingiz and North Tien Shan areas on Ordovician and Silurian rocks suggested relative rotations of ∼ 180°, whereas the Permian declination differences are of the order of 90° between the two areas. Thus, we assume that about 50% of the total post-Ordovician rotations are of pre-Late Permian age, with the other half of Late Permian–earliest Mesozoic age. The pre-Late Permian rotations are likely related to oroclinal bending during plate boundary evolution in a supra-subduction setting, given the calc-alkaline character of nearly all of the pre-Late Permian volcanics in the strongly curved belts.

An infrared investigation of inclusion-bearing diamonds from the Venetia kimberlite, Northern Province, South Africa : implications for diamonds from craton-margin settings

The Venetia kimberlites in the Northern Province of South Africa sampled diamonds from the lithosphere underlying the Central Zone of the Limpopo Belt. Given the general correlation of diamond-bearing kimberlites with old stable cratons, this tectonic setting is somewhat anomalous and, therefore, it is desirable to characterise the diamonds in terms of their infrared characteristics. A suite of diamonds of known paragenesis from the Venetia mine spans a large range of nitrogen concentrations from less than the detection limit to 1,355 ppm. Diamond nitrogen contents are, on average, higher in the eclogitic diamond population relative to the websteritic and peridotitic diamonds. Nitrogen aggregation states are variable, ranging from almost pure type IaA diamond (poorly aggregated nitrogen) to pure type IaB diamond (highly aggregated nitrogen). On a nitrogen aggregation diagram two distinct groups can be identified based on nitrogen content and nitrogen aggregation state. These are a minor population of diamonds with nitrogen contents generally higher than 500 ppm and nitrogen aggregation states of less than 40% IaB, and another, dominant population that is characterised by higher and more variable nitrogen aggregation. The unusually aggregated nature of the majority of the diamonds analysed is unique to Venetia relative to other intrusives on the Kaapvaal–Kalahari craton, but is similar to aggregation states observed for diamonds from other craton margin or adjacent mobile belt settings such as the Argyle lamproite and the George Creek kimberlite. This could be a consequence of diamond mantle residence at mantle temperatures higher than the norm for other kimberlites from the interior of cratons. Deformation of the mantle, associated with dynamic processes such as orogenesis or subduction, might also be responsible for accelerating the rate of nitrogen aggregation in these diamonds. Low numbers of diamonds with degradation of platelets at the Venetia kimberlite, relative to diamonds from the Argyle lamproite, indicate that deformation was at a significantly lower level. The comparatively low value of diamonds from Argyle (at approximately US$8/carat) as opposed to Venetia (US$90/carat) is in large part because of the very high abundance of brown diamonds at Argyle. Therefore, it is apparent that deformational history of the mantle in which the diamonds were resident prior to or during sampling by the host may have an important role to play in the profitability of a primary diamond deposit. The apparently consistent association of diamonds with unusually aggregated nitrogen with kimberlites, or lamproites intruded into craton margin or mobile belt settings suggests that it may be possible to recognise such contributory sources in alluvial diamond deposits, through the study of the infrared characteristics of the diamonds. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00410-002-0385-2

Regional mantle-rooted Discontinuities extending transversely to the margins of cratons and adjacent mobile belts. Metallogenic implications

The areas where marginal parts of cratons and the adjacent mobile belts are intersected by transregional, mantle-rooted discontinuities provide, during orogenic movements, favorable pathways for the ascent of heat, magmas and for concentration of metals. The intersection of the Appalachian foldbelt with a deep-seated transregional discontinuity, occurring between 40° and 42° N latitude at the eastern edge of the North American craton, has been chosen for a more detailed study. Morphologically, this intersection is manifested by a big curvature of the foldbelt, matching with the latitude of the intersecting transregional boundary. The magmatism and metal concentration occurred at different periods of time at different places of this structural intersection, being related to several phases of deformation : In the Precambrian, Paleozoic (especially in Cambrian-Ordovician) and Mesozoic. The orogenic belt of the Ural Mts., used for comparison with the Appalachians, reveals the biggest shortening of its width at the intersection with a major transregional discontinuity. The largest complexes of alkalic intrusives are related to a structural corridor which reflects the deep-seated discontinuity, extending transversely to the orogenic belt.

The difficulties of dating mafic dykes: an Antarctic example

Archaen gneisses of the Vestfold Hills of East Antarctica are transected by several compositionally discrete suites of tholeiitic dykes. A representative of one of those suites, which has been dated in the present study, shows that not only Rb−Sr whole-rock isochrons, but also U−Pb zircon techniques (if not properly applied) can produce erroneous crystallisation ages. Two zircon populations were recovered from the mafic dyke itself, one of which is 2,483±9 Ma in age and clearly of xenocrystic origin. The other yields an age of 1,025±56 Ma, which is not ascribed to the magmatic crystallisation of the dyke, but rather to the time that it underwent metamorphic/metasomatic alteration as a response to high-grade tectonism in the adjacent mobile belt. It is presumed that the zircon in question formed by the breakdown of another mineral or minerals (possibly magmatic baddeleyite), due either to ingress of a siliceous fluid, or more probably by the release of silica from the breakdown of pyroxene to amphibole. A cogenetic 1–2 cm wide felsic vein, of late magmatic/early hydrothermal origin, also contains two zircon populations. Again, most of the grains therein, which are interpreted as of xenocrystic origin, grew at 2,483±9 Ma. However, a few euhedral zircons with very high U and Th contents grew at 1,248±4 Ma, which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke.

Rifting evolution in geological history

Rift and rift-like zones, global linear extensional structures of the crust, have appeared during most periods of the Earth's history. Rifting as a geological process has undergone a complex evolution, with its historical “roots” in the early Precambrian. The development of the oldest linear mobile tectonic zones—Archean greenstone belts and granulite belts—combined the features of both rift and geosynclinal processes in the following epochs. Early Proterozoic mobile zones—protogeosynclinal regions and protorift zones—evolved under similar tectonic regimes of crustal destruction in the early stages and compression in the late stages of their development, but different in size, scale and intensity of the deformations and thermal processes experienced. Typical rift structures have existed since the late Proterozoic. In the late Proterozoic and Paleozoic, continental intracratonic rifting, in the form of aulacogen and aulacogeosynclinal zone development, was a process related and subordinated to a geosynclinal process in the adjacent mobile belts. In the Paleozoic, the first “epiorogenic” (postorogenic) rift zones appeared within the epigeosynclinal folded belts. During the Meso-Cenozoic epoch of the Earth's history rifting increased considerably and became independent of geosynclinal processes. It is believed to be one of the main forms of continental crust destruction and formation of the oceanic basins and probably occurred under conditions of some general expansion of the Earth.