The Role of Deep Fluids in Crustal Subsidence of the Cratonic Interior: The Moscow Sedimentary Basin during the Late Devonian

Doklady Earth Sciences - Slow crustal subsidence nonuniform in time and space occurred in the sedimentary basin of the Moscow Syneclise during 20 Ma in the Late Devonian. On the cool Precambrian...     

The Late Cretaceous-Paleogene active margin of Northeastern Asia: Geodynamic setting of terrigenous sedimentary basins in the Central Koryak terrane

The northeastern segment of the Late Cretaceous suprasubduction Okhotsk-Chukotka volcanic belt is not an analogue of Andean-type continental margin. During its formation, the belt was separated from the Paleopacific by a complexly built assembly that comprised the Central Koryak continental block and the Essoveem volcanic arc at its margin. Various types of independent terrigenous sedimentary basins were formed in the Late Cretaceous and Early Paleogene at the subsided portion of the microcontinent and its slope. The Uchkhichkhil-type basin was characterized by deposition of polymictic clastic sediments produced during erosion of the volcanic arc and pyroclastic material derived from active volcanic centers of this arc that extended along the microcontinent margin that faced the Okhotsk-Chukotka volcanic belt. The deposition of quartz-feldspathic flyschoid sequences as products of scouring of sialic basement of the continental block was inherent to the Ukelayat type of sedimentation. The closure of the minor oceanic basin that separated the Asian margin from microcontinent in the late Campanian resulted in the cessation of subduction-related activity of the Okhotsk-Chukotka volcanic belt and the Essoveem arc and initiated the formation of the Late Cretaceous accretionary margin of Asia. The deep structure of the central Koryak Highland deduced from the results of seismic surveying with the earthquake converted-wave method has corroborated the geotectonic interpretation.     

Different levels of ensimatic island-arc accretion

The possible scenarios of accretion of ancient ensimatic island arcs in the eastern and western frameworks of the Pacific Ocean are discussed. It is concluded that the accretion of ensimatic island arcs can occur at both the lithospheric and crustal (upper crustal) levels. In the case of lithospheric accretion, the subduction zone is jammed and the island-arc edifice is attached to the continent. During crustal-level accretion, the subduction of the lithosphere that underlies the island arc can develop further, thereby leading to the formation of a suprasubduction volcanic-plutonic belt at the continental margin.     

Cenozoic history of the Bering Sea and its northwestern margin

Tectonic reconstructions based on the geodynamic analysis of geologic, paleomagnetic, structural and kinematic data of Cenozoic age from the western Bering Sea region are proposed in the present paper. The most active tectonic and magmatic processes took place in the Komandorsky segment of the Bering Sea, exemplified by the Late Cretaceous–Early Eocene Olutorsky Arc and Eocene–Oligocene Govena–Karaginsky Arc, which was built on the structures of the Olutorsky Arc. A model of the complex collision of these two arcs with the paleocontinental margin, which considers rotations of the geological blocks from the various structural zones of the western margin of the Bering Sea in the horizontal plane (paleomagnetic data), was proposed by the authors. According to this model the collision of the flanks of the Olutorsky and Govena–Karaginsky arcs took place in the Eocene, before the collision of the central parts in the Miocene.     

厦门东海域鱼类食物网研究

本文分析了厦门东海域58种鱼类的营养关系.根据对它们的食性分析,并依其食料生物的生态类群,将厦门东海域的鱼类食性类型分为:浮游生物食性、底栖生物食性、游泳动物食性、浮游生物和底栖生物、底栖生物和游泳动物食性等5种.该海域鱼类的营养级可分为4级:杂食性鱼类、低级肉食性鱼类、中级肉食性鱼类和高级肉食性鱼类,其中低级肉食性鱼类占优势,为63.79%.本文还分析了厦门东海域鱼类的食物网及其能量流动途径,并提出合理利用和保护厦门东海域鱼类资源的建议.     

The Southern Urals. Decoupled evolution of the thrust belt and its foreland: a consequence of metamorphism and lithospheric weakening

An analysis is presented of the mechanisms of tectonic evolution of the southern part of the Urals between 48N and 60N in the Carboniferous–Triassic. A low tectonic activity was typical of the area in the Early Carboniferous — after closure of the Uralian ocean in the Late Devonian. A nappe, ≥10–15 km thick, overrode a shallow-water shelf on the margin of the East European platform in the early Late Carboniferous. It is commonly supposed that strong shortening and thickening of continental crust result in mountain building. However, no high mountains were formed, and the nappe surface reached the altitude of only ≤0.5 km. No high topography was formed after another collisional events at the end of the Late Carboniferous, in the second half of the Early Permian, and at the start of the Middle Triassic. A low magnitude of the crustal uplift in the regions of collision indicates a synchronous density increase from rapid metamorphism in mafic rocks in the lower crust. This required infiltration of volatiles from the asthenosphere as a catalyst. A layer of dense mafic rocks, 20 km thick, still exists at the base of the Uralian crust. It maintains the crust, up to 60 km thick, at a mean altitude 0.5 km. The mountains, 1.5 km high, were formed in the Late Permian and Early Triassic when there was no collision. Their moderate height precluded asthenospheric upwelling to the base of the crust, which at that time was 65–70 km thick. The mountains could be formed due to delamination of the lower part of mantle root with blocks of dense eclogite and/or retrogression in a presence of fluids of eclogites in the lower crust into less dense facies.

The formation of foreland basins is commonly attributed to deflection of the elastic lithosphere under surface and subsurface loads in thrust belts. Most of tectonic subsidence on the Uralian foreland occurred in a form of short impulses, a few million years long each. They took place at the beginning and at the end of the Late Carboniferous, and in the Late Permian. Rapid crustal subsidence occurred when there was no collision in the Urals. Furthermore, the basin deepened away from thrust belt. These features preclude deflection of the elastic lithosphere as a subsidence mechanism. To ensure the subsidence, a rapid density increase was necessary. It took place due to metamorphism in the lower crust under infiltration of volatiles.

The absence of flexural reaction on the Uralian foreland on collision in thrust belt together with narrow-wavelength basement deformations under the nappe indicate a high degree of weakening of the lithosphere. Such deformations took also place on the Uralian foreland at the epochs of rapid subsidences when there was no collision in thrust belt. Weakening of the lithosphere can be explained by infiltration of volatiles into this layer from the asthenosphere and rapid metamorphism in the mafic lower crust. Lithospheric weakening allowed the formation of the Uralian thrust belt under convergent motions of the plates which were separated by weak areas.     

Deep neogene structure and modern seismicity of the southern part of the Koryak Highland academician

The Northern Kamchatka and southern part of the Koryak Highland is considered to be an accretion-collision system in the Late Cretaceous and Cenozoic the development of which was caused by the subsequent accretion of various large terranes to the Asian continental margin. The Paleogene Goven Terrane accreted in the Miocene closes this system. Its boundary with the Olyutor Terrane is hidden under the Cenozoic sediments of the Il’pino-Pakhachino interarc trough. The destructive Khaily (March 8, 1991) and Olyutor (April 20, 2006) earthquakes are characterized by an aftershock area extended in the northeastern direction along the axial part of the Il’pino-Pakhachino trough. The aftershock area was intersected by a profile of the earthquake converted-wave method (ECWM) the interpretation of which reveals a correlation loss of the deep reflecting horizons under this area and three faults dipping to the southeast on seismograms.     

Cenozoic geodynamics of the Bering Sea region

In the Early Cenozoic before origination of the Aleutian subduction zone 50–47 Ma ago, the northwestern (Asian) and northeastern (North American) parts of the continental framework of the Pacific Ocean were active continental margins. In the northwestern part, the island-arc situation, which arose in the Coniacian, remained with retention of the normal lateral series: continent-marginal sea-island arc-ocean. In the northeastern part, consumption of the oceanic crust beneath the southern margin of the continental Bering shelf also continued from the Late Cretaceous with the formation of the suprasubduction volcanic belt. The northwestern and northeastern parts of the Paleopacific were probably separated by a continuation of the Kula-Pacific Transform Fracture Zone. Change of the movement of the Pacific oceanic plates from the NNW to NW in the middle Eocene (50–47 Ma ago) was a cause of the origin of the Aleutian subduction zone and related Aleutian island arc. In the captured part of the Paleopacific (proto-Bering Sea), the ongoing displacement of North America relative to Eurasia in the middle-late Eocene gave rise to the formation of internal structural elements of the marginal sea: the imbricate nappe structure of the Shirshov Ridge and the island arc of the Bowers Ridge. The Late Cenozoic evolution was controlled by subduction beneath the Kamchatka margin and its convergence with the Kronotsky Terrane in the south. A similar convergence of the Koryak margin with the Goven Terrane occurred in the north. The Komandorsky minor oceanic basin opened in the back zone of this terrane. Paleotectonic reconstructions for 68–60, 56–52, 50–38, 30–15, and 15–6 Ma are presented.