Late paleozoic-Early Mesozoic within-plate magmatism in North Asia: traps, rifts, giant batholiths, and the geodynamics of their origin

A number of large areas of igneous provinces produced in North Asia in the Late Paleozoic and Early Mesozoic include Siberian and Tarim traps and giant rift systems. Among them, the Central Asian Rift System (CARS) has the most complicated structure, evolved during the longest time, and is a large (3000 × 600 km) latitudinally oriented belt of rift zones extending from Transbaikalia and Mongolia to Middle Asia and including the Tarim traps in western China. CARS was produced in the Late Carboniferous, and its further evolution was associated with the lateral migration of rifting zones; it ended in the Early Jurassic and lasted for approximately 110 Ma. CARS was produced on an active continental margin of the Siberian continent and is noted for largest batholiths, which were emplaced simultaneously with rifting. The batholiths are surrounded by rift zones and compose, together with them, concentrically zoned magmatic areas, with crustal (granitoid) magmatism focused within their central portions, whereas mantle (rift-related) magmatism is predominant in troughs and grabens in peripheral zones. The batholiths show geological and isotopic geochemical evidence that their granitoids were produced by the anatexis of the host rocks at active involvement of mantle magmas. Zonal magmatic areas of the type are viewed as analogues of large igneous provinces formed in the environments characteristic of active continental margins. Large within-plate magmatic provinces in North Asia are thought to have been generated in relation to the overlap of at least two mantle plumes by the Siberian continent during its movement above the hot mantle field. In the continental lithosphere, mantle plumes initiated within-plate magmatic activity and facilitated rifting and the generation of traps and alkaline basite and alkali-salic magmatic associations. Because of the stressed states during collision of various type in the continental margin, the mantle melts did not ascend higher than the lowest crustal levels. The thermal effect of these melts on the crustal rocks induced anatexis and eventually predetermined the generation of the batholiths.     

Trace-element study and age dating of zircon from chromitites of the Bushveld Complex (South Africa)

The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5–4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760–930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051?±?9 Ma (2σ, MSWD?=?0.1) and 2,056?±?5 Ma (2σ, MSWD?=?0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity.     

The effect of long-range atmospheric transport of organochlorine compounds by soil studies from Mongolia to the Arctic

The results of studies on the distribution regularities are presented for polychlorinated biphenyls (PCBs) of different degrees of chlorination and for organochlorine pesticides (OCPs): hexachlorobenzene (HCB), hexachlorocyclohexane (HCCH), and DDT with its metabolites in the soils along the Lena River valley and in background areas of Eastern Siberia and Mongolia. A statistically reliable correlation is found between the PCB–OCP and C_org concentrations. The highest levels of HCB are registered in the soils between 50 and 60° N. The concentrations of αand γ-HCCH increase reliably, while those of pp′-DDT and pp′-DDE decrease from south to north. A trend of the decrease in concentrations from Mongolia to the Laptev Sea is revealed for tri-, tetra-, and penta-chlorine-substituted PCB congeners. However, the soils sampled northwards from 60° N show a reliable increase in the concentrations of tetra-, penta-, and hexachlorine-substituted congeners.     

Foidite and Meimechite Lavas of Polar Siberia (Some Questions of Petrogenesis)

Doklady Earth Sciences - For the Permian–Triassic foidite and meimechite lavas of Polar Siberia, both the whole-rock petrochemistry and geochemistry and that of melt inclusions in olivine...     

Tectonophysics and recent geodynamics

Important problems of tectonophysical analysis of observational results in recent geodynamics are considered. The paradox of low rates of recent horizontal deformations of the Earth’s surface is formulated. This paradox states that, according to GPS measurements, the annual mean rates of relative deformations are 10⁻⁹–10⁻¹⁰ per year, whereas, according to ground-based geodetic and deformographic observations, the rates of recent horizontal deformations of the Earth’s surface are on the order of 10⁻⁵–10⁻⁶ per year. This paradox is shown to appear as a result of the application of the procedure of averaging to the displacements obtained on large measuring bases with a low degree of details of the time of observations. It is established that, according to multiyear (over 40 years) data, the annual mean rate of deformation processes in the Kopet Dagh seismically active region is 4 × 10⁻⁸ per annum along the vertical and 3.2 × 10⁻⁸ per year along the horizontal.     

On the radio emission of Callisto

A model of an icy surface and interior for Callisto gives a predicted thermal radio emission in good agreement with experimental radio astronomical data. The radio brightness temperature of an icy surface will not depend on wavelength. This may be a method to test icy surface hypotheses. The brightness temperatures of other satellites with icy surfaces will be equal to 200–220°K and will not depend on wavelength.     

Recent geodynamics of dangerous faults

The analysis of the existing information concerning the present-day deformation activity of the fault zones in seismically active and aseismic regions suggests that the notions of an active fault and a dangerous fault should be distinguished. It is shown that a fault which is active for an expert in geotectonics will not be considered dangerous by an expert in geotechnical monitoring of buildings. The definition is given according to which a dangerous fault is understood as a zone of linear destruction which accommodates the contemporary short-period (a few months and years) pulsed and/or alternating motions with strain rates above 5 × 10^–5 per annum and earthquakes with M ≥ 5. A technique is developed for identifying the dangerous faults based on monitoring the recent ground surface displacements in accordance with a special protocol which ensures an increased degree of detail in time and space. Based on the idea of the probable accumulation of dangerous strains during the operating cycle of the objects, the criteria for assessing their geodynamical risks are formulated.     

Long-Term Geochemical Monitoring of the Source of the Angara River (Runoff from Lake Baikal)

The long-term data (1950–2017) on the chemical composition of the water from the source of the Angara River, which is the only run-off from Lake Baikal, a large and clean fresh water reservoir, are presented. The absence of water contamination despite the anthropogenic influence is shown. The similarity and constancy of the current composition of the waters in the lake and the Angara River are recorded. It is recommended to continue geochemical studies of this unique world heritage site.     

Recent Geodynamics of Tensile Faults

The results of the analysis of extensive data about the local anomalies of recent surface displacements in the fault zones estimated from repeated geodetic observations in seismically active and weakly seismic (platform) regions are presented. It is shown that the local, symmetric sags of the Earth’s surface in the vicinity of the faults are the predominant type of anomalies. The simultaneous recording of the vertical and horizontal displacements shows that the local subsidence is accompanied by horizontal deformations of elongation along the lines that are orthogonal to the fault’s strike. Different kinematic types of faults are considered, and it is shown that the revealed anomalies correspond to the recent activation of the local joints within the fault zones resulting in the subsidence of the Earth’s surface. Three variants of the models describing the formation of the joint-type anomalies are considered: the block model, the dislocation model, and the parametric model. The comparison of the calculated distributions of the horizontal and vertical displacements in the vicinity of the fault with the observations shows that the parametric model fits the field data best. The parametric model describes a fault as a zone with varying internal parameters in two variants: as an inhomogeneity with a reduced stiffness or as an inclusion with anomalous strain according to the terminology of John D. Eshelby. By the example of regularly shaped objects, the equivalence of both approaches is demonstrated.