Vibrations of the Earth crust and variations in the physical fields of the Earth atmosphere and ionosphere are continuously monitored by a variety of techniques and specialized facilities across the world. Nevertheless, most catastrophic earthquakes even in this century have occurred in “incidental” or “unexpected” places in “unpredicted” time. Earthquake predictions have errors as the current knowledge of focal mechanisms of strong (M≥8) earthquakes is still insufficient. It is believed today that the most common source of earthquakes is movement of rock blocks along a fault/megafracture. Such movements take place in a stepwise pattern with high or reduced friction, depending on the presence of fluids, hitches on the fault planes and other factors. Modern seismic forecasting is based on the concept of precursors.

The author considers geological and geophysical settings in areas of dynamic influence of faults, wherein 8>М>7.5 earthquakes took place. Based on earthquake recurrence curves constructed for such areas, four tectonic criteria for formation of strong earthquake sources are identified: structural (large seismically active faults), kinematic (large amplitudes of the fault wing’s displacements), rheological (physical properties of the fault infill material, such as low viscosity of the intra-fault medium) and dynamic (high rates of the fault wing’s displacements) criteria. These criteria should be in the focus of quantitative studies in order to provide a solid scientific basis for long-term forecasting of strong earthquakes. Curves constructed for the criteria can show changes in the physics of earthquake foci in case of strong seismic events.

With account of the tectonophysical features of faults associated with strong seismic events, the following conclusions are drawn. (1) In the continental lithosphere, catastrophic earthquakes (M≥8) occur in areas of dynamic influence of the major faults in the lithosphere in case of relatively high amplitudes of displacements of boundary blocks (i.e. fault wings). (2) In the relatively stable stress field, high amplitude displacements take place in case of reduced viscosity/quasi-viscosity of the medium comprising the internal structure of faults. (3) Reduced viscosity of the intra-fault medium is related to the physical conditions of transition of rocks in fault zones (mainly along the fault planes) in the state of quasi-plastic or plastic flow (unilateral pressure in excess of hydrostatic pressure, and relatively decreasing strength properties of the intra-fault medium with increasing length of the fault wings). (4) Reduced viscosity of the fault zone leads to an increase in the displacement rate of the fault wings in the constant stress field. The latter factor is the main one, transforming seismically active faults with M≤7.5 seismic events into faults of similar characteristics, but with earthquakes of higher energy, M≥8. Focal mechanisms of such earthquakes are associated with conditions for a potential increase of the displacement amplitude regardless of the presence of fluids, hitches on the fault planes and other poorly predictable factors. In-depth studies of the internal structure of faults with M≥8 earthquakes, their foci, conditions of the temporal regime of the seismic process before and after strong seismic events can discover a key to understanding the origin of earthquake sources, the criteria of energy release, and the occurrence of earthquakes with maximum energy. Further steps to develop the geological and geophysical (including tectonic) criteria for prediction of strong earthquakes should be focused on more detailed research of seismic zones wherein strong earthquakes were recorded.

Studying the density of both the crust and mantle is one of the topical problems in modern geophysics. Gravity modeling in combination with seismic tomography is an important tool for detecting density inhomogeneities in the crust and mantle, which can cause stresses and thus significantly impact the regional tectonics [Pogorelov, Baranov, 2010], especially in zones wherein continental margins actively interact with subducting oceanic plates and the entire depth of the tectonosphere is subject to stresses. Associated processes lead to considerable horizontal and vertical stresses that often cause catastrophic events on a global scale. The challenge of studying the global tectonic processes in the Earth’s tectonosphere can be addressed by gravity modeling in combination with seismic surveying.

Data from previous studies. I.L. Nersesov et al. [1975] pioneered in calculating the spatial pattern of mantle density inhomogeneities in Central Asia. Although the accuracy of their estimations was not high due to the limited database, their study yielded significant results considering the structure of the crust. Numerous subsequent geophysical projects have researched the crust to a level sufficient to develop regional models, that can give quite adequate information on the depths of external and internal boundaries of the crust and suggest the distribution patterns of seismic velocities and density values. With reference to such data, mantle density inhomogeneities can be studied with higher accuracy.

This paper reports on the estimations of gravity anomalies in the crust and upper mantle in Central and South Asia. The study region represents the full range of crust thicknesses and ages, as well a variety of crust formation types [Christensen, Mooney, 1995]. We used the 3D gravity modeling software package 3SGravity developed by Senachin [2015a, 2015b] that considers the spherical shape of the Earth's surface, and estimated gravitional anomalies using Baranov’s digital model of the crust, AsCrust [Baranov, 2010].

The study area includes the Alpine-Himalayan folded belt, the triple junction of rift zones in North Africa, and the marginal seas of Southeast Asia, which are framed by deep troughs with associated volcanic belts. Its relief ranges from the highest mountains in Himalayas to deepest troughs in Indonesia. In this region, the collision of the Indian and Asian plates causes thrusting at the Asian plate margin which results in thickening of the continental crust [Oreshin et al., 2011]. This process may be accompanied by the separation of the crustal layer of the Indian lithospheric plate from its mantle ‘cushion’, i.e. delamination, the mechanism of which is not fully understood [Jiménez-Munt et al., 2008; Krystopowicz, Currie, 2013; Ueda et al., 2012] (Fig. 1).

AsCrust, the digital model of the Earth's crust: depth to Moho map. A large volume of new data on reflection, refraction and surface waves from earthquakes and explosions was analyzed and integrated into the AsCrust model (1×1° grid). Ten digital maps were constructed: Moho depth, the upper, middle and lower crustal layers, as well as Vp velocities and densities in these layers [Baranov, 2010]. In our study, we calculated gravitational anomalies from the values of thicknesses and density of crustal layers at each point of the grid. The density in the layers was calculated from longitudinal wave velocities using the formula described in [Brocher, 2005] (Fig. 2).

The algorithm for gravity anomaly calculations. Modeling the gravity of large regional objects needs to take into account the curvature of the Earth's surface. Algorithms for calculating the gravity field from bodies bounded by spherical surfaces are proposed in [e.g. Kosygin et al., 1996; Starostenko et al., 1986; Strakhov et al., 1989; Jones et al., 2010; Li et al., 2011; Schmidt et al., 2007]. In this study, we used an algorithm based on equations for direct calculations of the gravity effect, which can be obtained for specific points located on the pole of the sphere. Such equations considerably simplify the algorithm, but require constant recalculation of the coordinate system for each calculation point, which complicates the task (Fig. 3).

Source data, and methods of gravity anomaly calculations. Our computational model includes seven layers: an water layer, three sedimental layers (depths of boundaries, and density values of the sedimental layers) from the model described in [Laske, Masters, 1997], and three crustal layers (depths of boundaries, and density values of the crust, which were estimated from velocities Vp) from the AsCrust model [Baranov, 2010], considering the territory covered by the model. For the surrounding regions, data on the structure and properties of the crust were taken from the CRUST 2.0 model [Bassin et al., 2000] and interpolated to the 1´1° grid. Thus, data with the resolution of 1´1° were used to describe the sediments and the crust, and data with the resolution of 0.1´0.1° characterized the water layer (batimetry).

Model GGM01 based on satellite observation data of the GRACE project (http://www.csr.utexas.edu) simulated the Earth's gravity field and was used to calculate anomalies in ‘free’ air across the entire surface of our model, which took into account the correction for the elevation of an observation object. The gravity field ranges from –250 to +260 mGal. The zone of collision of the Indian and Asian plates is marked by narrow parallel anomalies of different signs, reaching 200 mGal and more. The southwestern zone with negative anomalies corresponds, apparently, to the boundary of the junction zone of the two plates, wherein the Indian plate subducts underneath the Asian plate, as described in [e.g. He et al., 2010; Oreshin et al., 2011]. The gravity field of the study area quite clearly shows that Tibet is separated from the Tarim plate neigbouring it in the northeast. This separation is marked by a negative anomaly to –150 mGal, the boundaries of which are outlined by narrow zones of positive anomalies. The southern Caspian Sea is also characterized by a negative anomaly to –150 mGal, while Tien Shan is marked by a narrow band of positive anomalies up to 110 mGal. In most of the study area, the field is close to normal and varies within a few dozens of milligals. Moderately positive gravity (within 40¸80 mGal) is typical of the rest of the Alpine-Himalayan folded belt. A slight positive gravity field is revealed in the marginal seas of Southeast Asia, wherein there are two narrow zones of high-amplitude anomalies of different signs (up to 200 mGal), which are generated by isostatically uncompensated systems of island arcs and trenches (Fig. 6).

The gravity effect of the Earth's crust estimated for Asia shows the presence of major anomalies varying in the range of 940 mGal (from –380 to +560 mGal). The maximum positive anomaly is located in the vicinity of the African triple junction of the rift zones, wherein the anomaly reaches a positive maximum of about +560 mGal. Positive anomalies are also revealed in the Tarim Basin (+130 mGal), Southeastern China (+100 mGal), the Iranian plateau (+180 mGal), and back-arc subduction zones of the Indian and Pacific plates (+290 mGal). Large negative anomalies correspond to the Caspian and Black Seas (–380 mGal), Himalayas (–280 mGal), and eastern Tibet (–330 mGal). The Eastern Mediterranean is characterized by a negative anomaly (–310 mGal).

The eastern Arabian Peninsula and the Mesopotamian lowlands are characterized by negative anomalies up to –220 mGal. The map of calculated crustal gravity anomalies also shows submarine ridges (+280 mGal) that trend from south to north and seem to trace ‘hot spots’ that burn through the lithospheric plate (Fig. 7).

Gravitational anomalies in the mantle were calculated by subtracting the gravity effect of the crust from the observed gravity field. The anomalies range from –570 to +350 mGal, which is about twice the range of variations of this field. This directly indicates the presence of large density variations in the lithospheric mantle, which should compensate for the anomalous crustal masses. The largest positive mantle density inhomogeneities in the study region are revealed in the narrow band of the Himalayas (+330 mGal) and Eastern Tibet (+350 mGal). In the Caspian and Black Seas, the anomalies reach +250 and +300 mGal, respectively. The Eastern Mediterranean is characterized by a positive anomaly up to +280 mGal. The eastern Arabian Peninsula and the Mesopotamian lowlands are characterized by positive anomalies of up to +220 mGal. Negative anomalies are revealed in the Tarim Basin (–190 mGal), over submarine ridges in the Indian Ocean (–340 mGal), in Southeastern China (–120 mGal), the central Hindustan (–80 mGal), the Hindu Kush and Karakoram (–150 mGal). Subduction zones of the Indian and Pacific plates are also characterized by negative anomalies of up to –250 mGal. The triple junction zone (Red Sea, Gulf of Aden, the African Rift) in the northeastern African continent is the region of maximum negative anomalies in the mantle wherein gravity values are reduced to –570 mGal (Fig. 8).

Results and conclusion. By applying the 3SGravity software package and the AsCrust digital model, we revealed the spatial pattern of gravitational anomalies in the crust and mantle in Central and South Asia, which gives more precise information about the variations in density with depth in the study area. Our estimations show a significant variations of mantle gravity anomalies, several times larger than the changes in the observed anomalies.

Small earthquakes, often treated as “background seismicity”, are not distributed in space-time in a random manner. Often, space-time clustering is studied, that manifests itself as aftershock sequences and swarms. These phenomena can be described as a deviation (increase) of probability of short interevent distances and times as compared to the reference “pure random” or Poisson case; this tendency manifests itself in statistics of distances between epicenters. In the present work, we study the statistics of directions for vectors connecting pairs of epicenters of such small earthquakes which are close in space-time. Components of such pairs will be called “neighbors”, and the mentioned vectors will be called “link vectors”. A study of this kind is of interest from a number of viewpoints, such as: discovering new properties of statistical structure of observed fields of epicenters; establishing interactions between earthquake sources of small earthquakes, revealing geometrical properties of the pattern of active faults of a low rank. We will show that directions of link vectors clearly deviate from isotropy, and have instead non-uniform, often spiked, distribution of directions.

Pairs of neighbors are extracted from the catalogue of small (M_L=3.5–5.0) shallow earthquakes of the Kamchatka subduction zone. То define neighbors, bounds are set on the distance (10–60 km) and relative delay (0.5 day) between members of a pair. Before pair extraction, the work catalog was decimated to reduce space-time event density within dense clusters. With the catalog of pairs at hand, we constructed distributions of azimuths of link vectors (rose diagrams of directions). In Fig. 3 one can see example histograms and corresponding rose diagrams for two 10-year periods (see Table 1 for definitions and labels of the periods); processing was done using two variants of maximum delay: 0.5 and 5 days. Angles (modified azimuths, n) in all histograms and rose-diagrams are counted off from the direction with azimuth of 37° that represents the strike of the island arc. Before constructing rose diagrams, the modified azimuths were reduced to the [0° 180°] range by subtracting 180° when needed. One can see that with the stricter limit of 0.5 days, histograms and rose diagrams show more expressed deviations from the uniform (isotropic) distribution of angles. For both variants of the maximum delay, the along-arc oriented pairs manifest themselves (at n about 0° and 180°). At the less strict limit of 5 days, this orientation begins to dominate. Although this tendency formally means a break of isotropy, it is not of particular interest because it results from the fact that a large fraction of epicenters occupy a relatively narrow strip, well seen on Fig. 1; therefore the observed 0–180° preferred direction has no connection to epicenter distribution within narrow space-time neighborhoods that we intend to analyze.

To suppress the contribution of this interfering direction, a special normalization of angle histograms was performed. We additionally calculated similar histograms for larger delays, 100 to 150 days, marked T, considering these as representing pure effect of geometry of the epicenter field, and used them for normalization, performed in the following way. Values of the initial or raw (R) histograms are divided (point by point) by corresponding values of T-histograms. In this way the normalized (N) histograms are obtained, considered as most representative of preferred directions of neighbor pairs. To make the results more convincing, we performed statistical testing of the hypothesis “N-histogram differs from a constant”; actually, the equivalent hypothesis “the R-histogram differs from the T-histogram” was tested. The Pearson’s c² criterion was used. The significance value, Q, is indicated on plots, in most cases it is below 0.1 %. Such are the processing procedures employed; then the analysis of data was performed.

N-histograms have been determined for three circles of the 150-km radius shown on Fig. 1, and for five ten-year periods. For the corresponding R-, T- and N-histograms and rose diagrams see Fig 5, 4 and 6. One can see a clear and mostly significant deviation from isotropy; instead, narrow petals are seen in many cases. To see in the original map view how these petals are formed see Figs 7 and 8.

The following conclusions can be derived from this material. (1) The observed distributions of pair azimuths deviate significantly from the uniform law; in many cases, this deviation manifests itself as narrow petals. (2) In two out of three rose diagrams of N kind, there is an expressed petal oriented across the island arc, and along the maximum compression axis. Its formation is difficult to explain from the geomechanical viewpoint. (4) There is evident difference between the rose diagrams for the two southern circles SK and SP, located in the main part of the island arc, and that for the circle KG located near to the junction of Kurile-Kamchatka and Aleutian arc. (5) Clear temporal variations of rose diagrams are seen; these can reflect short-term evolution of parameters of seismotectonic deformation (of “seismic flow of rock masses” in terms by B.V. Kostrov [Kostrov, 1974, 1975]). We believe that the observed picture can be explained through propagation of pulses of aseismic slip along secondary faults. Such pulses are accompanied by small earthquakes; in this way, a pattern of oriented epicenter pairs arises, akin to the notion of migration of epicenters. The location of oriented pairs is tied to several hypothetic systems of subparallel (en-echelon) faults; each such system is manifested as an individual petal of a rose diagram. This interpretation is illustrated by Figs 7 and 8 where one can see in map view how a separate petal of a rose diagram is related to a set of subparallel links that formed it. The main result of the study is the design and testing of a new technique of investigation of hidden anisotropy of the field of epicenters, and detection of time variations of the revealed features. The technique has a potential for monitoring the stress regimen of the lithosphere.

The Global Positioning System (GPS) based on satellites and the networks of dual frequency receivers are actively used for geodetic and geophysical applications, as well as for studying the ionosphere and troposphere. The atmospheric water content is in the focus of research as a key parameter for determining of the accuracy of weather forecasting and hydrological monitoring. The precision of atmospheric water content calculations depends on the accuracy of determination of the delays of signals propagating from GPS satellites to ground-based GPS receivers when geodynamic measurements are conducted. This paper describes a technique that allows us to estimate the integrated water vapor (IWV) in the atmosphere from measurements of GPS satellite signal delays.

We consider remote sensing of the lower atmosphere by GPS measurements to detect the water vapor content in the conventional vertical column to the top level of the troposphere (up to 12 km above the Earth's surface). In studies of the propagation of signals from GPS satellites to ground receivers, the atmospheric water vapor is taken into account as a ‘wet’ component (ZWD) of the zenith tropospheric delay (ZTD). ZTD is the sum of ZHD (hydrostatic or ‘dry’ delay) and ZWD (‘wet’ delay). ZWD values can be converted with a very high confidence in integrated water vapor (IWV) values for each installed GPS receiver.

Introduction. Transtension is a system of stresses that tends to cause oblique extension, i.e. combined extension and strike slip. Syn-volcanic transtensional deformations of the lithosphere may provide two possible scenarios for control of magmatic processes. One scenario assumes ascending sub-lithospheric melts that mark the permeable lithosphere in a transtension area without melting of the lithospheric material; products of volcanic eruptions in such a zone show only the sub-lithospheric mantle material; components of magmatic liquids do not reveal any connection to the lithospheric structure. Another scenario yields a direct control of melting in lithospheric sources in an evolving transtensional structure. In this case, spatial-temporal changes of lithospheric and sub-lithospheric components are a direct indication of the evolving transtensional zone. In this paper, we present arguments in favor of the transtensional origin of the lithosphere-derived melting anomaly along the Wudalianchi volcanic zone, which are based on the study of components in the rocks sampled from the volcanic field of the same name.

Analytical methods. Trace elements were determined by ICP–MS using a mass-spectrometer Agilent 7500ce and isotopes using a mass-spectrometer Finnigan MAT 262. The methods used were described in the previous papers by Rasskazov et al. [2011] and Yasnygina et al. [2015]. Major oxides were measured by “wet chemistry”.

Structural setting of the Wudalianchi zone. This zone extends north-south for 230 km at the northern circuit of the Songliao basin, subsided in the Late Mesozoic – Early Cenozoic (Fig. 1).

Timing of volcanism and variations of K₂O contents in rocks from the Wudalianchi zone. Rocks, dated back to the Pliocene and Quaternary, show the stepwise increasing K₂O content interval along the Wudalianchi zone from the southernmost Erkeshan volcanic field (5.6–5.8 wt %) to the northernmost Xiaogulihe-Menlu volcanic field (2.0–9.5 wt %) (Fig. 2).

Spatial-temporal clustering of volcanoes in the Wudalianchi field. In terms of the general Quaternary evolution of volcanism in Asia [Rasskazov et al., 2012], spatial-temporal distribution and compositional variations of volcanic products, we distinguish three time intervals of the volcanic evolution: (1) 2.5–2.0 Ma, (2) 1.3–0.8 Ma, and (3) <0.6 Ma. The Central group of volcanoes showed persistent shifting of eruptions from Wohushan (1.33–0.42 Ma) to Bijiashan (0.45–0.28 Ma) to Laoheishan (1720–1721, possibly earlier) to Huoshaoshan (1721) (Figs 3, 4, 5). No spatial-temporal regularity of eruptions in volcanoes of the Erkeshan field and Western and Eastern groups of the Wudalianchi field reflected background activity.

Sampling. Representative sampling of rocks from the Wohushan–Huoshaoshan volcanic line was aimed to identify changing geochemical signatures along the whole volcanic line and in the course of eruptions in each volcano (Figs 3, 6, 7). For comparisons, the background volcanoes were also sampled.

Silica and alkalis oxides. On the total alkalis–silica (TAS) diagram (Fig. 8), data points of background rocks are distributed along the dividing lines between highly and moderately alkaline series mainly in tephriphonolite and trachyandesite fields with a few samples in the phonotephrite field. Background rocks from some volcanoes (e.g. Yaoquanshan and Weishan) are highly alkaline (phonotephrites and tephriphonolites). Background rocks from other volcanoes (Longmenshan, Jiaodebushan etc.) are moderately alkaline (mostly trachyandesites). In background rocks, Na₂O+K₂O range from 8.6 to 9.7 wt %, SiO₂ from 51.6 to 55.0 wt %. Phonotephrites from the Erkeshan field are comparable with the Wudalianchi background rocks of this type.

Data points of rocks from the Central group of volcanoes are also distributed along the discriminating line of highly and moderately alkaline series, mainly in the phonotephrite and trachyandesite fields. Almost all samples from the first volcano (Wohushan) fall within the data field of background rocks. Rock compositions of the second and third volcanoes (Bijiashan and Laoheishan) changed on each of them from similar to the background to the ones distinguished by the lower silica and alkalis contents. On the Bijiashan volcano, eruptions were exhibited by trachyandesites of a lava shield and by basaltic trachyandesites and phonotephrites of a volcanic cone. The trachyandesites were comparable to the background rocks, the basaltic trachyandesites and phonotephrites differed from them. On the Laoheishan volcano, rocks were subdivided into three groups: (1) basaltic trachyandesites and phonotephrites, (2) trachyandesites, and (3) phonotephrites. The first group was recorded in pyroclastic material from the late volcanic cone and lavas from the northern bocca, the second group in pyroclastic material from the northwestern edge of the late crater, and the third group in bombs from its southwestern edge. On the fourth volcano (Huoshaoshan), rocks are basaltic trachyandesites and phonotephrites.

In terms of Na₂O, K₂O, and SiO₂ contents, peripheral lavas of volcanic fans in the Bijiashan, Laoheishan, and Huoshaoshan volcanoes were close to background rocks. The contents of these oxides, differed from the background signatures, characterize rocks from volcanic cones in a linear progression that demonstrates the transition from compositions of the Wohushan volcano, close to background ones, through the intermediate values in the Bijiashan and Laoheishan volcanoes to the final compositions in the Huoshaoshan volcanic cone.

In the background rocks, K₂O concentrations range from 4.8 to 6.0 wt % with its relative decrease in the rocks of the beginning and end of volcanic evolution. Initial lava flows with K2O contents as low as 4.0 wt % erupted along the Laoshantou – Old Gelaqiushan north-south locus from 2.5 to 2.0 Ma and in the final cone of the Huoshaoshan volcano, erupted in 1721, fell to 3.2 wt %. Since 1.3 Ma, irregular spatial-temporal distribution of volcanic activity reflected dominated background processes. Between 1.3 and 0.8 Ma, eruptions took place at the South Gelaqiushan volcano and along the west-east locus of the Lianhuashan, Yaoquanshan, West Jaodebushan, West Longmenshan volcanoes. In the last 0.6 Ma, three groups of volcanoes erupted: Western (North Gelaqiushan, Lianhuashan, Jianshan-Jianshanzi, Central (Wohushan, Bijiashan, Laoheishan, Huoshaoshan), and Eastern (Weishan, East Jaodebushan, Xiaogoshan, West and East Longmenshan, Molabushan). Background eruptions continued in the Western and Eastern groups, whereas the Central group displayed stepwise shift of activity from the southwest to the northeast. Under such a regular volcanic evolution, relative reduction of K₂O abundances took place in final eruption products of the Huoshaoshan volcano (Fig. 9).

Other major oxides. Changes of rock compositions along the Wohushan-Huoshaoshan line, from the close to the background signatures at the first volcano (Wohushan) through the contrast major oxide contents at the Bijiashan and Laoheishan edifices to notably different from the background ones at the Huoshaoshan cone, are illustrated further by diagrams of SiO₂ vs. MgO, Al₂O₃ vs. MgO, CaO vs. MgO, and P₂O₅ vs. MgO (Figs 10, 11).

Trace elements. No sufficient difference is found between primitive mantle-normalized patterns plotted for rocks from different volcanoes (Fig. 13). Nevertheless, specific variations of rock compositions in the Central group of volcanoes close to the background and different from them are shown on the diagrams of Ni, Cr, Rb, Zr, Ba, Th, Sr, and La/Yb vs. MgO (Figs 12, 14, 15). A similar behavior was observed, on the one hand, for Rb and Zr, on the other hand, for Ba, Th, Sr, and La/Yb. In rocks from the Central group of volcanoes, which are compositionally close to the background ones, Rb concentrations increase from the first volcano (Wohushan) through the second (Bijiashan) to the third (Laoheishan). In rocks that differ from the background ones, Rb concentrations increase from the second to the fourth volcano and decrease in its final edifice. In rocks, close to the background ones, Zr concentrations decrease from the first to the second volcano and increase to the third volcano. In rocks, distinguished from background ones, relatively low concentrations of Zr at the first volcano change to elevated concentrations at the third and fourth volcanoes with relative decrease at the final Huoshaoshan edifice.

Discussion. Sub-lithospheric continuum of components under East Asia comprises a material from convective mantle domain with subducted slab (paleoslab) fragments of oceanic (paleooceanic) crust as well as delaminated lithospheric blocks of orogens. Volcanic rocks from the Wudalianchi field show a sub-lithospheric end-member, which belongs to this continuum. Lithospheric components of these rocks, however, have no connection with other sub-lithospheric components. We refer the Wudalianchi rocks to a sub-lithospheric–lithospheric cluster of components from the boundary between the lithosphere and sub-lithospheric convective mantle (Fig. 17). From the comparative analysis of K₂O, other major oxides, and trace elements in rocks of early and late eruption phases in the Central group of volcanoes, we infer that rocks were compositionally almost similar to the background ones in edifices of the first volcano (Wohushan), partially close to the background rocks and partly differed from them in edifices of the second and third volcanoes (Bijiashan, Laoheishan), and significantly different from the background rocks in the cone of the fourth volcano (Huoshaoshan) (Figs 18, 19). We suggest that magma generation under the Wudalianchi volcanic field was controlled by developing transtension of a layer at the base of the lithosphere that divided and shielded sources of the underlying homogeneous sub-lithospheric convective mantle and the overlying enriched heterogeneous lithosphere. The sub-lithospheric magma source had ⁸⁷Sr/⁸⁶Sr=0.7052, sources of the boundary shielding layer the same and lower Sr-isotopic ratios, and sources of the overlying region the same and higher ratios (Fig. 20). Through the extremely low row of data points for rocks from the Huoshaoshan volcanic cone in ⁸⁷Sr/⁸⁶Sr vs. ⁸⁷Rb/⁸⁶Sr plot, we get an estimate of about 98 Ma for the isotopic system closure at the base of the lithosphere with the initial ⁸⁷Sr/⁸⁶Sr apatite-related value 0.70485 and the underlying convective mantle domain with Rb/Sr=0.092 (Fig. 21). We infer that the development of transtension governed time and space of the locally introduced convective mantle component through the boundary shielding layer on background of melting enriched mantle material above the latter (Fig. 22). The 2.5–2.0 Ma local eruptions of sub-lithospheric liquids, derived from the axial part of the north-south zone of transtension, were followed by the 1.3–0.8 Ma background melts from a wider transtensional segment of the enriched lithospheric region. Afterwards, in the past 0.6 Ma, background melting of the enriched lithosphere sharply outlined edge portions of the transtensional segment, whereas simultaneous local sub-lithospheric melting propagated along a crack that originated within the boundary shielding layer due to concentrating tectonic forces at the central portion of the transtensional segment.

Introduction. One of the main tasks of paleomagnetic studies is to obtain a framework of reference poles for calculating the kinematic characteristics of lithospheric taxones as a basis for geodynamic reconstructions. Each paleomagnetic reference point must have a precise (±10 Ma) geochronological dating and a maximum paleomagnetic reliability index. A correct paleomagnetic pole (PMP) can be obtained from the data of geochronological and paleomagnetic studies conducted in one and the same geological object, such as a suite, an intrusive complex etc. In the Yakutian diamondiferous province (YDP), such objects include basalt nappes of the Upper Devonian Appainskaya suite, which stratigraphic position is undoubted (Fran, 385–375 Ma).

Geological setting (in brief). In the eastern segments of the Siberian platform, a powerful cycle of tectonic and magmatic activity in the Middle Paleozoic produced transgressive and sheet intrusions, volcanic pipes, lava and tuff formations comprised of basites, as well as all the currently known industrial diamondiferous kimberlite bodies. Magmatic activity of basites was associated with formation of paleorift systems, including the largest one, Viluyi paleorift (Fig. 1). In the Middle Paleozoic, the geodynamic setting for magmatism and rifting was determined by the plume-lithosphere interaction. The rise of the plume’s matter underneath the thinned lithosphere was accompanied by decompression melting and formation of basaltic magmas in large volumes.

We have studied basalts of the Appainskaya suite which were sampled from the Ygyatta and Markha river valleys (Fig. 2). In the coastal outcrops at the Ygyatta river, two nappes are observed, a (stratigraphically) lower outcrop 17÷23/10 containing plagiophyre palagonite basalts (upper five meters are outcropped), and an upper outcrop 16/10 containing olivinophyric palagonite basalts (upper three meters are outcropped). In the coastal outcrops of the Markha river, from the Enerdek loop to the M. Dyukteli river (outcrop 16÷20/14), only plagiophyric basalts of the lower nappe are developed. At this location, the total capacity of the basalts can reach 35–40 m. In view of the fact that the basalts lie subhorizontally at angles up to 5° (outcrop 17/14, Fig. 3), oriented samples were taken in the modern system of coordinates.

Formational features of the chemical composition typical of the Middle Paleozoic intrusive basites (higher contents of Ti, Fe and K) are less clear in derivatives of the effusive facies. By their chemical composition, the basalts are normal alkalinity rocks (the sum of alkali not higher than 3.05 %; SiO₂=48.1–49.7 %; rather moderate content of TiO₂=1.9–2.5 %) (Fig. 4 A, B). The amount of magnesia (Mg#) ranges from 46 to 56. The main carriers of natural remanent magnetization (NRM, In) are titanomagnetites that belong to titanomagnetite and hemo-ilmenite series (Fig. 4).

Research. Our research was conducted in specialized laboratories using modern equipment and facilities of Geo-Scientific Research Enterprise (NIGP) PJSC ALROSA (Mirny), Institute of the Earth's Crust SB RAS (Irkutsk), Kazan Federal University (Kazan) and Institute of Geology of Diamond and Precious Metals SB RAS (Yakutsk).

Research results. By magnetic (scalar and vector) parameters, basalts of the Appainskaya suite are characterized by the bimodal distribution of magnetic susceptibility values, NRM and æ: geometric means are 810·10^–5 Si-units and 225·10^–3 А/m, respectively, at the Ygyatta river, and 1470·10^–5 SI-units and 490·10^–3 А/m, respectively, at the Markha river (Table 1, Fig. 5). Factor Q is below 1. Results of the petrophysical observations are consistent with the geological materials and suggest that the basalts at the Ygyatta river occupy the upper stratigraphic horizon.

The studied outcrops of basalts of the Appainskaya suite have the following characteristic components of In^ch:

1. Component А – negative vectors of the characteristic NRM are clustered in the fourth sector of the stereogram (sample Igy179m1, Fig. 10, Fig. 14 А, Table 2). Found in outcrop 16/10. Component А is metachronic In^m that formed due to heating of basalts by dolerites of the Ygyatta sill, which suggests the dyke-type of the anisotropy of magnetic susceptibility (AMS) (Fig. 6 C) and a high oxidation level of titanomagnetites (sample 179, Fig. 8).

2. Component B – steep positive vectors of the characteristic In^ch (samples Igy224m2, Mrh142m2 and Mrh176t2, Fig. 10, Fig. 14 А, Table. 2). Found in outcrops 20/10 and 16÷18/14. Component В is typical of the outcrops with significant deviations of the axes of the AMS ellipse (Fig. 6 D, E), which suggests epigenetic changes in the basalts. New occurrences of titanomaghemites are observed in the studied outcrops (sample 228, Fig. 8), which leads to an almost complete destruction of vector In⁰ and formation of viscous NRM – In^v, which are oriented in the direction similar to the geomagnetic field. This conclusion is supported by the ‘artificial magnetization reversal’ tests (Fig. 11 А).

3. Component C – negative vectors of the characteristic NRM are clustered in the first sector of the stereogram at angles varying from –50 to –40° (Fig. 12, Fig. 14, Table 2). Found in four outcrops at the Ygyatta river (outcrops 17/10, and 21÷23/10).

4. Component D – positive vectors of the characteristic NRM are clustered in the third sector of the stereogram at angles varying from 40 to 50° (Fig. 13, Fig. 14, Table 2). Found in four outcrops at the Markha river (outcrops 20А, 20В, and 20С/14).

The primary origin of characteristic components C and D of the basalts is determined as follows:

- The ‘sedimentary’ type of AMS (Fig. 6 E, and Fig. 6 F);

- According to the differential thermomagnetic analysis (DTMA), the mineral carrier of magnetization is virtually unaltered titanomagnetite with the Curie point of ≈550°C (samples 254 and 204, Fig. 8);

- The presence of samples with negative NRM vectors (Table 1);

- The magnetically stable state of the components is confirmed by high values of hysteresis parameters (Fig. 7) and the ‘artificial magnetization reversal’ experiment (Fig. 11 B).

- The positive inversion test (Table 3, Fig. 14 B, and Fig. 14 C): γ/γс=5.1/6.2 at the sample level, and γ/γс=8.7/16.2 at the site level.

Discussion. Data on 12 sites and previously published values were used to calculate the reference paleomagnetic pole (PMP) (Fran) (Table 5, Fig. 15, А). The PMP coordinates are as follows: latitude j=1.7°, longitude l=92.8°, and confidence intervals dp/dm=3.7/5.9°. The PMP’s paleomagnetic reliability index is high enough, and the PMP can be thus considered as a reference for the Frasnian period (370±5 Ma). On this basis, taking into account the previous paleomagnetic data, paleomagnetic reconstructions of the Siberian platform, ranging from 420 up to 325 Ma, are obtained in our study (Fig. 15, B). In the above-mentioned period of time, the Siberian platform gradually moved in one direction, mostly latitudinal, from 11° to 25° N. After the Appainskaya time, the latitudinal movement was replaced by motions in the predominantly meridional eastward direction, and the average displacement velocity in these segments increased from 4.4 to 6.7 cm/year. It is possible that after the formation of the Appainskaya suite (Fran), the Siberian platform could pass the three hot spots representing the modern Atlantic islands near the northwestern coast of Africa (Canary, Madeira and Azores, i.e. the northern flank of the African superplume). These hotspots might have formed the tracks (Fig. 15) that controlled the intrusion of alkaline ultrabasic melts and formation of kimberlites in the Late Devon – Early Carbon.

Conclusion. In the lower stream composed of the palagonite plagiophyre basalts of the Appainskaya suite, the paleomagnetic studies reveal two primary components of the NRM vectors, from bottom to top, D and C, respectively, with the direct and reverse polarity. Their presence in the basalts is marked by the ‘sedimentary’ type of AMS, practically un-oxidized titanomagnetites, and the positive inversion test.

The reference PMP for the basalts of the Appainskaya suite, which is determined in our studies, provides for a more precise definition of the paleogeographic position and reconstruction of the drift of the Siberian platform in the Middle Paleozoic (from 420 to 325 Ma) and makes it possible to associate this drift with probable energy sources (i.e. hot spots), which might have been related to the intrusion of kimberlites.

We present results from petrographic and lithogeochemical studies of the Late Precambrian terrigenous rocks (sandstones, gravelites, and aleuritic sandstones) from the Oselkovaya series of Prisayanie. The studies were conducted to reconstruct the primary composition of the rocks in the source area. It has been found that the rocks in the lower part of the cross-section of this series (Marninskaya suite, and the lower part of the Udinskaya suites) are represented by more coarse-grained terrigenous rocks (gravelites, and sandstones) as compared to the upper part of the cross-section (the upper part of the Udinskaya suite, and the Aisinskaya suite) with sandstones and aleuritic sandstones. Gravelites and sandstones from the lower part of the Oselkovaya series show indicators of epigenetic changes that are less intensively expressed in the rocks from the upper part of the cross-section. The upper and lower parts of the Oselkovaya series are significantly different in terms of lithogeochemistry. The lower rocks show quite low contents of Na₂O and ratios K₂O/Na₂O ranging between 10 and 75. In the terrigenous sediments of the upper part, values of K₂O/Na₂O do not exceed 1–2. Sandstones and gravelites in the lower part of the Oselkovaya series are characterized by reduced concentrations of radioactive, rare-earth, and highly charged elements, as well as lower concentrations of Ni and Co relative to concentrations of these elements in sandstones and aleuritic sandstones of the Oselkovaya series. The petrographic and lithogeochemical characteristics of the terrigenous sediments of the lower and upper parts of the Oselkovaya series suggest different sources of the denudation of these rocks into the sedimentation basin. It is suggested that acid rocks were the denudation source of the terrigenous rocks in the lower part of the series, and the sandstones and aleuritic sandstones in the upper part of the series were sourced from rocks of a mixed (acid–base) composition. The composition of the rocks in the source area was reconstructed, and the published ages of detrital zircons from the sandstones of the upper and lower parts of the Oselkovaya series were taken into account. The reconstruction suggests that the lower part of the Oselkovaya series resulted from the destruction of the basement rocks in the Siberian craton. The upper part of the Oselkovaya series seems to have formed in the basin, wherein the denudation took place from the orogen formed as a result of the accretion of micro-continents and island arcs of the Paleo-Asian Ocean to the south-western margin of the Siberian craton.

We have studied the material composition of ore microparticles extracted from gold concentrates of operating quartz vein No. 30 located in the Irokinda deposit, Western Transbaikalia. We consider the origin of such microparticles in connection with our observation data and the previously published structural and geological features revealed in formation of the ore field, as well as tectonophysical conditions of formation of many gold-bearing quartz veins, including vein No. 30.

Gold-quartz veins, located in the allochthonous plate thrusted onto the Kelyano-Irokinda belt (Fig. 1), infill the NE-striking fault zones. E.A. Namolov conducted the tectonophysical analysis of the “elementary fracture – ore-bearing suture/joint” system, which provided a genetic explanation of the morphology of ore quartz veins (including vein No. 30) and conditions for formation of their host fault zones. Ore-bearing fractures are combinations of shear and cleavage cracks that occur in case of certain positions of the strain ellipsoid in conditions of horizontal compression. Due to repeated intra-mineralization displacements, the texture of the ores is strappy, and the quartz matrix of the veins contains numerous inclusions of host rocks.

The spherical particles have zonal structures and consist of metal nodes and external continuous or discontinuous shells, which thickness ranges from 10 to 400 microns (Fig. 2, Fig. 3). The nodes are composed mainly of native Fe with admixtures of Fe, Mn, Al (Table), the contents of which are typically less than 1.0–1.5 wt %.

Characteristic features of the mineral composition of shells of the spheroidal microparticles:

– The widespread graphite matrix consisting of minerals of different classes, except for native;

– Pyrite in the group of ore oxides of Fe, Mn, Cr, Ti;

– A large group of carbonate minerals;

– Feldspars and natrosilite among silicates;

– The mineral with CaBr₂ composition;

– Mono-mineral quartz rims.

The consequence of metamorphism, i.e. deformational or mechano-chemical transformations of rocks in Irokinda, as well as the autochthon (the rock bed of the Kelyano-Irokinda belt), is the gas-water (‘hydrothermal’) system capable of forming the spherical ore particles with low-temperature mineral rims.

The main feature of the structure of the spherical microparticles in Irokinda is a sharp contrast of the crystallization conditions of the metal nodes and their rims. Similar conditions leading to formation of contrasting mineral associations, that are similar in compositions to the discussed spherules, are characteristic of the gas-water-lithoclastitic and gas-water stages of mud volcanoes. For these stages, we suggest the cavitation mechanism of formation of spherical metal particles of Fe, Fe–Cr and other compositions, which is accompanied by combustion (pyrogenic melt) and pyrolysis of hydrocarbon components of the fluid. This mechanism, with the exception of the origin of the melt (in this case, of the friction type) seems to most closely correspond to the actual data.  The spheroids are likely to have formed in the pre-ore stage of formation of the quartz veins.

The high-temperature metal spherical microparticles revealed in our study can be regarded as specific indicators showing conditions in which the ore-forming system of the dynamo-metamorphic type was functioning to produce gold mineralization on the Irokinda deposit. The structure and composition of these microparticles differ from those of the microspherules from other gold deposits in Transbaikalia (black shale formation in Sukhoi Log, and low–sulphide gold–quartz ore formation in Pervenets), which also belong to the dynamogenic genetic type. However, the ore-forming systems of the compared deposits have two common factors that contribute to formation of spherical microparticles – high tectonic activity manifested by repeated (impulse-type) tectonic movements, and the associated unstable pressure conditions. The consequence of the latter is heterogenization of the gas-water fluid, which, in turn, leads to the cavitation and froth flotation mechanisms.

Introduction. The Lena gold province is one of the largest known gold resources in the world. The history of its exploration is long, but the genesis of gold mineralization hosted in black shales in the Bodaibo synclinorium still remains unclear. The studies face the challenge of discovering sources for the useful component and mechanisms of its redistribution and concentration. This study aims to clarify the time sequence of the ore mineralization in the Chertovo Koryto deposit on the basis of detailed mineralogical and geochemical characteristics of the ore, wallrock metasomatites and the Early Proterozoic host black shales, and to assess the applicability of the Sukhoi Log model for clarifying the Chertovo Koryto origin.

Geological setting. The Lena gold province is located in the junction area of the Siberian platform and the Baikal mountain region (Fig. 1). The main element of its geological structure is the Chuya-Tonoda-Nechera anticline. Its axial segment is marked by horsts composed of the Early Proterozoic rocks with abundant granitoid massifs. The Chertovo Koryto deposit is located within the Kevakta ore complex at the Tonoda uplift, the largest tectonically disturbed block between the Kevakta and Amandrak granitoids massifs. The 150 m thick and 1.5 km long ore zone of the Chertovo Koryto deposit is confined to the hanging wall of the fold-fault zone feathering the Amandrak deep fault (Fig. 2).

Composition. In the ore zone, rocks of the Mikhailovsk Formation include carbonaceous shales of the feldspar-chlorite-sericite-quartz composition with nest-shaped ore accumulations of the pyrite-quartz composition and quartz veinlets. In our study, we distinguish five mineral associations resulting from heterochronous processes that sequentially replaced each other:

- The earliest association related with the quartz-muscovite-sericite metasomatism and the removal of REE and other elements from the rocks and their partial redeposition;

- Metamorphic sulphidization presented by scattered impregnations of pyrrhotite, as evidenced by small lenses of pyrrhotite, which are considerably elongated (axes up to 0.7 cm long) along the foliation planes (Figs 3, a, b);

- Ore mineralization represented by a superimposed hydrothermal gold association with arsenopyrite (Fig. 3, d);

- Late chalcophilic mineralization formed at the final stage of hydrothermal-metasomatic process (Figs 3, e, f);

- Post-ore silification.

Geochemical characteristics. The geochemical study of rocks and ores from the Chertovo Koryto deposit show that the rocks of the Mikhailovsk Formation are characterized by higher contents of rock-forming elements, such as of Al₂O₃, Fe₂O_3total, MgO, K₂O, and P₂O₅, in comparison to the PAAS standards [Condie, 1993] and the black shale standard composition (SChS-1) [Petrov et al., 2004]. A characteristic feature of the ore zone is that the contents of practically all the oxides, except SiO_2, tend to decrease (Table 1). The distribution of rare elements repeats the pattern established for major elements. The least metamorphosed rocks of the Mikhailovsk Formation have higher contents (up to three times) of Cu, Mo, Ba, W, As, Pb relative to the values in the PAAS and SChS-1 standards. In the ore zone, the contents of almost all rare elements are considerably reduced (Table 2). The contents of elements in the siderophile group (Co, Ni) are clearly correlated with the ore processes and increased more than twice in the area of metamorphic changes. Samples with gold-ore grade contents show the highest concentrations of Co and Ni.

Conclusion. In our opinion, the Chertovo Koryto deposit was formed in five stages, the first two of which were pre-ore, with ore preparation, and probably considerably distant in time from the main ore-generating event. The staged formation of the Chertovo Koryto deposit correlates with the basic stages in the tectono-metamorphic history of the study region and is consistent with the model showing the formation of Sukhoi Log-type deposits [Nemerov, 1989; Buryak, Khmelevskaya, 1997; Large et al., 2007]. 

The Kara ore node is located within the Sretensk-Kara ore region of East Transbaikalia. The geological structure of this area is complex due to its location within the Mongol-Okhotsk suture, the zone wherein the Siberian and Mongolia-China continents collided into each other at the turn of the Early and Middle Jurassic. During the plate collision, intense magmatism was accompanied by the formation of focal-dome, dome-ring and other structures. The Kara ore node is controlled by the Ust-Kara focal dome-ring structure. The central part of latter is composed of Kara-Chacha granitoids from the Amudzhikan-Sretensk intrusive complex (J₃-K₁) with the system of subvolcanic and vein formations, including grorudites. It is suggested that gold mineralization in the study area is genetically related to grorudites; however, physical and chemical conditions for the formation of these alkaline rocks, their genesis and role in the hydrothermal gold-ore process still have not been sufficiently investigated. To this end, the authors of this paper have studied fluid inclusions (FI) in quartz from these rocks. It has been found that quartz porphyry phenocrysts in grorudite contain FI of diverse forms, the size of which ranges from 5 to 48 microns. Measured temperatures of ice melting (–2.5°C) and complete homogenization into liquid (350 °C) show that the concentration of salts in the fluid amounts to 4.2 wt % of eq. NaC, its density is 0.64 g/cm³, and the pressure is 1.6 kb. At LA-ICP-MS of individual FI, clear analytical signals were derived from Na and K. As, Mo, Sb, Cs, W, and Hg were traced in significant quantities. The Raman scanning showed the presence of N₂ in the primary (substantially gaseous) FI, and CO₂, N₂, and CH₄ in the primary-secondary FI.

The author continues to investigate additional planetary-level stresses that occur in the crust due to distributed tangential mass forces. Such forces may be related to the daily rotation of the Earth and movements of the relatively solid core relative to the geocenter. In [Rebetskii, 2016], he discusses how the tangential mass forces in the continental crust are influencing additional meridional and latitudinal stresses and attempted to explain regularities of planetary fracturing. In this paper, he considers the role of the tangential mass forces in the occurrence of lateral movement of the lithospheric plates.

The author proposes to estimate amplitudes of the tangential mass forces from the difference between the two global ellipsoids of rotation. The reference ellipsoid averages the level surface of the gravity potential, and the second ellipsoid averages the physical surface of the Earth, separately considering continents and oceans. The Earth’s dynamic compression factor estimated from satellite data is 1/305.5. This value corresponds well to the average polar compression of the two rotation ellipsoids, which approximately describes the shape of the Earth’s physical surface. Thus, in the first approximation, the polar compression of the Earth’s physical surface is less than that of the reference ellipsoid (1/298.25) that approximately describes the shape of the level surface of gravity (i.e. geoid).

Gravity vectors deviate from the normal to the physical surface of the Earth by relatively small angles, according to calculations from the data on rotation ellipsoids (a maximum value of 16.4 at the 45° latitude). Tangential mass forces are thus small (2.15×10^–4 G/cm³ at the 45° latitude). Due to small tangential forces, shear stresses about 0.3 MPa may occur at the base of the continental lithosphere (depths of 120–150 km). In their turn, such stresses can cause a shear flow in the asthenosphere, which provides for movements of the lithospheric plates at velocities of a few centimeters per year. The estimates in this study suggest that the tangential mass forces can be viewed as a possible source of the movements of the lithospheric plates.

Regional rotation ellipsoids, that average the physical surface of the continental and oceanic parts of the Earth, were estimated separately for the northern and southern hemispheres. The largest deviations of the ellipsoids from the reference ellipsoid were revealed for the oceanic parts of both hemispheres of the Earth. The regional ellipsoids for the oceanic parts show smaller polar compression (1/313.1 in the northern hemisphere, and 1/306.9 in the southern hemisphere) than that of the reference ellipsoid, and this predetermines the north-south orientation of the tangential mass forces from the poles to the equator. Compared to the reference ellipsoid, polar compression values estimated for the regional ellipsoids of the continental crust are larger (1/296.2) in the northern hemisphere and smaller (1/303.2) in the southern hemisphere. According to the calculations, the oceanic lithosphere makes the major contribution to submeridional movements of the continental plates.