V. A. Golubev

Paleogeographic Settings and Tectonic Deformations of the Barents Sea Continental Margin in the Cenozoic

The Barents Sea continental margin (hereafter, Barents margin) differs from other passive margins by the most extensive shelf, the giant thickness of sedimentary rocks in basins and troughs, and its unique tectonic position. The outer, almost rectangular promontory of the Barents margin juts out into its deepwater western and northern framing (Fig. 1), identified as the Norwegian‐Greenland and Eurasia basins, respectively. In this regard, the continental margin is affected by two, mutually perpendicular spreading zones (Knipovich and Gakkel ridges). The evolution of oceanic basins proceeded in the course of continuous tectonic and geodynamic interaction with the framing continental margins. In our case, this was expressed, first of all, in the separation and evolution of the Barents Sea shelf platform as an area of neotectonic transformations during the opening of young oceanic basins. Its structures were transformed against the background of breakup and block-shaped disintegration (destruction and fractalization) of the continental crust. This is indicated by Cenozoic volcanism in the Spitsbergen and Novaya Zemlya segments, development of tectonomorphic trenches (grabens), anomalous geophysical properties of the present-day Earth’s crust (including thermal and seismic activity), and specific deformations of the sedimentary cover. All the aforementioned allow us to make a judgement about the contribution of the Cenozoic ocean formation to the modern tectonics and architecture of the Barents margin. The initial breakup of the joint continental lithosphere located between the Barents margin, on the one hand, and Greenland and Lomonosov protoridge, on the other, most likely occurred in the region of the future divergence of plates during the Late Cretaceous‐ Early Paleocene. This is indicated by marine drilling and seismic profiling data suggesting that geological history of the Barents margin included a very important erosion and denudation phase related to the regional uplift before the rift stage. The amount of the material removed from only the inner shelf during the Cenozoic is estimated at 1.5‐2.0 km [1, 2]. In the peripheral zones adjoining the intercontinental rift systems in the Late Cretaceous‐Early Paleogene, the amount of eroded material increases to 3 km or more. However, up to one-half of the material was eroded by glacial processes.

Geothermal field and distribution for earthquake source depths in the Baikal Rift Zone

A comparison between values of thermal flux and the deep temperatures calculated by them and the depths of earthquake sources in three areas of the Baikal Rift Zone is made. It has been shown that during transit from the Baikal depression to the adjacent mountain massif, the thermal flux decreases almost 2–3 times. The corresponding deep temperatures decrease to a similar degree. The available data for these areas on earthquake depths show that their lower boundary both beneath the depression and the massif is located at almost the same depth, which is about 20 km. In this paper, the conclusion is made that the cause of the absence of an interrelation between thermal and seismic fields lies in the discrepancy between the measured values of thermal flux and its deep values. This discrepancy arises because conductive heat transfer in the upper part of the Earth’s crust, up to 5–10 km depth, is highly distorted by heat-and-mass transfer of ground waters. In the middle part of the crust, the difference in temperatures beneath depressions and ridges is leveled horizontally, which is reflected in almost the same depth for the basement of the seismogenerating layer beneath these main rift structures.

Geophysical data confirming lack of Late Cenozoic mantle intrusions in the Earth’s crust under the Baikal Depression

Young volcanoes and intrusions associated with them located within the depressions of the Baikal Rift zone are known only in the Tunkin Depression. The problem of the existence of cooling intrusion bodies in other depressions of this rift zone remains open. According to the geothermal data, the existence of large Late Cenozoic shallow intrusions was supposed in the Baikal Depression [1, 2]. At present, our knowledge of this depression in the geological and geophysical aspect increased strongly. The high density of geothermal, seismic, and other data in this region allows us to perform their joint analysis and make well-grounded conclusions on this basis related to the actual existence of such intrusions. The comparison of geothermal and seismic fields discussed here is based on the concepts that the location of earthquake sources is limited from below by the depths where brittle properties of the Earth’s crust disappear and their plasticity appears. Temperature is one of the main factors determining the depth of this transition. The temperature where this transition takes place reaches 350 ± 100°e at these depths, and the effect depends on the material composition, deformation rates, and pore pressure [3, 4]. In the zones of thermal influence of cooling intrusions, if they really exist, this isotherm, and consequently the lower boundary of the earthquake sources, should approach the Earth’s surface [5]. Figure 1 presents the measured values of heat fluxes obtained currently in the region of the Selenga River delta in the central part of the Baikal Depression [6]. The major part of measurements is located in 13 profiles that cross the basin of the lake. The other part of measurements was made in the boreholes along the coast of the lake. Figure 1 shows the projection of the intrusion body to the Earth’s surface. According to [1, 2], it is an elongated fissure intrusion of mantle substance extended along the axis of Baikal. It goes through the entire part of the Earth’s crust from the Moho boundary to the basement of sedimentary deposits of the lake located here at a depth of approximately six kilometers. The width of the intrusion is 8‐10 km, and it intruded 2.7 mln. yr. ago.

GEOLOGICAL FACTORS AND PHYSICOCHEMICAL PROCESSES OF GROUNDWATER FORMATION IN THE TUNKA DEPRESSION

The article describes the complex hydrogeological conditions of the Baikal rift zone viewed as a large struc‐ tural element and pioneers in distinguishing two independent hydraulic systems in the study area. Groundwater re‐ sources and compositions of groundwater in these two systems are generated in fundamentally different ways. In the deep sediment layers, groundwater generated due to sedimentation is at the stage of elision (exfiltration) water ex‐ change. Active phase transition of clay minerals to hydromica causes an additional water release, and sedimentary water and regenerated groundwater infiltrate from the condensed clay strata to sandy horizons. This process is ac‐ companied by decompaction, heaving sand, and high (extra‐high) reservoir pressures. Nitrogen‐rich water and car‐ bonic thermal water associated with faults and fault nodes are widespread in the basement of the Tunka depression. The thermal water result from infiltration and, together with fresh water, represents a uniform hydraulic system. Its development is determined by the dynamics of infiltration water in the water‐feeding area in the Tunka loaches. At different hypsometric levels of the hydrogeological section, nitrogen‐rich water descends, while carbonic thermal water ascends, and these processes occur simultaneously. Our study is focused on the physicochemical processes of the interaction between groundwater and sedimentary and crystalline rocks. It shows that the ion‐salt and gas com‐ positions of not only nitrogen‐rich thermal water, but also those of methane water and carbonic thermal water occur in the ‘water‐rock’ system without involving any additional substance from external sources. Compared to other thermal water, the composition of carbonic water is formed in a more complex way: first, it goes through the stage of the nitrogen‐rich thermal water while passing through the aluminosilicate rocks and only then interacts with the car‐ bonate rocks and become carbonic. The formation of carbonic water is accompanied by intensive karst processes at depths, which are ceasing closer to the surface. As a result of degassing, an opposite process is activated: authigenic minerals and travertines are formed on the surface. Groundwater and its gas phase are involved in the formation of rocks with a negative temperature, which are abundant in the Tunka depression, as well as large positive forms of the relief. It is shown that the activity of groundwater is not limited to the role of a filler in the host rocks and an interme‐ diary medium between different geospheres. Groundwater is an active agent that initiates, controls and implements many geological processes.

Surface and internal energy of hydrocarbon gas bubbles as a factor of formation of gas deposits and related heat anomalies

It is shown that, during coalescence of bubbles, the mechanical energy of the surface tension transits to the heat energy sufficient to increase the temperature of the bed–reservoir by several tens of degrees. The positive heat anomalies and anomalously high formation pressure in the petroleum regions may be caused by energy released during the amalgamation of a small-disperse gas phase into economic deposits.

Thermocapillary marangoni convection as a mechanism of migration and accumulation of hydrocarbon gases in regions of thermal anomalies

It is known that fluid can move under the influence of volume or surface (capillary) forces. The buoyancy force is the most widely spread force among volume forces. It acts in a gravity field and appears each time when inclusions of various density appear in the fluid, for example, gas bubbles. Capillary forces are applied tangentially to the free or interface fluid surface. They appear when the gradient of surface tension exists. These forces are directed in the direction of its increase. The main cause of variation in the surface tension in natural processes is its temperature depen dence. The existence of a temperature gradient in a two phase medium, for example, in water with gas bubbles, initiates the development of a convective cur rent, which was called thermocapillary Marangoni convection named after the first scholar who studied this phenomenon [2, 3].