Introduction. Determinations of (234 U/ 238 U) in groundwater samples are used for monitoring current deformations in active faults (parentheses denote activity ratio units). The cyclic equilibrium of activity ratio 234 U/ 238 U≈ ≈(234 U/ 238 U)≈γ≈1 corresponds to the atomic ratio ≈5.47×10-5. This parameter may vary due to higher contents of 234 U nuclide in groundwater as a result of rock deformation. This effect discovered by P.I.
The southern deformed edge of the Siberian paleocontinent is studied for zoning the disturbance of 234 U/ 238 U cyclic equilibrium in groundwaters discharged to the accreted Khamardaban terrane. A relationship is established between vertical variations of 234 U/ 238 U (ОА4/8) activity ratios and the hydrogeochemical zoning of groundwaters. Lateral ОА4/8 zoning of groundwaters in the basement rocks and sedimentary cover of the Siberian platform is described. It is emphasized that an important role is played by the water propagating from the South Baikal reservoir (SBR) (ОА4/8 = 1.95-1.99; U content = 0.44-0.46 μg/L) under the shore of Lake Baikal. It is suggested that the lateral advancement of the SBR water is facilitated by the development of low-angle (weakened by rifting) fractures of the Angara thrust fault, as well as subvertical fractures in the shear zone of the Main Sayan Fault. Monitoring data on the study area show that the ratios of the groundwaters components vary in time due to Cherdyntsev-Chalov deformational effect, chemical interaction of the waters and evaporites, and mixing of the groundwaters with contrasting hydrogeochemical signatures.
High 234U/238U activity ratio (AR4/8), identified in groundwater from the Elovka-Kultuk and Nilovka-Mondy inversion sections of the Tunka Valley, coincide with areas of earthquake concentrations. In order to substantiate an approach to earthquake prediction, spatial variations of this parameter were determined in natural water at the western termination of the valley and its temporal variations were monitored in water from the Mon-D well in the Mondy basin in 2013–2017. A recorded gradual decrease of AR4/8 values in water of this well, coeval with preparation and implementation of an earthquake with the energy class K=13.9 in the north of lake Khövsgöl, reflected crack closure that prevented deep water penetration in the Tumelik-Mondy aseismic zone. At the eastern termination of the valley, near the Kultuk village, decreasing AR4/8 values in groundwater were followed with their sharp increasing and transition to low-amplitude variations. Accordingly, crack closing was followed with their opening that facilitated the circulation of deep water and provided seismic events. Leveling of the Mondy anomaly in the Tumelik-Mondy aseismic zone, accompanied by a continuous activity of the Turan and Nilovka anomalies within the Nilovka-Mondy section, emphasized a specific role of the latter as an intermediate chain between the Khövsgöl segment of radial rifts, originated in front of the Hangay orogen, and the largest central basin of the Tunka Valley. The Kultuk, Zaktuy, and North-Tory AR4/8 and earthquake anomalies denoted boundaries of the Elovka-Kultuk section in the Khamardaban lithospheric block flattened at the edge of the Siberian platform basement.
Devonian dikes of the Urik-Belaya and Shagayte-Gol-Urik zones and Miocene lavas of the Urik volcanic field are spatially associated with each other at the structural junction between the Neoproterozoic Tuva-Mongolian massif and Siberian craton. The former dike belt is represented by basalts and basaltic andesites of tholeiitic series and the latter one by trachybasalts, trachyandesitic basalts of moderately alkaline series and trachybasalts, phonotephrites of highly alkaline one. The Urik volcanic field is composed of trachybasalts and trachyandesitic basalts of moderately alkaline series. A partial similarity between magmatic series of different age is found in terms of major oxides, trace elements, and Sr, Pb isotopes. The common component corrected for age was defined through its converging mixing trends with those of the lithospheric mantle and crust. The component identification was a basis for deciphering the nature of isotopic and geochemical heterogeneity of evolved magmatic sources. It was inferred that the common component characterizes either a modified (depleted) reservoir of the lower mantle or, more likely, a local region of the convecting asthenospheric mantle that underlies the Tuva-Mongolian massif. The latter interpretation assumes the formation of a locally convecting asthenosphere in the middle Neoproterozoic, along with the development of the Oka zone at the massif, and puts constrains on later sufficient processing of the asthenosphere due to rising plumes or subducting slabs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.