Tunka Rift Research Articles

Activity rating of Pliocene–Quaternary faults: a formalized approach ( example of the Baikal rift system)

It is suggested to estimate the Pliocene–Quaternary fault activity in a formalized way from synthesis of different data. The respective database consists of two main sections: (i) general information and basic fault parameters and (ii) geomorphic, structural, paleoseismic, seismological, geophysical, geodetic, engineering-geological, hydrological, and meteorological data. The fault characteristics are scored according to their significance, and the cumulative score measures the fault activity. With this approach, the faults in the Barguzin and Tunka rift basins and in the northeastern flank of the Baikal rift system have been divided into five activity classes (low, medium, relatively high, high, and very high activity) and mapped correspondingly. It has been recommended that the concept of an hazardous fault, as updated with regard to the activity rating, refers to faults of relatively high, high, and very high activity. Thus identified hazardous faults within the study area are quite few (4–8%), though this percent may increase slightly as more input data become available. The underestimation cannot be dramatic because all known seismological and structural characteristics of faults essential for the activity rating have been already taken into account. The new approach may be useful in seismic risk assessment and in choice of sites for instrumental monitoring of seismicity.

Relief structure of the Tunka rift system (Lake Baikal region)

Relief structure of the Tunka rift system (Lake Baikal region)

Slavin Yar: a new Upper Cenozoic reference section in the Tunka rift valley (Southwestern Baikal region)

The biostratigraphic study of a new Upper Cenozoic reference section in the Tunka rift valley (southwestern Baikal region) accompanied by radiocarbon measurements made it possible to date its lithological units. It is established that the section is largely composed of Upper Pleistocene fluvial sediments resting with distinct angular unconformity uapon Pliocene conglomerates. The revealed structural features of the section confirm the views that the directed development of the Tunka depressions was complicated by local inversions, when the sedimentation area became reduced. The main sedimentation features during the Late Cenozoic and its stages are reconstructed for the studied area.

Conditions of formation of ore-geochemical specialization of modern hydrotherms in the Baikal Rift Zone

Modern hydrotherms of the Baikal Rift Zone (BRZ) are divided according to contents of ore and rare elements and gas composition into: (1) carbonaceous, forming in carbonate rocks, (2) hydrosulfuric and H 2S-free nitric and methane, discharged in Cenozoic deposits, and (3) nitric, produced in granitoids. Carbonaceous hydrotherms are enriched in Sr, Ba, Y, Co, and Cr; nitric ones, in Li, Rb, Cs, Mo, W, and other granitophile elements; and hydrosulfuric hydrotherms are saturated with complex metals. High contents of U and associated elements are specific for oxidized nitric alkaline thermal waters and are atypical of carbonaceous ones, and high contents of Au are observed in methane hydrotherms. Separation of ore elements between organic and clayey matter takes place as early as the initial stage of sediment formation. During sediment accumulation, these elements pass into hydrothermal solution in the same separated associations. In oxidizing (Eh ranges from +50 to +260 mV) alkaline medium, U and, obviously, other elements of uranium association, present not only in granitoids but also in the organic matter of sediments, pass into the solution most actively. Alkaline medium (7 < pH < 10) favors the transition of Mo and W from igneous rocks and sediments into the solution. Methane hydrotherms (8 < pH < 9, Eh < –70 mV) can accumulate gold. In strongly reducing (Eh ranges from –200 to –360 mV) alkaline (pH > 8) hydrosulfuric anoxic medium, chalcophile complex metals of the clayey matter of sediments are the most active to pass into the solution. Thus, as Eh decreases and H 2S concentration increases, hydrothermal solution extracts various element associations from igneous and sedimentary deposits in the sequence U–Mo, W–Au–Sn, Cu, Pb. In the solution the differentiation of elements continues. Hence, the separation of ore-element associations at the initial stage of sediment formation, long before the hydrothermal activity, is the first track in a chain of processes leading to the differentiation of ore deposits according to formational types. Hydrotherms confined to deep-fault zones contain an endogenous fluid with mantle 3He/ 4He values close to those in methane and carbonaceous hydrotherms of the Tunka rift depression in the BRZ. According to results of thermodynamic modeling, the fluid consists of CH 4, H 2O, and CO 2 and obviously lacks mantle ore elements, as it was initially of hydrocarbon composition.

Variations of stress fields in the Tunka Rift of the southwestern Baikal region

The stress fields in the Tunka Rift at the southwestern flank of the Baikal Rift Zone are reconstructed and analyzed on the basis of a detailed study of fracturing. The variation of these fields is of a systematic character and is caused by a complex morphological and fault-block structure of the studied territory. The rift was formed under conditions of oblique (relative to its axis) regional NW-SE extension against the background of three ancient tectonic boundaries (Sayan, Baikal, and Tuva-Mongolian) oriented in different directions. Such a geological history resulted in the development of several en echelon arranged local basins and interbasinal uplifted blocks, the strike-slip component of faulting, and the mosaic distribution of various stress fields with variable orientation of their principal vectors. The opening of basins was promoted by stress fields of a lower hierarchical rank with a near-meridional tension axis. The stress field in the western Tunka Rift near the Mondy and Turan basins is substantially complicated because the transform movements, which are responsible for the opening of the N-S-trending rift basins in Mongolia, become important as Lake Hovsgol is approached. It is concluded that, for the most part, the Tunka Rift has not undergone multistage variation of its stress state since the Oligocene, the exception being a compression phase in the late Miocene and early Pliocene, which could be related to continental collision of the Eurasian and Indian plates. Later on, the Tunka Rift continued its tectonic evolution in the transtensional regime.

Rockfall-Dammed lakes in the Baikal region : Evidence from the past and prospects for the future

The mountain province of East Siberia, which includes the Baikal Rift system, is a zone of high tectonic and seismic hazard. Earthquakes and coseismic faulting are dangerous not only by themselves but also as far as they initiate rock collapse and downslope movement of unconsolidated deposits, which may block river valleys and produce rockfall-dammed lakes. Within some rifts of the rift system, evidence of past dammed lakes was discovered that arose instantly, in a geological sense, and flooded large areas of forest. In mountains around some rift basins, small living dammed lakes were encountered, as well as traces of catastrophic debris flows that may have accompanied breaching of earlier collapse-produced dams. Analysis of geomorphological setting in the region, especially in the Muya Rift Basin, revealed conditions favourable to hazardous origination of rockfall-dammed lakes. A large dammed lake may come into existence due to the collapse of bedrock over the narrow antecedent valley of Vitim in the Muya Rift. Preliminary estimates based upon data on the Vitim River discharge showed that the lake might form in as short as 27 days, though the rapidity of its formation, and hence the degree of the risk, can vary as a function of the highly variable amount of summer discharge of the river. Rockfall-dammed lakes may also originate in the floors of Chara and Tunka Rift Basins. Due to their rapid formation, lakes will bring about extensive flooding and cause danger to the taiga, railways and constructions in this populated developing area, and will cause degradation of the permafrost.

Helium Isotopic Data for the Ground Waters in the Baikal Rift Zone

Abstract The paper considers regularities of distribution and concentration of total helium and its isotopes in the solute gas in the groundwaters of the Baikal rift zone. The 3He/4He ratios obtained for 31 sites showed geographical variability throughout the Baikal rift zone, a variation according to types of groundwaters (methane, carbon dioxide or nitric hydrotherms, and cold fresh waters) and certain relationship with total concentration of helium. In the south-western termination of the Baikal rift zone, high 3He/4He ratios align along the rift axis in an area where mantleborn helium is brought by ascending flows in hot and cold springs, with its maximum percentage (above 50%) within the Tunka rift basin which is one of few volcanic provinces in the rift. The highest contributions of mantle-born helium are attributed to methane and carbon dioxide hydrotherms, and to cold fresh-water springs at the boundaries of the high 3He/4He area. The groundwaters throughout the rest of the Baikal rift zone contain mainly radiogenic (crust-born) helium where the mantle component contributes under 10%, that is consistent with the hypothesis of the infiltrational origin of deep groundwaters. Outside the high 3He/4He area the isotopic ratio is higher in sulphate nitric hydrotherms than in those of carbonate chemistry. The 3He/4He ratios decrease at the boundaries of the rift zone with the Siberian platform and the Trans-Baikal fold area. The concentrations of mantle-born helium in the Baikal rift zone was compared to that in the Eger rift (West Europe).