RESULTS OF COMPREHENSIVE STUDIES OF THE UNDERGROUND HYDROSPHERE WITHIN THE WESTERN SHOULDER OF THE BAIKAL RIFT (AS EXEMPLIFIED BY THE BAYANDAI – KRESTOVSKY CAPE SITE)

The subject of comprehensive studies is the underground hydrosphere of the upper crust of the western shoulder of the Baikal rift, being characterized by high tectonic activity in the recent stage of tectogenesis. The studies were focused on the Bayandai – Krestovsky Cape site, considering it as a benchmark for the territory of the Western Pribiakalie (Fig. 1). The available hydrogeological survey database is used to study underground waters circulating at depth of several kilometers. Analyses of deeper waters are conducted on the basis of geophysical data. According to results of initial geological and geophysical studies [Семинский и др., 2010], the crust at the junction of the Siberian crater and the SayanBaikal folded belt is characterized by a hierarchic zoneblock structure (Fig. 2). Regardless of the scale of studies, the territory under study can be divided into sections of two types, that alternate from NW to SE and represent wide highly destructed zones and relatively monolithic blocks of the crust. The Obruchev fault system is distinguished as the main interblock zone (the 2nd hierarchic level in the study area). It represents the 50 km long NW shoulder of the Baikal rift (the1st hierarchic level) and includes the Morskaya, Primorskaya and Prikhrebtovaya interplate zones (the 3rd hierarchic level). These zones are traced from depth of dozens of kilometers; at the surface, they are represented by fault structures of the highest hierarchic levels.Specific features of the current zoneblock divisibility of the crust serve as the structural basis for interpreting the materials obtained by hydrogeological studies conducted on the Bayandai – Krestovsky Cape site to research the distribution, mineralization and macrocomponent compositions of waters which represent the subsurface part of the underground lithosphere in the study area. The research is based on analyses of the underground water samples from 46 observation points scattered within the study area. The available amount of sample data can actually be considered as a complete base of standard hydrogeological data for the study area. The known specific features of hydrogeology of the Western Pribaikalie are confirmed by the general analysis of the available data. In general, fresh waters of subsurface origin of the midand submontane regions are dominant. In the anionic composition of the waters, hydrocarbon ion is predominant. Calcium and magnesium are abundant among basis ions, while potassium and sodium are rare. With the detail data on the Bayandai – Krestovsky Cape site, the above conclusions are more precisely determined, and it is generally concluded that the structural tectonic factor controls the hydrogeology of the region.The analysis of mineralization, composition, temperature and other characteristics of the underground waters (Fig. 3–6) show that at the low hierarchic level the NW boundary of the Obruchev interblock zone performs a function of a distinct hydrogeological barrier which represents the marginal suture of the Siberian platform. The marginal suture is active at the currently stage of tectogenesis. It separates lowmineralized and fresh waters of the platform block from fresh and ultrafresh waters of the mountainous area. The Predbaikalskaya, Prikhrebtovaya, Primorskaya and Morskaya interplate zones are distinguished within the limits of these structures. At the given hierarchic level, they control the positions of the corresponding anomalies in terms of water exchange modes, general mineralization and water cut ratios. The magnetotelluric sounding data (Fig. 2 and 7) suggest that the anomalies are subsurface manifestations of deep sections of conductivity; such sections have fluid origin in the region under study. Specific features of the structure of such anomalies are controlled by the presence of interblock zones of the fault level of the hierarchy. The interblock zones are composed of tectonic dislocations of various ranks. Depending on their specific internal structures, such dislocations can act as impermeable screens or channels for migration of fluids. Based on the joint interpretation of the results of the hydrogeological studies and the earlier geological and geophysical studies on the Bayandai – Krestovsky Cape site, a conceptual model showing specific features of fluid saturation of the upper crust of the western shoulder of the Baikal rift is developed (Fig. 8). Generally speaking, the underground hydrosphere of the Western Pribaikalie is a uniform system at the subsurface and deeper levels, and its structure and content are mainly determined by the active zoneblock structure of the crust in the region under study. Its nature is controlled by the development of the Baikal rift, which western shoulder (and also the adjacent platform region) is occupied by a hierarchy of subvertical zones in the upper crust that are anomalous in terms of permeability concerning underground waters. In the tectonically active segment of the region, these zones are larger than the blocks and thus cause more significant dislocations and higher watercut of the crust. The relationship between dislocations and watercut allows determining specific features of the zoneblock structure by regions by applying methods of hydrogeological surveys for studies of shallow horizons and geophysical methods, which are more sensitive to fluid contents, for studies of deeper layers. On the other side, geophysical and hydrogeological data (at least on tectonically active regions) should be interpreted with account to the concept that the fields under study depend on the structural status of the rock massif, rather than on the lithology