Tab. 2: Gravity measurements of the Inntal underground at Thaur

A cross-section of the Inn-valley has been surveyed by refraction- and refiection-seismic and gravimetrie methods. The thickness of the Inn-va.!ley sediments is 340- 390 m. At the northern edge of the valley an intermediate layer between sediments and basement has been detected, which is up to 300 ITl thick. This zone seems to mark the boundary of the northern calcareous alps.

137Cs and 90Sr depositions, and 137Cs/90Sr ratios of samples from the Kraton-3 underground nuclear explosion site, Russia

Strontium-90 activity concentrations in surface soils and areal deposition densities have been studied at a site contaminated by an accidental release to atmosphere from the underground nuclear explosion 'Kraton-3' conducted near the Polar Circle (65.9°N, 112.3°E) within the territory of the former USSR in 1978. In 2001-2002, the ground surface contamination at 14 plots studied ranged from 20 to 15000 kBq/m**2, which significantly exceeds the value of 0.44 kBq/m**2 deduced for three background plots. The zone with substantial radiostrontium contamination extends, at least, 2.5 km in a north-easterly direction from the borehole. The average 137Cs/90Sr ratio in the ground contamination originated from the 'Kraton-3' fallout was estimated to be 0.55, which is significantly different from the ratio of 2.05 evaluated for background plots contaminated mostly from global fallout. Although vertical migration of 90Sr in all undisturbed soil profiles studied is more rapid than that for 137Cs, the depth of percolation of both radionuclides into the ground is mostly limited to the top 10-20 cm, which may be explained, primarily, by permafrost conditions. The horizontal migration rate of radiostrontium in the aqueous phase exceeds the radiocaesium migration rate by many times. This phenomenon seems to be a reason for the significant enrichment of the soil surface layers by radiostrontium at some sites, with variations occurring in accordance with small-scale irregularities of landscape.

(Table 1) Isotopic and geochemical parameters of gas phase from underground fluids of the Baikal rift and its framing

Results of studying isotopic composition of helium in underground fluids of the Baikal-Mongolian region during the last quarter of XX century are summarized. Determinations of 3He/4He ratio in 139 samples of gas phase from fluids, collected at 104 points of the Baikal rift zone and adjacent structures are given. 3He/4He values lie within the range from 1x10**-8 (typical for crustal radiogenic helium) to 1.1x10**-5 (close to typical MORB reservoir). Repeated sampling in some points during more than 20 years showed stability of helium isotopic composition in time in each of them at any level of 3He/4He values. There is no systematic differences of 3He/4He in samples from surface water sources and deeper intervals of boreholes in the same areas. Universal relationship between isotopic composition of helium and general composition of gas phase is absent either, but the minimum 3He/4He values occurred in methane gas of hydrocarbon deposits, whereas in nitrogen and carbon dioxide gases of helium composition varied (in the latter maximum 3He/4He values have been measured). According to N2/Ar_atm ratio nitrogen gases are atmospheric. In carbonic gas fN2/fNe ratio indicates presence of excessive (non-atmogenic) nitrogen, but the attitude CO2/3He differs from one in MORB. Comparison of helium isotopic composition with its concentration and composition of the main components of gas phase from fluids shows that it is formed under influence of fractionation of components with different solubility in the gas-water system and generation/consumption of reactive gases in the crust. Structural and tectonic elements of the region differ from the spectrum of 3He/4He values. At the pre-Riphean Siberian Platform the mean 3He/4He = (3.6+/-0.9)x10**- 8 is very close to radiogenic one. In the Paleozoic crust of Khangay 3He/4He = (16.3+/-4.6)x10**-8, and the most probable estimate is (12.3+/-2.9)x10**-8. In structures of the eastern flank of the Baikal rift zone (Khentei, Dauria) affected by the Mz-Kz activization 3He/4He values range from 4.4x10**-8 to 2.14x10**-6 (average 0.94x10**-6). Distribution of 3He/4He values across the strike of the Baikal rift zone indicates advective heat transfer from the mantle not only in the rift zone, but also much further to the east. In fluids of the Baikal rift zone range of 3He/4He values is the widest: from 4x10**-8 to 1.1x10**-5. Their variations along the strike of the rift zone are clearly patterned, namely, decrease of 3He/4He values in both directions from the Tunka depression. Accompanied by decrease in density of conductive heat flow and in size of rift basins, this trend indicates decrease in intensity of advective heat transfer from the mantle to peripheral segments of the rift zone. Comparing this trend with data on other continental rift zones and mid-ocean ridges leads to the conclusion about fundamental differences in mechanisms of interaction between the crust and the mantle in these environments.