Fig. 1. The layout chart of the magnetotelluric observation sites:

1 - site of sounding, 2 – site of long-period sounding, 3 - number of regional submeridian MT-profile, 4 – designation of sublatitudinal deep MT Sounding-profile, 5 - basic faults, 6 - frontier of Kyrgyzstan.

     The reliable two-dimensional geoelectric model of the deep structure of tectonically active region was constructed using the original methodical approaches in quantitative interpretation of these data. Basic elements of the model are the heterogeneous conducting layer in the middle bottom crust and the structures of low resistance in the upper part of the crust which are related to the zones of active faults at different levels. The zones of basic faults of the Tien Shan have proved to be electric structures both of low resistance (such faults as Liniya Nikolayeva, Central–Terskeysky and Atbashy-Inylcheksky), and of high resistance (the Talas-Fergana fault). The detailed geoelectric research in the area of the Talas-Fergana fault shows that the zone of the fault separates large (regional) blocks of the Earth's crust with different types of physical properties.

Fig. 2. Geoelectric model of submeridian MT-profile I-I (740 East longitude). In the upper part of the figure you can see the diagrams of modeling and observed values of tipper at the period of 1600 sec.

Fig. 3. Geoelectric model of submeridian MT-transect III-III Balkhash-Naryn-Kashgar (760 East longitude). In the upper part of the figure you can see the diagrams of modeling and observed values of tipper at the period of 1700 sec.

     The layer of high electroconductivity spatially correlates with the layer of low rates of longitudinal seismic waves according to the data of deep seismic sounding and method of inverse vertical seismic sounding. The analysis of allocation of the earthquake hypocenters in the Earth's crust of the central Tien Shan shows that the seismic centers are mainly concentrated above the horizon with low electrical resistivity and seismic rates. Thus, the upper boundary of a conducting layer constrains the depth of distribution of the earthquake hypocenters in the region.

 2.

Magnetotellurics has developed a new line of investigations which applies the method of "quasi-continuous" MT-profiling for the detailed study of the buried conducting structures in active thrust zones of faults of the Tien Shan. The pioneering results evidence the great potential and efficiency of the given method for the assessment of structural and kinematic characteristics of the "latent" geological objects.

Fig. 4. А) Structural geological cross-section along the line AB, by the data of A. Mikolaychuk and M. Gubrenko B) Geoelectric cross-section along the Issyk-Ata profile. Isolines show the lg of specific resistance in Ohm.m Circles show the distribution of the earthquakes hypocenters with magnitude > 5 for the period of 1968-2000 in a belt of 25 km wide along the line of MT-profile (the data provided by the Seismological Network of the Institute of Seismology, National Academy of Sciences of the Kyrgyz Republic).

 3.

     The obtained long-period MT-data evidence the asymmetric distribution of electric properties of the upper mantle not only in meridian, but also in latitudinal direction. The first results of the analysis have shown the immersion of the surface of electric asthenosphere to the east and to the west from 760 East longitude, and also the sharp change in conductivity in the Talas-Fergana fault. Model tests estimate the resolution of MT-inversion about 100 km regarding the horizontal size of three-dimensional conducting structure for the depths of upper mantle of the Tien Shan. Thus, deep MT-research evidences a much more complicated, than it was thought before, structure of electroconductivity of the upper mantle of the Tien Shan, which, in fact, has an essentially three-dimensional distribution.

 4.

     There are two basic hypotheses chosen to explain the nature of revealed conducting structures of different scales: fluidization of the porous fractured spaces and the development of a young (in geological understanding) mantle process, which has caused the melting of the upper mantle substance and its lifting in the bowels of the Tien Shan.