The results of reflection CMP seismic profiling of the Central Tien Shan in the meridional tract 75–76° E from Lake Song-Köl in Kyrgyzstan to the town of Kashgar in China are considered. The seismic section demonstrating complex heterogeneous structure of the Earth’s crust and reflecting its near-horizontal delamination with vertical and inclined zones of compositional and structural differentiation was constructed from processing of initial data of reflection CMP seismic profiling, earthquake converted-wave method (ECWM), and seismic tomography. The most important is the large zone of underthrusting of the Tarim Massif beneath the Tien Shan.

The implications of recent seismological and resistivity data for the geometry and orientations of neotectonic faults are generally consistent with the morphotectonic model of Gorny Altai as an area of crustal failure at the junction of two relatively stable blocks. The model predicts motions under general NW compression mainly on right-lateral strike-slip faults accompanied by systems of pinnate reverse and extensional faults. The locations and mechanisms of aftershocks that followed the 2003 Chuya earthquake (Gorny Altai) indicate long seismic activity generated by a neotectonic NW right-lateral strike-slip fault which separates the North Chuya and South Chuya ranges from the Kurai-Chuya system of intermontane basins. The plane of the northwestern termination of the active fault zone dips in the SE direction, beneath the ranges, at about 70°. MT data show two types of conductors that reach the surface, namely, nearly vertical zones along neotectonic faults between the blocks not involved into vertical motion, according to morphotectonic evidence, and inclined zones between the uplifted (subsided) blocks. We interpret the former as strike-slip faults and the latter as reverse or reverse oblique faults, which always dip beneath the uplifted blocks and record the general compressional setting.

At the Bishkek geodynamic test area in the Northern Tien Shan, complex electromagnetic monitoring was conducted, including a wide frequency range by magnetotelluric sounding in various modifications and induction sounding with a controlled impulse source (method of sounding in the far zone, SFZ). The geoelectrical sections of the lithosphere were constructed for the study area. Based on the geophysical monitoring data and the profile MT monitoring data on the Baitik depression, the main faults belonging to the Northern Tien Shan fault system were detected: Shamsi-Tyundyuk (Predkirgizsky), Baitik, Chonkurchak, and Issyk-Ata. From the geoelectrical data, we obtained new information that is independent of other geophysical methods used for studying the deep structure of the Earth - the hidden fault structures and the geoelectrical segmentation of the study area were revealed. The latter reflects the main elements of the block structure of the junction zone of the Chuya basin and the Kirghiz ridge. This information needs to be taken into account when constructing a comprehensive model showing the geological, geophysical and geodynamical features of the development of the Tien Shan, a vivid example of an intracontinental orogen. The MTS, FS and SFZ data from the Ak-Suu and Chon-Kurchak electromagnetic monitoring stations were interpreted. Based on the representative array of experimental electromagnetic data, we analyzed the effective depths of field penetration, which are most sensitive to changes in the electromagnetic parameters of the medium for the stationary and routine observation sites with respect to the clusters of seismic events. This paper discusses the development of the azimuth MT monitoring technique and the analysis of the time series of electromagnetic parameters, which were used for determining the contribution of each component of the impedance tensor to the informativeness of the monitoring studies. The data from the 2004-2016 KNET Catalogue of the Research Station of RAS were analyzed in order to investigate the relationship between the space-time seismicity distribution and the electrical conductivity variations that correspond to the features of the seismicity pattern at depth. Examples discussed are the time-frequency series of MT monitoring in 2007 and 2016 at the Ak-Suu and Chon-Kurchak stations that identified anomalous electromagnetic parameters corresponding to the fluid redistribution model for the pore-cracked medium: syn-phase decreasing and increasing values of the parameter by the orthogonal azimuths. Thus, a phenomenological model has been tested. It relates the change in the stress-strain state of the medium with the redistribution of fluids between the systems of fractures, which causes variations in the active and reactive components of the electrical resistance.

Magnetotelluric soundings (MTS) in the Kyrgyz Tien Shan along 74° and 76° E profiles reveal conductors in the crust which delineate the boundaries of the At-Bashi accretionary-collisional zone and the Issyk-Kul microcontinent. Correlated to earthquake converted-wave patterns (vP) along the MANAS profile collected in 2007, the geoelectric model for the At-Bashi zone lends support to the hypothesis that the position and dip of large thrust sheets, as well as the way and direction of exhumation of eclogites in this zone, are similar to those in Northwest China. Petrological analysis, geothermobarometry, and elastic P-wave velocities measured in laboratory on lower-crust and upper-mantle xenoliths indicate that at the time when the xenoliths were dragged to the surface about 70 Ma ago, the Moho was 20 km shallower than now (35 km against 55 km) and the heat flux was 20 mW/m2 higher (80 against 60 mW/m2).

High-density array MT soundings of the crust in the seismically active northern Tien Shan were performed using Phoenix MTU-5 stations in the Bishkek Geodynamic Polygon, at the junction of the Chu basin and the Kyrgyz Range. The MT transfer functions were determined to an accuracy of 1–2% (amplitude) and about 0.5–0.8 deg (phase) in most of 145 soundings. Preliminary analysis of the collected data aimed at estimating the geoelectrical dimensionality. The Bahr decomposition analysis indicated the presence of local 3D structures in the crust of the area superposed on the regional 2D structure.