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The paper presents a geodynamic interpretation of the deep structure and active tectonics of the northern Tien Shan, with particular emphasis on strike-slip motions, which produced a pull-apart in the centre of the Issyk-Kul basin. The study is based on a detailed interpretation of satellite imagery, fault plane solutions of earthquakes, seismic, and geodetic data.Seismic and magnetotelluric studies show tectonic layering of the Tien Shan lithosphere, with several nearly horizontal viscoelastic layers and the lower layer underthrust northward in the northern Tien Shan. This active process may be responsible for the intricate present-day tectonic framework of the northern Tien Shan.The recent tectonics of the northern Tien Shan inherits the earlier structure: The lens-shaped Issyk-Kul microcontinent comprising Precambrian-Palaeozoic metamorphic and magmatic rocks is surrounded by thick shear zones which have been involved in the activity over most of the Cenozoic. In the Quaternary the strain propagated as far as the central part of the Issyk-Kul basin.

The paper presents new data on geochemistry (major and trace elements, isotopes) and petrology (composition of phenocrysts and mineral thermometry) of basalts from the Early Cambrian Katun’ accretionary complex of the Kuznetsk–Altai island arc (Gorny Altai). We also discuss the geodynamic settings of the formation of basalts based on petrologic, geochemical and isotope data taking into account their relationships with associated sedimentary rocks of oceanic origin and with terrigenous sedimentary rocks of the accretionary complex. The Late Neoproterozoic basalts associated with siliceous sediments are characterized by medium TiO2 and Zr/Nb, flat REE patterns and Nb/Lapm < 1 (MORB-type oceanic floor basalts). The oceanic floor basalts are represented by high- and low-magnesium varieties. The Early Cambrian basalts (Manzherok Formation) were formed in an oceanic island setting (OIB-type). They occur in sections consisting of paleo-oceanic island basalts alternated with siliceous-terrigenous-carbonate sediments of slope facies. The Manzherok basalts are characterized by high crystallization temperatures (1175 °C), and enriched incompatible elements (LREE, Ti, Nb), whose variations suggest a mantle plume source and variable degrees of partial melting in the spinel and garnet stability fields. The low to high-Mg Middle Cambrian basalts of the Ust’-Sema Formation occur as subparallel dikes and lava flows, which cut and overlap the accretionary complex. The basalts are characterized by low TiO2, Nb, LREE, high Zr/Nb and, compared to the OIB-type basalts, lower temperatures of crystallization, 1047 °C and 1138 °C for the low- and high-Mg varieties, respectively. Geochemically, the low-Mg basalts of the Ust’-Sema Formation are close to MORB, and the high-Mg basalts—to island-arc tholeiites. Such a combination of geochemical and geological data suggest that the low-Mg basalts of the Ust’-Sema Formation formed during ridge subduction, and the high-Mg basalts formed in a suprasubduction setting, at the crust-mantle boundary.

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