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The Kokchetav Complex is a tectonic mega‐melange consisting of seven pre‐Ordovician units (units I‐VII) of contrasting lithologies and P–T conditions of metamorphism, overlain and/or intruded by four post‐recrystallization entities. Most of the constituent rock types display affinities with continental crust; paraschists and paragneisses, which carry biogenically produced carbon, clearly were laid down near the surface of the Earth. Microdiamond (and rare coesite) inclusions are contained in strong, refractory garnet, zircon, clinopyroxene, and kyanite, some of the constituent neoblastic phases of this metasedimentary unit. Systematic mineral parageneses and textural relationships support the hypothesis that the metamorphic assemblages represent a close approach to chemical equilibrium at the time of formation. Metamorphism of diamond‐bearing paragneisses and schists transpired at 535 ± 5 Ma; physical conditions included minimum pressures of 40 kbar and temperatures exceeding 900 °C. Other associated units contain mineralogic evidence of somewhat lower to considerably lower pressures and temperatures: observed magnesite + diopside pairs, coesite, grossular‐pyropic garnet, potassic clinopyroxene, Si‐rich phengite, barroisite‐crossite(?), aluminous titanite and/or Al‐rutile, and the assemblage talc + kyanite + garnet all testify to relatively elevated pressures of formation. The metamorphosed lithotectonic units represent individual, discrete stages in what initially may have been a continuous P‐T series, but intense post‐metamorphic dislocation has resulted in the preservation of a chaotically mixed sequence rather than an unbroken gradation in preserved conditions of metamorphism. Only units I‐III, and probably VIb may represent portions of a dismembered subduction zone lithologie assemblage. The uplift to mid‐crustal levels and cooling of the mega‐melange took place by about 515–517 Ma, at which time the complex was stabilized as a part of the Kazakhstan microcontinental collage. An hypothesized Late Vendian‐Early Cambrian subduction of the Kazakhstan‐North Tianshan(?) microcontinental salient to depths exceeding 125 km, followed by decoupling from the descending oceanic crust‐capped lithospheric plate is held responsible for the ultrahigh‐pressure metamorphism of the Kokchetav Complex. Inasmuch as vestiges of a calc‐alkaline volcanic/plutonic arc of approximately Early Cambrian age are preserved as only scattered relics in the general region, the plate‐tectonic setting may have involved an intra‐oceanic, Marianas‐type, incipient arc which was subsequently removed through transform faulting or erosion.

The Paleo-Asian ocean is defined by units located between the Russian (East European), Siberian, Tarim, and Sino-Korean (North China) continents. The study of the composition, age, and structural position of island-arc magmatic rocks, ophiolites, and high-pressure metamorphic assemblages and their mutual correlations made it possible to identify similarities and differences in the evolution of the Paleo-Asian and Paleo-Pacific oceans. The initial stage of the evolution of the Paleo-Asian ocean defined its opening at 900 Ma, whereas opening of the PaleoPacific took place at 750 to 700 Ma. Closing of the Paleo-Asian ocean in the Carboniferous (NE branch) and the Permian corresponds to the main stage of reorganization and re-opening of the Paleo-Pacific.The maximal opening of the Paleo-Asian ocean occurred after or simultaneously with the first accretion-collision event at 600 to 700 Ma, resulting from the collision of microcontinents and the Siberian continent. Vendian-Early Cambrian boninite-bearing island-arc complexes occur as lavas, sheeted dikes, and sill-dikes associated with gabbro-pyroxenites and ultramafics. These complexes are widely distributed in the Gornyy Altay, East Sayan, and West Mongolian regions and can be considered fragments of a giant boninite-bearing belt.In the late Early Cambrian, collision of seamounts with an island arc caused the squeezing of the subduction zone and return flows within the accretionary wedge. Serpentinite melange within fragments of ophiolites and high-pressure rocks are typical components of the Late Paleozoic accretionary wedges. Because of Middle Cambrian-Early Ordovician collisional events, two new oceans (Junggar-Irtysh-Kazakhstan and Uralian-South Tien Shan-South Mongolian) were formed. The junction of both oceans in East Mongolia opened to the PaleoPacific.

A 3-D model of the seismic heterogeneities of P- and S-velocities has been constructed down to 1100 km beneath the Kurile–Kamchatka and Aleutian subduction zones on the basis of the regional tomographic inversion of data from global seismic catalogs. Particular attention is paid to verifying the data by different tests. A clear image of a classic subducting oceanic slab is observed along the entire Kurile–Kamchatka arc, which coincides in the P- and S-models and with the distribution of deep seismicity. These data served as a basis for a parametric model of the upper and lower slab boundaries beneath the Kurile–Kamchatka arc. According to this model, the slab has various thicknesses and maximum penetration depths in different arc segments. In the southern part of the arc, between depths of 600 and 700 km, the slab moves horizontally and does not penetrate the lower mantle. Beneath the North Kuriles and southern Kamchatka, it subducts down to 900 km. These data suggest that the subducting slab becomes a viscous and nonelastic body and the changes in its shape may be due to phase transitions with increasing temperature and pressure. We attribute its gentler dipping and thickening beneath the South Kuriles to the oceanic “pushing” mechanism. The lithospheric thinning, steeper subsidence, and penetration into the lower mantle beneath the North Kuriles are due to the predominant “gravity sinking,” or “slab pull,” mechanism. Unlike some other researchers, we have obtained a high-velocity anomaly beneath the western Aleutian arc (not as clear as beneath the Kurile–Kamchatka arc, yet quite reliable). It suggests the presence of a slab subducting down to 200–250 km. In the eastern Aleutian arc, we clearly observe the Pacific slab subducting down to 500–600 km (somewhat deeper than in the previous studies).

We present seismic images of the mantle beneath East Russia and adjacent regions and discuss geodynamic implications. Our mantle tomography shows that the subducting Pacific slab becomes stagnant in the mantle transition zone under Western Alaska, Bering Sea, Sea of Okhotsk, Japan Sea, and Northeast Asia. Many intraplate volcanoes exist in these areas, which are located above the low-velocity zones in the upper mantle above the stagnant slab, suggesting that the intraplate volcanoes are related to the dynamic processes in the big mantle wedge above the stagnant slab and the deep slab dehydration. Teleseismic tomography revealed a low-velocity zone extending down to 660 km depth beneath the Baikal rift zone, which may represent a mantle plume. The bottom depths of the Wadati–Benioff deep seismic zone and the Pacific slab itself become shallower toward the north under Kamchatka Peninsula, and the slab disappears under the northernmost Kamchatka. The slab loss is considered to be caused by the friction between the slab and the surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then the slab loss was enlarged by the slab-edge pinch-off by the hot asthenospheric flow and the presence of Meiji seamounts.

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