Geodynamic and seismic activation in the southeastern part of Far Eastern Russia is associated with nucleation of strain waves in the buffer zone of the Amurian Plate when it moves northeastward. The highest crustal seismicity is typical of Lower Priamurye, where a zone of М ≥ 4.5 earthquakes originated in its central part that was related to reactivation of the Bureya Orogen. The maximum of the regional compression is confined to the meridional dome, under which the depth of the Moho discontinuity increases from 36 to 40 km. The area of lower rock density in the central part of the Bureya Orogen is exhibited by an intense negative gravity anomaly. A contemporary uplift of the orogen initiated the centrifugal migration of seismicity during 1983–2009, when epicenters of М ≥ 5.0 earthquakes migrated from the center of the dome to its periphery. Strong earthquakes (М ≥ 5.0) are also concentrated south of the Bureya Orogen in the transregional Tan-Lu Fault zone, where the earthquake foci migrate in the northeastern direction. Geodynamic activation is controlled here by the tangential rotation (pressure) of the Amurian Plate, forcing the Central Sikhote-Alin tectonic block to move in the southern direction. North of the block, a 500 × 950 km crustal plate is formed due to extension, which is inclined along the Tan-Lu Fault. In the compression area, a mountain uplift of the same name is formed at the opposite boundary of the Central Sikhote-Alin block. Being rotated clockwise, the block interacts with the subduction zone of the Pacific Plate at a depth of 400–600 km, where deep-focus earthquakes with М ≥ 6.0 are generated due to bilateral pressure.

The paper presents the results from a study of original and published data on the chemical composition and age of mantle peridotites from Sakhalin Island ophiolites. The material and genetic proximity of peridotites from the Berezovsky and Shelting plutons, on the one hand, and mélange zone serpentinites, on the other, have been established. In composition and Fe2O3 and MgO variations, Sakhalin peridotites differ radically from those of the Northeast Asia ophiolite complexes (Krasnogorsky Massif, Karaginsky Island Massif, etc.), which are fragments of Pacific Plate mantle. Conversely, Sakhalin peridotites have a subcontinental genesis and are compositionally close to xenoliths of lherzolites from Hankai Microcontinent mantle (southern Sikhote-Alin). The rythmics of alternation in compression and expansion at the margin of the Asian continent in the last 180 million years, caused by cyclical changes in the Pacific spreading rate, have been considered. According to data obtained by U-Pb dating of zircons, the formation of the Berezovsky Massif took place 169–154 Ma ago during Jurassic expansion of the continental margin. Matching age and composition data demonstrate that the Sakhalin ophiolites formed within the marginal sea basin during riftinduced destruction of the periphery of the Hankai Craton. The assumed tectonic setting was close to that reconstructed for the Jurassic Josephine ophiolites of the California margin of the North American continent. The continental genesis of the studied ophiolites agrees with the age and tectonic mode of ophiolite formation for Sakhalin Island.

The Bureya orogen is a special object among the geodynamic factors determining the high seismicity of the Lower Amur region. Its location and deep structure are studied on the basis of comprehensive geophysical and tectonic data. This orogen is a low-density lithospheric domain expressed by an intensive negative gravity anomaly and Moho sunken down to 40 km depth. Within the limits of this lithospheric structure, contemporary uplifting takes place to form a meridional dome peaking at more than 2000 m altitude. The position of the orogen in the regional structure gives us grounds to think that the Bureya orogen formed in the Paleogene, at the finishing stage of tectonic block movement along the Pacific margin represented by the NE-trending strike-slip faults of the Tang Lu Fault Zone. Compression was concentrated at the triple junction between the Central Asian, Mongolian–Okhotian, and Sikhote Alin tectonic belts. The meridional orientation of the Bureya orogen is associated with the parallel elongated Cenozoic depressions in the region. The united morphotectonic system may have formed resulting from lithospheric folding under horizontal shortening in the Paleocene–Eocene. The wavelength of the Lower Amurian fold system is 250 km, which is consistent with the theoretical estimates and examples of lithospheric folds in other regions. The contemporary activation of the Bureya orogen began in the Miocene, under the effect of the Amurian Plate front moving in the northeastern direction. As a result of shortening, the meridional cluster of weak (M ≥ 2.0) earthquakes formed along the western boundary of the orogenic dome. The most intensive deformations caused another type of seismicity associated with the activation-related uplift of the mentioned orogen. As a result, the so-called Bureya seismic zone formed above the apex of the dome, and it is here that the strongest regional earthquakes (M ≥ 4.5) occur.

The compositional and isotope–geochemical features of zircons from wehrlite of the Feklistov massif, which formed platinum coastal placers, are discussed in this paper for the first time. Zircons from wehrlite of the Feklistov massif, similarly to worldwide zoned clinopyroxenite–dunite massifs, are characterized by different morphology, composition and a wide spectrum of ages (from 2.717 to 0.373 Ga). The Late Devonian age (373.2 ± 7.5 Ma) of zircons allows us to characterize the timing of the formation of wehrlite from the Feklistov massif and to correlate its emplacement with a significant superplume event, which covered the Siberia and Laurussia continents. The geological meaning of this dating refers to limiting the lower age boundary for emplacement of the Feklistov clinopyroxenite–dunite massif into the Earth’s crust, which does not contradict geological observations.

In Turonian/Coniacian time a major plate reorganization occurred in Pacific Ocean that resulted in synchronous climax of compression in Eastern Asia. At this time the spreading rate in the Farallon-Pacific-Izanagi center increased to its maximum, the Kula plate replaced the Izanagi plate, and the convergence angle between the Pacific and Eurasia plates experienced fast rotation. This geodynamic culmination was caused by the peak of overall extension of Pacific lithosphere which could be precisely fixed at similar to 89-87 Ma, based on the age/spatial patterns of the Cretaceous seamounts. The maximum of Pacific stretching induced the coeval westward compression both along subduction boundaries and in intraplate environment of Eastern Asia. We summarize and reevaluate the reliable geochronological data of the main Late Cretaceous geological events in Eastern Asia caused by the peak of Pacific dynamics. Increasing westward subduction and compression produced: (a) maximum of high-pressure metamorphism in the Cretaceous accretion units of Taiwan, Japan and Sakhalin islands, (b) immense subduction related volcanic belts extending along the northeast Asian boundary, (c) vast volumes of granitoids which were emplaced during the 95-85 Ma interval everywhere from Chukotka to Sikhote-Alin in Russia, throughout Japan, Korea and SE China. Thermochronological data suggest that (d) the Pacific-induced compression triggered uplift, exhumation and cooling of East China granites with the peak age of orogeny in Great Xing'an Range at 89-87 Ma. Simultaneously, as thermal history results revealed, (e) subsidence and burial heating in Cretaceous sedimentary basins inverted to cooling. This compressional peak is also well documented by (f) the similar to 88-86 Ma Qingshankou/Yaojia unconformity in the postrift successions of the lacustrine Songliao Basin. Seismostratigraphy study of the main (T11) unconformity shows huge, gentle folds whose apices were truncated by exposure and denudation caused by the westward compression. Thus the Pacific-induced deformation at similar to 87-89 Ma encompassed the whole eastern Asia from subduction boundary into the hinterland. (C) 2015 Elsevier Ltd. All rights reserved.

The T11 unconformity lies between the Qingshankou and Yaojia Formations in the post-rifting sequence of the Cretaceous Songliao Basin, NE China. It is intimately associated with petroleum reservoirs and considered to be a disconformity forming in the tectonic quiet stage. We present the interpretations from new seismic surveys and cored sections of the Cretaceous Continental Scientific Drilling borehole (CCSD-SK-1) in order to resolve the nature and origin of T11 unconformity. The T11 is often a low-angle unconformity with underlying Qingshankou Formation having been deformed and eroded prior to deposition of the Yaojia Formation. In the post-rift evolution of the basin it marks an abrupt change from a deep lake to shallow lake or subaerial environment, documented by reddening of the lacustrine mudstone, extinction of the ostracod assemblages and a great increase of coarse detrital inputs. The sharp change of depositional environment, the truncation of gentle folds and the cluster of volcanic and paleoearthquake activities, all happened simultaneously, immediately before the development of T11 unconformity, indicating a significant regional compressional uplift event in the basin. The timing of the T11 unconformity formation is within the interval 88–86.2Ma. Correlations with coeval unconformities in other Cretaceous sedimentary basins in eastern Asia indicate that this compressional uplift coincided with an episode of global plate reorganization between the Eurasian and Paleo-Pacific plates that culminated at 88–87Ma. During this short interval the northeast Asian margin, in eastern China, South Korea, Japan and Russian Far East experienced widespread violent volcanic and granite emplacement activity triggered by compression resulting from rapid and orthogonal slab subduction. The post-rift basin tectonic inversion occurred during T11 (Coniacian) time; thereafter the basin again evolved in an extension regime. Two subsidence phases in post-rift history took place as the direct consequence of Coniacian compression peak, which defined the distribution of oil sources and reservoirs.

The evolution and specific features of seismogynamics of the Baikal zones are reviewed in the context of interactions between deep deformation waves and the regional structure of the lithospheric mantle. The study is based on a model of the mantle structure with reference to chemical compositions of mantle peridotites from ophiolotic series located in the south-western framing of the Siberian craton (Fig. 1). The chemical zonation of the lithospheric mantle at the regional scale is determined from results of analyses of the heterogeneity of compositions of peridotites (Fig. 2, Table 1) and variations of contents of whole rock major components, such as iron, magnesium and silica (Fig. 3). According to spatial variations of the compositions of peridotites, the mantle has the concentric zonal structure, and the content of SiO2 is regularly decreasing, while concentrations of FeO∑ and MgO are increasing towards the centre of such structure (Fig. 4). This structure belongs to the mantle of the Siberian craton, which deep edge extends beyond the surface contour of the craton and underlies the north-western segment of the Central Asian orogenic belt.Results of the studies of peridotites of the Baikal region are consistent with modern concepts [Snyder, 2002; O’Reilly, Griffin, 2006; Chen et al., 2009] that suggest that large mantle lenses underlie the Archaean cratons (Fig. 5). The lenses are distinguished by high-density ultrabasic rocks and compose high-velocity roots of cratons which have remained isolated from technic processes. Edges of the mantle lenses may extend a few hundred kilometers beyond the limits of the cratons and underlie orogenic belts that frame the cratons, and this takes place in the south-western segment of the Siberian craton.The revealed structure of the lithospheric mantle is consistent with independent results of seismic and magmatectonical studies of the region. The Angara geoblock is located above the central part of the mantle lense (Fig 6, A); it is one of four main tectonical units that compose the basement of the Siberian craton [Mironyuk, Zagruzina, 1983]. As evidenced by the zonal composition of the mantle lense, the centre of the lense is highly dense, and this explains the location of a seismic anomaly there (Fig. 6, B) which is determined to a depth of about 50–60 km [Pavlenkova G.A., Pavlenkova N.I., 2006]. The high-velocity root located in this segment of the craton is traced by seismic tomography [Koulakov, Bushenkova, 2010] to a depth of about 600 km (Fig. 7). The southward-stretching edge of the sub-cratonic mantle has played a major role in the evolution of the Central Asian orogenic belt. In the Paleozoic, the position and the configuration of the accretional margin of the Siberian paleocontinent were determined by the hidden boundary of the craton (Fig. 8, A). Along the craton’s boundary, rifting zones of various ages are located, and intrusions are concentrated, which genesis was related to extension settings (Fig. 8, B). The Cenozoic sedimentary basins are located above the hidden edge of the Siberian craton, which gives evidence of involvement of the deep lithospheric structure in the formation of the recent destruction zone. The basin of Lake Baikal is located along the mantle edge of the Siberian craton, and the basin’s crescent shape accentuates the strike of the mantle edge.In the region under study, the wave nature of seismicity is most evidently manifested by the cyclicity of the strongest earthquakes in the Baikal zone (Table 2). Three seismic cycles are distinguished as follows: (1) at the turn of the 20th century (earthquakes in the period from 1885 to 1931, M=6.6–8.2), (2) the middle of the 20th century (earthquakes from 1950 to 1967, M=6.8–8.1), and (3) at the turn of the 21st century (earthquakes from 1991 to 2012, M=6.3–7.3). While moving in the mantle, the deformation front collapses with the craton’s basement, partially releases its energy to the lithosphere and involves the fragmented edge of the crust overlying the craton’s edge into deformation (Fig. 9, A). This interaction resulted in the formation of the Mongolia-Baikal and the Altai-Baikal seismic sutures whereat all the strong earthquake took place in seismic cycles (1) and (3), respectively (Fig. 9, B). The third, West Amur seismic suture framing the boundary of the Amur plate comprises locations of strong earthquakes that occurred in cycle (2) (Fig. 10). An important specific feature of the Baikal seismic zone is orthogonal migration of earthquakes within seismic sutures. In each of the sutures, epicenters of strong earthquakes (M>6.0) migrated in the transverse direction, which established the orientation of maximum compression during interaction of deformation waves with the mantle structures (Fig. 9, and 10). The less strong seismic events (М<6.0) (Fig. 11) migrated along the seismic sutures. At the western flank of the zone, in the Altai-Baikal and Mongolia-Baikal sutures, latitudinal migration took place in the direction from west to east with account of the trajectory of the deformation wave. In the northern part of the West Amur suture, latitudinal migration was directed from east to west, and its direction was gradually changed to meridional in the southern part, which reflected the anticlockwise rotation of the Amur plate. This conclusion can explain a paradox of counter migration of seismicity in the Baikal zone, which is revealed by S.I. Sherman [Sherman, Zlogodukhova, 2011].In each of the three seismic/deformation sutures, stresses are released via orthogonal multi-directional migration of earthquakes (Fig. 12), and the sutures are regularly combined to compose a complex structure of the stress field in the Baikal seismic zone. Their positions predetermine locations of the major riftogenic structures, primarily sedimentary basins from Tunka to Ubsunur (Fig. 9, B). The three seismic sutures join and overlap each other in the area of Lake Baikal and thus set up the maximum intensity of deformation in this area. Apparently, each of the deformation sutures corresponds to one of the three basins of Lake Baikal (Fig. 13, A). Their depths are correlated with widths of the sutures, which is explained by ‘weakening’ of the deformation wave in the successive cycles of its interaction with the deep structure of the lithosphere. Seismicity of the Baikal zone and its Cenozoic rifting reflect the character of the stress field generated by interaction between the deep deformation wave and the organization of the lithospheric mantle.

Space-and-time regularities of seismicity of the North China (Tan-Lu) zone are studies, and tectonic nature of strong earthquakes is analyzed. The concept of its genesis is still a matter of debate as this zone is located in the centre of the ancient SinoKorean craton, i.e. thousand kilometers away from convergent margins of Eurasia and the Pacific оcean and IndoAustralian plates (Figure 1). The information on the regional cycling dynamics [Xu, Deng, 1996] is updated. Two cycles, in which strong earthquakes (14 shocks with М≥7.0) occurred in the region under study, are distinguished, i.e. from 1500 to 1700, and from 1800 to 1980 (Figure 2). The seismodynamics of the North China zone is consistent with the Circum Pacific оcean deformation wave that occurs once in 300 years at the margin between Asia and the ocean and thus causes the strongest earthquakes (М≥8.8) and eruptions of volcanoes in the Pacific оcean belt [Vikulin et al., 2009, 2010]. This wave came to the northern regions of China in the years of 1500 and 1800 (Figure 3) and triggered seismic activity cycles. The second factor predetermining the seismicity of the Northern China is a specific structure of the region which can manifest seismic activity due to the impact of deformation waves. The genesis of the metastable structure of the region is related to tectonic restructuring of the lithosphere of the SinoKorean craton due to shear displacements in the Tan-Lu megazone. Regional variations of compositions of mantle xenoliths of the Sikhote Alin orogeny demonstrate that the latent strike of the Tan-Lu faults can be traced across the south-eastern areas of Russia to the Tatar Strait. These faults are borders of the Vshaped mantle block (400 x 1500 km) (Figure 5), which composition is characterized by an anomalous content of iron and a low depletion of peridotites. The tectonic mantle block maintains its activity; being impacted by compression from the west, it is squeezed out towards the Sakhalin Island and simultaneously subject to the clockwise rotation. As a result, along and above the margins of the covered lithospheric block in the southern Far East of Russia, main seismic zones have formed (Figure 5, B), wherein earthquakes of M≥5.0 are recorded. The anomalous mantle block at the base of the Sikhote Alin used to be a part of the SinoKorean craton; it was cut out in the JurassicCretaceous period and moved in the northeastern direction along the Tan-Lu shear fault. The lithosphere of the craton was significantly extended during closure of the remaining area, and an evident consequence of extension was formation of two Cenozoic rifting systems. In the Paleogene, the Hebei rift system occurred westward of the Tan-Lu megazone; it hosted earthquakes of the latest seismic cycle. The Shanxi rift system strikes in the northeastward direction and separates the western block of the craton (called Ordos) from the western block (called Hebei); it hosted earthquakes of the earlier seismic cycle.Recent geodynamics. During restructuring of the lithosphere, rotations of tectonic blocks were of importance, along with the lithospheric extension. The specific features of the craton structure suggest two tectonophysical mechanisms of rotation. Firstly, when the triangleshaped zone westward of Tan-Lu was being closed, the lithospheric segment rotated clockwise (Figure 5, С). Consequently, at the mobile front, a compression zone was formed; it has two maximums located NE and SW of the rotation centre. This structural pattern is typical of the lithosphere of the central part of the craton. Within the limits of two conjugated maximums westward of Tan-Lu, the crustal thickness is reduced, and the depth to the asthenosphere is sharply decreased (Figures 4, B and 4, C). The rotation of the blocks in the lithosphere resulted in formation of the gigantic anticline fold, where at the eastern area of rifting is located. Secondly, the clockwise rotation of the Hebei tectonic block triggered the counter clock rotation of the Ordos block that is located west of Hebei (Figure 6, A). At the border of the two blocks rotating in the opposite directions, grabens of the Sshaped Shanxi rift system were formed. The rotation of the tectonic blocks is evidenced by changes of strikes of ancient dykes of the craton (Figure 6, B). Regularities of migration of earthquakes in the North China zone reflect specific features of the tectonic structure of the craton (see Figure 4, А). The earthquakes of the latest cycle were caused by increasing compression of the lithospheric fold. The seismic events of the earlier cycle were associated with the rotation of the Hebei and Ordos blocks. The tectonic mechanism, that were triggered during restructuring of the lithosphere in the early Cretaceous – early Cenozoic, are still actively controlling seismicity in the North China zone.

The role of the lateral structure of the lithospheric mantle in the seismotectonics and seismicity of the southern part of the Russian Far East has been investigated. The positions of the epicenters of all the major earthquakes in Sakhalin (M ≥ 6.0), as well as in the Amur region and the Primorye zones (M ≥ 5.0), are defined by the boundaries of the Anyui block of highly ferruginous mantle, which lies at the base of the Sikhote-Alin area. Three cycles of large earthquakes are recognized in the region: the end of the 19th-beginning of the 20th century, the mid-20th century, and end of the 20th-beginning of the 21st century. In the seismic zone of the Amur region (hereafter, the Amur seismic zone), the epicenters of the large earthquakes in each cycle migrate from the SW to NE along the Tan-Lu fault megasystem at a rate of 30–60 km/yr. The specific features of the seismicity of the region are explained by the repeated arrival of strain waves from the west. The waves propagate in the upper part of the mantle and provoke the activation of the deep structure of the region. The detailed analysis of the earthquakes in the Sikhote-Alin area (M ≥ 4.0) in 1973–2009 confirmed the clockwise tectonic rotation of the mantle block. The characteristics of the Primorye zone of deep-focus seismicity at the Russia-China boundary are stated. Since 1973, 13 earthquakes with M ≥ 6.0 have been recorded in the zone at a depth of 300–500 km. This number of earthquakes is at least twice as many as the number of large deep-focus earthquakes elsewhere in the Sea of Japan-Sea of Okhotsk transition zone. The unique genesis of the Primorye seismic zone is related to the additional compression in the seismofocal area due to the creeping of the Anyui mantle block onto the subduction zone during its rotation. The geodynamic implications of the seismotectonic analysis are examined, and the necessity of division of the Amur plate into three geodynamically independent lithospheric blocks is substantiated.

The deformation history of the Pacific plate is recorded in the age/spatial variation patterns of the seamounts in the western part of the Pacific Ocean. Based on (40)Ar/(39)Ar data, they formed from 120 to 65 Ma ago and have united into 3 linear parallel zones up to 5 thousand km long. The seamounts studied become progressively younger in the southeastern direction along the strike of these zones. The growth rate of seamounts zones was relatively low (2-4 cm/yr) during the first (120-90 Ma) and third stages (85-65 Ma), whereas during a short period in the middle of Cretaceous (90-85 Ma) the zones were propagating stunningly fast up to similar to 80-100 cm/yr. Also at similar to 87-88 Ma there is rapid increase in the magmatic activity during the seamount formation. Age/spatial variation identified best of all can be explained, if the Pacific plate experienced diffuse extension during its northwestward motion, the consequence of which was the formation of impaired zones of decompression melting. The direction of extension (325-340 degrees NW) calculated from the strike of seamounts zones is consistent with the drift of the Pacific Plate (330 degrees NW) in the Late Cretaceous. The dynamics of seamounts building is in good agreement with the spreading acceleration in the mid-Cretaceous and the pulses in the development of the Ontong Java, Manihiki, and Caribbean-Colombian oceanic plateaus. All these processes reflect on different sides the tension of the Pacific lithosphere, which culminated at the age of 88-87 Ma coeval with the short period when the Izanagi Plate ceased its existence and the Kula Plate was formed. At the same time, i.e., 95-85 Ma ago, the vast volumes of tin granitoids were emplaced everywhere in northeastern Asia, from Chukotka to Sikhote-Alin region. Simultaneously, the immense volcanic belts were formed along the Asian continental boundary. The main Okhotsk-Chukotsk volcanic belt extends at a right angle to compression vector and it's major pulse of extrusive activity, based on new (40)Ar/(39)Ar data, occurred 89-87 Ma ago. The mid-Cretaceous maximum of compression of the continental margin correlates remarkably well with the culmination in tension of the Pacific Plate. The linear parallel zones of seamounts in the west of the ocean and the complementary system of great transform faults in its eastern part probably indicate to the general process responsible for the deformation of the Pacific lithosphere.

It is shown that the deep structure of the lithosphere played a decisive role in the recent deformations and seismicity in the Far East. The regional variations in the composition of the mantle xenoliths and Neogene-Quaternary basalts provided grounds for mapping the NE-extending wedge-shaped block of the Fe-rich mantle at the base of Sikhote Alin. Its boundaries continue the Yilan-Yiton and Fushun-Mishan strike-slip faults of the Tan-Lu zone, along which this mantle block was displaced along the continental margin in the Jurassic-Cretaceous. The localization of strong (M ≥ 5.0) earthquake epicenters in the Amur region shows that such a mantle structure determines the key features of the regional deformations and seismotectonics. Under the dominant western compression due to the Amur Plate’s motion, the mantle wedge is extruded in the northeastern direction to provide an additional stress at the Okhotsk Plate boundary. This process resulted in the formation of the Sakhalin high-seismicity zone at the front of the mantle block. In its characteristics, the zone is similar to the convergence area between the Indian and Eurasian plates. In both cases, the main deformation and seismicity features were caused by the horizontal pressure of the tectonic block, the frontal part of which is marked by regularly alternating compression and extension zones. In Sakhalin, strong earthquakes with M ≥ 6.0 are confined to the seismic suture 50 km wide with concentrated compression. This structure is discordant relative to the main faults of the island, being parallel to the front of the mantle wedge. The two migration cycles established for the Sakhalin earthquakes with M ≥ 6.0 correspond to periods of 1907–1971 and 1995–2007. During both cycles, the first shocks occurred in the north and subsequently migrated in the southeastern direction simultaneously decreasing in the depths of the earthquake foci. The systematic migration implies that asymmetrical compression is responsible for both the extrusion of the mantle wedge and its southeastward clockwise rotation. The latter plays the decisive role in the initiation of strong earthquakes on Sakhalin.

Changes in the spreading rates in the Pharallon-Pacific-Izanagi (Kula) triple junction during the Cretaceous and Cenozoic are revised using new data of the dynamics of the Pacific plate. The cyclic character of the spreading is recognized, and the stages of its acceleration and deceleration are distinguished. Approximately 130, 87, and 42 My B.P., at the culminations of the cycles, when maximal spreading rates were reached, the principal rearrangements in the tectonic evolution of the ocean occurred. The spreading rates were minimal about 140, 120, 65, and 15 My B.P. The latter periods are marked by pulses of basalt magmatism in the west, east, and northeast of the Pacific Ocean. The study recognized no signs of the intrusion of the Middle Cretaceous superplume, which was suggested by Larson. Both the cycling revealed and the pulsations of the intraplate volcanism are most probably related to the regularities of the deformations of the oceanic lithosphere and reflect the periodic alternations of regimes of compression and extension of the Pacific plate during the last 180 My.

A correlation between the age and position of 25 seamounts in the West Pacific Ocean formed, judging from the 40 Ar/ 39 Ar data, in the period from 120 to 65 My B.P. was recognized. The seamounts studied are joined into linear zones with extensions up to 5000 km; the age of the seamounts decreases in the southeastern direction. In the interval 93–83 My B.P., the seamount formation was extremely rapid; this interval coincides with the period of acceleration in the Pacific plate movements. In the middle of this interval, 87 My B.P., an intensification of the magmatic activity accompanying the seamount formation was observed simultaneously with the extinction of the Isanagi plate and the appearance of the Kula plate. The results of this study are in the best agreement with the hypothesis of diffuse tension of the Pacific plate at its displacement in the northwestern direction, which led to the formation of weak zones of decompressional melting. The complementary character of the system of tension zones in the western part of the ocean with respect to the system of major transform faults in its eastern part, which probably reflects the common general process of deformation of the Pacific lithosphere in the Cretaceous, is shown.

The dynamics of the Pacific Plate is recorded in the systematic variation of location and the 40 Ar- 39 Ar age of seamounts in the western Pacific from 120 to 65 Ma ago. The seamounts are grouped into three linear zones as long as 5000 km. The seamounts become younger in the southeastern direction along the strike of these zones. Correlation between age and location of seamounts allows division of the history of their formation into three stages. The rate of seamount growth was relatively low (2–4 cm/yr) during the first and the third stages within the intervals of 120–90 and 85–65 Ma, whereas during the second stage (90–85 Ma), the seamounts were growing very fast (80–100 cm/yr). In the midst of this stage, at ∼87 Ma ago, the magmatic activity increased abruptly. The dynamics of seamount building is in good agreement with (1) pulses in the development of the Ontong Java, Manihiki, and Caribbean-Colombian oceanic plateaus; (2) the age of spreading acceleration in the mid-Cretaceous; and (3) the short period when the Izanagi Plate ceased to exist and the Kula Plate was formed. The variation of the seamounts’ age and location is in consistence with the hypothesis of diffuse extension of the Pacific Plate in the course of its motion with formation of impaired zones of decompression melting. The direction of extension (325°–340° NW) calculated from the strike of seamount zones is consistent with the path of the Pacific Plate (330° NW) in the Late Cretaceous. The immense perioceanic volcanic belts were formed at that time along the margin of the Asian continent. The Okhotsk-Chukchi Peninsula Belt extends at a right angle to the compression vector. Three stages of this belt’s evolution are synchronous with the stages of seamount formation in the Pacific Plate. The delay in the origination of the East Sikhote-Alin Volcanic Belt and its different orientation were caused by counterclockwise rotation of the vector of convergence of oceanic and continental plates in the mid-Cretaceous. At the same time, i.e., 95–85 Ma ago, the volcanic activity embraced the entire continental margin and the tin granites were emplaced everywhere in eastern Asia. This short episode (90 ± 5 Ma) corresponds to the mid-Cretaceous maximum of compression of the continental margin, and its age fits a culmination in extension of the Pacific Plate well.