Scholarly article on topic 'The latest geodynamics in Asia: Synthesis of data on volcanic evolution, lithosphere motion, and mantle velocities in the Baikal-Mongolian region'

The latest geodynamics in Asia: Synthesis of data on volcanic evolution, lithosphere motion, and mantle velocities in the Baikal-Mongolian region Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Sergei Rasskazov, Irina Chuvashova

Abstract From a synthesis of data on volcanic evolution, movement of the lithosphere, and mantle velocities in the Baikal-Mongolian region, we propose a comprehensive model for deep dynamics of Asia that assumes an important role of the Gobi, Baikal, and North Transbaikal transition-layer melting anomalies. This layer was distorted by lower-mantle fluxes at the beginning of the latest geodynamic stage (i.e. in the early late Cretaceous) due to avalanches of slab material that were stagnated beneath the closed fragments of the Solonker, Ural-Mongolian paleoceans and Mongol-Okhotsk Gulf of Paleo-Pacific. At the latest geodynamic stage, Asia was involved in east–southeast movement, and the Pacific plate moved in the opposite direction with subduction under Asia. The weakened upper mantle region of the Gobi melting anomaly provided a counterflow connected with rollback in the Japan Sea area. These dynamics resulted in the formation of the Honshu-Korea flexure of the Pacific slab. A similar weakened upper mantle region of the North Transbaikal melting anomaly was associated with the formation of the Hokkaido-Amur flexure of the Pacific slab, formed due to progressive pull-down of the slab material into the transition layer in the direction of the Pacific plate and Asia convergence. The early–middle Miocene structural reorganization of the mantle processes in Asia resulted in the development of upper mantle low-velocity domains associated with the development of rifts and orogens. We propose that extension at the Baikal Rift was caused by deviator flowing mantle material, initiated under the moving lithosphere in the Baikal melting anomaly. Contraction at the Hangay orogen was created by facilitation of the tectonic stress transfer from the Indo-Asian interaction zone due to the low-viscosity mantle in the Gobi melting anomaly.

Academic research paper on topic "The latest geodynamics in Asia: Synthesis of data on volcanic evolution, lithosphere motion, and mantle velocities in the Baikal-Mongolian region"

Geoscience Frontiers xxx (2016) 1—20

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The latest geodynamics in Asia: Synthesis of data on volcanic evolution, lithosphere motion, and mantle velocities in the Baikal-Mongolian region

Sergei Rasskazova,b'*, Irina Chuvashovaa,b

aInstitute of the Earth's Crust, Siberian Branch ofRAS, Irkutsk, Russia b Irkutsk State University, Irkutsk, Russia

ARTICLE INFO

ABSTRACT

Article history:

Received 20 November 2015

Received in revised form

30 May 2016

Accepted 13 June 2016

Available online xxx

Keywords:

Volcanism

Geodynamics

Cenozoic

Asthenosphere Lithosphere

From a synthesis of data on volcanic evolution, movement of the lithosphere, and mantle velocities in the Baikal-Mongolian region, we propose a comprehensive model for deep dynamics of Asia that assumes an important role of the Gobi, Baikal, and North Transbaikal transition-layer melting anomalies. This layer was distorted by lower-mantle fluxes at the beginning of the latest geodynamic stage (i.e. in the early late Cretaceous) due to avalanches of slab material that were stagnated beneath the closed fragments of the Solonker, Ural-Mongolian paleoceans and Mongol-Okhotsk Gulf of Paleo-Pacific. At the latest geodynamic stage, Asia was involved in east—southeast movement, and the Pacific plate moved in the opposite direction with subduction under Asia. The weakened upper mantle region of the Gobi melting anomaly provided a counterflow connected with rollback in the Japan Sea area. These dynamics resulted in the formation of the Honshu-Korea flexure of the Pacific slab. A similar weakened upper mantle region of the North Transbaikal melting anomaly was associated with the formation of the Hokkaido-Amur flexure of the Pacific slab, formed due to progressive pull-down of the slab material into the transition layer in the direction of the Pacific plate and Asia convergence. The early—middle Miocene structural reorganization of the mantle processes in Asia resulted in the development of upper mantle low-velocity domains associated with the development of rifts and orogens. We propose that extension at the Baikal Rift was caused by deviator flowing mantle material, initiated under the moving lithosphere in the Baikal melting anomaly. Contraction at the Hangay orogen was created by facilitation of the tectonic stress transfer from the Indo-Asian interaction zone due to the low-viscosity mantle in the Gobi melting anomaly.

© 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

From a long-lasted discussion on origin of mantle magmatism (Morgan, 1971; Hofmann, 1997; Anderson, 2007; Maruyama et al., 2007; Foulger, 2010; Karato, 2012), it follows that magmatic sources might originate at different levels of the mantle (Fig. 1). Local decreasing of seismic velocities may be interpreted in terms of: (1) a plume, starting from the lower thermal boundary layer of the mantle, (2) a counterflow from the lower mantle after an avalanche of slab material from the transition layer through its lower

* Corresponding author.

E-mail address: rassk@crust.irk.ru (S. Rasskazov).

Peer-review under responsibility of China University of Geosciences (Beijing).

boundary 660 km, (3) a melting anomaly of a domain that extends above the transition layer at depths of 200—410 km, (4) a melting anomaly of a domain that occurs beneath the lithosphere at depths of 50—200 km, (5) a melting anomaly of the lower part of the lithosphere, activated due to rifting, and (6) a melting anomaly at the crust—mantle boundary originated through delamination of an orogenic root. A melting anomaly in the upper mantle may be associated, on the one hand, with rifting or orogenesis in the lithosphere, on the other hand, with a plume or a counterflow from the lower mantle. The definition of each type of melting anomaly requires high-resolution seismic tomography and synthesis of the identified low-velocity anomalies and mantle volcanism evolution.

From the geochemical difference between mid-ocean basalts (MORB) and oceanic island basalts (OIB), it was proposed that the source of the latter occurs in the deep mantle (Morgan, 1971). This

http://dx.doi.org/10.1016/j.gsf.2016.06.009

1674-9871/® 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

S. Rasskazov, ¡. Chuvashova / Geoscience Frontiers xxx (2016) 1—20

Figure 1. Systematics of mantle melting anomalies in continents.

hypothesis has been confirmed by high-resolution seismic tomography of the mantle beneath Hawaii and other islands (Montelli et al., 2004; Zhao, 2009). Another kind of geodynamic regime seems to operate due to the accumulation of subducted oceanic slab material in the transition layer. After some residence time, the material collapses into the lower mantle, inducing a reverse flow of hot material (Mitrovica et al., 2000; Yoshioka and Sanshadokoro, 2002; Zhao, 2009). In the case of hot material rising from the lower to the upper mantle through the transition layer, the latter thins from the top and bottom as inferred from the opposite Clapeyron slopes (Ito and Takahashi, 1989; Anderson, 2007; Maruyama et al., 2007). Studies of hotspots generally confirm the anticorrelated transition zone thinning (Shen et al., 1998; Li et al., 2000, 2003; Owens et al., 2000; Hooft et al., 2003), although in some regions (for instance, in the western North America), the 410 and 660 km discontinuities show no anti-correlation (Houser et al., 2008).

Under Central and East Asia, the transition layer is "cool" and the lower mantle has relatively high velocities (Castillo, 1988; Montelli et al., 2004; Maruyama et al., 2007) that makes rising plumes here doubtful. One can assume only deep dynamics governed by slab avalanches from the transition layer to the lower mantle. Meanwhile, these regions reveal upper mantle low-velocity anomalies comprised into the shallow (depth less than 200 km) Sayan-Mongolian and deeper (depth up to the transition layer) Trans-baikalian upper mantle domains, the origin of which has been explained by processes related to convergent interaction between India and Asia in the south and the Pacific plate subduction in the east, respectively (Rasskazov et al., 2003a,b, 2004).

The mantle dynamics in Asia was an object for speculations in terms of (1) interpreting geophysical data on present-day mantle structure without examination of volcanism (Zorin, 1971; Zorin et al., 2003, 2006), (2) ascertaining spatial or spatial-temporal distribution of late Mesozoic and Cenozoic volcanism without examination of deep mantle structure (Rasskazov, 1994; Yarmolyuk et al., 2007), (3) recording low-speed anomalies in the mantle under Quaternary volcanic fields (Lei and Zhao, 2005; Zhao, 2009; Wei et al., 2012), and (4) study of relations between low-velocity mantle anomalies and volcanism evolution (Rasskazov et al., 2003a,b, 2004, 2012; Rasskazov and Taniguchi, 2006).

Unlike the previous studies, we consider the present-day state of Asia in this paper first of all as a result of the latest geodynamic stage, and decipher deep geodynamics using basic models of seismic tomography of the mantle and evidence on spatial-temporal distribution of volcanic activity in key areas, along with

motions of the lithosphere and structural reorganizations at the boundary between the Pacific plate and Asia.

2. Global and regional expressions of the latest geodynamic stage

The latest geodynamic stage comprises processes of the Earth's unidirectional evolution. This stage likely began at the last Phan-erozoic (mid-Cretaceous, 118—83 Ma) paleomagnetic superchron. "The Quiet Cretaceous period" corresponded to a high-temperature ("superplume") state of the mantle, reflected in the maximum rate of oceanic crust formation, extreme greenhouse conditions, sea level rise, and enhanced organic productivity (Larson, 1991a,b; Tatsumi et al., 1998; Larson and Erba, 1999; Condie, 2001; Jenkyns et al., 2004; Courtillot and Olson, 2007; Trabucho et al., 2010).

An initial global reference point of the final geodynamic stage is defined through marine 87Sr/86Sr records that exhibit the net effect of continental (crustal) and oceanic (mantle) processes. The predominated dissolution of crustal material, which is enriched by 87Sr, was provided in convergent conditions and was displayed by the upper envelope line of the main evolutionary trend. An episodic increasing dissolution effect of oceanic material, exhibited by relative decreasing 87Sr/86Sr, was due to divergent events that resulted in the lower envelope line of the main trend. This main trend demonstrates a general decreasing role of convergence in the early—middle Phanerozoic and its increasing in the late Phanero-zoic (Fig. 2). The contribution of the crustal and oceanic components was changed drastically at the main trend bend. The divergent 87Sr/86Sr minimum of 160 Ma corresponded to the global structural reorganization that launched formation of the new (Pacific) lithospheric plate from the center of a former ridge triple junction between the Kula, Farallon, and Phoenix plates in Southern Paleo-Pacific (Hilde et al., 1977) and closing the Mongol-Okhotsk Gulf in Northwestern Paleo-Pacific (Parfenov et al., 2003). At about 90 Ma, relative variations of 87Sr/86Sr were negligible, i.e. the lower and upper limiting lines of the main trend converged. The descending early—middle Phanerozoic portion of the main trend changed to the late Phanerozoic one at ca. 90 Ma. The unique meaning of this time was matched also by the episode of komatiite magmatism in Gorgona Island (Arndt et al., 2008).

The global transition from the early—middle to the late Phan-erozoic geodynamic stage had regional consequences in Asia. Due to accommodation of interplate convergent processes in Central Mongolia, high-K latites from crustal sources changed to moderate-K mantle-derived basalts. From trace-element and Nd—Sr—Pb isotope signatures, it was inferred that the 91—31 Ma basalts were derived from sources related to the reactivated Gobi system of paleoslab fragments that stagnated in the mantle after closing the Solonker and Ural-Mongolian paleoceans (Rasskazov et al., 2012; Rasskazov and Chuvashova, 2013). Afterwards, the 32—0 Ma basalts were spatially related to the development of the Hangay orogen. The alternation of high- and moderate-potassic lava eruptions demonstrated cycles as long as 20 million years at the 91—31 Ma time interval, relatively short cycles of 2.5 million years between 32 and 2 Ma, and more short ones of 0.7—0.3 Ma in the past 2 Ma (Rasskazov et al., 2010).

A change of magmatism was recorded at ca. 90 Ma in Shandong Peninsula (Xu et al., 2004). There was a magmatic lull here, lasting from 90 to 75 Ma, that separated late Cretaceous and Cenozoic basaltic eruptions from the early—middle Phanerozoic granitic and basic intrusions. The Nd and Sr isotopic signatures of these older intrusions indicate their origin from enriched mantle and crustal sources. Some 75 Ma basaltic lavas still possess these isotopic

S. Rasskazov, I. Chuvashova / Geoscience Frontiers xxx (2016) 1—20

Figure 2. The main sea water trend of 87Sr/86Sr variations in the early—middle and late Phanerozoic (a). Bending the upper envelope line (convergent line) of the trend at ca. 90 Ma corresponds to the initial reference point of the latest global geodynamic stage. Bending of the lower envelope line (divergent line) at ca. 160 Ma means the preceded global structural reorganization. Inserts (b) and (c) illustrate descending and ascending parts of the main trend. The line of marine records is adopted after McArthur et al. (2001, 2012).

signatures, but later on changed to those of the depleted mantle sources.

3. Motion of the Asian lithosphere, structural reorganizations at the Asian-Pacific boundary

The style of present-day deformations in Asia was modeled in experiments on plasticine (Tapponnier et al., 1982). Extrusion tectonics in front of the Indian indenter was simulated using a plate confined on only the left-hand side, leaving the right-hand side free deformed. The resulting extrusion of the material to the right was similar to the large-scale deformation in the China and Indochina blocks (Fig. 3).

In recent publications (Halim et al., 1998; Rasskazov et al., 1998; Calais et al., 2003, 2006; Kreemer et al., 2003; Rasskazov and Taniguchi, 2006; Jin et al., 2007; Sankov et al., 2011), the hypothesis on eastward motions of Asian blocks was supplemented by data on spatial-temporal distribution of Cenozoic volcanism as well as paleomagnetic and GPS data that showed a general division of Asia into a stable part and tectonically unstable areas — diffused deformational zones of considerable width and length. In terms of dominated Cenozoic extensional or compressional forces, tectoni-cally active (mobile) systems were subdivided to rift and orogenic, respectively. The former occur in Inner Asia (Baikal and Circum Ordos or Shanxi) and at the continental margin (East China), the latter (Central Asian and Olekma-Stanovoi) in front of the Indo-Asian and Asia-North American collision zones. In fact, the Baikal Rift Zone is situated between the orogenic systems related to different collision zones. The Shanxi and East China rift systems occur between the Indo-Asian collision zone and regions of rifting and spreading at the Pacific-Asia convergent zone (Okhotsk-Japan-Philippine and South China Sea) (Fig. 4).

The character of the tectonic deformation is reflected in present-day motions of the crust. The Irkutsk GPS station, which is located at the southeastern margin of the Stable Asia (Siberian Platform), moves southeastwards with a local rate of ca. 3 cm yr-1 (Rothacher et al., 1996). The Stable Asia rotates clockwise around a pole located to the south of Eurasia (Gatinskii, Rundquist, 2004). Some large relatively rigid blocks of Asia (Amurian, South China, and Sunda) also rotate. The Amurian block moves relative to the Stable Asia with anticlockwise rotation with a rate of 0.03° Ma-1 at a pole located in the northern margin of the Okhotsk Sea (58.8° N, 157.7°E). A similar rotation with the faster and better constrained rate of 0.22° Ma-1 is determined for the South China block, with respect to Eurasia about a pole at North China (47.3° N, 126.8° E). The Sunda block rotates clockwise with a rate of 0.13° Ma-1 about a pole at the Indian ocean west of Australia (26.0° S, 80.4° E) (Kreemer et al., 2003). The Ordos block underwent general counterclockwise rotation in the late Cenozoic (Xu and Ma, 1992). GPS measurements demonstrate the present-day opening the South Baikal basin with a rate of 3.4 ± 0.7 mm yr-1 (Sankov et al., 2011).

It is noteworthy that different parts of Asia underwent complicated deformation with temporal changes of extension and compression. Boundaries of relatively stable blocks were occasionally affected with strike-slip motions. For instance, an early Holocene change of stress field was found through a study of fissure eruptions in the northeastern Baikal Rift Zone (Rasskazov, 1999). Similarly, changes of deformation were registered by long-term GPS observations. No clear sign of current extension was recorded across the northeastern Shanxi Rift although the extension rate was as high as 4 ± 2 mm yr-1 in 1992—1996 (He et al., 2003).

At the East Asian continental margin, tectonic conditions change quickly from the south to the north in the area of the Korean Peninsula and Sakhalin Island. In the former area and to the south of it, the Philippine Sea plate has never been closely linked with

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Figure 3. Schematic map of Cenozoic extrusion tectonics in East Asia. Brown arrows represent qualitatively major block motions with respect to Siberia (without rotations). Black arrows indicate direction of extrusion-related extension. Numbers refer to assumed extrusion phases: 1 - ca. 50 to 20 Ma; 2 - ca. 20 to 0 Ma; 3 — most recent and future. Arrows on faults in western Malaysia, Gulf of Thailand, and southwestern China Sea (earliest extrusion phase) do not correspond to present-day motions. Red arrows demonstrate counter motions of the Pacific Plate and Stable Asia as inferred from hotspot frame (Pacific Ocean) and GPS data (Irkutsk station). Modified after Tapponnier et al. (1982).

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Figure 4. Mercator projection of late Cenozoic mobile systems with predominating compression or extension at the southeastern part of Eurasia and Pacific-Eurasia convergent zone. Collision zones: IACZ — India-Asia, ANACZ — Asia-North America. Orogenic systems: CAOS — Central Asian, OSOS — Olekma-Stanovoi. Rift systems: BRS — Baikal, CORS — Circum Ordos (Shanxi), ECRS — East China. Regions of rifting and spreading at a convergent zone: OJP — Okhotsk-Japan-Philippine, SC — South China, AS — Andaman Sea. Modified after Rasskazov et al. (1998).

Asia. Along this plate, there was the boundary of "the free continental margin" that allowed extrusion tectonics from the Indo-Asian collision zone as it was proposed by Tapponnier et al. (1982). In contrast, the Okhotsk Sea plate joined Asia in the middle Eocene and became one of the Asian blocks by the Middle Miocene (Rasskazov et al., 2005; Rasskazov and Taniguchi, 2006). The Pacific plate subduction, expressed by late Cenozoic island arc volcanism, was accompanied by back-arc extension of crust along the Philippine, Japan, and Okhotsk seas (Fig. 4).

In the middle Cenozoic, the Pacific plate moved along the Asian margin. Late Cenozoic structural reorganization of East Asia was accompanied by accretion of the Kula-Izanagi plate fragments to the continental margin and transition to the Pacific plate subduction. At the Miocene—Oligocene boundary, volcanism was limited to weakened zones of the lithosphere and in the early Miocene was widespread fromJapan to the Baikal area. This spatial redistribution of volcanism appeared to reflect intraplate consequences of structural reorganization at the eastern interplate boundary of Asia that led to the back-arc extension with rollback of Japanese islands towards the ocean (Fig. 5).

Initiation of subduction was accompanied by back-arc opening of the Sea of Japan at about 15 Ma, with quick clockwise rotation of Southwest Japan and formation of the oblique Honshu-Korea slab

flexure. The direct Hokkaido-Amur slab flexure, which coincides with the convergence direction between the Pacific plate and Asia, resulted from subsequent subduction of the Pacific plate moving at a speed of ca. 10 cm yr_1 and Asia, the current rate of which, according to the GPS-geodesy, is 3 cm yr_1. The Hokkaido-Amur flexure extends from the junction between Japan and Kuril trenches under the Southwestern Hokkaido to the southern part of the Middle Amur basin. Another slab flexure starts from the junction of the Izu-Bonin and Japan trenches and extends beneath the Central Honshu to the south-eastern margin of Korean Peninsula (Fig. 6a).

Insert 6b demonstrates a plot of the slab configuration modified relative to a common straightened trench taken as the abscissa. The isoline of 550 km that corresponds to the middle part of the transition layer was chosen to plot the slab projection at the surface relative to the common straightened trench. This parameter increases, as the slab dipping decreases. The graph is indicative for relative comparisons of slab dipping. This west-northwest projection is compiled along the Hokkaido-Amur flexure that directs in accordance with the long-term Pacific-Asia convergence. The measured distances are 700 km at the northeastern tip of the South Okhotsk basin, increase to 1820 km at the Honshu-Korea flexure, and drastically decrease along the Shikoku basin. The Hokkaido-

S. Rasskazov, ¡. Chuvashova / Geoscience Frontiers xxx (2016) 1—20

Figure 5. Spatial redistribution of volcanism in Asia accompanied the early Miocene launch of the rollback motions in the Japan Sea back-arc region. Compilation of geochro-nological data that fall at the time interval of 23—17 Ma is from (Zhou et al., 1988; Scharer et al., 1990; Rasskazov, 1993; Pouclet et al., 1995; Xu et al., 1996; Takeuchi, 1997; Okamura et al., 1998; Wu et al., 1998; Rasskazov et al., 2000, 2003, 2005; Zheng et al., 2002; Barley et al., 2003; Kudryashova et al., 2006; Rasskazov and Taniguchi, 2006).

Amur flexure is interpreted as a superimposed structure expressed with a relative high of the projection up to 1760 km (relative decrease of dipping). A gentle slope of the 550-km line from the South Okhotsk basin to the Honshu-Korea flexure is attributed to a slab configuration that existed before northwestern propagation of the Hokkaido-Amur flexure. Hence, this part of the line is

characteristic of the boundary between the Pacific plate and Okhotsk Sea-Asia sector that existed just after quick clockwise rotation of Southwest Japan at ca. 15 Ma. The descending line towards the Mariana trench shows a transform-like shift of the Pacific plate boundary between the Okhotsk Sea-Asia and Philippine Sea sectors.

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Figure 6. Slab flexures in the Pacific-Asia Convergent zone. a - Contours of the depth (km) to the top of seismically active subducting Pacific slab (Gudmundsson and Sambridge, 1998). b - Graph showing geometrical relations between trenches (violate lines on (a) and (b)) and slab flexures (green lines on (a) and (b)) and also between trenches and back sides of the late Cenozoic deep basins (pink line on (b)). Trench junctions: KJ — Kuril-Japan, JI — Japan-Izu Bonin, IM — Izu Bonin-Mariana. Direction of the Pacific plate motion on (a) coincides with direction of the Hokkaido-Amur slab flexure on (b) (shown by an open arrow). Orientation of the Honshu-Korea slab flexure, shown by a blue line on (a), resulted from the Sea of Japan opening. Approximate shift of the Japan-Izu Bonin trench junction is shown on (a) by blue dashed line along an arrow directed from the former location of the eastern tip of SW Honshu at the northwestern part of Sea of Japan (JI' within a square) to its current position JI. This shift took place in opposite direction relative to the Pacific plate motion. Vertical green arrows on (b) emphasize relative elevation of the Hokkaido-Amur flexure above the main facet of the Pacific slab extending along the Asia-Okhotsk Sea sector of relatively steady subduction. The oppositely directed arrows demonstrate transform-like relation at the Honshu-Korea flexure between the Eurasia-Okhotsk Sea and Philippine Sea sectors of the Pacific slab. Modified after Rasskazov et al. (2004).

To define the spatial relation between the Pacific plate shape and subduction-related extension, we plotted also distances from trenches to back-side boundaries of deep basins (Fig. 6b). The distances gradually increase along the profile, as slab dipping decreases. The largest distance is defined between the Izu Bonin trench and the western boundaries of the Sea of Japan and Shikoku basins. This correlation may reflect growing rollback effect from the middle part of the Kuril trench through the Japanese trench to the Izu Bonin trench.

The Honshu-Korea flexure appears to originate due to retreat of the Japan-Izu Bonin junction during opening of the Sea of Japan. The timing of these processes was constrained by paleomagnetic data. Northeast Honshu was subjected to counterclockwise rotation as long as back-arc opening proceeded, and Southwest Honshu underwent quick clockwise rotation at about 15 Ma (Otofuji, 1996). More recent paleomagnetic data on the forearc side of Northeast Japan show a pronounced change in declination from 300° at 21 Ma to 0° at 18 Ma.

The amount of presuming counterclockwise rotation of 60° and more is much larger than expected from reconstructions based on geological data (40—30°). This difference was interpreted as an indication of differential rotation of the blocks (Hoshi and Takahashi, 1999).

Paleomagnetic studies in the Sikhote Alin volcanic belt revealed counterclockwise rotation of the late Cretaceous — early Paleogene stratigraphic units. This phenomenon was explained firstly in terms of left-lateral motion along north—south trending strike-slip faults. Further analyses of data showed, however, contradiction of this interpretation to paleomagnetic results on the eastern margin of the North China Block. The Bogopolye Formation was deposited at the southeastern margin of the ridged Mongolian block without sufficient rotation relative to Eurasia since 53—51 Ma. The strati-graphically lower volcanic rocks of the Sijanov Formation and Kisin Grope, the earliest ignimbrites of the late Cretaceous Mon-astyrskaya and Primorskaya Series of the East Sikhote Alin volcanic belt, were emplaced when this block rotated counterclockwise up

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to 41° ± 16°. The rotation was directed oppositely relative to the one of the eastern part of North China Block reconstructed for the latest early Cretaceous, late Cretaceous, and post-Cretaceous units. This spatial relationship was suggested to be analogous to the one associated with the early—middle Miocene opening Sea of Japan (Otofuji et al., 2003).

4. Selecting basic seismic tomography models for definition of melting anomalies

Available velocity models of Asia, accented mostly on China, show melting anomalies projected from the mantle to the Quaternary volcanic fields (Lei and Zhao, 2005; Zhao, 2009; Wei et al., 2012). Some models (Huang and Zhao, 2006; Li et al., 2006; Chen et al., 2015b) yield an overview of the mantle structure beneath a large area of Asia at depths of the transition layer and underlying region. The small-scale generalized sections provide no links

between low-velocity anomalies and Cenozoic volcanic fields. Other models concentrate on limited areas of South Baikal in Siberia (Gao et al., 1994a,b; Zhao et al., 2006) or Hangay in Central Mongolia (Mordvinova et al., 2007; Chen et al., 2015a). Optimal focusing on the territory of the Baikal-Mongolian region demonstrate the S-wave velocity images (Yanovskaya and Kozhevnikov, 2003; Kozhevnikov et al., 2014) that are selected in this study as the basic models for registration of melting anomalies related to the latest geodynamic stage (Figs. 7 and 8).

The model by Yanovskaya and Kozhevnikov (2003) was compiled to the depth of 300 km using digital data from the IRIS system, with extra data, obtained by temporal digital stations of the Russian-American project "Teleseismic tomography of the Baikal rift" in 1992—1993, and also by digitized analog seismic records of 1975—1987 at the Irkutsk, Novosibirsk, and Yuzhno-Sakhalinsk stations. The model displays a high-speed region at depths of 200—300 km beneath the Siberian craton to the west of the

Figure 7. Model of S-wave tomography for the upper mantle of Asia after Yanovskaya and Kozhevnikov (2003).

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Figure 8. Model of S-wave tomography of the upper mantle and transition zone after (Kozhevnikov et al., 2014). Shear-wave velocity variations are shown by isolines in percent. The depth and average velocity (Vs av) is indicated above each map.

meridian 105° E, consistent with the similar region in the previous high-resolution model of Bijwaard et al. (1998). From the analysis of relations between local low-velocity anomalies of the Yanovskaya & Kozhevnikov model and distribution of Cenozoic volcanism, large low-velocity Sayan-Mongolian and Transbaikalian upper mantle domains were defined (Rasskazov et al., 2003a).

The model by Kozhevnikov et al. (2014), which involves the transition layer (Fig. 8), was compiled from a representative set of the group-velocity dispersion curves of the basic mode of Rayleigh waves with the oscillation periods in the range of 10—250 s. On a background of increasing speed of shear waves with depth, an alternation of low- and high-speed layers with significant lateral variations was recorded. Generalized anomalies at the shallow mantle in this model differ from the more detailed image in the model by Yanovskaya and Kozhevnikov (2003). In this sense, the model of Fig. 7 is used here for the registration of low-velocity anomalies in the shallow mantle and Fig. 8 for detecting transition layer anomalies.

5. Discussion

5.1. Identifying melting anomalies at the transition layer

The presence of low-velocity columns that might extend from the different levels of the mantle to the Quaternary volcanic field is examined as a relevant criterion for definition of the melting anomaly including the patterns started from the core—mantle boundary (Montelli et al., 2004; Zhao, 2009; Foulger, 2010). Meanwhile, a Neogene or older volcanic field may be shifted

relative to a mantle low-velocity anomaly due to motion of the lithosphere. Comprising such a volcanic field with any low-velocity mantle inhomogeneity into a single melting anomaly will depend on a direction and speed of the lithosphere motion.

At the transition layer of the Baikal-Mongolian region, we examine three low-velocity anomalies (Gobi, Baikal, and North Transbaikal) that have key significance for decryption of the latest geodynamics of Asia in connection with distribution of late Cretaceous volcanic fields.

Beneath Southwest Gobi, S-wave velocities are locally reduced in the lower part of the mantle transition layer (at depth of 600 km) up to 4.5%. In the middle portion of the transition layer (at depth of 500 km), the image is smoothed. At a higher level, the velocity is reduced locally just above the transition layer (at depth of 400 km). A center of the two minima, associated with the transition layer, is located at coordinates 44° N, 95° E. No late Cretaceous and Cenozoic volcanic rocks are known in this area. The late Cretaceous 91—71 Ma rocks of the Durveldzhin-Borzogiyn group of volcanic fields are offset east of this anomaly over 600 km (Fig. 9).

A similar transition layer anomaly is identified by local decreasing velocities at depths of 600 and 400 km beneath the Lake Baikal area with the center at the coordinates 52° N, 108° E. No late Cretaceous and Cenozoic volcanic rocks are known here. The 100—90 Ma rocks of the Khushinda volcanic field are shifted to the east of the Baikal low-velocity anomaly over 350 km. The North Transbaikal transition layer anomaly is exhibited by locally reduced waive-speed at 600 km (55°N, 116°E) and 400 km (55°N, 123°E). Late Cretaceous volcanic rocks associated with this low-velocity anomaly are not identified. Only the late Cenozoic Udokan

Figure 9. Relations between the key melting anomalies of the transition layer and late Cretaceous volcanic fields. Shear-wave velocity variations are shown by isolines in percent. (a) and (b) are the slices (f) and (h) in Fig. 8.

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volcanic field occurs in the area between the 600-km and 400-km inhomogeneities.

5.2. Relations between the transition layer low-velocity anomalies, late Cretaceous volcanism, and sutures of the closed Phanerozoic paleoceans

From paleomagnetic data, geological terranes of Mongolia, occurred far to the north of the North China block during the late Paleozoic and early Mesozoic. Mongolia, as well as Siberia, moved from south to north in the Paleozoic, from north to south from the late Triassic to the end of the Jurassic, and did not move north—south in the Cretaceous and Cenozoic. Paleolatitudes of bimodal rift-related magmatic units of Mongolia are not statistically different from paleolatitudes of the Siberian craton at least since the end of the Permian (250—275 Ma) (Kovalenko, 2010). No north—south paleo-pole changes in the Cretaceous and Cenozoic means no substantial north—south movement of Asia since that time.

The Gobi transition layer melting anomaly is located between the fragments of the suture zones traced the closed Solonker and Ural-Mongolian paleoceans. The Baikal and North Transbaikal anomalies are spatially associated with a fragment of the closed Mongol-Okhotsk Gulf of Paleo-Pacific (Fig. 10).

The Ural-Mongolian suture divides the territory of Mongolia to the North and South megablocks. The stable structural ensemble of Precambrian and Caledonian terranes in the North megablock, which formed by the end of the Ordovician, includes the Tuva-Mongolian microcontinent. The megablock includes the Mongol-Okhotsk suture that marks the Gulf of Paleo-Pacific, which closed sequentially from west to east during the late Paleozoic and Mesozoic, with quick mutual approaching of shores at the end of the Jurassic — beginning of the early Cretaceous (Sengor and Natal'in, 1996; Kravchinsky et al., 2002). The Hercynian (Sillurian—Carbonif-erous) island arcs of the South megablock (as part of the South Mongolian-Hinggan or Tianshan-South Hingganling zone) are separated from the North China craton and accreted terranes by the Solonker suture (Yanshin, 1974; Belichenko and Boos, 1990; Tseden et al., 1992; Kovalenko et al., 1996; Parphenov et al., 2003; Kozakov et al., 2007; Windley et al., 2007; Wan, 2010). According to various estimates (Ruzhentsev et al., 1989; Sengor and Natal'in, 1996; Qin et al., 2013; Zhou and Wilde, 2013; Han et al., 2015), the Solonker paleocean closed in the late Permian or early Triassic, between 275 and 248 Ma.

In the latest geodynamic stage, volcanism enveloped an area between the Solonker and Ural-Mongolian sutures in the time interval of 91—31 Ma and shifted northwards (to area of the Tuva-

Figure 10. Spatial relations of the transition zone low-velocity anomalies with the sutures of the closed Phanerozoic paleocean fragments. The sutures are modified after (Sengor and Natal'in, 1996; Parfenov et al., 2003).

Mongolian microcontinent and western end of the Mongol-Okhotsk suture) in the last 32 Ma. The lithosphere of the microcontinent was affected by compression resulted in growing of the Hangay orogen, whereas the lithosphere of the western end of the Mongol-Okhotsk suture underwent extension that led to subsiding of basin in the Central Mongolian rift segment (Fig. 11). It was shown (Rasskazov et al., 2012) that magmatic liquids of alkaline basalt and andesite compositions erupted in the Hangay orogen from various sources. The temporal change of the sources demonstrated delamination processes at the thickened lithospheric keel, unlike the Central Mongolian rift segment area, where erupted potassic basalt melts, derived from lithospheric-asthenospheric sources, were indicative for the lithosphere thinning.

Subduction of slab material from the closed paleocean fragments under Asia was an important precursor for the formation of the transition layer melting anomalies of the latest geodynamic stage. Slab material stagnated at the transition layer of the mantle. Specific conditions of relatively low viscosity in the lower mantle in the Cretaceous Quiet Period of 119—83 Ma favored the

distortion of the 660-km boundary and mass transfer between the transition layer and the lower mantle. We speculate that the Gobi, Baikal, and North Transbaikal melting anomalies of the mantle transition layer were due to avalanche collapses of this slab material into the lower mantle. As a result, hot reverse fluxes penetrated through the transition layer from the lower mantle at the beginning or just before the latest geodynamic stage (i.e. ca. 90 Ma).

5.3. Rate estimates of the lithosphere motion

The resulting vector of east-southeastern shift of the lithosphere with the late Cretaceous volcanic fields relative to the Gobi transition layer low-velocity anomaly, with the amplitude of 600 km, was provided mostly by general motion of the Asian lithosphere. A small northward drift was supplemented due to the Indo-Asian convergence. Late Cenozoic volcanism in the Hangay-Belaya orogenic zone was derived from the shallow melting anomalies of 50—150 km that belongs to the Sayan-Mongolian low-velocity

Figure 11. Spatial distribution of late Cretaceous through Cenozoic volcanic fields relative to suture fragments of the closed Phanerozoic paleoceans in Central Mongolia (Yanshin, 1974; Belichenko, Boos, 1988; Tseden et al., 1992; Kovalenko et al., 1996; Zorin, 1999; Kozakov et al., 2007; Windley et al., 2007; Parphenov et al., 2003; Wan, 2010). Hangay orogen belongs to the southern part of the Hangay-Belaya orogenic zone.

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domain. The resulting east-southeastern vector of the lithosphere motion relative to these low-speed anomalies coincided with the one in the South Gobi with the amplitude reduction to 300 km. The average speed of the lithosphere motion is estimated to be ca. 2 cm yr_1 in the past 15 Ma. Assuming the same velocity of the lithosphere motion in South Gobi in the past 15 Ma, we obtain the average velocity estimate of the lithosphere motion in Central Mongolia of ca. 0.4 cm yr_1 for the preceding period from 90 to 15 Ma (Fig. 12).

The resulting amplitude of 350 km and eastward displacement of the lithosphere with the Khushinda and Irenga late Cretaceous—Paleogene volcanic fields relative to the Gobi transition layer low-velocity anomaly is due to east-southeast general motion of Asia and nearly the same additional north-northeastern drift. The latter was more effective than the convergent drift of the lithosphere in Central Mongolia progressed at the same direction.

The initial 16—14 Ma high-Mg eruption at the Bereya volcanic center of the Vitim Plateau reflected the local late Cenozoic adia-batic ascent of hot mantle material beneath Western Transbaikal (Chuvashova et al., 2016). The Bereya volcanic center shifted eastwards over 300 km relative to the low velocity anomaly located at the depth of 250—300 km. This shift yields the average speed estimate for late Cenozoic lithosphere motion to ca. 2 cm yr_1. From the amplitude of the Western Transbaikal displacement, over 300 km in the last 15 Ma for the Irenga volcanic field, the 300-km distance of its movement should be limited by this time interval and explained by a short (50 km long) shift of volcanism from Khushinda to Irenga by movement of the lithosphere at the same

direction. The average speed of this motion, referred to the time range of 90—50 Ma, is as low as 0.1 cm yr_1 (Fig. 13).

5.4. Transition layer distortion under North Transbaikal and its possible cause

A key assumption of plate tectonics is that plates are moved due to drag driven by mantle flow acting on the base of the lithosphere and boundary forces acting along the edges of plates. In models of plate tectonics, flow in the asthenosphere is examined as a consequence of the plate motion. Counterflow can be induced by drag from the lithosphere and pressure-gradient in the asthenosphere due to its displacement either by a subducting slab or return flowing mantle towards a mid-ocean ridge (Forsyth and Uyeda, 1975; Smith and Lewis, 2003; Turcotte and Schubert, 2002). How a subduction begins remains poorly understood (e.g. Niu et al., 2003; Stern, 2004). In the context of this study, we propose that there was a dynamic connection between mantle melting anomalies and subduction processes in East Asia.

On the one hand, the model of progressive west-northwest slab subduction assumes a pool down of oceanic material under the continental margin, accompanied by a reverse flow directed from a

Figure 12. Motion of the lithosphere with volcanic fields in Central Mongolia relative to the mantle melting anomalies, located: (a) in the transition layer after model by Kozhevnikov et al. (2014), (b) in the upper level of the upper mantle after model by Yanovskaya and Kozhevnikov (2003).

Figure 13. Motion of the lithosphere with volcanic fields in the Baikal area relative to the mantle melting anomalies: (a) in the transition layer after the model by Kozhevnikov et al. (2014), (b) in the lower level of the upper mantle after the model by Yanovskaya and Kozhevnikov (2003).

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continent to an ocean. Due to the reverse flow, created above the directly-subducted Hokkaido-Amur slab flexure, the lithosphere of the northeastern Baikal Rift Zone was stretched at the border between the Archean basement of the Siberian craton and accreted terranes. On the other hand, a similar flow was created above the Honshu-Korea slab flexure, while Southwest Japan was rotated and rolled back from the continental edge allowing the Sea of Japan opening (Fig. 14).

From the hypothesis on temperature-dependent phase control of transition layer boundaries, it follows that a thermal (thermo-chemical) flux from the lower mantle should be identified both at the base and top of the transition layer. The spatially coincided low-velocity anomalies related to the lower and upper boundaries of the transition layer beneath both Gobi and Baikal are indicative of the absence of differential lateral movement of the material at the transition layer of this part of Inner Asia during the latest geodynamic stage. And vice versa, the mutual spatial separation of the North Transbaikal anomalies of 600 km and 400 km reflects lateral motion of the material in the transition layer beneath East Asia.

In North Transbaikal, the resulting displacement vector of the low-velocity anomaly at the top of the transition layer relative to its bottom is directed from the west to the east. The offset is made up of the east-southeast movement provided by the Asia-Pacific convergence and the transversal north—northeast shift. The total range of motion (about 400 km) and constituent trends is

comparable to the lithosphere movement relative to the Baikal transition layer melting anomaly. Apparently, the dynamics of the Baikal and North Transbaikal melting anomalies was due to a common cause, although movement took place at the level of the upper mantle in the Lake Baikal area and at the level of the transition layer in North Transbaikal (Fig. 14).

Distortion of the North Transbaikal melting anomaly is indicative of lateral movement of the material in the transition layer under East Asia that is combined with concentrated pull down of material into the transition layer along the directly-subducted Hokkaido-Amur flexure of the Pacific slab. It is possible that the transition layer was modified in the late Cretaceous—early Paleo-gene due to the subducting Kula-Izanagi slab before and during the Okhotsk plate accretion to the Asian margin. So far, there is no evidence on separation of effects produced by the Kula-Izanagi and Pacific slabs.

The peculiar structure of the transition layer beneath North Transbaikal, highlighted by the lack of distortion in the Gobi and Baikal transition layer melting anomalies, is indicative of a possible cause of the observed difference. The Gobi anomaly coincides with the direction of the obliquely-subducted Honshu-Korea flexure, whereas the Baikal one occurs between the directions of obliquely-and directly-subducted flexures (Honshu-Korea and Hokkaido-Amur, respectively). We suggest that the Gobi and Baikal melting anomalies differ from the North Transbaikal one in connection with

Figure 14. Spatial relations between the selected transition zone melting anomalies in the Baikal-Mongolian region and flexures of the subducted Pacific slab.

Figure 15. The sequence of events from generating primary thermal heterogeneity of the Baikal transition layer melting anomaly (panels a-c) and counterflow structure in the asthenospheric mantle under the moving lithosphere (panels d-f) (explanations in the text).

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different dynamics of the Pacific slab flexures: the passive formation of the former during the Sea of Japan opening and the active formation of the latter through progressive pull down of the Pacific slab material into the transition layer at the direction of the Pacific Plate and Asia convergence (Fig. 6).

5.5. The Baikal melting anomaly as a cause of lithospheric extension at the Baikal Rift

The subduction factor of mantle dynamics in the Baikal-Mongolian region is complemented by displaying asthenospheric counterflows under the moving lithosphere.

It has long been drawing attention to the asymmetric rise of the asthenosphere in the Baikal Rift Zone, its location under the Siberian platform ".at a much greater depth than under the Transbaikal region" and has been inferred that ".horizontal flowing within the asthenosphere should be directed from the northwest to southeast" (Zorin, 1971, p. 143). The same flowing in the asthenosphere was assumed in later papers (Gao et al., 1994a,b; Lebedev et al., 2006; Sankov et al., 2011). The movement of the lithosphere in these hypotheses, however, was not taken into account.

Extension of the lithosphere beneath the Baikal basin was likely provided by a dynamic effect of the asthenospheric counterflow originated in the transition layer melting anomaly. The possible sequence of events is presented in Fig. 15.

In the transition layer beneath the closing Mongol-Okhotsk Gulf of Paleo-Pacific, slab material stagnated until mid-Cretaceous (Fig. 15a). By the beginning of the latest geodynamic stage (i.e. ca. 90 Ma), this material collapsed from the transition layer into the lower mantle with ascent of reverse hot flow resulted in volcanism of the Khushinda field (Fig. 15b). The hot flux provided local heating of the upper mantle beneath Western Transbaikal. In the time interval

of the late Cretaceous (90—65 Ma), movement of the Asian lithosphere was negligible with the volcanic lull (Fig. 15c). At the late Cretaceous—early Paleogene, movement of the lithosphere was initiated and accompanied with volcanic activity in the Irenga field. At this time, East Asia was affected by a rollback mechanism that was repeated at the beginning of the Pacific slab subduction in the early—middle Miocene and resulted in opening of the Sea of Japan. Respectively, the motion of the lithosphere, initiated in the time interval of 65—50 Ma, got a new impulse at 22—15 Ma (Fig. 15d and e). The early—middle Miocene structural reorganization facilitated the development of a melting anomaly at the upper mantle level to the northeast of the primary flow at the transition layer, with a corresponding shift of volcanism from Khushinda and Irenga to Vitim Plateau (in Fig. 15, this episode is not shown). At present, due to the lithosphere motion, the primary transition layer melting anomaly occurs under the Baikal, a shallow tail stretches towards Transbaikal and a deeper reverse flow towards the Siberian craton. The lithosphere of the Baikal Rift is affected by a deviator flow (i.e. from one point in opposite directions) in the Baikal melting anomaly (Fig. 15f).

5.6. The Gobi melting anomaly as a cause of lithospheric contraction in the Hangay orogen

A counterflow is identified under the moving lithosphere in Gobi (Fig. 16). Low-velocity anomalies in the Sayan-Mongolian domain (150—200 km) and underlying upper mantle with slab fragments (300 km) are shifted westwards relative to the primary transition layer melting anomaly, centered at depths of 400 and 100 km. Overall decreasing velocity is characteristic for the bottom of the lithosphere and underlying asthenosphere (at depth of about 50 km).

Figure 16. Counterflow in the asthenosphere under the moving lithosphere in Gobi. The interpretation is based on Fig. 8 and space-time distribution of volcanic activity in the past 90 Ma (Rasskazov et al., 2010, 2012).

Similar to the Baikal area, migrating volcanism in Central Mongolia marked a melting anomaly that propagated north-northeastwards in the upper mantle relative to the primary Gobi transition layer hot flux. In the time interval of 91—31 Ma, volcanic activity was focused in Gobi (Fig. 11, segment BC in Fig. 16). Then, after the structural reorganization of ca. 32 Ma, volcanic activity shifted northeastwards to the Hangay-Belaya orogenic zone that remained active in the last 22 Ma (Fig. 11, segment CD in Fig. 16) (Rasskazov et al., 2010; Rasskazov and Chuvashova, 2013).

No extensional structure in the Gobi region, such as the Baikal Rift Zone, is explained by acting asthenospheric flow, associated with the subduction of the Honshu-Korea flexure, along the strike

of west—east geological structures in this area, i.e. along the Solo-nker and Ural-Mongolian sutures (Fig. 10). Lateral west—east flowing material in the upper mantle under Gobi took place in conditions of prevailing orthogonal north—south contraction of the lithosphere governed by Indo-Asian interaction. Unlike the "free" eastern continental margin of Asia that caused eastward movement of blocks accompanied by subduction, the northern edge of Asia did not provide such a scenario. As a result, limited northward promotion of the Gobi lithosphere and low-viscosity mantle material at the Gobi melting anomaly caused enhanced transfer of tectonic stress to the Hangay orogen with its delamination and respective development of the orogen-related melting anomaly (Fig. 17).

6. Conclusion

Figure 17. Facilitating tectonic stress transfer from the Indo-Asian zone of interaction to the Hangay orogen due to the low-viscosity mantle at the Gobi melting anomaly (a) and the development of shallow mantle processes through delamination of the orogen root (b).

From the synthesis of data on volcanic evolution, movement of the lithosphere, and mantle velocities in the Baikal-Mongolian region, we infer that the latest geodynamics of Asia were governed by processes related to transition layer melting anomalies. We have identified the Gobi, Baikal, and North Baikal transition layer melting anomalies that are spatially related to west-east trending sutures of the closed Solonker, Ural-Mongolian paleoceans and Mongol-Okhotsk Gulf of Paleo-Pacific. We suggest that by the beginning of the latest geodynamic stage at ca. 90 Ma, the transition layer was destroyed by lower-mantle fluxes due to avalanches of slab material that were stagnated beneath the closed fragments of the paleoceans.

The Gobi and Baikal melting anomalies underwent changes by upper mantle counterflows, in contrast to the North Transbaikal one that showed a distorted low-velocity anomaly at the transition layer. We connect these features with subduction processes in East Asia and suggest an explanation that takes into account the different origins of the Honshu-Korea and Hokkaido-Amur flexures of the subducted Pacific slab. The former flexure resulted mostly from the Sea of Japan opening in the early—middle Miocene, the latter formed due to progressive pull down of the Pacific slab material into the transition layer at the direction of the Pacific plate and Asia convergence.

At the latest geodynamic stage, Asia was involved in east-southeast movement accompanied by opposite subduction of the Pacific plate. The currently active subduction process began in the early—middle Miocene structural reorganization. In regions of primary transition layer melting anomalies, this change was reflected in formation of upper mantle low-velocity domains associated with the development of rifts and orogens. We suggest that extension at the Baikal Rift was caused by deviator flowing mantle material, initiated under the moving lithosphere in the Baikal melting anomaly. Contraction at the Hangay orogen was created by facilitation of the tectonic stress transfer from the Indo-Asian interaction zone due to the low-viscosity mantle in the Gobi melting anomaly. Rate estimates of the east—southeastward motion of the Asian lithosphere relative to the Gobi and Baikal transition layer low-velocity anomalies vary from values as low as 0.1 cm yr_1 in the time range of 90—50 Ma to 2 cm yr_1 in the time interval of the past 15 Ma and 3 cm yr_1 at Present. The latter estimate is obtained by GPS-geodesy.

The considered patterns of late Cretaceous through early—middle Cenozoic and late Cenozoic volcanism, possibly related to low-velocity anomalies of the transition layer and upper mantle in the Baikal-Mongolian region, show evidence on connections between evolution of inter- and intraplate mantle processes displayed on a continental scale at the latest geodynamic stage. In order to develop a comprehensive geodynamic model for the whole of Asia, this type of connections need to be tested in other regions.

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Acknowledgments

This work was supported by the Russian Science Foundation for Basic Research (project 14-05-313228) and completed in frame of studies in the Chinese-Russian Wudalianchi-Baikal Research Center on volcanism and environment. We are grateful to Nick Roberts and two anonymous reviewers for constructive discussion of the manuscript.

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S. Rasskazov, I. Chuvashova / Geoscience Frontiers xxx (2016) 1—20

Rasskazov Sergei: Head of Laboratory for isotopic and geochronological studies at the Institute of the Earth's Crust SB RAS, Irkutsk, Russia (since 1996), Head of Dynamic Geology Chair at the Irkutsk State University (since 2012), M.S. (1976) from the Irkutsk State University, Ph.D. (1982), Dr. (1992), and Professor (2005) of Petrology and Volcanology from the Institute of the Earth's Crust SB RAS. He has studied Cenozoic rifts in Central and East Asia, North America, and East Africa. His main interest is origin of volcanic rocks, erupted at the latest geodynamic stage of the evolving Earth. He has published more than 500 works that include 17 monographs and 132 peer-reviewed papers.

Chuvashova Irina: Research Scientist at the Institute of the Earth's Crust SB RAS, Irkutsk, Russia (since 2004), assistant professor of Dynamic Geology Chair at the Irkutsk State University (since 2015), M.S. (2006) from the Irkutsk State University, Ph.D. (2010) from the Institute of the Earth's Crust SB RAS. She worked in late Mesozoic and Cenozoic volcanic fields of Mongolia, China, Kyrgyzstan, Sakhalin, Amur region, and Siberia. Results of her research in geochemistry and petrology of volcanic rocks have been presented in 118 publications that include 6 monographs and 18 peer-reviewed papers.