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

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.

Mid-Miocene thermal impact on the lithosphere by sub-lithospheric convective mantle material:Transition from high-to moderate-Mg magmatism beneath Vitim Plateau,Siberia

High-Mg lavas are characteristic of the mid-Miocene volcanism in Inner Asia.In the Vitim Plateau,small volume high-Mg volcanics erupted at 16-14 Ma.and were followed with voluminous moderate-Mg lavas at 14-13 Ma.In the former unit,we have recorded a sequence of（1） initial basaltic melts,contaminated by crustal material,（2） uncontaminated high-Mg basanites and basalts of transitional（K-Na-K） compositions,and（3） picrobasalts and basalts of K series;in the latter unit a sequence of（1） initial basalts and basaltic andesites of transitional（Na-K-Na） compositions and（2） basalts and trachybasalts of K-Na series.From pressure estimation,we infer that the high-Mg melts were derived from the sublithospheric mantle as deep as 150 km,unlike the moderate-Mg melts that were produced at the shallow mantle.The 14-13 Ma rock sequence shows that initial melts equilibrated in a garnet-free mantle source with subsequently reduced degree of melting garnet-bearing material.No melting of relatively depleted lithospheric material,evidenced by mantle xenoliths,was involved in melting,however.We suggest that the studied transition from high-to moderate-Mg magmatism was due to the mid-Miocene thermal impact on the lithosphere by hot sub-lithospheric mantle material from the Transbaikalian low-velocity（melting） domain that had a potential temperature as high as 1510℃.This thermal impact triggered rifting in the lithosphere of the Baikal Rift Zone.

Responses of a ^(234)U/^(238)U activity ratio in groundwater to earthquakes in the South Baikal Basin,Siberia

In the western part of the South Baikal Basin,spatial-temporal distribution of earthquake epicenters shows quasi-periodic seismic reactivation.The largest earthquakes that occurred in 1999(M_(W)=6.0)and 2008(M_(W)=6.3)fall within seismic intervals of 1994-2003 and 2003-2012,respectively.In the seismic interval that began in 2013,the ^(234)U/^(238)U activity ratio(AR)in groundwater was monitored assuming its dependence on crack opening/closing that facilitated/prevented water circulation in an active boundary fault of the basin.Transitions from disordered,high-amplitude fluctuations of AR values to consistent,low-amplitude fluctuations in different monitoring sites were found to be sensitive indicators of both small seismic events occurring directly on the observation area,and of a large remote earthquake.The hydroisotopic responses to seismic events were consistent with monitoring data on deformation and temperature variations of rocks.The hydroisotopic effects can be applied for detecting a seismically dangerous state of an active fault and prediction of a large future earthquake.