Mantle plume,large igneous province and continental breakup——Additionally discussing the Cenozoic and Mesozoic mantle plume problems in East China

Based on the former workers' study results such as numerical simulation of fluid mechanics, seismic tomography of the whole earth and igneous rocks, the basie characteristics of mantle plumes are summarized in detail, namely the mantle plume, from the D" layer near the core-mantle boundary (CMB) of 2900 km deep, is characterized by the shape of large head and thin narrow conduit, by the physical property of high temperature and low viscosity. The LIP (large igneous province) is the best exhibition when the mantle plume ascends to the surface. According to the basie characteristics of the mantle plumes and the LIP, as well as the temporal-spatial relationships between the mantle plume and Continental breakup, the detailed research on petrology, geochemistry, temporal-spatial distribution, tectonic background of the Cenozoic-Mesozoic igneous rocks and gravity anomaly distribution in East China has been done. As a result, the Mesozoic igneous rocks in Southeast China should not be regarded as an example of typical LIP related to mantle plumes, for their related characteristics are not consistent with those of the typical LIPs related to mantle plumes. The Cenozoic igneous rocks in Northeast China have no the typical characteristics of mantle plumes and hotspots, so the Cenozoic volcanism in Northeast China might have no the direct relationships with the activity of mantle plumes.

Plate tectonics in the twenty-first century

Plate tectonics was originally established as a kinematic theory of global tectonics,in which the Earth’s rigid outer layer,the lithosphere,consists of different size plates that move relative to each other along divergent,convergent or transform boundaries overlying the ductile asthenosphere.It comprises three elements:rigid lithosphere plates,ductile asthenosphere,and coupled movement systems.It operates through the interlinked processes of continental drift,seafloor spreading and lithospheric subduction,resulting in the generation,modification and demise of lithospheres throughout geological time.The system of lithospheric plates in horizontal and vertical movements forms the spatiotemporal linkages of matter and energy between the surface and interior of Earth,advancing the kinematic theory with a dynamic explanation.While top-down tectonics through lithospheric subduction plays a key role in the operation of plate tectonics,it is balanced for the conservation of both mass and momentum on the spherical Earth by bottom-up tectonics through asthenospheric upwelling to yield seafloor spreading after continental breakup.The gravity-driven subduction of cool lithosphere proceeds through convergence between two plates on one side,and rollback of the subducting slab makes the vacancy for upwelling of the hotter asthenosphere to form active rifting in backarc sites.Plate convergence is coupled with plate divergence between two plates along mid-ocean ridges on the other side,inducing passive rifting for seafloor spreading as a remote effect.Thus,plate tectonics is recognizable in rock records produced by tectonic processes along divergent and convergent plate margins.Although the asthenospheric upwelling along fossil suture zones may result in continental breakup,seafloor spreading is only induced by gravitational pull of the subducting oceanic slab on the remote side.Therefore,the onset and operation of plate tectonics are associated with a series of plate divergent-convergent coupling systems,and they are critically dependent on whether both construction and destruction of plates would have achieved and maintained the conservation of both mass and momentum on the spherical Earth.Plate margins experience different types of deformation,metamorphism and magmatism during their divergence,convergence or strike-slip,leaving various geological records in the interior of continental plates.After plate convergence,the thickened lithosphere along fossil suture zones in intracontinental regions may be thinned by foundering.This causes the asthenospheric upwelling to reactivate the thinned lithosphere,resulting in superimposition and modification of the geological record at previous plate margins.The operation of plate tectonics,likely since the Eoarchean,has led to heat loss at plate margins and secular cooling of the mantle,resulting in the decrease of geothermal gradients and the increase of rheological strength at convergent plate margins.Modern plate tectonics is characterized by the predominance of rigid plate margins for cold subduction,and it has prevailed through the Phanerozoic.In contrast,ancient plate tectonics,that prevailed in the Archean and Proterozoic,is dominated by relatively ductile plate margins for collisional thickening at forearc depths and then warm subduction to subarc depths.In either period,the plate divergence after lithospheric breakup must be coupled with the plate convergence in both time and space,otherwise it is impossible for the operation of plate tectonics.In this context,the creation and maintenance of plate divergent-convergent coupling systems are responsible for the onset and operation of plate tectonics,respectively.Although a global network of mobile belts is common between major plates on modern Earth,it is difficult to find its geological record on early Earth if microplates would prevail at that time.In either case,it is important to identify different types of the geological record on Earth in order to discriminate between the different styles of plate tectonics in different periods of geological history.

Key geodynamic processes and driving forces of Tethyan evolution

Tethys tectonic system has experienced a long-term evolution history,including multiple Wilson cycles;thus,it is an ideal target for analyzing plate tectonics and geodynamics.Tethyan evolution is typically characterized by a series of continental blocks that separated from the Gondwana in the Southern Hemisphere,drifted northward,and collided and accreted with Laurasia in the Northern Hemisphere.During this process,the successive opening and closing of multistage Tethys oceans(e.g.,Proto-Tethys,Paleo-Tethys,and Neo-Tethys)are considered core parts of the Tethyan evolution.Herein,focusing on the life cycle of an oceanic plate,four key geodynamic processes during the Tethyan evolution,namely,continental margin breakup,subduction initiation(SI),Mid-Ocean Ridge(MOR)subduction,and continental collision,were highlighted and dynamically analyzed to gather the following insights.(1)Breakup of the narrow continental margin terranes from the northern Gondwana is probably controlled by plate subduction,particularly the subduction-induced far-field stretching.The breakup of the Indian continent and the subsequent spreading of the Indian Ocean can be attributed to the interactions between multiple mantle plumes and slab drag-induced far-field stretching.(2)Continental margin terrane collision-induced subduction transference/jump is a key factor in progressive Tethyan evolution,which is driven by the combined forces of collision-induced reverse push,far-field ridge push,and mantle flow traction.Moreover,lithospheric weakening plays an important role in the occurrence of SI.(3)MOR subduction is generally accompanied by slab break-off.In case of the considerably reduced or temporary absence of slab pull,mantle flow traction may contribute to the progression of plate subduction.MOR subduction can dynamically influence the overriding and downgoing plates by producing important and diagnostic geological records.(4)The large gravitational potential energy of the Tibetan Plateau indicates that the long-lasting India-Asia continental collision requires other driving forces beyond the far-field ridge push.Further,the mantle flow traction is a good candidate that may considerably contribute to the continuous collision.The possible future SI in the northern Indian Ocean will release the sustained convergent force and cause the collapse of the Tibetan Plateau.Based on the integration of these four geodynamic processes and their driving forces,a“multienginedriving”model is proposed for the dynamics of Tethyan evolution,indicating that the multiple stages of Tethys oceanic subduction provide the main driving force for the northward drifting of continental margin terranes.However,the subducting slab pull may be considerably reduced or even lost during tectonic transitional processes,such as terrane collision or MOR subduction.In such stages,the far-field ridge push and mantle flow traction will induce the initiation of new subduction zones,driving the continuous northward convergence of the Tethys tectonic system.