A Sheeted Dike Complex in the Protrozoic Miaowan Ophiolite Complex on the Northern Yangtze Craton: Recording Seafloor Spreading

The Proterozoic Miaowan Ophiolite Complex is a highly dismembered ophiolitic complex cropping out near the northern margin of the Yangtze Craton(Peng et al.,2012).The rocks of this complex consist of,from bottom

Elemental Compositions of Clinopyroxenes from Southern Okinawa Trough Basalt:Implications for Magmatism and Structural Environment in an Initial Back-Arc Basin

The major and trace elemental compositions of clinopyroxene from basalt were used to characterize the nature of the primitive magma and structural environment beneath the southern Okinawa Trough(SOT),which is an initial back-arc basin at a continental margin.The clinopyroxenes in the basalt were augite with variable Mg^(#)contents(73.37-78.22).The regular variations in major oxide contents(i.e.,CaO,FeO,TiO_(2),and Cr)with decreasing Mg#implied that the clinopyroxenes evolved from being enriched in Mg,Ca,and Cr to being enriched in Fe and Ti.The clinopyroxenes had relatively low rare earth element concentrations(7.51×10^(-6) to 12.68×10^(-6))and negative Eu anomalies(δEu=0.67-0.95).The Kd_(cpx) values of clinopyroxenes(0.2-0.26),which were used to examine whether the clinopyroxene was equilibrated with its host basalt,demonstrate that these clinopyroxene phenocrysts were not captured crystals but were instead produced by crystallization differentiation of the magma.The calculated clinopyroxene crystallization temperatures showed a narrow range of 990-1061℃,and their crystallization pressures ranged from 2.0 to 3.2 kbar.The geochemistry features of these clinopyroxenes indicated that the parent magma belonged to the subalkaline tholeiitic magma series and suggested that the magma experienced crystallization differentiation of olivine,plagioclase,and clinopyroxene,where the crystallization of plagioclase occurred earlier than that of clinopyroxene.Combined with geophysical data,this research on primitive magma and its crystallization differentiation from clinopyroxene indicates that the SOT is in the stage of‘seafloor spreading’and that basaltic rocks produced from tholeiitic magma represent the generation of oceanic crust.

Reconstructing seafloor age distributions in lost ocean basins

Reconstructions of past seafloor age make it possible to quantify how plate tectonic forces,surface heat flow,ocean basin volume and global sea level have varied through geological time.However,past ocean basins that have now been subducted cannot be uniquely reconstructed,and a significant challenge is how to explore a wide range of possible reconstructions.Here,we investigate possible distributions of seafloor ages from the late Paleozoic to present using published full-plate reconstructions and a new,efficient seafloor age reconstruction workflow,all developed using the open-source software GPlates.We test alternative reconstruction models and examine the influence of assumed spreading rates within the Panthalassa Ocean on the reconstructed history of mean seafloor age,oceanic heat flow,and the contribution of ocean basin volume to global sea level.The reconstructions suggest variations in mean seafloor age of~15 Myr during the late Paleozoic,similar to the amplitude of variations previously proposed for the Cretaceous to present.Our reconstructed oceanic age-area distributions are broadly compatible with a scenario in which the long-period fluctuations in global sea level since the late Paleozoic are largely driven by changes in mean seafloor age.Previous suggestions of a constant rate of seafloor production through time can be modelled using our workflow,but require that oceanic plates in the Paleozoic move slower than continents based on current reconstructions of continental motion,which is difficult to reconcile with geodynamic studies.

Relationship between Formation of Zhongyebei Basin and Spreading of Southwest Subbasin, South China Sea

The Zhongyebei （中业北） basin （ZYBB） is an NE-striking, narrow and small sedimentary basin superimposing the southern 1/2 segment of the proposed spreading axes of the SW subbasin of the South China Sea （SCS）. More than 4 500 m strata were identified in the Zhongyebei basin, including the Paleogene lower structure layer and the Neogene upper structure layer. The SW subbasin of the South China Sea has been regarded as an oceanic basin opened by seafloor spreading, as evidenced by the flat and deep （〉 4 000 m mostly） seafloor with linear magnetic anomalies, and by the shallow Moho depth of 〈 12 km as estimated from gravity modeling. The classic model of seafloor spreading predicts that sediments on the oceanic crust are younger and thinner towards the spreading axes. But in the southwestern segment of the SW subbasin, contradictions appear. Firstly, the thick sedimentation in the ZYBB is along the proposed spreading axes. Secondly, the sediments are thinner （500-1 500 m） and younger away from the proposed spreading axes. Thirdly, geological elements of the two sides of spreading axes develop asymmetrically in the southwestern SW subbasin. Two models, the early opening model and the limited modeling model, are suggested for resolving this paradox. The former suggests that the opening of the SW subbasin was in Late Eocene and earlier than the oldest sediment in the ZYBB. The latter proposes that the opening of the SW subbasin was limited to its northeastern portion, and did not extend to the southwest portion. The ZYBB is a rift basin survived from the spreading but subjected to severe syn-spreading magmatic disturbance. The SW subbasin and the ZYBB of the SCS provide a unique opportunity for studying the structural evolution and dynamic mechanism at the tip of a propagating seafloor spreading. Both models have unresolved questions, and further studies are needed.

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.