Upper-mantle Density Structure Beneath The Typical Slow-ultraslow Spreading Ridges And Its Geological Implications

Mid-ocean ridge is a huge active tectonic belt with the most frequent volcanic and seismic activity,massive magma upwelling and formation of new oceanic crust in the earth system,and also a kind of tectonic boundary of the plates.The research on the structural of the ridge has great significance for understanding the tectonic evolution and dynamic process of oceanic lithosphere.As a manifestion of intraplate mantle magmatism,the adjacent hotspots of ridges can affect them,making some segments of them to present anomaly.Moreover,as another represent of large igneous province(LIP)in oceanic area and intraplate mantle magmatism,the anomalies of oceanic plateaus indicate their formation and evolution mechanisms are probably related to hotspot and ridge.Therefore,ridge especially the ultraslow spreading ridge has become the focus in research on the Marine science in recent years.However,due to the limitations of scale of current research on ridge and regional ship-borne geophysical data,there are still some shortcomings in the research on scientific issues related to ridge and its adjacent area.Therefore,the mantle gravity anomalies which can reflect the heterogeneous uppermantle density distribution beneath the ultraslow spreading Southwest Indian Ridge(SWIR)and slow spreading South Atlantic Ridge(SMAR)and their adjacent areas are firstly calculated in this thesis.Then the upper-mantle density structures beneath the study areas are imaged based on geophysical inversion theory.Subsequently the inverted uppermantle density variation is approximately divided into reference density anomaly and residual density anomaly which can reflect thermal and compositional effects respectively.The density features of different structures in study area show the ridges have low reference and inverted density anomalies and negative residual density anomaly at shallower depths.The oceanic plateaus and rises which belong to LIP present obvious low inverted density anomaly and negative residual density anomaly at deeper depths.The oceanic basins present high inverted density anomaly and positive residual density anomaly.This thesis obtains the following geological knowledge through density features combined with other data:(1)The SWIR has three segments with higher degree of mantle melting.Combined with residual bathymetry and isotopic ratios of the MORB.This thesis suggests the segment between the Prince Edward and Discovery II Fracture Zones(FZs)is influenced by the melt supply from the Marion hotspot.But no evidence indicates the segment between the Indomed and Gallieni FZs is affected by the Crozet hotspot.For the SMAR,the segment south of the Agulhas-Falkland FZ also has high degree of mantle melting and is suggested to be influenced by the residual plume materials from the Discovery hotspot.Moreover,the density and temperature structures beneath hotspots indicate they generally present thermal anomaly above 300 km and melting anomaly distributed within transition zone.(2)The Madagascar Plateau,Del Cano Rise in the southwest Indian Ocean and Rio Grande Rise,Walvis Ridge in the South Atlantic Ocean are taken as examples to discuss the formation mechanism of oceanic plateaus in this thesis.Firstly the low-density anomaly beneath the oceanic plateaus reflect they belong to LIP and their mantle plume origins.When they gradually leave away from plume with plate movement,the original enriched plume materials cool down and experience melting extraction,resulting in current cold and depleted mantle with negative residual density anomaly.However,differences of reference density,temperature and lithospheric susceptibility between oceanic plateaus suggest the Del Cano Rise and southwest part of the Walvis Ridge are still influenced by mantle plume materials and present high mantle temperature.(3)Various data is used to compare ridges with different spreading rates,where the difference of velocity and temperature anomalies reflects the difference of mantle temperature and upwelling of hot materials beneath ridges with different spreading rates.The results show the EPR has higher mantle temperature than other ridges.Combined with subduction zone of global plate boundaries,this thesis suggests the far-field tectonics also affects the spreading rate of ridge besides the upwelling of mantle materials,which makes the EPR has fast spreading rate.Moreover,this thesis compares degree of mantle meling beneath two segments of the SMAR and SWIR respectively,suggesting the SMAR has higher degree of mantle melting than the SWIR.Therefore,this thesis suggests degree of mantle melting is a significant index which can reflect the spreading rate of ridge besides oceanic crust age,LAB depth and mantle temperature.This thesis obtains the upper-mantle density structures beneath the slow and ultraslow spreading ridges and their adjacent areas.Based on our results and other data,some previous opinions are proved and new points are proposed.These results and points provide important references for the research on scientific issues about ridge,hotspot and oceanic plateaus.Moreover,they have significance on understanding the tectonic evolution and deep dynamic processes of oceanic lithosphere.