Deep Structure And Thermal Structure Of The Southwest Indian Ridge 48°-52°E

The Southwest Indian Ridge(SWIR)is about 7700 km long,with a small axial variation in spreading rate of about 14 mm/a.It is a super-slow spreading mid-ocean ridge and is mostly spreading obliquely.More than 10 transform faults and non-transformation discontinuities(NTD)of different scales divide the SWIR into multi-level ridge segments.A series of explorations have been carried out along SWIR during the Chinese Dayang cruises.More than 10 active and inactive hydrothermal vents and numerous hydrothermal anomalies have been identified between the Indomed and Gallieni fault zone(FZ).They may be controlled by various geologic settings,undergo diverse processes of hydrothermal circulation,host unique ecology systems,and form hydrothermal deposits in various morphologies with higher mineral resource potential.The magma at depth provides the heat source,and the detachment fault provides the channel for the hydrothermal circulation.Therefore,the deep structure and thermal structure of midocean ridges(MOR)are very important,and they are closely related to the formation of seafloor massive sulfide deposits and hydrothermal activity,which have the potential to form larger-scale seafloor massive sulfide deposits.In this thesis,shipborne gravity data and bathymetry data obtained from the Chinese Dayang cruises 20,21,30,34,39,and 40 to obtain seafloor gravity anomalies that redefine the MOR spreading axis in the 48°-52°E of SWIR,and extract the linear structure features in the study area.Subsequently,the density structure of the upper mantle in the study area is inverted,and the three-dimensional density structure of the top of the lithosphere in the SWIR Longqi and Duanqiao hydrothermal fields is inverted with the three-dimensional Ocean bottom seismometer data.Finally,the thermal structure of the study area is analyzed and discussed.The main research contents and results of this thesis can be summarized as follows:(1)A total of nearly 200 linear structures were inferred in the study area based on the vertical derivative of horizontal gradient,normalized standard deviation,tilt angle,and local wavenumber method.The inferred linear structures are mainly divided into two groups.One group distributes along the MOR spreading axis,including the fault and lithologic interface.The other group of nearly north-south distribution is a secondary interruption,probably NTD.The MOR spreading axis is separated by these nearly northsouth distributed linear structures and divided into several segments.(2)The three-dimensional density inversion method in the rectangular coordinate system is adopted,and a clear low-density anomaly is observed along the MOR spreading axis of the study area,reflecting that the old oceanic crust was pushed away from the spreading center during the process of the MOR to form a new oceanic crust.At the same time,the eastern end of the study area near the Gallieni FZ has a cold and thick lithosphere.The Longqi hydrothermal field is dominated by tectonism,while the Duanqiao hydrothermal field is dominated by magmatism.Different structural features reveal the hydrothermal field with different structure backgrounds and hydrothermal circulation mechanisms in SWIR,while the constrained density structure has obvious advantages reflecting more structural information,especially reflected in the deep anomalies.(3)The reference thermal structure of oceanic lithosphere with age and depth of oceanic crust is obtained by the latest oceanic lithospheric reference thermal structure equation considering the effects of radiogenic heating and long-term cooling,after which the density anomalies were converted to temperature anomalies and the two were summed to obtain the absolute lithospheric temperature in the study area.The results indicate that the MOR exhibits a significant high-temperature anomaly,especially between the Indomed and Gallieni FZ,and the temperature gradually decreases away from the MOR spreading axis,like the characteristics of the reference thermal structure,which may be the result of the spreading of the MOR axis.In this thesis,the linear structure and mid-ocean ridge spreading axis of SWIR 48°-52°E are inferred.In addition,the density structure and temperature structure of the upper mantle in this study area are obtained,and the top density structure of the lithosphere in the typical hydrothermal area were established.Based on the results of this thesis and combined with other related data,some new insights are proposed while verifying some of the previous viewpoints.These results and insights provide important references to study the scientific problems related to the deep structure and thermal structure of the ultraslow-spreading MOR and help to understand the magma supply and hydrothermal circulation mechanisms of the ultraslow-spreading MOR.