Seismic Oceanic Crustal Structure Of The Southwest Indian Ridge Segment In Duanqiao Hydrothermal Active Field

The lithosphere of Mid-Ocean Ridge is the place where the material and energy from the earth's multiple spheres exchange with each other.The Southwest Indian Ridge(SWIR),characterized by anomalous low magmatic productions,low mantle temperature and long transform fault,is considered as a representative and also a key research area of an ultra-slow spreading mid-ocean ridge.Two outstanding scientific problems regading the SWIR need to be solved:(1)With small amount of magma supply,what are the mechanics and structures of the dense hydrothermal active area(energy material exchange area)in this ultra-slow spreading ridge?(2)Is the crust of this ultra-slow spreading ridge composed of the exposed serpentine mantle,oceanic core complex or just normal ophiolite suite? One of the key technologies to solve these problems is the refraction seismic experiment.From January to March in 2010,during the 6th leg of voyage DY115-21,a 3-D Ocean Bottom Seismograph(OBS)exploration experiment was carried out on the “Longqi” and “Duanqiao” hydrothermal active field of SWIR.This experiment aims to obtain a velocity structure of the oceanic crust in the hydrothermal area with the inversion of the seismic tomographic imaging method.The results can be used to indicate a spreading and evolution mechanism of the ultra-slow spreading ridge and provide basis for the exploration and development of the submarine hydrothermal mineral deposits in this field.The “Duanqiao” hydrothermal field,with anomalous shallow bathymetry,locates between the Indomed and Gallien transform faults of SWIR.Its full spreading rate is ?14 mm/a,and an extinct hydrothermal vent was found in the spreading center.In this study,we used the seismic data of line-y5y6(crossing the spreading center within the “Duanqiao” hydrothermal field)from the 3-D OBS exploration experiment to obtain a 2-D P-wave(Vp)and S-wave(Vs)structure models.Our results show that the thickness of the layer2 is relatively uniform(? 2km),with a lateral difference in Vp;the interface between layer2 and layer3 is generally consistent with the isoline of 6.4 km/s(Vp).The thickness of layer 3(?6 km)is extremely higher and the Vp(6.3-6.9 km/s)is lower than the “Normal oceanic crustal”.In addition,the Vs of the layer2 is 1.3-3.2 km/s with generally uniform depth in the transverse direction,while the Vs of the layer3 increases from 3.3km/s at the top interface to 4.3 km/s at the bottom boundary with depth,and no significant change is found along the transverse direction as well.The origin of the anomalously thick oceanic crust and low velocity of layer3 in the study region is most likely due to the heterogeneity of mantle.Integrating the velocity structure,Vp/Vs and Poisson's ratio(?)with previous studies,we suggested that the oceanic crust of the ridge within the “Duanqiao” hydrothermal active field belongs to the normal oceanic crust,where the layer2 and layer3 consist of basalt and gabbro,respectively,and both layers have not been affected by serpentinization.