Using OBS Teleseismic Receiver Function To Study The Deep Structure Of The Southwest Indian Ridge And Interaction Of The Ridge-hotspot System

The possibility and limitations of extracting the receiver function from the teleseismic dataset recorded by Ocean Bottom Seismograph(OBS)via synthetical modeling are analyzed.Then the problem of applying receiver function on OBS data was finally solved.In this study,the impacts of water-layer multiples and sedimentary on calculating receiver function from OBS data were discussed.According to different situations,routines were proposed to solve the existed problems.Based on the analysis of extracting receiver function from OBS data,the teleseismic OBS dataset collected from the southwestern subbasin in the South China Sea in 2011 was applied on the examination of proposed routines and reliable results were achieved.Furthermore,these routines were used to process the teleseismic OBS data,which is sourced from the 3-D OBS exploration experiment at the Southwest Indian Ridge(SWIR at 50°E)in 2010.The magma supply and mechanism of hydrothermal activities were deduced by the inverted results of receiver functions,which were extracted from OBS teleseismic events.Simultaneously,a hot mantle transition zone(MTZ)beneath the mid-ocean ridge was inferred by the thinned MTZ thickness computed from the receiver functions.For discussing the debate of off-axis interaction between the SWIR and Crozet hotspot,the dataset recorded by CRZF station on Crozet islands was supplemented for the receiver function and regional waveform inversion.According to the results of receiver function and regional waveform inversion,regional positive thermal anomalies were deduced and give strong geophysical evidence to the indirect interaction between SWIR and hotspot.Under these processes,the following 3 main points were achieved:(1)It is feasible to calculate the receiver function from OBS data and its difficulties were solved.Due to the special seafloor environment of OBS,water-layer multiples and sedimentary were considered in synthetic modeling,their impacts on receiver function were discussed.The first conclusion is that the water-layer multiples affect the hypothesis of regarding vertical component as the equivalent source.But the polarization waves of multiples on both radial and vertical components have a strong correlation,so the influence of multiples on the receiver function will be mainly suppressed by deconvolution.This results in ignoring the influence of multiples.Consequently,it does not affect the subsequent processing of the receiver function,for instance,using H-k stacking to obtain the Moho depth and the average Vp/Vs ratio of ocean crust,adopting common conversion point(CCP)stacking to obtain the images of the discontinuity.Because the velocity and thickness of seawater can be easy to achieve,it also does not affect the inversion of the S-wave velocity structure of ocean crust-upper mantle.The second conclusion is that the dissipation effect of the sedimentary layer will make the waveform of the receiver function change a lot and the multiple phases of the Moho are difficult to identify,so it is hard to use H-k stacking to achieve the Moho depth and the average Vp/Vs ratio of the oceanic crust.Due to similar reasons,CCP stacking can not be used to image the discontinuity.But the inversion methods can change the models with the corresponding gradient to match the waveforms,so the inversion method can still be used to obtain the S-wave velocity structure of sedimentary layer,oceanic crust and upper mantle.Finally,the synthetic modeling results are verified by the observed OBS data in the southwestern subbasin in the South China Sea,and the S-wave velocity structure and Moho depth are obtained.They suggest that there is a magma chamber under the remnant spreading ridge.(2)The receiver function routines were applied on the observed OBS dataset collected in the SWIR and the S-wave velocity structure,Moho depth and thickness of MTZ beneath the near “Longqi” hydrothermal vent area were achieved.Firstly,a low velocity(?2.6 km/s)layer exists at 4?6 km under the seafloor in the inverted S-wave velocity model.It suggests that maybe there is the magma chamber or partial melting existed beneath the SWIR(49°39?E)and perhaps the lasting heat supply of the hydrothermal activities is sourced from this.Secondly,the Moho depths obtained from receiver function and P-wave tomography at the same place are quite different(e.g.the southwestern subbasin in the South China Sea and the Southwest Indian Ridge),the former is always deeper than the latter.The reason for this difference is analyzed from the seismic raypath of teleseismic events and active sources.We suggest that the different depth between receiver function and P-wave tomography result from the Moho transition zone with a thickness of 2?3 km,which is not a simply one-order seismic velocity discontinuity.Thirdly,the positive thermal anomaly of 182?237 K beneath the ridge was deduced from thinned MTZ.Comparing with the previous studies,the thinned MTZ and positive thermal anomaly beneath the hydrothermal vent at SWIR are the same as the situation beneath the normal spreading ridge rather than the influence of hotspot.(3)The results of CCP stacking of receiver functions extracted from a permanent station on Crozet islands reveal a large number of low-velocity layers in the mantle and the big undulation of 410-and 660-km discontinuities with various thickness of the MTZ,which has a strong lateral heterogeneity.Low-velocity layers beneath the Crozet hotspot are imaged by receiver functions and are interpreted to result from magma upwelling caused by elevated temperatures and mineral dehydration.Besides,regional waveform inversion indicates that the thickness of the MTZ beneath the area between SWIR and Crozet hotspot is thinned and a thermal anomaly of 124.97?165.72 K is deduced.In conclusion,both the thicknesses of SWIR and Crozet hotspot are thinned,there is no direct evidence for off-axis interaction between them.But the positive thermal anomalies in the mantle beneath both SWIR and hotspot lead to mineral dehydration in MTZ.The added water results in lower melting point,due to the decompression at the mid-ocean ridge,the melting materials of the mantle beneath SWIR and Crozet hotspot migrated to the spreading ridge and formed the mid-ocean basalt.