Deep Seismic Detection And Crustal Structure Imaging In The Middle Section Of The 90°E Ridge In The Indian Ocea

The Ninety East Ridge(NER)is the world’s longest linear structure developed in the eastern Indian Ocean,extending southward from the continental shelf of the Bengal Basin across the equator to the southern Indian Ocean’s Broken Ridge.The formation of the ridge is related to the multi-stage evolution of the Late Cretaceous to Cenozoic in the eastern Indian Ocean.Due to its crossing of the equator and its length exceeding 5,000 kilometers,the tectonic background varies in different regions,leading to distinct geophysical characteristics in different segments.This paper is based on the analysis of two deep seismic reflection profiles acquired during the 2021 IND-DWZP01 geophysical survey expedition in the eastern Indian Ocean.The data preprocessing includes decoding and truncating single-station Ocean Bottom Seismometer(OBS)data,clock drift correction,shot and secondary time corrections,and conversion of Common Receiver Gathers(CRGs)to profile displays.The seismic phase characteristics of the 90°E Ridge and the adjacent oceanic ridges are analyzed.A twodimensional initial crustal structure model is constructed by combining existing research results.Forward modeling using the RAYINVR software package is performed to obtain synthetic images.The model results show that the sediment layer thickness in the middle section of the90°E Ridge is relatively small,with a maximum thickness of approximately 1.0-1.5kilometers in the surrounding basins.The lateral heterogeneity of the crust is significant.The seismic velocities in the ridge range from 3.7-5.0 kilometers per second at the top to7.0-7.7 kilometers per second at the bottom.The upper crust exhibits a strong velocity gradient,while the lower crust shows a weak velocity gradient.The Moho interface exhibits distinct segmented characteristics,with significant variations in Moho depth.The maximum depth increases from 12.5 kilometers in the central Indian Ocean Basin to 28.5 kilometers at the ridge,and reaches 14.5 kilometers in the Wharton Basin to the east.The uppermost mantle layer has velocities ranging from 7.8-8.0 kilometers per second at the top to 8.2kilometers per second at the bottom.Based on a comparison with existing research results,the following understandings are obtained:(1)The crustal thickness in the basins on both sides of the 90°E Ridge matches the average thickness of oceanic crust,while the crust at the ridge itself is thicker.(2)From the central Indian Ocean Basin to the 90°E Ridge and then to the eastern Wharton Basin,the Moho depth changes from shallow to deep and then to shallow again.The maximum Moho depths beneath the ridge are 28.5 kilometers and 22.0 kilometers,respectively,and the variations in the Moho interface mirror the variations in the seafloor interface.(3)The upper crustal velocities at the ridge increase from 3.7 kilometers per second to 6.5kilometers per second,indicating thickening of extrusive rock layers and lower velocities compared to adjacent seafloor basalts,suggesting lower density and higher porosity.This is likely related to the high pressure during shallow-water intrusive magmatism and lava cooling.The 6.0 kilometers per second velocity contour line beneath the ridge is located approximately 3-4 kilometers below the sedimentary basement,suggesting a transitional zone of basalt and ultramafic rock materials.(4)The lowermost crustal velocities at the ridge exceed 7.2 kilometers per second,indicating the presence of melt products from hot spot volcanism intruding into or beneath the crust.The thicknesses of the intrusive layer beneath the ridge are approximately 9 kilometers and 6 kilometers for the two seismic profiles,respectively,with a gradual thinning towards both sides of the ridge.(5)The ridge exhibits segmented characteristics,and each segment may indicate different causal mechanisms: the northern segment may have formed due to intraplate volcanic activity,the middle segment is influenced by ridge-plume interactions,and the southern segment may have formed due to the transformation fault between the Indian Plate and the Antarctic Plate.