Tectonic-magmatic Processes Of The Indian Ocean: Evidence On The Residual Mantle Bouguer Gravity Anomaly

For the Indian Ocean, three classes of spreading rates have been identified: theultra-slow spreading Southwest Indian Ridge (SWIR), the slow spreading CentralIndian Ridge (CIR) and the Southeast Indian Ridge (SEIR). Many local geologicalfeatures such as hotspots or plumes, large-scale fracture zones, and ocean corecomplexes have been found. Such systematic and regional complex geologicalsettings provide good conditions for studying on magmatic processes at mid-oceanicridges. Using the Indian Ocean as an example, we used the Euler Rotation method tomodel the initial symmetric RMBAs to discuss on-axis magmatic processes and theasymmetric RMBAs to discuss off-axis magmatic processes. We obtained thefollowing results:(1) Three main factors are considered to explain variations of RMBAs in ourstudy, including crustal thickness (Hc), potential mantle temperature (T), and mantledepletion (X). The gravity anomaly induced by5%mantle depletion is16mGal, theeffect of200℃thermal variation on gravity anomalies is46mGal, and oceanic crusthas to be taken into account in regions with RMBAs variations larger than46mGal.(2) Based on our results, we partition the Indian Ocean into four categories:symmetric negative RMBA (<0mGal) and symmetric positive RMBA (>0mGal),and asymmetric negative RMBA (<0mGal) and asymmetric positive RMBA (>0mGal). Symmetric negative RMBAs indicate areas that are lighter than normal due toexcessive on or near-axis magmatic supplies at the initial stage of oceanic crustalformation. Asymmetric negative gravity signals then occur from the resultingenhanced off-axis melt processes. While symmetric positive RMBAs indicate areasthat are heavier than normal due to reduced on or near-axis magmatic supplies, andasymmetric positive signals then express due to reduced off-axis melt.(3) Symmetric negative RMBAs and asymmetric negative RMBAs areassociated with the Bouvet, Marion, Reunion, Kerguelen, and Balleny hotspots or plumes in the Indian Ocean:1) When the hotspot gets close to or gets away from theridge, the off-axis magmatism is stronger than the on-axis magmatism (theasymmetric negative RMBAs are more negative than the symmetric RMBAs), and theasymmetric negative RMBAs are observed along the hotspot tracks.2) When thehotspot is on or near the ridge, the on-axis magmatism is stronger than the off-axismagmatism (the symmetric negative RMBAs are more negative than the asymmetricRMBAs).(4) The negative RMBAs in the range of-46mGal~0are result from the weakactivities of the Bouvet, Marion, Reunion, Kerguelen, and Balleny hotspots or plumesin the Indian Ocean. The negative RMBAs less than-46mGal are result from thestrong activities of these hotspots or plumes, accompanied by hotspot-related crustalthickening and hotspot-related hydrothermal mineralizing system.(5) Symmetric positive RMBAs and asymmetric positive RMBAs are associatedwith the oceanic core complex (OCC) in the Indian Ocean, accompanied byOCC-related crustal thinning and OCC-related hydrothermal mineralizing system.(6) Widespread obvious asymmetric positive RMBAs are observed in thenorthwestern flank of the SEIR and the southern flank of the SWIR around theRodriguez Triple Junction (RTJ) in our results, where no OCC has been investigated.We predict the exist of the OCC and the OCC-related hydrothermal mineralizingsystem in these areas.(7) On the basis of the previous geology, chronology and geochemicalachievements, we constructed the plate frame of the Indian Ocean since120Ma bythe Gplates software, to reconstruct the processes of the breakup of theGondwanaland, the extinct of the Neo-Tethys Ocean, and the formation of the IndianOcean.(8) Correlation analysis between along axis geochemistry and geophysics wereconducted in our study: a positive correlation exists between the degree of mantledepletion and RMBA in areas (-16mGal <RMBA <16mGal) that may be affectedby mantle depletion, a negative correlation exists between the mantle temperature andRMBA in areas (-46mGal <RMBA <46mGal) that may be affected by mantle temperature, and a positive correlation exists between the crustal thickness andRMBA in areas (RMBA<-46mGal and RMBA>46mGal) that must be affected byoceanic crust.