东南印度洋中脊地质构造特征及研究进展

The geotectonic features of the Southeast Indian Ridge and its current research progress

东南印度洋中脊(Southeast Indian Ridge, SEIR)是印度洋中扩张速度最快的洋中脊,由SEIR增生的洋壳占印度洋总面积的50%以上,它是塑造印度洋现今构造格局的关键要素.相对西南印度洋中脊和西北印度洋中脊, SEIR具有更复杂的地质构造特征和演化过程.综合SEIR及邻区海底高原的地形地貌特征、重磁异常特征和玄武岩地球化学特征,探讨了SEIR的分段、洋中脊演化过程和地幔不均一性,以及板内火山作用与洋中脊的成因关系等.本文将有助于深入理解东南印度洋区域的构造演化历史,全面理解整个印度洋的洋中脊系统和大地构造格局,增进对冈瓦纳大陆裂解和印度洋演化过程的认识.初步研究认为东南印度洋区是多期洋中脊演化的结果,经历了北西向扩张、南北向扩张直至北东向扩张的三期洋壳增生过程.东南印度洋脊下的地幔源区存在不均一性,尤其是阿姆斯特丹-圣保罗海底高原和澳大利亚-南极错乱带两个区域.东南印度洋中的海底高原与热点火山作用密切相关,同时部分存在热点-洋脊相互作用或残留陆壳物质的影响.

The Southeast Indian Ridge(SEIR)is the fastest spreading ridge in the Indian Ocean,with an average half spreading rate of25-50 mm/a.The crust produced by the SEIR accounts for more than half of the total Indian oceanic crust.It represents a pivotal element in shaping the current tectonic settings of the Indian Ocean.Compared with the Southwest Indian Ridge(SWIR)and Northwest Indian Ridge(NWIR),the temporal and spatial evolution of the SEIR has been more complex,as reflected by the present-day tectonic and geomorphological diversity of its segmentation pattern.In this study,we integrate the topographic and geomorphological features,the gravimetric and geomagnetic signatures,and the MORB geochemical characteristics from the SEIR and the adjacent areas,in order to define the segmentation of the ridge,to trace its evolution through time,to characterize the heterogeneity degree of the mantle source beneath the ridge and to explore potential genetic links between regional intraplate volcanisms and the spreading ridge.The study shows that the axial bathymetry of SEIR undulates along the ridge,with the shallowest depth at the Amsterdam-St.Paul Plateau(ASP)and the deepest ones at the Australian-Antarctic Discordance(AAD).Using bathymetric,gravity and geomagnetic data,thirty-one first-order ridge segments can be distinguished.The boundaries of each segment coincide with transform faults.The eastern Indian Ocean expanded owing to three main successive phases of oceanic accretion each characterized by a distinct orientation:northwestward phase before 84 Ma,a north-south one between 84 and 38 Ma and a northeastward phase after 38 Ma.The mantle source of the SEIR is heterogeneous,especially in ASP and AAD regions.While the ASP results from a hotspotridge interaction,the AAD represents the isotopic boundary between Indian and the Pacific Ocean mantle domains.In the eastern Indian Ocean,there are many off-axis oceanic plateaux,such as the Kerguelen Plateau,Broken Ridge,Ninetyeast Ridge,Naturaliste Plateau and Wallaby-Cuvier Plateau.The buildings of these off-axis oceanic plateaux are either related to plume activity,and/or to presence of remnants of continental crust material embedded in the shallow mantle.The SEIR integrated with the NWIR at^38 Ma,while the SWIR wedged into the NWIR-SEIR sometime after or before 38 Ma at the Rodrigues Triple Junction.The heavy isotopic compositions of SEIR are similar to NWIR and the northeast end of SWIR.Compared with Pacific-Antarctic Ridge,the Indian ridge systems show obvious Dupal anomaly,indicating the larger-scale mantle heterogeneities.This study can help better understand the evolution of the eastern Indian Ocean,the ridge systems and tectonic patterns of the entire Indian Ocean.It can also enhance our knowledge on Gondwana continent rifting and the evolution of Indian Ocean.