| The Mid-ocean ridge system is one of the most important geological units of the whole earth.The ultra-slow spreading ridges, whose spreading rate is generally as slow as less than20mm/a,account for36%of the global mid-ocean ridges. Generally speaking, ultra-slow spreading ridgeshave a very limited magma supply, but tend to supply in focus pattern, and leading to adiscontinuous and uneven crust along the axis. Focus magmatism builds volcanic centers, andregion between those centers, the crust is very thin, even mantle peridotite exposed. Forultra-slow spreading ridges, tectonic extension plays a more important role in seafloor spreading,and leading to a thinning crust along the spreading direction, even exposed lower crust or uppermantle at the seafloor usually resulted from detachment faults. As the unique magmato-tectonicpattern, ultra-slow spreading ridges perform very distinctly at topography, geophysical fields andgeochemistry, and its spreading pattern should be a complement and amendment to thetraditional seafloor spreading model. Thus, study on ultra-slow spreading ridges is a hot topic inthe field of earth sciences today.Southwest Indian Ridge (SWIR) has a spreading rate of only14~16mm/a, and is an idealplace to study ultra-slow spreading ridges. However, as its remote location, compared toMid-Atlantic Ridge (MAR) and East Pacific Rise (EPR), research on SWIR is still poor.Indomed-Gallieni Segment, located at the middle SWIR, shows an axial highland with volcanoswidely distributed, indicating its volcanogenic. This segment also shows very low residualmantle bouguer gravity anomaly (RMBA), indicating a very thick crust. Especially near50.5°Eof Segment27(Cannat et al,1999), preliminary OBS inversion shows a crust as thick as8~10km, suggesting a very adequate magma supply here. This segment shows very richgeological phenomenons, and of great scientific value. First, axial rift is absent at Segment27,and occupied by volcanic eruption high. Second, at south of Segment28, a oceanic core complex(OCC), as large as40km2, was identified both by topography and OBS data. Third, at theposition of37°47′S/49°39′E, also the south side of Segment28, reported the frst active high-temperature hydrothermal feld on ultra-slow spreading ridges worldwide. All thse bring outa series of scientific problems. Why such an axial highland can be built at ultra-slow spreadingridges whose magma supply is low? Is it effected by anything others? How magma increase willeffect the evolution of ultra-slow spreading ridges? Why under such a high magma supply candetachment faults develop which generally mean strong tectonic extension? The locations of thedetachment fault and the hydrothermal vent are very close, so is that just a coincidence, or thereis a link?To discuss problems mentioned above, Using the high-resolution and full-coveragemultibeam bathymetric data of SWIR (49°~51°E) mainly collected by voyage DY115-21ofChina in2010, and combined with relevant research results from regional gravity and magneticdata and3D OBS detection, mainly by the method of morphotectonics, this thesis tries to talkabout its magmato-tectonic evoluton, and draws following main understandings.Segment28and29are experiencing an asymmetry spreading under the control of deepasymmetrical faults. The north flank is ancient volcanic ridge accretion zone as the axialvolcanic constructions are mainly migrating to the north. There are lots of blocks at the southflank as the production of strong tectonic extension. Detachment faults may widely distribute atthe south flank, and the OCC is resulted from one detachment fault, whose influence range mayextend farther westward, covering the location of the hydrothermal vent. This detachment faultmay provided the material and energy access for the development of the hydrothermal vent.By comparing the geomorphological characteristic between axial area and the flank, it isbelieved that the volcanic seafloor of the north flank is the trace of the axial volcanic ridges(AVRs), while the smooth seafloor corresponds to the nontransform discontinuous (NTDs).However, the extending direction of such geomorphological partition is inconsistent with thespreading direction. This should suggest that the loaction of AVRs is migrating, and so thepositon of focus magmatism is migrating, too. Judged from the geomorphological characteristicof the north flank, the positon of focus magmatism here shows a westward migration.The axial topography, which is alternated with ridge and trough and arranged in en echelonpattern, is often owned by ultra-slow oblique spreading ridges, which suggests a strong focusmagmatism pattern. The volcanic seafloor at the north flank is divided into several phases bylarge long faults, and this indicates that the axial magmatism also has some kind of cycle duringwhich magma supply gradually becomes less. During the magma supply high time, AVRs arebuilt, while splited when becomes much lower. Such magmatism cycle at Segment28and29is about1.5Ma. The several conjugate uplifts of Segment27distributed axial symmetricallyindicates a similar magmatism cycle here, too. However, it may be because Segment27hasmuch larger magma flux, it behaves quite differently from Segment28,29in splitting pattern,period length and strength of tectonism.It is deduced a magma increased event started to effect Segment Indomed-Gallieni at about10~8Ma ago, and had built the axial highland here. However, this a magma increased event nowonly still has an effect on Segment27. Since4Ma ago, other part of Segment Indomed-Gallienihad restored to normal ultraslow spreading evolution, just as Segment28and29, and this is whythey now have evry strong tectonic activity. During the earlier time, most part of SegmentIndomed-Gallieni may have experienced the same evolution as today’s Segment27. Segment27is the only area still affected by this magma increased event, and shows some characteristicusually owned by faster spreading ridges in spite of an ultraslow spreading rate. Nevertheless itsintensity and extent are decreasing, and the magma increased event may be already at the end ofits life. After an analysis of topography, geophysical field and geochemical characteristics, it isthought that this magma increased event is not because of effect from Crozet hotspot, and maybe resulted from a local heterogeneous mantle. |
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