The morphotectonics and its evolutionary dynamics of the central Southwest Indian Ridge (49° to 51°E)

The morphotectonic features and their evolution of the central Southwest Indian Ridge （SWIR） are dis- cussed on the base of the high-resolution flfll-coverage bathyraetric data on the ridge between 49°-51°E. A comparative analysis of the topographic features of the axial and flank area indicates that the axial topogra- phy is alternated by the ridge and trough with en echelon pattern and evolved under a spatial-temporal mi- gration especially in 49°-50.17°E. It is probably due to the undulation at the top of the mantle asthenosphere, which is propagating with the mantle flow. From 50.17° to 50.7°E, is a topographical high terrain with a crust much thicker than the global average of the oceanic crust thickness. Its origin should be independent of the spreading mechanism of ultra-slow spreading ridges. The large numbers of volcanoes in this area indicate robust magmatic activity and may be related to the Crozet hot spot according to RMBA （residual mantle Bouguer anomaly）. The different geomorphological feature between the north and south flanks of the ridge indicates an asymmetric spreading, and leading to the development of the OCC （oceanic core complex）. The tectonic activity of the south frank is stronger than the north and is favorable to develop the OCC. The first found active hydrothermal vent in the SWIR at 37°47＇S, 49°39＇E is thought to be associated with the detach- ment fault related to the OCC.

Geophysical Evidence for Carbonate Platform Periphery Gravity Flows in the Xisha Islands,South China Sea

High-resolution bathymetric and partial seismic data were utilized to determine the characteristics and distribution of mass-transport deposits(MTDs)in the Xisha area,North of the South China Sea.Many isolated carbonate platforms are found in this area,providing mass-wasting materials as a sediment source for MTDs.The MTDs in the study area were divided into three types(slide,slump,and debris flow)based on their rheological properties.The MTDs were mostly distributed at the toe of the carbonate platform slope,where a large amount of carbonate debris accumulated.A total of 32 MTDs were identified on the carbonate platform slopes.A systematic assessment of the MTD morphometric parameters showed that their total area covered 2225.2 km2,with the largest MTD covering 354.1 km2.These MTDs differ largely from the MTDs of other areas,especially in terms of sediment compositions and deposit processes.Gullies and channels on the slope and reefs on the top of the platform act as sediment conduits and carbonate-producing sources,respectively,playing vital roles in the distribution and generation of gravity flow deposits.The development model proposed in this work states that reefs disintegrate into carbonate debris and mix with seawater to form gravity flows,which scour the seafloor and accumulate at the downslope,eventually resulting in failure.Sediments from these failures turn into gravity flows,flowing into submarine canyons and channels.The results of this research further our understanding of the development pattern of MTDs in a carbonate setting.

Geophysical mapping of Vercelli Seamount： Implications for Miocene evolution of the Tyrrhenian back arc basin

Since the Tortonian, the geodynamic evolution of the Tyrrhenian Sea has been driven by an eastward roll-back of the entire Apennine subduction system, triggering distinct episodes of back-arc basin formation with spots of oceanic crust. Major structural differences are observed between northern and southern portions of the Tyrrhenian Sea, reflecting two distinct evolution stages of the Ionian slab retreat. In the central portion of the Tyrrhenian Sea, the seafloor morphology is characterized by a set of magmatic intrusions and structural highs associated to an Ee W magnetic lineament along the 41 st geographical parallel. The Vercelli seamount represents one example of structural highs correlated to Miocene magmatic episodes along the 41 st parallel zone. In this study, we discuss the results of new high resolution magnetic data and morphological mapping of the Vercelli seamount acquired during the VER2010 cruise. The seamount represents the relict part of a granitic intrusion emplaced during the Tortonian phase of Tyrrhenian rifting. Tectonic and deep-sea erosive processes have jointly modified the seamount structure that can be observed nowadays. Cumulative gradient analysis highlights an asymmetric morphology of the flanks as a result of erosive action of opposite water mass gyres which modelled the southern portion of the seamount. The joint interpretation of magnetic and bathymetric datasets identifies a high magnetized source laying close to the base of the seamount and located in correspondence to a small basin. This structure has been modelled as a post-Tortonian lava sequence emplaced between structural highs in correspondence of Ne S elongated flat sedimentary basin.Modelling of new geophysical data highlights the relationship between crustal setting and magnetic evidences of the central Tyrrhenian Sea, providing a new interpretation of the 41 st magnetic lineament.