Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 87–95 DOI: 10.1007/s13131-013-0394-1 http://www.hyxb.org.cn E-mail: [email protected] The morphotectonics and its evolutionary dynamics of the central Southwest Indian Ridge (49° to 51°E) LIANG Yuyang1,2,3, LI Jiabiao3*, LI Shoujun3, RUAN Aiguo3, NI Jianyu3, YU Zhiteng3, ZHU Lei4 1 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Key Laboratory of Submarine Geosciences, the Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China 4 China Ocean Mineral Resources Research and Development Association, Beijing 100860, China Received 12 May 2013; accepted 18 August 2013 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2013 Abstract The morphotectonic features and their evolution of the central Southwest Indian Ridge (SWIR) are dis- cussed on the base of the high-resolution full-coverage bathymetric 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. Key words: ultra-slow spreading, multibeam bathymetry, morphotectonics, oceanic core complex, Southwest Indian Ridge Citation: Liang Yuyang, Li Jiabiao, Li Shoujun, Ruan Aiguo, Ni Jianyu, Yu Zhiteng, Zhu Lei. 2013. The morphotectonics and its evolutionary dynamics of the central Southwest Indian Ridge (49° to 51°E). Acta Oceanologica Sinica, 32(12): 87–95, doi: 10.1007/ s13131-013-0394-1 1 Introduction of hydrothermal vents are not reduced in ultra-slow spreading Ultra-slow spreading ridges have been a global hot research ridges. In 2007, the first active high-temperature hydrothermal field because of their scientific and resource significance. A typ- field on ultra-slow mid-ocean ridge was discovered at the loca- ical ultra-slow spreading rate usually is lower than 12 mm/a, but tion 37°47′S, 49°39′E of the Southwest Indian Ridge (SWIR). It sometimes it is defined as being no more than 20 mm/a (Dick was also found that the distribution density of hydrothermal et al., 2003). Previous seafloor spreading models of mid-ocean vents between 49°–52°E was as high as 2.5/100 km, close to that ridges were mainly based on the studies about slow-fast spread- of 36°–38°N of the MAR where there is a sufficient magma sup- ing ridges such as the EPR (East Pacific Rise) and MAR (Mid-At- ply (Tao et al., 2011). Since the polymetallic sulfide deposit as- lantic ridge), so it is necessary to consider the ultra-slow spread- sociated with hydrothermal activity is one of important poten- ing ridges in order to understand some unique phenomena tial mineral resources, ultra-slow spreading ridges also have an such as the extremely thinning oceanic crust, oblique spread- important resource significance. ing, non-transform discontinuities and distinct segmentation The SWIR, whose spreading rate is only 14 mm/a with its pattern. This is also important for improving our knowledge western part being slightly faster than the eastern, has a num- about the dynamic mechanism of global seafloor spreading and ber of unique characteristics of ultra-slow spreading. Following the interaction between different spheres of the earth. Studies a series of scientific cruises (Fig. 1), progress has been made on slow-fast spreading ridges showed that hydrothermal vents in study on the morphotectonics of this area. Dick et al. (2003) are mainly controlled by the heat sources of melt bodies and, depicted the axial topography of 9°–25°E, and combining with therefore, correlated with spreading rate. It might be inferred gravity and magnetic data and geological sampling illustrated that the hydrothermal activity would decrease on an ultra-slow the structural features of the oblique-orthogonal spreading, spreading ridge because of its relatively limited magmatic ac- the magmatic accretion and amagmatic accretion. Cannat et tivity. However, recent investigations revealed that the activities al. (2006) analyzed the topography of both flanks of 61°–66°E Foundation item: The National Natural Science Foundation of China under contract No. 91028006; the Dayang 115 under contract No. DYXM- 115-02-3-01. * Corresponding author, E-mail: [email protected] 88 LIANG Yuyang et al. Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 87–95 40° 45° 50° 55° 60° 65° 70° 75° E a 25° S 30° 35° 36° 40° 44° 48° 52° E 34° S b 38° 40° 42° 46° 45° 50° RMBA −120−80 −40 04080 mGal −4 000 −3 000 −2 000 −1 000 0 Depth/m Fig.1. Bathymetry map (ETOPO2) of SWIR and the study area, the shaded relief image illuminating from N45°E at an angle of 45° (a) and regional RMBA of SWIR (b) (Zhang et al., 2013). and identified three distinct types of seafloor: (1) volcanic tonics and discuss their structural characteristics and dynamic seafloor, with bounding horsts and grabens in a similar man- mechanism. ner to the abyssal hills described at faster spreading seafloor; (2) smooth seafloor, probably associated with amagmatic ac- 2 Data and methods cretion; and (3) corrugated seafloor, probably associated with The bathymetric data mainly come from the full-coverage detachment fault-caused OCCs. Sloan et al. (2012) focused on multibeam survey during the leg for 3D OBS seismic explora- the flank topography of 54°–67°E and discussed the effect of tion at the SWIR (49°–51°E) of cruise DY115-21 in 2010, with the spreading rate, sub-axis mantle temperature, and volcanic or integrated multibeam data collected from other cruises of this non-volcanic crust on topographic parameters such as root area. The data was collected by Simrad EM120 multibeam echo mean square height, lineament azimuth and characteristic sounder system on board the R/V Dayang I. It was mounted width of abyssal hills. Based on analysis of the on-axis deep tow under the hull with a 12 kHz transmitting/receiving transducer side scan sonar data and off-axis multibeam bathymetric data and contains 191 beams arrayed over an arc of 150°and each of both 58°30′–60°12′E and 63°23′–65°45′E, Mendel et al. (2003) beam width up to 1°. The maximum depth of the survey was thought that there was a magmato-tectonic cycle of the axial 11 000 m. Multibeam processing systems were set up on the evolution. vessel to carry out real-time processing for the sound veloc- Ultra-slow spreading ridge is an important frontier in the ity, heave, roll, pitch and heading offered by the velocity pro- deep sea research, but so far publications are rare. The features files and motion sensor unit (U-Phins & Octans). The drafts at of magmatic and amagmatic segments at the central rift valley start and end of the survey were recorded, and the linear draft of ultra-slow spreading ridges, the tectonic role the detachment correction for depth was made in post-processing. A full-cov- fault played in the spreading process, and the distinct tectonic erage multibeam sounding bathymetric survey was carried out evolution and dynamic mechanism of ultra-slow spreading over the region of 49°–51°E of the SWIR, with a total measuring ridges are some of the un-answered questions. length of 5 930 km. According to the principles of topographi- With little sediment cover at the spreading center of the cal continuous variation and adjacent swath comparison (Li, SWIR, the seafloor topography is basically a direct reflection of 1999), all the data were examined and edited to remove errone- regional tectonics and deep response. Using the high-resolu- ous points with the CARISHIPS 6.0 software. The depth data was tion multibeam bathymetric data of the SWIR (49°–51°E) col- exported as an ASCII file with the HIPS, and was subsequently lected during voyages over several years, in combination with gridded at 50 m spacing using universal Kriging, shaded with relevant research results from regional gravity-magnetic data artificial illumination, and displayed on a workstation using and 3D OBS detection, we analyze the segmental morphotec- Global Mapper V 7.01 (Fig. 2). LIANG Yuyang et al. Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 87–95 89 49.0° 49.2° 49.4° 49.6° 49.8° 50.0° 50.2° 50.4° 50.6° 50.8° E 37.4° S 37.6° 37.8° 38.0° −4 000 −3 000 −2 000 −1 000 Depth/m Fig.2. Multibeam shaded relief image of the study area.