Magmatic activities on the Southwest Indian Ridge between 35°E and 40°E, the closest segment to the Marion hotspot
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We conducted geophysical surveys, including bathymetry, gravity, and magnetism, within a first‐order segment of the Southwest Indian Ridge (SWIR) between the Prince Edward and Eric Simpson fracture zones (FZs) (latitude 35°–40°E, segment PE), in the vicinity of the Marion hotspot. Segment PE includes four orthogonally spreading second‐order segments (PE‐1, PE‐2, PE‐3, and PE‐4) and a long, oblique axial valley (NTD‐1). Segments PE‐1, PE‐2, and PE‐4 are magmatic, whereas segment PE‐3 and NTD‐1 are characterized by low magmatic activity. Segment PE‐3 is a nascent segment and NTD‐1 contains three tiny magmatic sections. Each low‐magmatic interval along the axis of segment PE lies between two magmatic segments. This segmentation pattern is similar to the SWIR between the Gallieni and Melville FZs; therefore, a strong melt‐focusing process can be expected. Different characteristics of second‐order magmatic segments suggest that the magmatic activity in each segment varies among each other as well as that of the other segments of SWIR. Continuous seafloor morphology and isochrons over off‐axis areas of segment PE‐1 and NTD‐1 suggest that PE‐1 shortened after the C2An chron. The V‐shaped bathymetric structure between segment PE‐1 and NTD‐1 suggests that the melt supply center has migrated westward. This westward melt migration would have reduced magmatic activity at NTD‐1 after C2An. Ridge obliquity may also have reduced magmatic activity. Geophysical characteristics of second‐order segments suggest that magmatic activity of segment PE is mainly controlled by a strong melt‐focusing process and a comparatively low contribution of melt supply from Marion hotspot.Keywords:
Hotspot (geology)
Seafloor Spreading
Isochron dating
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Classification of discontinuities
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Abstract We establish a high‐resolution magnetic isochron pattern in the East Subbasin (ESB) of the South China Sea (SCS) based on recently collected magnetic data, which provides an updated age of seafloor spreading in the ESB and reveals a new type of ridge reorientation. Seafloor spreading in the ESB initiated at Chron 11n.1r (29.7 Ma) and ceased shortly after Chron 5Br (15.6 Ma). Successive ridge jumps occurred between Chrons 9r and 7n, which explains the substantial asymmetric geometry of the ESB. Furthermore, the ridge reorientation associated with ridge jumps highlights a new ridge reorientation model in which the ridge jumps off‐axis and reorients synchronously to adapt to the new direction of seafloor spreading. In the ESB, this type of reorientation responds more rapidly to changes in the direction of plate motion than gradual ridge rotation.
Seafloor Spreading
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Bedforms are common features in shallow marine environments, and their presence evokes questions regarding the spatial and temporal stability of the seafloor. Though observation of bedform dynamics from multibeam bathymetry and its derived products enhances understanding of seafloor stability, the ability to successfully detect bedform migration depends on (1) the survey resolution and positioning uncertainty, and (2) the establishment of an optimum survey-repetition rate.
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Seafloor Spreading
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Multibeam bathymetry data could represent nearly continuous coverage depth measurements of the seafloor and reveal geomorphological regions. Recent studies have utilized multibeam bathymetry data to provide geological maps, but their delineations were done manually. Manual classification and delineation are inherently subjective and therefore can be inaccurate. In this paper, we try to develop one strategy to explore seafloor stretching in Mariana trench arc via squeeze and excitation network, combining data clustering, slope and gradient. In our experiments, we use the high-resolution multibeam bathymetric data collected by NOAA Office of Ocean Exploration and Research (OER). The geomorphological seabed in the Mariana region is automatically classified into different classes. The experimental results demonstrate that geomorphological seabed classification strategy achieves a robust, automated delineation approach.
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Long-range acoustic experiments done in the mid-Atlantic ridge region show strong backscatter from many ocean bottom features. One type of feature characteristic of this region, and a possible contributor to this acoustic backscatter, takes the form of a seafloor ridge corner formed by two ridge faces intersecting at near-right angles. We have investigated through computer simulations the effects of an anelastic seafloor corner on the backscattered acoustic field. The acoustic source insonifying the seafloor corner was near the sea surface and approximately 4 km away in range. The ocean depth at the seafloor corner was also 4 km. Simulations were made using cw computer models. Time-domain calculations were obtained from FFTs of the cw fields. Examples are presented that show large backscatter near the base of the ridge while relatively low backscatter near the ridge peak. This is attributed to the angle of the insonifying field and multiple scattering effects. Backscatter from a longer range (10 - 20 km), gently sloped sedimented seafloor is also presented and discussed.
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For decades, sidescan sonars have been the primary tool to obtain acoustic images of the seafloor. Such images provide qualitative information on the seafloor surveyed based on amplitude variations of the backscattered acoustic signals received. In the 1980s, bathymetric sidescan sonar systems, capable of simultaneously producing acoustic images and measuring depth at numerous points across the swath, added a quantitative description of the seafloor in the form of a depth contour map. Similar claims can be made with multibeam echo sounders well known for their high-resolution swath bathymetry capabilities. Taking advantage of this high bathymetric resolution, the beamformed acoustic backscatter data can also be displayed as a geometrically correct acoustic image of the seafloor and provide textural information not available in the contoured bathymetry of the same area. Likewise, knowledge of the bathymetry, particularly bottom slopes, is needed to correct for the angular dependence of seafloor acoustic backscatter and construct a map of acoustic backscattering strength over the area. Such a map will give clues to regional variations in lithelogies.
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