17:15 〜 18:45
[SCG48-P17] Late Cretaceous triple junction trace and an extinct spreading ridge around Del Cano Rise, Conrad Rise, and Crozet Plateau in the southern Indian Ocean
キーワード：地磁気異常、海嶺三重点、拡大停止海嶺、インド洋
Revealing the dynamics of the triple junction is essential for considering hotspot-ridge interactions and the formation and splitting of oceanic plateaus. In the central part of the Indian Ocean, there is an RRR-type triple junction known as the Rodriguez Triple Junction. The triple junction can be traced back to the off-axis part of the middle part of the Southwest Indian Ridge (SWIR) around C31ny, based on satellite gravity and magnetic anomalies. Additionally, a recent study proposed that it was located north of the Conrad Rise around C34ny. In this area, an extinct ridge between the Del Cano and Conrad Rises is estimated from the vertical gravity gradient. While this ridge was mentioned in recent models of the triple junction transition, its spreading history wasn't much considered in those models, and the hypothesis of this extinct ridge itself was not based on the seafloor topography or magnetic observations. We conducted magnetic surveys around the Del Cano Rise, Conrad Rise, and the Crozet Plateau. By combining our data with previous magnetic datasets, we evaluated the presence of the extinct ridge and trace of the triple junction around these oceanic plateaus.
The total magnetic anomaly profiles over the extinct ridge segment generally show a basically symmetrical pattern with respect to the axis. Additionally, the distinct magnetic boundaries parallel to the axis estimated from the vector magnetic anomaly analysis suggest two-dimensional magnetic sources. These magnetic features indicate that the extinct ridge hypothesis is plausible.
Magnetic anomalies were inferred to correspond to C34ny and C33no. The shape of the extinct ridge axis shows a curvature on the western side, while on that the eastern side is relatively linear. The total spreading rate on the west segment is ~62 km/Ma with 5-9% asymmetric spreading in the south. On the east segment, the rate is ~51 km/Ma, with 2-7% asymmetric spreading in the north. The spreading directions were different in each segment and changed with age. The different spreading styles imply that the extinct ridge is comprised of two segments.
Based on the shapes of the isochrons for C34ny and C33no as well as the spreading rates and asymmetricity, the formation history of the extinct ridge can be outlined as follows: 1. Just before C34ny, the eastern segment seemed to propagate to the western segment, and there was an offset between the eastern and western segments, 2. Subsequent asymmetric spreading or ridge jumps may resolve the offset, leading to the formation of a single spreading axis, 3. westward narrowing of the isochrons of C33no indicates ridge propagation, and 4. the extinct ridge may have ceased at ~78 Ma.
The eastern segment was aligned with the Southeast Indian Ridge (SEIR) at C34ny, implying that the segment was part of the SEIR. However, by the time the spreading ceased, the orientation of the Extinct Ridge differed from that of the SEIR, suggesting separation into distinct spreading segments. This situation indicates the presence of another branch of the spreading ridge between the Del Cano Rise and Crozet Plateau (SEIR segment), that is, the existence of a triple junction. Several magnetic anomalies can be explained by this hypothesis. The triple junction was located near the Del Cano Rise and Conrad Rise at C33no. After the cessation of the extinct ridge, the triple junction migrated northward, and the seafloor spreading transitioned to the SEIR and SWIR.
The total magnetic anomaly profiles over the extinct ridge segment generally show a basically symmetrical pattern with respect to the axis. Additionally, the distinct magnetic boundaries parallel to the axis estimated from the vector magnetic anomaly analysis suggest two-dimensional magnetic sources. These magnetic features indicate that the extinct ridge hypothesis is plausible.
Magnetic anomalies were inferred to correspond to C34ny and C33no. The shape of the extinct ridge axis shows a curvature on the western side, while on that the eastern side is relatively linear. The total spreading rate on the west segment is ~62 km/Ma with 5-9% asymmetric spreading in the south. On the east segment, the rate is ~51 km/Ma, with 2-7% asymmetric spreading in the north. The spreading directions were different in each segment and changed with age. The different spreading styles imply that the extinct ridge is comprised of two segments.
Based on the shapes of the isochrons for C34ny and C33no as well as the spreading rates and asymmetricity, the formation history of the extinct ridge can be outlined as follows: 1. Just before C34ny, the eastern segment seemed to propagate to the western segment, and there was an offset between the eastern and western segments, 2. Subsequent asymmetric spreading or ridge jumps may resolve the offset, leading to the formation of a single spreading axis, 3. westward narrowing of the isochrons of C33no indicates ridge propagation, and 4. the extinct ridge may have ceased at ~78 Ma.
The eastern segment was aligned with the Southeast Indian Ridge (SEIR) at C34ny, implying that the segment was part of the SEIR. However, by the time the spreading ceased, the orientation of the Extinct Ridge differed from that of the SEIR, suggesting separation into distinct spreading segments. This situation indicates the presence of another branch of the spreading ridge between the Del Cano Rise and Crozet Plateau (SEIR segment), that is, the existence of a triple junction. Several magnetic anomalies can be explained by this hypothesis. The triple junction was located near the Del Cano Rise and Conrad Rise at C33no. After the cessation of the extinct ridge, the triple junction migrated northward, and the seafloor spreading transitioned to the SEIR and SWIR.