09:15 〜 09:30
[SCG45-02] The new tectonic model between the Madagascar Ridge and the Del Cano Rise in the southern Indian Ocean
The Indian Ocean is among the best locations worldwide to understand the supercontinent fragmentation process because the seafloor spreading history following the breakup of Gondwana has mostly remained. Focusing on the south Indian Ocean, the seafloor is dominated by the Southwest Indian Ridge (SWIR), and several aseismic ridges and hotspots are emplaced. Several aseismic ridges have been recognized as oceanic large igneous provinces. Many studies have already presented seafloor-spreading models of the Indian Ocean; however, previously reported magnetic survey lines are sparse and not adequately aligned to detect the seafloor spreading direction. Therefore, the detailed spreading history, including these aseismic ridges, is still an open question.
Based on our newly obtained magnetic data (total and vector magnetic field data) as well as using an open dataset of magnetic data, we re-interpreted seafloor ages in the SWIR segments between the Discovery Ⅱ and Gallieni fracture zones. The Madagascar Ridge, Del Cano Rise, Crozet plateau, and Conrad rise are emplaced on both off-axis areas. The identified oldest isochron was Chron 31o (71.4 Ma). Before Chron 21y (45.7 Ma), the spreading rate was ultraslow (18–19 km/Ma). Following Chron 21y, the spreading rate was changed to slow (~25 km/Ma). The spreading direction was changed from N10°W to N05°E around Chron 21. The segmentation pattern at Chron 30 (66.5 ~ 68.4 Ma) differs from the present SWIR. The reorganization had occurred around Chron 24 (52.6 ~ 57.1 Ma) and then changed into the present pattern around Chron 21. Revised seafloor ages suggest that the southern Madagascar Ridge and the Del Cano Rise had once formed a single bathymetric high at least before Chron 30. These two rises have thick crusts and have been recognized as formed by excess volcanism on or near the spreading axis of SWIR, or Marion hotspot-SWIR interaction; however, isochrons and linear magnetic anomalies implying the seafloor spreading process do not observe on the two rises. Therefore, we propose that excess volcanism related to seafloor spreading is not the only factor for their gravity anomaly and thick crust, the other factor, such as continental fragments, most likely contributes to the formation of the rises.
Based on our newly obtained magnetic data (total and vector magnetic field data) as well as using an open dataset of magnetic data, we re-interpreted seafloor ages in the SWIR segments between the Discovery Ⅱ and Gallieni fracture zones. The Madagascar Ridge, Del Cano Rise, Crozet plateau, and Conrad rise are emplaced on both off-axis areas. The identified oldest isochron was Chron 31o (71.4 Ma). Before Chron 21y (45.7 Ma), the spreading rate was ultraslow (18–19 km/Ma). Following Chron 21y, the spreading rate was changed to slow (~25 km/Ma). The spreading direction was changed from N10°W to N05°E around Chron 21. The segmentation pattern at Chron 30 (66.5 ~ 68.4 Ma) differs from the present SWIR. The reorganization had occurred around Chron 24 (52.6 ~ 57.1 Ma) and then changed into the present pattern around Chron 21. Revised seafloor ages suggest that the southern Madagascar Ridge and the Del Cano Rise had once formed a single bathymetric high at least before Chron 30. These two rises have thick crusts and have been recognized as formed by excess volcanism on or near the spreading axis of SWIR, or Marion hotspot-SWIR interaction; however, isochrons and linear magnetic anomalies implying the seafloor spreading process do not observe on the two rises. Therefore, we propose that excess volcanism related to seafloor spreading is not the only factor for their gravity anomaly and thick crust, the other factor, such as continental fragments, most likely contributes to the formation of the rises.