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The Central Indian Ocean, namely the Central Indian, Crozet, and Madagascar basins, formed by rifting and subsequent drifting of India (now Capricorn), Antarctica, and Africa (now Somalia). We gathered a comprehensive set of sea surface magnetic anomaly profiles over these basins and revisited location and identification of magnetic isochrons between C34ny (83.0 Ma) and C20ny (42.536 Ma) using the objective analytic signal technique. We present high‐resolution magnetic isochrons for 29 periods based on ~1,400 magnetic anomaly picks. From the conjugate sets of picks, we derive two‐plate finite rotation parameters for both the Capricorn‐Antarctica and Capricorn‐Somalia motions. These finite rotations are compared to three‐plate reconstructions of the plate boundaries between the Capricorn, Antarctica, and Somalia plates, constrained by the closure of the Indian Ocean Triple junction. In general, the three‐plate reconstructions slightly overrotate the reconstructed isochrons with respect to the Capricorn‐Antarctica and Capricorn‐Somalia two‐plate reconstructions. Conversely, the two‐plate reconstructions for Somalia‐Antarctica slightly underrotate the isochrons compared to the Capricorn‐Antarctica‐Somalia three‐plate reconstructions. We suspect that the discrepancies between the two‐plate and three‐plate methods result from the recent seafloor deformation in the Capricorn‐India diffuse plate boundary and/or from the contrasted nature and geometry of magnetic isochrons at different spreading rates (i.e., magnetic structure of the three spreading centers). Three‐plate reconstructions better constrain the closure of the triple junction but spread any misfit among all three plate boundaries. When enough quality data are available, two‐plate reconstructions may lead to more realistic plate motion estimates from which additional geological problems can be identified and solved. Key Points: We mapped high‐resolution magnetic isochrons in the Central Indian Ocean between 83.0 and 42.5 MaWe derived two‐plate solutions to the relative motion of Capricorn‐Antarctica and Capricorn‐Somalia plate pairs at higher resolutionTwo‐plate reconstructions may lead to more realistic plate motion estimates when enough quality data are available [ABSTRACT FROM AUTHOR]

This paper presents the results from the Deflo-hydroacoustic experiment in the Southern Indian Ocean using three autonomous underwater hydrophones, complemented by two permanent hydroacoustic stations. The array monitored for 14 months, from November 2006 to December 2007, a 3000×3000 km wide area, encompassing large segments of the three Indian spreading ridges that meet at the Indian Triple Junction. A catalogue of 11 105 acoustic events is derived from the recorded data, of which 55 per cent are located from three hydrophones, 38 per cent from 4, 6 per cent from five and less than 1 per cent by six hydrophones. From a comparison with land-based seismic catalogues, the smallest detected earthquakes are mb 2.6 in size, the range of recorded magnitudes is about twice that of land-based networks and the number of detected events is 5-16 times larger. Seismicity patterns vary between the three spreading ridges, with activity mainly focused on transform faults along the fast spreading Southeast Indian Ridge and more evenly distributed along spreading segments and transforms on the slow spreading Central and ultra-slow spreading Southwest Indian ridges; the Central Indian Ridge is the most active of the three with an average of 1.9 events/100 km/month. Along the Sunda Trench, acoustic events mostly radiate from the inner wall of the trench and show a 200-km-long seismic gap between 2 °S and the Equator. The array also detected more than 3600 cryogenic events, with different seasonal trends observed for events from the Antarctic margin, compared to those from drifting icebergs at lower (up to 50°S) latitudes. Vocalizations of five species and subspecies of large baleen whales were also observed and exhibit clear seasonal variability. On the three autonomous hydrophones, whale vocalizations dominate sound levels in the 20-30 and 100 Hz frequency bands, whereas earthquakes and ice tremor are a dominant source of ambient sound at frequencies <20 Hz. [ABSTRACT FROM AUTHOR]