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From slow to ultraslow: A previously undetected event at the Southwest Indian Ridge at ca. 24 Ma

¹ Laboratoire de Géosciences Marines, CNRS-UMR 7154, Institut de Physique du Globe de Paris, 4 place Jussieu 75252 Paris cedex 05, France
² Environmental, Geographic, and Geological Sciences, Lehman College, City University of New York, 250 Bedford Park Blvd., Bronx, New York 10469, USA
³ Institut de Physique du Globe de Strasbourg, UMR7516 CNRS-ULP, Ecole et Observatoire des Sciences de la Terre, 5 rue Descartes 67084 Strasbourg cedex, France

View larger version (43K): [in this window] [in a new window]   Figure 1. Magnetic anomaly profile md47 compared with variable-spreading-rate synthetic anomaly profiles. Profile md47 magnetic anomaly forms A21 to A8 correspond to synthetic profile calculated with a spreading rate of 29 km/m.y. (bottom). The 15 km/m.y. synthetic profile (top) matches profile md47 A6 to A0. Shading indicates the period during which the spreading rate decrease occurred. Map shows location of profile md47, Figure 3, and Figure DR1 (see footnote 1). CIR—Central Indian Ridge; RTJ—Rodriguez triple junction; SWIR—Southwest Indian Ridge; SEIR—Southeast Indian Ridge.

View larger version (39K): [in this window] [in a new window]   Figure 2. Magnetic anomaly identifications along four profiles (see Fig. 3 for locations) using variable rate model (bottom): 29 km/m.y. for A21 to A6C (24 Ma) and 14.5 km/m.y. for A6 to A1 (present). Shading and model as in Figure 1.

View larger version (100K): [in this window] [in a new window]   Figure 3. Magnetic anomaly identification A6, A8, and A13 plotted on the Southwest Indian Ridge tectonic map. Background bathymetry derived from satellite sea-surface altimeter (Smith and Sandwell, 1997). Anomaly identifications appear as solid symbols, and conjugate rotated anomaly identifications appear as open symbols. Dashed lines indicate location of profile md47 and Figure 2 profiles. a—profile mdop04_TA; b—profile md66; c—profile ata9510_TR; d—profile th99a; FZ—fracture zone; RTJ—Rodriguez triple junction. Inset shows location of finite rotation poles and 95% confidence ellipse for Africa-Antarctica relative plate motions. Poles and ellipses for A6, A8, and A13 were calculated for this work; the A5 pole and ellipse are from Lemaux et al. (2002).

View larger version (71K): [in this window] [in a new window]   Figure 4. Predicted and observed changes at the Rodriguez and Bouvet triple junctions in response to rate change at the SWIR. A: Location map. The Rodriguez triple junction is to the east where the SWIR meets the CIR and the SEIR. The Bouvet triple junction is to the west where the SWIR meets the SMAR and the SAAR. B: Triple junction velocity triangles predict spreading direction change at the CIR and SAAR, constant plate motion for the SMAR and SEIR assumed. The period A13–A8 is shown in red and A6–A5 in black. C and D: Evidence for plate motion change between A8 and A6 in satellite-derived gravity maps of CIR (C) and SAAR (D). CIR—Central Indian Ridge; SEIR—Southeast Indian Ridge; SWIR—Southwest Indian Ridge; SMAR—Southern Mid-Atlantic Ridge; SAAR—South American–Antarctic Ridge.