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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

Changes in plate motion are thought to be recorded in the trend of fracture zones, even though fracture zones provide no information about the spreading rate. Using newly compiled published and unpublished magnetic data from the Southwest Indian Ridge, we calculated finite rotation poles for A13, A8, and A6, from which we determined a 50% decrease in spreading rate from slow to ultraslow at ca. 24 Ma not accompanied by a significant change in spreading direction. This spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries. Finally, we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca. 24 Ma at the global scale.

Key Words: mid-ocean ridges • global tectonics • kinematics • magnetic anomalies