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Modes of seafloor generation at a melt-poor ultraslow-spreading ridge

¹ Equipe de Géosciences Marines, CNRS-UMR 7154, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris cedex 05, France
² Institut de Physique du Globe, 5 rue Descartes, 67084 Strasbourg cedex, France
³ Géosciences Azur, CNRS-UMR 6526, 250 rue A. Einstein, Sophia Antipolis, 06560 Valbonne, France
⁴ Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan
⁵ Equipe de Géosciences Marines, CNRS-UMR 7154, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris cedex 05, France
⁶ Institut de Physique du Globe, 5 rue Descartes, 67084 Strasbourg cedex, France

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years.

Key Words: mid-ocean ridges • avolcanic • ultraslow spreading • tectonic asymmetry

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