On spreading modes and magma supply at slow and ultraslow mid-ocean ridges

Type Article
Date 2019-08
Language English
Author(s) Cannat MathildeORCID1, Sauter Daniel2, Lavier Luc3, Bickert Manon1, Momoh EkeabinoORCID1, Leroy Sylvie4
Affiliation(s) 1 : Marine Geosciences, Institut de Physique du Globe de Paris, UMR 7154 - CNRS, Université de Paris, France
2 : Institut de Physique du Globe de Strasbourg, UMR 7516 - CNRS, Université de Strasbourg, France
3 : Department of Geological Sciences, Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, United States
4 : Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre de Paris, ISTeP, France
Source Earth And Planetary Science Letters (0012-821X) (Elsevier Science Bv), 2019-08 , Vol. 519 , P. 223-233
DOI 10.1016/j.epsl.2019.05.012
WOS© Times Cited 45
Keyword(s) slow and ultraslow mid-ocean ridges, divergent plate boundaries, detachment faults, melt supply, axial lithosphere, tectonic and magmatic seafloor spreading processes

The ultraslow eastern Southwest Indian Ridge (SWIR) offers an opportunity to study the effect of magma supply on an ultraslow mid-ocean ridge starting from quasi-melt-free detachment-dominated spreading, and transitioning to volcanic spreading as one nears prominent axial volcanos. Detachments in the quasi melt-free mode extend along-axis 60 to 95 km and have a lifetime of 0.6 to 1.5 myrs. They cut into their predecessor's footwall with an opposite polarity, causing part of the footwall lithosphere to experience further deformation, hydrothermal alteration, sparse magmatism and possibly thermal rejuvenation, in a hanging wall position. The accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults. We examine the transition from this nearly amagmatic detachment-dominated mode to the more common volcanic mode of spreading, showing that it occurs along-axis over distances <= 30 km. It involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults, as the magmatic contribution to the divergence of the two plates increases. We develop a conceptual model of this transition, in which magma plays a double role: it fills the space between the diverging plates, thus reducing the need for displacement along faults, and it modifies the thermal state and the rheology of the plate boundary, affecting its thickness and its tectonic response to plate divergence. Based on a comparison of the ultraslow eastern SWIR, with the faster spreading Mid-Atlantic Ridge, we show that the activation of the volcanic, or of the detachment-dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation, over a range of axial lithosphere thickness, and therefore over a range of the M ratio defined by (Buck et al., 2005) as the relative contribution of magma and faults to plate divergence (M is smaller, for a given volume of melt per increment of plate separation, if the plate is thicker). We therefore propose that M does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role (melt itself is weak, hydrothermally altered gabbro-peridotite mixtures are weak, and melt heat sustains more vigorous hydrothermal circulation), resulting in contrasted potentials for strain localization, footwall flexure on faults and the development of detachment faults. (C) 2019 Elsevier B.V. All rights reserved.

Full Text
File Pages Size Access
11 3 MB Access on demand
Author's final draft 37 2 MB Open access
Top of the page