Séminaire de Mathilde Cannat

Résumé :

Plate divergence at mid-ocean ridges results from the combination of three main processes: magma emplacement, faulting and hydrothermalism. Melt fluxes at fast and most intermediate spreading ridges are high enough that the axial lithosphere remains thin, or at least thinner than the cumulative melt thickness (melt flux divided by expansion rate) per unit length of ridge. In this configuration, magma emplacement fully accommodates plate divergence. This is not the case for slow and ultraslow ridges, where melt fluxes are lower, resulting in colder axial geotherms and an axial lithosphere that is commonly thicker than the cumulative melt thickness per km of ridge. In this configuration, normal faults must accommodate a significant part of plate divergence, while magma may be emplaced over the full thickness of the lithosphere, ie not just at crustal level. This creates the conditions for the exhumation of mantle-derived peridotites and the formation of a composite (variably serpentinized peridotites and magmatic rocks) oceanic crust. Variable depths of magma emplacement also have predictable consequences in terms of hydrothermal fluxes from the solid Earth to the ocean. Melt fluxes at slow-ultraslow mid-ocean ridges are highly variable, both in time and along-axis. This results in spatially and temporaly variable interactions between magma and faults, and between magma and hydrothermal circulation. In this presentation, I illustrate these interactions and point to several remaining questions.