EMPLACEMENT OF MANTLE ROCKS IN THE SEA-FLOOR AT MIDOCEAN RIDGES

This paper discusses the geological and geophysical data available on mid-ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow-spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long-lasting magma chamber: gabbroic magmas should instead form short-lived dike or sill-like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill-like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle-derived peridotites form screens for numerous gabbroic intrusions. This discontinuous magmatic crust is expected to form in magma-poor ridge regions, where there is not enough magma to produce a 4-to 7-km-thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle-derived ultramafic screens may be smaller than scismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3-type scismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle-derived rocks.