NATURE OF THE MOHO TRANSITION ZONE IN THE OMAN OPHIOLITE

The Moho Transition Zone of ophiolites is dominantly composed of dunit e, with various types of segregations (gabbros, pyroxenites, and chrom itites). Representing a level of magmatic exchange between asthenosphe ric mantle and the constructing ocean crust, it records active melt ci rculation below a spreading ridge axis and offers the opportunity of o bserving the distribution of melt locally percolating and pending in a deforming porous matrix. In the Oman ophiolite, the Moho Transition S one has a thickness varying from ten to hundreds of meters; its thickn ess and composition are related to the geometry of the asthenospheric mantle flow: thick Moho Transition Zones are on top of mantle diapirs characterized by vertical flow, whereas thin Moho Transition Sones are present in areas of horizontal mantle flow. A large high-temperature plastic strain is recorded in thin Moho Transition Sones, in contrast to thick ones where the strain is weaker and heterogeneous. Thick Moho Transition Zones display an intense magmatic activity expressed by di ffuse melt impregnations, dikes and sills. In these thick zones, we ha ve studied the geometry of the melt circulation at various scales. We present here the analysis of textures and lattice fabrics which record high-temperature plastic strain and allow us to quantify it. Melt cir culates within the dunites and can locally destroy the solid framework , in relation to a viscosity drop and the sharp overturn of mantle flo w observed in this type of transition zone.