Nature of the Moho Transition Zone in the Oman Ophiolite

The Moho Transition zone of ophiolites is dominantly composedof dunite, with various types of segregations (gabbros, pyroxenites,and chromitites). Representing a level of magmatic exchangebetween asthenospheric mantle and the constructing ocean crust,it records active melt circulation below a spreading ridge axisand offers the opportunity of observing the distribution ofmelt locally percolating and ponding in a deforming porous matrix.In the Oman ophiolite, the Moho Transition Zone has a thicknessvarying from ten to hundreds of meters; its thickness and compositionare related to the geometry of the asthenospheric mantle flow:thick Moho Transition Zones are on top of mantle diapirs characterizedby vertical flow, whereas thin Moho Transition Zones are presentin areas of horizontal mantle flow. A large high-temperatureplastic strain is recorded in thin Moho Transition zones, incontrast to thick ones where the strain is weaker and heterogeneous.Thick Moho Transition Zones display an intense magmatic activityexpressed by diffuse melt impregnations, dikes and sills. Inthese thick zones, we have studied the geometry of the meltcirculation at various scales. We present here the analysisof textures and lattice fabrics which record high-temperatureplastic strain and allow us to quantify it Melt circulates withinthe dunites and can locally destroy the solid framework, inrelation to a viscosity drop and the sharp overturn of mantleflow observed in this type of transition zone. KEY WORDS: Oman; ophiolite; Moho Transition Zone; textures ^*Corresponding author