Many numerical models have stressed the key importance of processes operative within magma-hydrothermal transition zones of spreading centers, but few studies have attempted a geologic characterization of the key physical parameters of this zone. With this in mind, fracture-related data have been collected in the Samail ophiolite (Oman) within gabbro/dike transition zones and major plagiogranite plutons, and at each site the nature, abundance, orientation, filling, and attitude of the fractures were determined. Two main hydrothermal vein groups can be distinguished: an amphibole vein system and a quartz-epidote-sulfide vein system. Amphibole veins are restricted to the gabbro, whereas quartz-epidote-sulfide veins mainly appear in the sheeted-dike complex and plagiogranite. The amphibole vein system is strongly anisotropic (perpendicular to the layering and subparallel to the sheeted dikes) and heterogeneous. The quartz-epidote-sulfide veins, too, are subparallel to the sheeted dikes. They are characterized by a sharp density decrease of macroscopic veins: from an average 0.5 m-1 (sheeted dikes) to 0.2 m-1 (transition zone) to less than 0.01 m-1 (gabbro). In plagiogranite, where no concentric or radiating vein systems were seen, they are 0.1 to 0.3 m-1. The ubiquitous presence of ridge parallel hydrothermal veins implies a major contribution of regional tectonic stresses associated with spreading in addition to other mechanisms involved in fracture generation: thermal cracking, volatile-rich magma expansion, differential expansion of pore fluids. ¿ American Geophysical Union 1994