The Tydeman Fracture Zone was identified 160 km SE of the Azores in 1977 from a detailed bathymetry and magnetic survey between 22° 30' and 23°40'W, (Twigt et al. 1979). The fracture zone forms a linear trough here with a width of 20 km, water depth of 4.7 km and orientation of 085°. This study extends the path of this fracture zone between 18° and 30°W though not yet to the ridge crest, using a larger dataset of bathymetric, seismic reflection and magnetic profiles. A discontinuity in the fracture zone valley indicates an interruption in its formation, possibly during the formation of the Azores platform. Two separate wide-angle seismic experiments carried out over old crust in the active section of the Tydeman Fracture Zone show low crustal velocities compared with normal oceanic crust of comparable age, an absence of an identifiable layer 3, and a shallower than normal Moho overlying a region of anomalously low upper mantle velocities. Estimates of Poisson's ratio suggest that hydrothermal circulation may continue for as long as 70 Ma after crustal formation.