We present results from WKBJ and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles (ESPs), along flow lines parallel to the Blake Spur fracture zone across 140 Ma Atlantic crust. These profiles provide detailed constraints on variations in crystal structure at the fracture zone. Seven profiles on either side of the fracture zone show a normal structure for old oceanic crust. A 2--3 km thick upper layer with a steep velocity gradient is underlain by 4--5 km of crust with velocities of 6.5--7.2 km/s and low gradient and a sharp transition to upper mantle velocities of 8 km/s. Several velocity discontinuities were detected within the upper 2--3 km, but these do not generally coincide with intractustal reflectors detected by simultaneous normal incidence reflection profiles. This structure shows little regional variation toward the fracture zone. Despite the small magnetic anomaly offset (~12 km) and the indistinct topographic signature of the fracture zone, three ESPs within a 10--20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure, relative to normal oceanic crust. A 2--4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7.2--7.6 km/s and high Poisson's ratio (probably at least 0.29), which is consistent with 15--30% serpentinization of upper mantle peridotites. This interpretation requires the action of off-axis hydrothermal circulation, penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km. The original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent ''normal'' crust, which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments. ¿ American Geophysical Union 1991