The Mid-Atlantic Ridge at 8--9¿S is characterized by a transition from axial valley to axial high and recent episodes of ridge jumping and ridge propagation. We present constraints on the structure of 0--4 Ma crust in this region on the basis of the analysis of wide-angle seismic data from a grid of profiles across and parallel to the current and abandoned spreading centers. A 350--800 m thick oceanic layer 2A, interpreted as high-porosity extrusive basalts, is underlain by a ~2.0--2.5 km layer 2B with velocities which increase with age and decrease in the vicinity of the pseudofaults. Layer 3 velocities are uniform across the area except for a possible localized anomaly at the ridge axis. The crustal thickness varies from 6--7 km near the pseudofaults formed by ridge propagation to 9--10 km at the segment center of the recently (~0.3 Ma) abandoned spreading center. Seismically determined crustal thickness and density variations and age-related lithospheric cooling can plausibly account for all observed variations in gravity across the area, and there is no requirement for the thicker crust at the segment center to be underlain by hot mantle. The transition from axial valley to axial high occurs at a crustal thickness of ~8 km.