To investigate the influence of inherited inhomogeneity in a lithosphere under extension, we studied the deep structure of Corsica, an island in the Mediterranean sea, at the boundary of the extensional Tyrrhenian basin, using a temporary array of eight broad-band seismographs. Between the stations in the western Hercynian part of the island and the stations in eastern Alpine Corsica, an average static station delay of 0.4 s was observed for P waves, and 0.7 s for S waves. The crustal structure is obtained through seismic data, gravity modeling and the receiver function method. The difference in crustal structure between the west and east of Corsica can explain only about half of the anomaly. Consequently we postulate a sharp increment in the thickness of the lithosphere to account for the remaining 0.2 s of P anomaly. ¿ 1999 American Geophysical Union