The Porcupine Basin southwest of Ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup. Profiles across the basin image a bright reflection that appears to represent a detachment fault, and may in part be a décollement at the top of partially serpentinized mantle. Although overall the basin appears symmetric, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension was asymmetric. Farther south, the Porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment. Despite no evidence for synrift magmatism, this high has previously been interpreted as a basaltic structure. However, it may represent a serpentinite-mud volcano or diapir: we suggest that such structures produce the serpentinite breccias found within the rifted continent-ocean transition of nonvolcanic margins.