Asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults. Nevertheless, cogent evidence for their existence remains limited. Furthermore, even when inferred detachment faults are imaged seismically, it is difficult to demonstrate that differential displacement has occurred across these surfaces. We present tectonic and stratigraphic evidence from the Northern Carnarvon basin, northwest Australia, that documents the existence of an intracrustal eastward dipping detachment. By integrating sequence stratigraphy with kinematic and isostatic models of basin development, we conclude that the Northern Carnarvon basin was formed as a consequence of four extension events: (1) a broadly distributed late Permian event, (2) a predominantly localized Rhaetian event responsible for the inception of the Barrow and Dampier subbasins, (3) a localized Callovian fault reactivation within the Barrow-Dampier subbasins, and (4) a Tithonian-Valanginian event that generated large post-Valanginian regional subsidence across the Northern Carnarvon basin with only minor accompanying brittle deformation and erosional truncation. The regional distribution and amplitude of the post-Valanginian subsidence are not consistent with the minor amounts of Tithonian-Valanginian brittle upper crustal extension observed on the margin. Large portions of the platform were emergent or at very shallow water depths prior to the Tithonian-Valanginian extension. To match the distribution and magnitude of the post-Valanginian thermal-type subsidence requires significant lower crustal and mantle extension across the Northern Carnarvon basin. Such a distribution of extension implies the existence of an eastward dipping, intracrustal detachment having a ramp-flat-ramp geometry that effectively thinned the lower crust and lithospheric mantle. The detachment breached the surface close to the position of the continent-ocean boundary, west of the Exmouth Plateau. The flat component of the detachment occurred at midcrustal depths (~15 km) across the plateau and ramped beneath the Australian continent. Lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation, which may explain the paradox that both sides of many conjugate margins appear to be the upper plate. ¿ 1998 American Geophysical Union