Deep crustal seismic observations from northwest Australia demonstrate that the Cuvier margin is a volcanic passive margin that formed as Greater India rifted away from Australia in the Early Cretaceous. Formation of the Cuvier Basin and initial sea-floor spreading resulted in the emplacement of exceptionally thick oceanic crust, while contemporaneous spreading off the adjacent Exmouth Plateau formed normal-thickness oceanic crust. Crust of normal thickness was generated in the Cuvier Basin within 3-4 m.y. after spreading began. There is no evidence for hot-spot activity with the appropriate timing or in the appropriate spatial pattern to account for the difference in volcanism along the margin. We believe that rapid rifting of the Cuvier margin created strong lateral temperature gradients in the subjacent upper mantle at the time of breakup. This induced small-scale convection in the upper mantle that increased the rate of upwelling and, thus, the volume of melt generated by decompression partial melting. In turn, this resulted in a larger volume of magma at the initiation of sea-floor spreading, and thicker oceanic crust was initially emplaced. Rifting on the Exmouth Plateau happened over a much longer period of time, and no such convective partial melting occurred. The volume of melt associated with rifting was therefore entirely the result of passive upwelling, and a normal thickness of oceanic crust was emplaced immediately adjacent to the Exmouth Plateau.