Re-examination of the magnetic lineations of the Gascoyne and Cuvier Abyssal Plains, off NW Australia

The Exmouth and Cuvier margins of NW Australia and the adjacent Gascoyne Abyssal Plain (GAP) and the Cuvier Abyssal Plain (CAP) formed when Greater India rifted and separated from Australia during the Late Jurassic and Early Cretaceous. Using new and existing data, we re-examine the magnetic lineations of the GAP and CAP in order to better determine the timing of final breakup, the early seafloor-spreading history and the origin of several bathymetric features. The time of final continental breakup is similar along the middle Exmouth (at M10N or M11) and Cuvier (at M10N) margins, and cannot be the cause of their different (wide versus narrow) rift architecture. In contrast, in the intervening southern Exmouth margin, spreading did not occur until M7 to M4 time. The initial seafloor spreading of the GAP (the area older than M6 in the middle and older than M4 in the south) is associated with excess magmatism. In the CAP, Sonja Ridge was a spreading axis that propagated northward from M6 to M4 time at the expense of Sonne Ridge. Both ridges became extinct during M4 to M3 time when another spreading centre propagated south near the Greater India ocean—continent boundary. This eliminated the 300-km-long-offset Cape Range Fracture Zone and left formerly Indian lithosphere on the Australian plate. An extinct ridge and pseudofault indicate M4 to M0 southward propagation within oceanic crust of the GAP as part of the same early spreading reorganization that simplified the spreading geometry after the rifted margins were fully separated.