Geological aspects of the opening of the Tasman Sea

Geophysical interpretation of marine magnetic anomalies indicates that the Tasman Sea formed by sea floor spreading, opening between about 82-60 Ma. Geological data indicates that an extensional regime over much of the New Zealand region was initiated in the mid to late Albian (about 105 Ma). On the future Tasman Sea margin in the South Island, extensional half-graben basins of this age are oriented WNW-ESE, essentially parallel to the future spreading axis associated with the opening of the Tasman Sea, suggesting that initial rifting occurred 20-25 Ma before sea floor spreading began. Less well-constrained data from the Tasman margins and sea floor suggest that a separate extensional event took place between 95-100 Ma (Cenomanian). Sea floor spreading in the proto Tasman Sea, beginning at about 82 Ma, resulted in unconformities, subsidence and marine transgression in the region, and the reactivation of the WNW- ESE fracture system in southwestern New Zealand. A new rift system was initiated in a NNE direction in the late Maastrichtian (ca. 70 Ma) along the west coast of the South Island of New Zealand and in the South Taranaki Basin. The new rifts were probably formed along reactivated mid Cretaceous NNE-trending transfer faults which transferred the opening of the New Caledonia Basin along the west coast of New Zealand to link with the Tasman Sea spreading axis. The final cessation of spreading in the Tasman Sea at about 60 Ma is reflected by virtual halting of sedimentation and development of local unconformities by late Paleocene time.