TECTONIC SUBSIDENCE AND EUSTASY IN THE CANTERBURY BASIN, NEW ZEALAND: A COMPARISON OF 1-D BACKSTRIPPING AND SEQUENCE STRATIGRAPHIC APPROACHES

The Canterbury Basin is located off the central east coast of the South Island of New Zealand. It formed when New Zealand rifted from Antarctica around 85 Ma. This passive margin setting may have been subject to a number of plate reorganizations related to motion changes within Antarctica, the separation of New Zealand from Australia and the development of the Alpine Fault.

We have analyzed the Clipper-1 well, which was drilled in the early 1980's and penetrated 4500 m of sediment, ending in metamorphic basement rocks. The precision of age and water depth data generated in the 1980's is limited. However, backstripping results suggest that the Canterbury basin was not impacted by regional tectonics for the first 50 m.y. after rifting. In fact, the long-term sea-level history indicated by the difference between backstripped tectonic subsidence and theoretical oceanic thermal subsidence is remarkably similar to the long-term eustatic results reported from the New Jersey margin. However, beginning about 25 Ma, and coinciding to the time of initiation of motion on the Alpine Fault, tectonic subsidence rates in the Canturbury Basin increased. Subsidence rates accelerated again at about 15 Ma and show a final increase at 5 Ma. A total of about 520 m of tectonic subsidence was calculated during this period of increasing subsidence rates.

We also performed a detailed, semi-quantitative analysis of the seismic line Ewing 01-74 (EW74), provided by Craig Fulthorpe and Hongbo Lu. The seismic section analyzed is well defined to the Marshall Paraconfomity (about 30 Ma) but most of the section is less than 15 Ma. Thus, this analysis only covers the portion of the subsidence that is strongly influenced by increasing tectonic subsidence rates. By assuming a similar water depth for clinoform rollover points, an overall sea level rise throughout this period was observed. The magnitude of the rise is about 0.8 seconds two way travel time, which would amount to about 1200 m of relative rise assuming an average velocity of about 3 km/s in sandstone/shale and carbonate sediments.

The subsidence obtained from sequence stratigraphy is more than twice the tectonic subsidence suggested for the same time interval by backstripping. A major source of the discrepancy is likely to be the inclusion of sediment loading in the sequence stratigraphic result.