Changes in plate boundary kinematics: Punctuated or smoothly varying - Evidence from the mid-Cenozoic transition from lithospheric extension to shortening in New Zealand

- Major plate motion changes occur smoothly and systematically, not punctuated.
- High resolution tectonic fingerprint of plate motion changes determined
- Plate boundary transition migrates at rates higher than plate velocities.

The marine magnetic anomaly record and plate kinematics derived from that data provide evidence of numerous cases of significant changes in plate motions and plate interactions. However, in most cases the temporal resolution provided by the marine record does not discriminate between punctuated (abrupt) or smooth variations in plate motions. During a 10 million year period (~ 30 Ma to 20 Ma), the Euler poles (stage poles) describing Australia-Pacific relative plate motion migrated more than 20° in latitude. The New Zealand segment of the plate boundary is close to the location of the Euler poles and their migration making the tectonics of its plate boundary structures particularly sensitive to the history of plate kinematics. Based on the rapid southward migration of the Euler (stage) poles from north of New Zealand to south of the landmass, there is an expected rapid, migrating transition from an extensional (stage pole north of the site) to transpressional (stage pole south of the site) plate boundary system. The geologic signature of this rapid change in plate boundary kinematics is preserved in the stratigraphic record of a suite of basin deposits that span the latitudinal sweep of the migrating Euler poles. Analyses of these deposits indicate that the timing of the transition from extensional to transpressional tectonics shows a continuous and systematic southward sweep, indicating that the changes in Australia-Pacific plate motions during the 30 Ma-20 Ma interval were smoothly continuous and not punctuated.