LARGE TECTONIC ROTATIONS SINCE THE EARLY MIOCENE IN A CONVERGENT PLATE-BOUNDARY ZONE, SOUTH-ISLAND, NEW-ZEALAND

A palaeomagnetic study in part of the New Zealand prate-boundary zone provides new constraints on the temporal and spatial distribution of N eogene and Quaternary tectonic rotations. Thermal demagnetization of s amples from Cretaceous basaltic dykes, palaeocene-Oligocene micritic l imestone, and Miocene and Pliocene siltstones in the Marlborough regio n, South Island, have defined stable, high-temperature magnetic compon ents, which are interpreted as the primary magnetization. Declination anomalies, after tectonic corrections, are interpreted as rigid body r otations about a vertical axis of sample sites relative to the Pacific plate. All palaeomagnetic data from Marlborough cluster into three ma in groups. A 60-100 degrees clockwise rotation affected Palaeocene to Middle Miocene sedimentary sequences across Marlborough between simila r to 18 Ma and similar to 8 Ma, coeval with a phase of low-angle thrus ting. The absence of this rotation in a Late Cretaceous dyke swarm def ines the present western limit of the early rotating zone. A regional similar to 20 degrees clockwise rotation occurred in the last 4 Ma dur ing the development of the Marlborough Fault System in a zone of dextr al transpression, although locally clockwise rotations less than or eq ual to 40 degrees may have occurred near some of the major dextral str ike-slip faults. However, a negligible rotation is observed in the sam e period in the region to the southeast of the major Kekerengu dextral strike-slip fault, which appears to have acted as a hinge zone, accom modating relative rotation by dextral strike-slip on an arcuate fault, bending, and internal deformation The observed tectonic rotations rec ord the overall clockwise rotation of the trend of the southern end of the Hikurangi margin from W to NW in the Early Miocene to similar to NE today, determined independently from the long-term relative plate m otion data for the Pacific and Australian plates.