Large tectonic rotations since the Early Miocene in a convergent plate-boundary zone, South Island, New Zealand

Abstract

A palaeomagnetic study in part of the New Zealand plate-boundary zone provides new constraints on the temporal and spatial distribution of Neogene and Quaternary tectonic rotations. Thermal demagnetization of samples from Cretaceous basaltic dykes, Palaeocene-Oligocene micritic limestone, and Miocene and Pliocene siltstones in the Marlborough region, South Island, have defined stable, high-temperature magnetic components, which are interpreted as the primary magnetization. Declination anomalies, after tectonic corrections, are interpreted as rigid body rotations about a vertical axis of sample sites relative to the Pacific plate. All palaeomagnetic data from Marlborough cluster into three main groups. A 60–100° clockwise rotation affected Palaeocene to Middle Miocene sedimentary sequences across Marlborough between ∼ 18 Ma and ∼ 8 Ma, coeval with a phase of low-angle thrusting. The absence of this rotation in a Late Cretaceous dyke swarm defines the present western limit of the early rotating zone. A regional ∼ 20° clockwise rotation occurred in the last 4 Ma during the development of the Marlborough Fault System in a zone of dextral transpression, although locally clockwise rotations ≤ 40° may have occurred near some of the major dextral strike-slip faults. However, a negligible rotation is observed in the same period in the region to the southeast of the major Kekerengu dextral strike-slip fault, which appears to have acted as a hinge zone, accommodating relative rotation by dextral strike-slip on an arcuate fault, bending, and internal deformation. The observed tectonic rotations record the overall clockwise rotation of the trend of the southern end of the Hikurangi margin from W to NW in the Early Miocene to ∼ NE today, determined independently from the long-term relative plate motion data for the Pacific and Australian plates.