Large tectonic rotations in a wide zone of Neogene distributed dextral shear, northeastern South Island, New Zealand

Abstract

The New Zealand plate-boundary zone in the northern part of South Island, New Zealand, comprises a series of active dextral strike-slip faults, referred to as the Marlborough Fault Zone (MFZ), which link oblique continental convergence farther south, along the Alpine Fault, to subduction of the Pacific plate along the Hikurangi margin. New paleomagnetic data are presented which, together with previous studies, suggest coherent Neogene clockwise rotation up to ~ 130° about a vertical axis of crustal blocks in the MFZ, starting at ~ 20 Ma. There is a striking correlation between the amount of observed rotation and the trend of Mesozoic basement structures, creating this part of the New Zealand orocline in a zone of distributed dextral shear, about 100 km wide, which has persisted for about 20 Ma. In the southern part of the MFZ, crustal blocks appear to be on a scale of 1–10 km, with boundaries that are subparallel to the basement strike. This way, deformation is accommodated by a combination of fault slip and rotation in a zone of more pervasive dextral shear, resulting in coherent bending of the basement structure. In the central part of the MFZ, this rotation appears to be that of elongate crustal blocks, ~ 50 km × 10 km in size, rotating ~ 80° clockwise at an average rotation rate ~ 4°/Ma. In detail, the rotation rate has decreased towards the present, from ~ 6°/Ma at ~ 20 Ma to < 2/Ma today. The overall effect of all this deformation is a ‘straightening-out’ of the major faults, so that the northern segments have evolved from ~ NW trending thrusts in the early Miocene to ~ NE dextral strike-slip faults today. A zone of higher shear strain farther north has resulted in an additional clockwise rotation 40–50°, with a total of ~ 130° clockwise rotation. The combination of rotation and fault displacement has acted as a hinge at the southern end of the Hikurangi margin, accommodating a drastic swing in trend from ~ NW in the early–middle Miocene to ~ NE trend.