Tectonic rotations about vertical axes during the last 4 Ma in part of the New Zealand plate-boundary zone

Abstract

The declinations of the primary magnetization of sedimentary rocks in the northern part of the New Zealand plate-boundary zone, after thermal or alternating field cleaning, have been used to determine tectonic rotations about vertical axes of rigid crustal blocks. The pattern of rotations during the last 4 Ma, combined with structural style and continuity, defines seven structural domains each ca 100 × 100–200 km across, which contain crustal blocks up to 100 km across and 20 km thick. Large crustal blocks (tens of km across) in two of these domains have rotated clockwise more than 20° relative to one of the margins of the plate-boundary zone in the last 4 Ma. Their behaviour appears to be controlled by the nature of the plate boundaries, such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge. Small crustal blocks (< 10 km across) may have rotated clockwise through angles greater than 20° during the last 4 Ma, floating on an underlying zone of more distributed deformation. The tectonic rotations of the large crustal blocks, and the nature of the deformation at their boundaries, combined with an interpolation of the relative plate positions, can be used to reconstruct the plate-boundary zone at ca 4 Ma.