Distribution and mechanism of Neogene to present‐day vertical axis rotations, Pacific‐Australian Plate Boundary Zone, South Island, New Zealand

Abstract

Remarkably little knowledge exists about mechanisms of vertical axis rotation in continental crust. Steeply dipping basement rocks in South Island, New Zealand, provide an opportunity to map the distribution of rotations across the Pacific‐Australian plate boundary zone, and to delineate boundaries of rotated blocks in unusual detail. We synthesize new structural data with new and existing paleomagnetic data, with geodetic data, and with patterns of Neogene‐Quaternary faulting in the strike‐slip Marlborough fault system. For the past 20 m.y., vertical axis rotations have been hinged about two crustal‐scale boundaries near the east coast. The NE hinge accommodated ∼50° of early‐middle Miocene clockwise rotation, which caused deformation of the eastern ends of the Alpine‐Wairau and Clarence strike‐slip faults. The SW hinge has accommodated a further 30°–50° of finite clockwise rotation since ∼4 Ma and deflects active fault traces. The locus of rotation has shifted southwestward astride a subduction margin that is lengthening in that direction. Rotating rocks are pinned to the south against a locked collision zone where the continental Chatham Rise impinges against the margin. Slip on inland strike‐slip faults is transformed seaward across a zone of fault termination into rigid body rotation of a large continental block that has been thrust eastward over the downgoing subducted slab of the Pacific plate. The rotation mechanism is a “migrating hinge,” which resembles a flexed telephone book. Strike‐slip faults are translated through a brecciated hinge region that does not coincide with a fixed material line in the rock.