Age and origin of the New Zealand Orocline in relation to Alpine Fault movement

Abstract

The Alpine Fault, and the recurved arc structures (orocline) in the late Palaeozoic-Mesozoic Rangitata Orogen, are two of the first-order structures of the New Zealand subcontinent. A commonly held view is that the orocline originated as a gigantic drag fold with Alpine Fault movement. In view of the post-Oligocene age of the Alpine Fault, however, this concept, involving a single recurved arc, cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the Rangitata Orogen and its cover rocks. These data indicate at least 62° of dextral rotation during each of the Mesozoic Rangitata Orogeny and the late Cenozoic Kaikoura Orogeny. A new model presented here postulates that there are actually two dextral recurved arcs. A Western Arc, which lies mainly outside the Australia–Pacific plate boundary zone, has a multiple origin having formed chiefly during the late Triassic phase of the Rangitata Orogeny by the subduction accretion of the exotic Rakaia terrane. An Eastern Arc coincides with the area of the present plate boundary zone. It formed as a mega, brittle–ductile, shear zone with the late Cenozoic development of the modern Australia–Pacific plate boundary, which includes the Alpine Fault. This solves the problem of the nature and location, north of the Alpine Fault, of the relative plate movement evident as 480 km of displacement on the Alpine Fault. A major implication of this model for plate tectonic reconstruc­tions of the SW Pacific is that only approximately 500 km of dextral displacement has occurred through New Zealand during the late Cretaceous–Cenozoic compared with the commonly adopted figure of 1000 km.