Northland allochthon: An alternative hypothesis of origin

Abstract

In 1979, Ballance & Spörli presented the first comprehensive account of the Northland Allochthon, which they considered was emplaced from the north or northeast by thrusting and gravitational gliding. A key problem with the gravity emplacement hypothesis has been the origin of the large scale (>5 km) uplift in the early Miocene that is necessary to emplace bathyal to oceanic rocks onto the autochthonous continental crust of the Northland Peninsula. The commonly invoked tectonic “flake” hypothesis has not been fully explained for the Northland situation and presents some problems. An alternative two-stage process is proposed in which the rocks of the allochthon first formed a thick accretionary wedge at the subduction zone east of the Three Kings Ridge. The southern end of the subduction zone terminated at a triple-junction with a major transform fault. This left the weak rocks of the accretionary wedge unsupported at the southern end. A change in the convergence direction in the early Miocene, signalled by the initiation of subduction under Northland, perturbed the accretionary wedge. Lacking a buttress to the south, the accumulated rocks were free to flow southwards in a succession of gravity slides to form the Northland Allochthon. © 2004 Taylor & Francis Group, LLC.