Anatomy of a continent-backarc transform ? The Vening Meinesz Fracture Zone northwest of New Zealand

Abstract

The northwestern continental margin of New Zealand offers one of the finest examples of a continent-backarc transform. This transform, part of the Vening Meinesz Fracture Zone (VMFZ), accommodated about 170 km of sea-floor spreading in the Norfolk backare basin together with eastward migration of a volcanic arc, the Three Kings Ridge, in the Mid- to Late Miocene. Before the onset of spreading, strain along the VMFZ may have been linked to a major Early Miocene obduction event — the emplacement of the Northland Allochthon. The transform is manifested by a belt up to 50 km wide of left-stepping, linear fault scarps up to 2000 m high within an approximately 100 km-wide deformed zone. A marginal ridge, the Reinga Ridge, which includes a faulted, folded and uplifted Miocene sedimentary basin, occurs within the high-standing continental side of the deformed zone, whereas a narrow strip of linear detached blocks occupies the deep backarc oceanic side. Prespreading uplift and erosion of crust in the proto-backarc region, are volcanism, and obduction of the allochthon, supplied clastic sediments to the basin on the continental side. This basin was complexly deformed as the transform evolved. The transform was initiated as a dextral strike-slip fault zone, which developed right-branching splays and left-steps along its length, uplifting and cutting the continental margin into left-hand, en echelon blocks and relays. Folds formed locally within relay blocks and at the distal ends of the splays. Only the high continental side of this zone (the Reinga Ridge) remains, the formerly adjacent crust (the Three Kings Ridge) having been displaced towards the southeast. As the Three Kings block moved and the Norfolk Basin opened, opposing rift margins of the backarc basin foundered to form terraces. The oceanic side of the transform also subsided to produce the belt of detached blocks (some laterally displaced by strike slip) and linear troughs along the main escarpment system.