A review of the location and significance of the boundary between the Western Province and Eastern Province, New Zealand

Abstract

New Zealand is a rifted continental fragment of the Gondwana Cretaceous subduction margin that is composed of multiple tectonostratigraphic terranes. An outstanding issue is how the continent grew by terrane accretion and, in particular, what the relationship is between the Early Palaeozoic Western Province terranes and the allochthonous Late Palaeozoic–Mesozoic Eastern Province terranes. At a regional scale, the contact between provinces is located within the Median Batholith: a high-level assortment of Palaeozoic–Mesozoic contiguous plutonic bodies that invade the Western Province Takaka and Buller terranes and the proximal Eastern Province Brook Street Terrane. When viewed at higher resolution, however, the eastern edge of the Western Province is demarcated by contractional Early Cretaceous ductile shear zones, post-kinematic felsic plutons or Cenozoic brittle faults. The contractional shear zones outline a fundamental lithologic, isotopic and metamorphic boundary that was in place by c. 115 Ma. A key feature of this Cretaceous boundary is that it does not juxtapose the Takaka Terrane directly against the Brook Street Terrane. Instead, intervening between these two terranes but enclosed by the Median Batholith, are slivers of Mesozoic meta-volcano-sedimentary rock that unconformably overlie Carboniferous plutons and have been intruded by the dominantly dioritic Mesozoic Darran Suite. These rocks are collectively assigned to the Drumduan Terrane, which is bound to the west by the contractional shear zones and Cenozoic faults, and separated from the Brook Street Terrane to the east by Cenozoic faults. The Drumduan Terrane is assigned to the Eastern Province. Cretaceous convergent deformation at the Western Province–Eastern Province boundary was a two-stage process that first localised on the eastern side of the Takaka Terrane at c. 128–115 Ma during juxtaposition with the Drumduan Terrane. Transpression may have aided terrane accretion. Syn-kinematic Separation Point Suite magmatism was focused along, and adjacent to, first-phase shear zones. The Eastern Province–Western Province boundary was partially reworked during c. 115–100 Ma by a second phase of convergent deformation, although these ductile shears appear to have had only minor displacement.