EVIDENCE FOR EARLY MIOCENE WRENCH FAULTING IN THE MARLBOROUGH FAULT SYSTEM, NEW-ZEALAND - STRUCTURAL IMPLICATIONS

In New Zealand, the Marlborough strike-slip faults link the Hikurangi subduction zone to the Alpine fault collision zone. Stratigraphic and structural analysis in the Marlborough region constrain the inception of the current strike-slip tectonics. Six major Neogene basins are inv estigated. Their infill is composed of marine and freshwater sediments up to 3 km thick; they are characterised by coarse facies derived fro m the basins bounding relief, high sedimentation rates and asymmetric geometries. Proposed factors that controlled the basins' generation ar e the initial geometry of the strike-slip faults and the progressive s trike-slip motion. Two groups of basins are presented: the early Mioce ne (23 My) basins were generated under wrench tectonics above releasin g-jogs between basement faults. The late Miocene(ll My) basins were in itiated by half-grabens tilted along straighter faults during a transt ensive stage. Development of faults during Cretaceous to Oligocene tim es facilitated the following propagation of wrench tectonics. The Plio cene (5 My) to current increasing convergence has shortened the basins and distorted the Miocene array of faults. This study indicates that the Marlborough Fault System is an old feature that connected part of the Hikurangi margin to the Alpine fault since the subduction and coll ision initiation. (C) Elsevier, Paris.