EARLY MIOCENE THIN-SKINNED TECTONICS AND WRENCH FAULTING IN THE PONGAROA DISTRICT, HIKURANGI MARGIN, NORTH-ISLAND, NEW-ZEALAND

The Pongaroa-Akitio area, Northern Wairarapa, North Island, New Zealan d, is part of the exposed East Coast Deformed Belt at the obliquely co nvergent plate boundary of the Hikurangi margin. The sedimentary succe ssion includes an allochthonous unit of Early Cretaceous greywacke bas ement resting on latest Cretaceous rocks. Since the unit's basal conta ct is subparallel to the bedding of the strata it overlies, the alloch thon is inferred to be an unrooted gliding nappe similar to allochthon ous outliers described in Northland and the Raukumara Peninsula. The s outhward emplacement of this ''Greywacke Nappe'' is supported by struc tural markers in the body of the nappe and is well dated as earliest M iocene by the youngest rocks involved, which are earliest Miocene (Wai takian; c. Aquitanian), and because Otaian-Altonian (c. Burdigalian) f aults postdate nappe emplacement. This thin-skinned tectonic phase imm ediately preceded inception of dextral strike-slip faulting along nort heast-trending Otaian-Altonian (Burdigalian) faults. The present 300 k m offset of similar allochthonous outliers on both sides of the faults of the western coastal ranges results from cumulative dextral strike- slip movement on these faults through the Miocene.