Neogene development of the onland part of the forearc in northern Wairarapa, North Island, New Zealand: a synthesis

Northern Wairarapa, North Island, New Zealand, lies in the onland part of t he forearc of the Hikurangi margin, which formed since early Neogene time w hen the Pacific plate commenced westward subduction beneath the Australian plate. The onland part of the forearc is now 75 km wide, and there are two dextral fault zones: the Wellington Fault and the Alfredton-Pleckville Faul ts. However, in early Miocene time, a zone of dextral faulting lay adjacent to the inboard part of the trench slope break. At various times during its development, the onland part of the forearc comprised the following units: (1) Coastal Block (outboard); (2) Tawhero Basin; (3) Forearc Platform; (4) Mt Bowen Basin; (5) Mauriceville Ridge; (6) Eketahuna Basin, a successor b asin with two subunits; and (7) Axial Ranges (Tararua Range) (inboard). How ever, currently only the Coastal Block, the widespread Eketahuna Basin (com prising units 2, 3, 4, 5 and 6), and the Axial Range units are developed. A t various times the following NNE- and NE-trending faults bounded the units : Tinui (outboard), Waihoki, Mauriceville, Pleckville-Hastwells, and Wellin gton (inboard). The Coastal Block developed in early Miocene time in an area where there is now a gradient change in the subduction zone. In post-Waitakian time, c. 2 2.4 Ma, the Coastal Block and the Forearc Platform (which lies inboard of t he Coastal unit) were commonly uplifted and shed sediment into the c. 8 km wide Tawhero Basin which lay between them. Oblique subduction probably comm enced at c. 16.7 Ma, causing dextral transpression in the Coastal Block and the Tawhero Basin. At c. 11.3 Ma, the Forearc Platform of the mapped area was submerged by the sea. Part of the inboard part of the forearc, destined to become the Mt Bowen Basin, was hilly or mountainous before c. 8 Ma, whe n it represented a late Miocene axial range, but it was then downwarped, th us increasing the width of the forearc in North Wairarapa. At c. 7 Ma the W ellington Fault was active whereas the Waihoki Fault became inactive at c. 6.6 Ma. During early Pliocene time, the largely bathyal Eketahuna Group (max. 1.5 k m thick) was deposited in the 30 km wide NNE-trending Eketahuna Basin. Subs equently, probable underplating of the onland part of the forearc occurred at c. 3.7 Ma and c. 1.6 Ma, causing widespread uplift. Tectonics similar to that of the present commenced at c. 3.7 Ma or somewhat earlier. The Tawhero Basin was shortened at least 9% in the period from early Tongap orutuan (11.3 Ma) to latest Kapitean (5.2 Ma) time, due to increasing compr ession at that time. In the Forearc Platform there was at least 10% crustal shortening (shown by the development of the Spring Hill Anticline) during the same period. However, crustal shortening was greatest (>33%) in the 8 k m wide belt adjoining the eastern side of the Wellington Fault (since c. 7 Ma).