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

Abstract

Northern Wairarapa, North Island, New Zealand, lies in the onland part of the forearc of the Hikurangi margin, which formed since early Neogene time when 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 Faults. 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 basin with two subunits; and (7) Axial Ranges (Tararua Range) (inboard). However, currently only the Coastal Block, the widespread Eketahuna Basin (comprising units 2, 3, 4, 5 and 6), and the Axial Range units are developed. At various times the following NNE‐ and NE‐trending faults bounded the units: Tinui (outboard), Waihoki, Mauriceville, Pleckville‐Hastwells, and Wellington (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. 22.4 Ma, the Coastal Block and the Forearc Platform (which lies inboard of the Coastal unit) were commonly uplifted and shed sediment into the c. 8 km wide Tawhero Basin which lay between them. Oblique subduction probably commenced 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, when it represented a late Miocene axial range, but it was then downwarped, thus increasing the width of the forearc in North Wairarapa. At c. 7 Ma the Wellington 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 km thick) was deposited in the 30 km wide NNE‐trending Eketahuna Basin. Subsequently, 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 Tongaporutuan (11.3 Ma) to latest Kapitean (5.2 Ma) time, due to increasing compression 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 km wide belt adjoining the eastern side of the Wellington Fault (since c. 7 Ma).

Keywords:

• Neogene
• forearc basins
• oblique subduction
• northern Wairarapa
• tectonics
• positive half‐flower structures
• crustal shortening
• sedimentation
• geological history
• hydrocarbon prospectivity