Structural analysis of a middle Cretaceous accretionary wedge, Wairarapa, New Zealand

Abstract

The middle Cretaceous (Motuan) Mangapokia Formation near Te Awaiti, southeast North Island, New Zealand, is a complexly deformed, weakly metamorphosed, submarine fan turbidite sequence that represents the youngest part of the Torlesse accretionary wedge.

Two fold events, several episodes of shear fractures, and local development of melange fabric were associated with Cretaceous subduction and accretion. F₁ folds occur within melange fragments and predate regional stratal disruption and melange formation. The melange fragments were rotated during formation of melange. Isoclinal to tight, predominantly asymmetric, overturned, mesoscopic F₂ folds developed during accretion but after pervasive stratal disruption and melange formation. Fold axes trend northeast-southwest and plunge variably to the northeast or southwest.

Two later fold events are post-Torlesse, probably late Cenozoic responses to Miocene-Recent subduction at the Hikurangi Margin. Mesoscopic and macroscopic, upright, predomin-antly close to open F₃ folds overprint and interfere with the Cretaceous folds and associated structural fabrics, and dominate the regional structural grain. F₃ hinge planes trend northeast-southwest, axes plunging to the northeast or southwest; their trend is spread by up to 80° as a result of an F₄ fold event. Mesoscopic and macroscopic, open to gentle, upright F₄ folds, best developed in the east, strike predominantly northwest-southeast, and plunge variably to the northwest or southeast. Dome and basin structures result from interference between F₄ and F₃. One macroscopic F₄ fold, the Te Awaiti Syncline, has affected most of the Te Awaiti area. Late Cenozoic folds with northwest-southeast orientations have not previously been recorded in eastern Wairarapa. The folds are most likely crossfolds that formed almost simultaneously with F₃ folds during a single phase of deformation. Quaternary deformation includes tilting of the region on northeast-southwest-trending growing folds.

Keywords:

• Cretaceous
• structure
• subduction
• accretionary wedge
• Wairarapa
• Torlesse
• Mangapokia Formation
• geometric analysis
• folds
• melange
• New Zealand