Tectonic evolution of Pegasus Basin and the Hikurangi Trench, offshore New Zealand, The

Abstract

The Pegasus Basin overlies part of the tectonic transition between oblique southwest subduction of the Pacific Plate below the East Coast of the North Island of New Zealand, and the strike-slip faulting that dominates the majority of the South Island of New Zealand. The transition from this strike-slip zone into the actively subducting Hikurangi Trench requires a significant translation of plate motion from margin parallel to margin normal within the Pegasus Basin. The purpose of this research was to understand the distribution of strain along this complex transition, and to identify how shortening is manifested on structures through time. The different stress regimes along the coast may correspond to different shortening amounts absorbed on a variety of structures that translate strain accommodation through the major tectonic transition from compressional subduction to strike-slip displacement. Interpretations of 2D seismic profiles guided by models of margins with comparable tectonic settings ensure geologically restorable interpretations and reasonable shortening values within this transition zone. Restorations of depth converted seismic cross sections constrain ages of the faults and establish controls on their timing and activation. Complexities such as a rotating forearc, a southern migrating subduction zone, and a strike-slip zone further complicate restorations past the late Pliocene-Quaternary aged sediments. Shortening values acquired from restorations align with shortening values from other studies in the area, and also align with estimates based off of the current plate motion vector movement. Economic interest in the Pegasus Basin is primarily petroleum based. Though the basin has not been tested, active seeps, pockmarks, gas chimneys, and surface slicks are numerous within the Pegasus Basin. Structural interpretation, and modelling of potential hydrocarbon accumulations and the implied fluid pathways impact the feasibility of exploration and development of hydrocarbons in the Pegasus Basin.