The last 2 Myr of accretionary wedge construction in the central Hikurangi margin (North Island, New Zealand): Insights from structural modeling

Abstract

AbstractThree depth‐converted and geologically interpreted seismic profiles provide a clear image of the offshore outer accretionary wedge associated with oblique subduction of the Pacific Plate beneath the central Hikurangi margin. Plio‐Quaternary turbidites deposited over the pelagic cover sequence of the Hikurangi Plateau have been accreted to the margin by imbrication along E‐verging thrust faults that propagated up‐section from the plate boundary décollement. Growth stratigraphy of piggy‐back basins and thrusting of progressively younger horizons trace the eastward advance of the leading thrust front over ∼60 km in the last 2 Myr. Moderate internal shortening of fault‐bounded blocks typically 4–8 km wide reflects rapid creation of thrust faults, with some early formed faults undergoing out‐of‐sequence reactivation to maintain critical wedge taper. Multistage structural restorations show that forward progression of shortening involves: (1) initial development of a ∼10–25 km wide “proto‐thrust” zone, comprising conjugate sets of moderately to steeply dipping low‐displacement (∼10–100 m) reverse faults; and (2) growth of thrust faults that exploit some of the early proto‐thrust faults and propagate up‐section with progressive break‐through of folds localized above the fault tips. The youngest, still unbreached folds deform the present‐day seafloor. Progressive retro‐deformations show that macroscopic thrust faults and folds account for less than 50% of the margin‐perpendicular shortening imposed by plate convergence. Arguably, significant fractions of the missing components can be attributed to mesoscopic and microscopic scale layer‐parallel shortening within the wedge, in the proto‐thrust zones, and in the outer décollement zone.