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

Abstract

Three 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.