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 decollement. 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 breakthrough 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 decollement zone.

Splay fault branching from the Hikurangi subduction shear zone: Implications for slow slip and fluid flow

“…[] and extended to our study from Ghisetti et al . []. Main structural features imaged in the ∼120 km long prestack depth‐migrated section include the plate interface with associated subducted sedimentary zone, a step‐down in the décollement possibly accommodating sediment underplating, and splay fault branching at the décollement.…”

“…[], and adapted from Ghisetti et al . []. HKB, the top of the Hikurangi margin basement; MES, Mesozoic sediments; Sequence Y (Seq.…”

Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

“…The plate interface itself is marked by both low‐ and high‐amplitude reflections, and in places its position is inferred from frontal thrust fault intersections (Figure c). Seismic reflection data reveal that several large splay faults exist within the accretionary wedge, which sole into the subduction plate boundary fault (Barker et al, ; Barnes et al, ; Barnes & Mercier de Lépinay, ; Ghisetti et al, ; Lewis & Pettinga, ; Mountjoy & Barnes, ; Plaza‐Faverola et al, ; Plaza‐Faverola et al, ). Splay faults across the Hikurangi margin are primarily northwest dipping, and some have large measurable displacements within the frontal wedge (Barnes et al, ; Barnes et al, ; Ghisetti et al, ).…”

“…Seismic reflection data reveal that several large splay faults exist within the accretionary wedge, which sole into the subduction plate boundary fault (Barker et al, ; Barnes et al, ; Barnes & Mercier de Lépinay, ; Ghisetti et al, ; Lewis & Pettinga, ; Mountjoy & Barnes, ; Plaza‐Faverola et al, ; Plaza‐Faverola et al, ). Splay faults across the Hikurangi margin are primarily northwest dipping, and some have large measurable displacements within the frontal wedge (Barnes et al, ; Barnes et al, ; Ghisetti et al, ). Fluid flow along splay faults may be expected due to widespread active fluid seepage observed at the seafloor on the crests of thrust‐related bathymetric ridges offshore of Hawke's Bay and Wairarapa (Barnes et al, ; Fagereng et al, ; Greinert et al, ; Plaza‐Faverola et al, ).…”

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“…At the deformation front, the plate interface thrust is developed at about 5 km below sea level and about 2 km below the seabed, at least locally in the upper part of the Hikurangi Basement Sequence, thought to comprise volcaniclastics and/or chert/limestone rocks (Davy et al, 2008). The décollement position at northern Hikurangi is stratigraphically deeper than at the southern Hikurangi margin, where it is believed to occur in the inferred pelagic sequence above Paleogene carbonates Ghisetti et al, 2016).…”