Megathrust propagation and accretionary wedge development at the front of a sediment-rich subduction system, central Hikurangi Margin, New Zealand

Abstract:

The central Hikurangi margin imbricated wedge is characteristic of wide (>100 km), low taper (4-5°) accretionary thrust systems associated with a relatively smooth subducting plate, thick input sedimentary sequence (~4 km in the trench), moderate convergence rate (~ 40 mm/yr), and a relatively weak interplate fault. The >65 km-wide frontal part of the wedge comprises late Cenozoic accreted turbidites and the upper pelagic sequence of the subducting Hikurangi Plateau. Whilst the deeper (10-30 km) part of the subduction interface beneath land is interseismically strongly coupled (with slow slip events reported beneath the adjacent continental shelf), the style of megathrust slip beneath the outer accretionary wedge is unknown. To support numerical forward modelling of interplate rheological and frictional properties, in a related study, we use 2D seismic reflection profiles to define the geometry of the shallow portion of the interplate megathrust, frontal wedge, and a spectacular protothrust zone outboard of the stepped frontal thrust.

We reconstruct the evolution of the frontal wedge and the trench-ward propagation of the megathrust fault through progressive restoration, decompaction, and back-stripping of depth-converted seismic sections, in four stages (~0.6, 1.0, 2.0, and ≥3.5 Ma old. Folding in the fault hanging wall sequences is restored by mechanisms of fault-propagation folding and trishear. Whilst protothrusts develop as conjugate arrays with up-dip and down-dip propagation, back-thrusting on major thrust faults is relatively minor. Our preliminary results indicate spatial variations in finite shortening (~15-30%) and timing of megathrust propagation, with occurrence of out-of-sequence thrusting. Megathrust propagation may have occurred earlier than suggested in previous interpretations, with the highest shortening rates in the interval ~ 1-2 Ma. Estimated shortening rate is <10 mm/yr across the outer wedge, representing ~10-30% of total convergence.