Numerical Experiments Into the Style of Accretion and Megathrust Behavior Along the Hikurangi Margin, New Zealand

Abstract:

GPS data show that the northern portion of the Hikurangi subduction thrust fault beneath the eastern North Island of New Zealand is creeping steadily, while the southern segment is locked and appears capable of producing great earthquakes. This change is mirrored by variations in fluid chemistry from springs and seeps, with faster fluid expulsion and less water-rock interaction in the central and northern margin compared to the southern margin. Wedge morphology and deformation also change along-strike, with a wide accretionary imbricate wedge in southern and central Hikurangi transitioning to a non-accreting, steep wedge in the north that experiences periodic subduction erosion from seamount subduction.

We use results from restoration of depth-converted seismic sections along the margin to constrain the initial conditions for numerical models that link mechanical wedge development to fluid flow. These models are used to investigate the effect of sediment inputs, lower plate roughness (including seamounts), and fracture permeability on megathrust strength. We test model predictions for wedge morphology, fault development, and fluid-flow rates along localised pathways over time against current wedge structure and fluid chemistry.

Our preliminary results show that the central and southern Hikurangi accretionary wedges approximate growth of a critical wedge geometry, while northern Hikurangi margin morphology episodically cycles as seamounts enter the margin. Sediment subducted around the seamounts provides a fluid source that can drive fluid overpressure on and around the subduction interface. Whether this overpressure causes significant frictional weakening of the megathrust depends on the development of fracture permeability and its interactions with upper plate faults. To produce high rates of fluid flow consistent with fluid chemistry in the central and northern margin, significant permeable pathways must develop, locally limiting megathrust overpressure.