Décollement strength and mechanical segmentation along the frontal wedge of the Hikurangi convergent margin (New Zealand): Insights from critical taper analysis

To better understand the mechanics of outer forearcs, which is influenced to a large degree by the frictional properties of the plate interface, is a pre-requisite to address topics like natural hazard such as large earthquakes and slumping. Previous research including geodetic and geophysical measurements as well as critical taper analyses (CTA) has revealed that the mechanics of the Hikurangi margin (HM) offshore New Zealand may vary along the margin. However, the relationship between the morphology of the submarine margin and effective décollement strength of the Pacific-Australian plate boundary is not fully understood. From areal gridded data of surface slope (α) and plate dip (β) we characterise the central HM as an intermediate accretionary wedge and the northern HM as a non-accretionary wedge sensu Lallemand et al. (1994). Our areal CTA, comprising the entire region of the submarine margin from the transition to strike-slip faulting in the south to subduction erosion in the northernmost part of HM reveals significant variation of the effective décollement strength of the Pacific-Australian plate boundary. Altogether the décollement strength shows significant regional variability along the HM (0.02 to 0.21), increasing to the toe of the accretionary wedge, and in regions with seamount subduction. Furthermore, mean values of the décollement strength are increasing from the central (0.05) to the northern HM (0.09). Also, our CTA suggests moderate to high fluid overpressure along the plate interface. Finally, out approach of an areal computation of the décollement strength provides a low-effort and on the same hand helpful tool to characterise convergent margin mechanics.