The mechanics of a forearc, a wedge-shaped part of the overriding plate between the trench and the volcanic arc, are elegantly described in terms of the critical taper (CT) concept. Based on the Mohr-Coulomb failure criterion and applying an elasto-plastic rheology, CT describes the state (sub-critical, stable, super- critical) of any point within the wedge as a function of its geometry (slope and dip), basal and internal friction as well as basal and internal fluid pressure. Subduction erosion and the subduction of seamounts and other lower plate topographic features such as basement ridges may temporarily increase surface slope and therefore facilitate local to regional mechanical instability. Here we study the causes of local and regional failure at the central Hikurangi wedge offshore New Zealand's North Island and the Peruvian margin. The geometry of both margins is well known from both seismic studies and swath bathymetry coverage and allows quantification of local slope gradients and other curvature attributes. The Rock Garden area at the central Hikurangi margin is characterized by the presence of an accretionary wedge with a relatively low overall taper of 7°, which would be critical or stable if basal friction is 5° or larger. However, compared to other accretive margins, the Rock Garden ridges have steep flanks with the landward flanks being as steep as the seaward ones. Local slope gradients of more than 10° are widely found. In terms of CT this would be unstable even if the wedge and its base would be largely at hydrostatic pressure. However, there is evidence for at least moderate overpressuring. Here, the wedge locally is unstable and numerous, but local slumps are found along the flanks of the accretionary ridges, predominantly at their crests and close to their base. A subducting seamount and a close-by ridge shaped lower-plate feature, which causes a pronounced re-entrant in the outermost accretionary ridge, lead to regional slope gradients of more than 15°. Here, slumping of an area as large as 100 km2 could occur in future. In contrast, the Peruvian margin, which undergoes fast subduction erosion, is characterized by a large taper of 13° to 16°, and a narrow 15 km margin wedge. Here, removal of upper plate material through the subduction channel makes the lower slope mostly super-critical and leads to gravitational failure. The strength of the plate interface and the amount of overpressuring play a crucial role for the mechanical stability of both margins. Fluid pressure fluctuations within the seismic cycle are well capable of pushing large parts of the lower and middle slopes outside the taper stability field. Our comparison highlights that, while the causes of individual slumps are different at both types of margins, seismic behaviour and local as well as regional mechanical stability may well be intimately linked. |
