The accretionary prism widens from south (S, C1) to north (N1), with a corresponding increase in frontal accretion of sediments to the Australian plate (Lewis and Pettinga, 1993; Barnes and Mercier de Lepinay, 1997; Nicol and Beavan, 2003).

The southern end of the subduction zone is located where buoyant continental crust of the Chatham Rise collides with the northern South Island . To the north is the anomalously thick oceanic crust of the Hikurangi Plateau (ca. 15 km thick at C1 and 10 km thick at N2 (Davy and Wood, 1994).

Examination of the profiles shown in Figure 3 indicates that:

•  normal and transverse components are strain-partitioned over geological timescales, with thickening occurring nearer the trench and strike-slip deformation further inland near the axial range;

•  the two southernmost sections (S and C1) have narrow accretionary wedges whereas those in the centre (C2 and N1) have wide, low-taper accretionary wedges

•  the northern-most section (N2) has a narrower wedge with steep taper, subduction erosion, and subduction of sediment to depth below the ranges

Figure 3: Figure 3: Schematic cross-sections showing interpreted structure across the Hikurangi subduction margin. Slab depths constrained by data from Reyners (1998) and Ansell and Bannister (1996). (a) Northern profile N2, based on Eberhart-Phillips and Chadwick (2002) and Collot et al. (2001); (b) profile through Hawke Bay N1, simplified from Barnes et al. (2002). Leading edge of Hikurangi Plateau inferred from Reyners et al. (submitted); (c) central profile C2, modified from Nicol and Beavan (2003); (d) central profile C1, and (e) southern profile S, modified from Barnes et al. (1998) and Eberhart-Phillips and Henderson (2004). All sections approx. 1:1. Red arrows indicate approximate location of trench.