Plate interface and forearc

The forearc of the Hikurangi margin (Fig. 2.1) can be subdivided into an offshore imbricate wedge of deforming Cenozoic sediments, with an actively accreting Qua- ternary wedge in the southeast; an outer arc high formed by the Coastal Ranges; and a subaerially exposed inner forearc, containing Early Miocene to Pliocene marine sediments (Cole and Lewis, 1981; Lewis and Pettinga, 1993). Uplifted Cretaceous greywackes of the Axial Ranges act as a backstop to the deforming zone. Back-arc extension in the Taupo Volcanic Zone is linked to offshore exten- sion in the Havre Trough to the north of New Zealand (Fig. 1.1), and does not extend south of the central North Island.

The margin can also be divided laterally, according to structural changes along the subduction margin revealed by geophysical surveys (Collot and Davy, 1998). These divisions can also be correlated to changes in structure of the overlying plate, and the degree of coupling at the plate interface (Reyners, 1998).

Southern Kermadec margin

Subduction of the Hikurangi Plateau continues in an intra-oceanic setting to the north of New Zealand, until the Kermadec Trench intersects with the NW- trending, 1000 m-high Rapuhia scarp at 36◦_{S (Fig. 1.1). There is an abrupt change}

to normal-thickness Pacific crust north of this feature (Davy and Wood, 1994); the resulting buoyancy contrast is inferred to have caused a tear fault in the subduct- ing plate (Davey and Collot, 2000). Sediment transport from the New Zealand landmass to the trench north of New Zealand is impeded by a volcanic ridge near East Cape, resulting in limited active accretion at the inner trench (Collot and Davy, 1998). Dextral strike-slip faults have been identified on the middle and upper slope of the margin (Collot and Davy, 1998; Davey et al., 1997) (Fig. 1.5), behind which is a forearc basin containing up to 10 km of mainly Cenozoic sedi- ment fill (Davey et al., 1997). The basin is floored by Cretaceous oceanic crust, which seismic surveys have shown to be obducted onto the Raukumara Peninsula as part of the East Coast Allochthon (Davey et al., 1997) (Fig. 2.1). Back-arc rift- ing, at rates of 15–20 mm/yr in the Havre Trough began at about 5 Ma (Wright, 1993); extension is slightly oblique to the plate motion vector (Delteil et al., 2002).

Northern Hikurangi Margin - the Raukumara Peninsula

Between 37◦_{40’S and 40}◦_{20’S, there is no actively accreting wedge. Instead, the}

structural trench is indented by 10–25 km (Fig. 2.1), and the inner trench wall has a much steeper slope than other parts of the margin (12◦ _{as opposed to 2.5}◦_{). In}

this section of the plate boundary, the subducting Hikurangi Plateau has a rougher topography due to a higher abundance of volcanic seamounts (the Northern Vol- canic region of Wood and Davy, 1994) (Fig. 2.1). Slumping and gravitational

collapse resulting from multiple collisions of these seamounts have led to the re- moval of the lower margin by tectonic erosion (Collot et al., 1996).

Significant changes in the structure of the overlying plate also occur along this section of the margin. In the northeast, 3D seismic models derived from inversion of earthquake arrival times (Reyners et al., 1999) reveal a low velocity zone in the lower crust, which thins abruptly north of Gisborne, and appear to represent a thick (>20 km) accumulation of subducted sediment (Fig. 2.1). Sediment un- derplating has previously been hypothesised to explain the uplift and extension through gravitational collapse of the Raukumara Range in the last 6 Ma (Thorn- ley, 1996; Walcott, 1987), and is supported by the observation that the low velocity zone is most extensive beneath the most rapidly uplifting part of the range.

Large thicknesses of underplated sediments are often linked to tectonic erosion at a subduction margin (von Heune and Scholl, 1991). However, south of Gisborne there is a 50 km section of the indented (tectonically eroding) part of the margin with no significant underplating, suggesting additional controls on the accumula- tion of sediments beneath the forearc. The thickness of the over-riding Australian crust also increases abruptly in the Gisborne region, from 17–19 km in the north to 36–37 km in the south (Davey et al., 1997; Reyners et al., 1999). It is possible that thinner crust on the over-riding plate in the northeast allows the sediment to pond against stronger upper mantle (Reyners et al., 1999).

The increased subduction of sediments beneath Raukumara appears to have reduced coupling at the plate interface. Earthquake waveform modelling indicates 1–2 km of subducted sediments along the shallow interplate thrust, made weak by elevated pore fluid pressure (Eberhart-Phillips and Reyners, 1999). Velocity fields derived from geodetic data (Walcott, 1984b; Beavan and Haines, 2001; Wallace et al., 2004) and geological slip rates on major faults (Beanland and Haines, 1998) also indicate a major discontinuity in velocities across the plate boundary off Raukumara, supporting the idea that significant horizontal stresses are not being transferred into the overlying plate.

Central Hikurangi margin - Hawke Bay and the Wairarapa

From 40◦_{30’S to 42}◦_{S, the presence of a Plio-Pleistocene accretionary wedge, con-}

sisting of an imbricate fold and thrust belt (Collot et al., 1996), indicates that active accretion is occuring at the trench. Outbuilding of this wedge has acceler- ated during the Quaternary (Barnes and Mercier de Lepinay, 1997). This section of the Hikurangi margin is arcuate, with the structural trend changing from 15◦ _in

the north to 50–70◦ _{in the south. Three NE-trending active fault scarps, the most}

landward of which appears to have accommodated significant dextral strike-slip, have been imaged by sidescan sonar on the upper margin behind the active wedge

(Barnes et al., 1998) (Fig. 1.5).

The topography of the plate interface beneath the Wairarapa region has been imaged in detail, using PS and SP converted arrivals to further constrain hypocen- tres (Reading et al., 2001). The depth to the plate interface increases by 2–4 km in the southern Wairarapa, but this change appears to be gradual rather than abrupt. More of these converted phases originate beneath the northern Wairaraipa, where the crust is thinner, suggesting that a small thickness (<2 km) of underplated sediments may be present (Fig. 2.1.)

Geodetic measurements indicate that up to 50% of contemporary inter-plate convergence is being accommodated on the Australian plate (Nicol and Beavan, 2003), and clustering of thrust earthquakes near the plate interface at depths of 20– 25 km indicate the down-dip edge of a locked region (Ansell and Bannister, 1996; Reyners et al., 1997; Reyners, 1998). Much lower convergence rates within the Australian plate are indicated by the geologically derived velocity field (Beanland and Haines, 1998), suggesting that much of the short-term deformation is related to the accumulation of elastic strain and that this locking is not permanent.

Southern Hikurangi margin - Marlborough

Beneath the Marlborough region, strong coupling across the subduction interface is indicated by the absence of low angle thrust earthquakes at the interface itself, and by focal mechanisms of upper plate earthquakes being dominated by sub- horizontal compression (Reyners et al., 1997). Marlborough marks the transition on the Pacific plate from thickened oceanic crust to the 23–26 km thick continental crust of Chatham Rise (Fig. 2.1). Partial subduction of the Hikurangi Plateau beneath Chatham Rise is indicated by seismic sections across this boundary (Wood and Davy, 1994), which is probably a result of its collision with the Gondwana margin at _∼105 Ma (Sutherland and Hollis, 2001).

Subduction of continental material to depths of at least 50 km is indicated by the presence of a low velocity slab in 3D seismic velocity models (Eberhart- Phillips and Reyners, 1997). However, velocity fields derived from GPS measure- ments (Beavan and Haines, 2001) and estimates of Quaternary fault slip rates (Holt and Haines, 1995) indicate that >80% of relative plate motion is currently being accommodated in the Marlborough Fault Zone. 3–4 km of turbidites have been deposited without the development of a significant accretionary wedge in the subduction trench off the Marlborough coast (Collot et al., 1996), and Quater- nary slip rates at the subduction thrust are estimated at <1 mm/yr (Barnes and Mercier de Lepinay, 1997). These observations suggest that the underthursting of buoyant continental crust has permanently locked the interface in this region (Collot et al., 1996).