The North Island Dextral Fault Belt, Hikurangi Subduction Margin, New Zealand

Abstract

The active North Island Dextral Fault Belt comprises two main strands and many associated faults. The western strand is 450 km long and extends from Cook Strait to the Bay of Plenty. From Cook Strait to the Waikaremoana Region the faults strike northeast and are predominantly dextral, whereas north of the Waikaremoana Region the faults strike north and are predominantly normal with secondary dextral motion. The western strand merges northwards with the margin of the Taupo Volcanic Zone; it is continuous through the bend and through the Axial Ranges because it follows a major pre-existing fault. The 260 km long eastern strand extends from Cook Strait to Hawkes Bay; it becomes discontinuous and changes progressively from dextral to reverse-dextral to the northeast as it steps eastward and merges with reverse faults of the imbricate frontal wedge in Hawkes Bay. Late Quaternary slip rates on the North Island Dextral Fault Belt decrease to the northeast from approximately 2l mm/yr in the Wellington Region to 8.5, 5.0, 6.6 and 4.6 mm/yr in the Eketahuna, Hawkes Bay, Mohaka and Waikaremoana Regions and 1.6 mm/yr in the Bay of plenty Region. In western Hawkes Bay, an on-lap unconformity aged 2.5±0.3 Ma is offset 7±1 km dextrally and 120 m vertically, up to the NW, by the Mohaka fault. The derived slip rate is 3.0±0.8 mm/yr, equivalent to the late Quaternary slip rate on this fault. Total slip on the Dextral Fault Belt is in the order of tens rather than hundreds of kilometres. Currently active dextral faulting commenced at 4-2 Ma as the North Island Dextral Fault Belt developed from a suite of Mio-Pliocene reverse faults in the inner forearc of the Hikurangi margin. Back-arc spreading and increased rotation rates of the Hikurangi forearc also commenced in this period. Uplift and emergence of the southern Axial Ranges and inner forearc, caused by fault-related crustal thickening, commenced after 1 Ma. Active reverse faults occur only east of the eastern strand of the Dextral Fault Belt, which lies along the 18 km structural depth contour of the Hikurangi subduction interface. Seismicity is sparse in the upper plate and shows only a slight NE grain in the Wellington Region. Only the M8 1855 Wairarapa and 193l Hawkes Bay earthquakes were associated unequivocally with the Dextral Fault Belt (eastern strand); both appear to branch up from the subduction interface. Several moderate magnitude, plate interface earthquakes show horizontal slip vectors that are rotated toward the trench-normal. Geodetic analysis of large scale conventional survey data generally shows relative contraction in a NW-SE direction in the south and in a NE-SW direction in the north, consistent with clockwise rotation of the Hikurangi margin. Strain rates are not detectably higher near the North Island Dextral Fault Belt than elsewhere. Rotation of the Hikurangi forearc relative to the Australian Plate is associated with crustal contraction in the south and extension the north. Rotation of the forearc increased the obliquity between the direction of relative plate motion and Mio-Pliocene reverse faults and led to the development and subsequent rotation of the North Island Dextral Fault Belt south of the bend in the Waikaremoana Region, which corresponds to the boundary between rotated and un-rotated parts of the forearc. New paleomagnetic data in the Manawatu Strait area at the eastern margin of the Wanganui Basin indicate 4±2° rotation since 2.3 Ma with respect to the Australian Plate. These data support the inference that the North Island Dextral Fault Belt is itself rotating. Plate tectonic reconstructions indicate that relative plate motion was approximately perpendicular to the Hikurangi margin until 4-2 Ma, therefore there is no requirement for earlier dextral fault belts in the Hikurangi margin. Along the Hikurangi margin, subduction geometry and obliquity and rates of relative plate motion vary little. Obliquity is probably sufficient for partitioning everywhere and rotated plate interface earthquake slip vectors suggest that partitioning is occurring. Nevertheless, the component of margin-parallel motion accommodated on the North Island Dextral Fault Belt only approaches predicted values in the Wellington Region and decreases markedly to the north. An explanation for kinematic variation and rotation in the Hikurangi forearc involves the degree of dynamic coupling across the Hikurangi subduction interface. Dynamic coupling is largely controlled by the depth extent of subducted Pacific Plate, which increases to the north and is at least twice the depth of the main zone of seismicity. Dynamic coupling is at a maximum in the Marlborough Region, at the south end of the Hikurangi margin, where the subduction interface is locked and all relative plate motion is accommodated in the upper plate. In the Wellington Region, the North Island Dextral Fault Belt accommodates about 75% of margin-parallel relative plate motion, but lies within, rather than at the back edge of, the forearc wedge. Between Wellington and Hawkes Bay the degree of dynamic coupling decreases, as does the proportion of deformation transferred to the upper plate. North of Hawkes Bay, the subduction interface is dynamically uncoupled, back-arc spreading prevails and regional uplift, caused by deep-seated crustal processes such as sediment under-plating, is accompanied by extensional deformation. The Dextral Fault Belt traverses northwestward to lie above the 70 km structural depth contours on the subduction interface in the Bay of Plenty, where it ceases to accommodate margin-parallel relative plate motion.