Southern Hikurangi Margin Research Articles

On factors controlling the depth of interseismic coupling on the Hikurangi subduction interface, New Zealand

We apply a finite difference numerical thermal model to the shallow region of the Hikurangi subduction margin, North Island, New Zealand. This subduction zone is characterised by subduction of thick and buoyant crust of the Hikurangi Plateau and along-strike variations in subduction obliquity, onshore surface heat flow (low in the south, high in the north), inferred pore-fluid pressure and seismic style (locked in the south, aseismic in the north). The model reproduces low onshore surface heat flow observed in the southern Hikurangi margin. We propose that greater surface heat flow in the north is caused by shear heating and convective heat flow by fluids escaping from the interface. The model predicts that the Hikurangi megathrust is comparatively cold, so that if the lower limit of the interface seismogenic zone is thermally controlled, it should occur near the intersection with the hangingwall Moho. This agrees with observation in the southern, locked segment, but thermal control alone cannot explain the shallow interseismic locking depth in the central and northern margin. In crustal strength curves, changing fluid pressure is capable of moving the brittle-viscous transition vertically by tens of kilometres. We propose that subduction zone seismic style depends on the fluid pressure regime both in the hangingwall and along the interface, and define four end-member types of megathrust fault segments. Contrary to many previous studies, we suggest that the high fluid pressure, low shear strength, end-member is fully coupled, with potential for large interplate earthquakes. In the hydrostatic case the fault is stronger, but the brittle-viscous transition occurs significantly shallower than in the near-lithostatic case, inferred to promote aseismic creep at relatively shallow levels. Along-strike variation in seismic style on the Hikurangi megathrust can therefore be explained by variation in fluid pressure state along the margin.

Strain partitioning in the transition area between oblique subduction and continental collision, Hikurangi margin, New Zealand

Spatial differences in late Quaternary structural style and deformation rates indicate a complex pattern of strain partitioning within the transition area from oblique subduction beneath southern North Island to oblique continental collision in northern South Island, New Zealand. The late Quaternary structure of the offshore southern Hikurangi margin is determined here using seismic reflection profiles, MR1 side‐scan sonar swath images, EM12Dual multibeam swath bathymetry and backscatter images, and sediment core and dredge samples. Where oblique (50°) subduction of the oceanic Hikurangi Plateau is occurring beneath southern Wairarapa region, the steep, dissected, upper margin consists of a wedge of rocks that are being deformed by strike‐slip and reverse faults. Beneath the lower margin an accretionary wedge of Quaternary age has developed rapidly by frontal accretion of a thick succession of turbidites on the Pacific Plate. Estimates of late Quaternary geological strain rates have been made on two margin‐perpendicular transects across southern North Island and the offshore margin to budget the predicted NUVEL‐1A plate motion. More than 33–55 % of the total plate motion is expressed in the offshore part of the upper plate above the subduction detachment in the central Hikurangi margin between 40°S and 41°S. This offshore deformation could potentially include margin‐parallel fault slip of up to 14–15 mm/yr. Across the upper plate, strain is partitioned into zones of folding and thrust faulting and possibly more than one zone of strike‐slip faulting, but the partitioning may be incomplete with oblique‐slip thrusting an important component of the deformation. The wide distribution of deformation within the upper plate partially reflects strong coupling between the subducting oceanic plateau and the leading edge of the overriding plate. This coupling increases toward the southwest, where the continental part of the subduction zone beneath Marlborough may now be inactive or very strongly coupled. Toward the southwest the transpressive deformation style of the upper Wairarapa margin continues into the upper Marlborough margin and onshore into the strike‐slip Marlborough Fault System of northern South Island. In contrast, the accretionary wedge narrows to the southwest and terminates in southeastern Cook Strait, approximately above the transition from subducted oceanic crust to subducted continental crust of the Pacific Plate. Farther southwest beneath the lower Marlborough margin, low‐strain folding and thrust faulting is occurring, but the style of deformation is not typical of the accretionary wedge. This shortening results either from a small amount of subduction slip beneath Marlborough and/or from contractional strain partitioned to the southeast of the continental transpression zone. Whereas the initial onset of continental collision occurs in Marlborough, the surface expression of deformation of the Pacific Plate occurs south of 42°30′S. Normal faulting of the northwestern margin of the continental Chatham Rise possibly reflects flexural extension of the Pacific Plate crust at the southern end of the subduction zone, whereas folding and thrust faulting of the north Canterbury coastal region results from NW‐SE contraction of the delaminated upper crust entering the Southern Alps collision zone.

Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi margin, New Zealand

Analysis of seismic reflection profiles, swath bathymetry, side‐scan sonar imagery, and sediment samples reveal the three‐dimensional structure, morphology, and stratigraphic evolution of the central to southern Hikurangi margin accretionary wedge, which is developing in response to thick trench fill sediment and oblique convergence between the Australian and Pacific plates. A seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby, by coccolith and foraminifera biostratigraphy of three core and dredge samples, and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment. Structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds, on the growth and mechanics of frontal accretion under variable convergence obliquity, and on the amounts and rates of horizontal shortening. The data place constraints on the partitioning of geological strain across the entire southern Hikurangi margin. The principal deformation front at the toe of the wedge is discontinuous and represented by right‐stepping thrust faulted and folded ridges up to 1 km high, which develop initially from discontinuous protothrusts. In the central part of the margin near 41°S, where the convergence obliquity is 50°, orthogonal convergence rate is slow (27 mm/yr), and about 75% of the total 4 km of sediment on the Pacific Plate is accreted frontally, the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening. At least 80% of this shortening has occurred within the last 0.4±0.1 m.y. at an average rate of 12±3 mm/yr. This rate indicates that the frontal 30 km of the wedge accounts for about 33–55% of the predicted orthogonal contraction across the entire plate boundary zone. Despite plate convergence obliquity of 50°, rapid frontal accretion has occurred during the late Quaternary with the principal deformation front migrating seaward up to 50 km within the last 0.5 m.y. (i.e., at a rate of 100 km/m.y.). The structural response to this accretion rate has been a reduction in wedge taper and, consequently, internal deformation behind the present deformation front. Near the southwestern termination of the wedge, where there is an along‐the‐margin transition to continental transpressional tectonics, the convergence obliquity increases to >56°, and the orthogonal convergence rate decreases to 22 mm/yr, the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward‐verging folds. These structures have accommodated not more than 0.5 km of horizontal shortening at a rate of < 1 mm/yr, which represents < 5% of the predicted orthogonal shortening across the entire plate boundary in southern North Island. The landward‐vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin, to the northern South Island region where the upper and lower plates are locked or at least very strongly coupled.

From oblique subduction to intra-continental transpression: Structures of the southern Kermadec-Hikurangi margin from multibeam bathymetry, side-scan sonar and seismic reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.

Paleomagnetic constraints on the tectonic rotation of the southern Hikurangi margin, New Zealand

Abstract Plate reconstructions of the New Zealand region indicate substantial Cenozoic tectonic rotation of the Hikurangi margin. This study was undertaken to determine the rotational history of the northern Marlborough paleomagnetic domain in the southern part of the Hikurangi margin. Most Cenozoic rocks in the Marlborough region are unstably magnetised, and useful results were obtained only from late Miocene ‐ early Pliocene strata (c. 8–4 Ma). Clockwise rotations, with respect to the Pacific plate, between the Wairau and Kekerengu Faults are consistently about 20° and occurred after c. 4 Ma. No rotation is observed in the interval between 8 and 4 Ma, in contrast with the northern part of the Hikurangi margin where rotation has been continuous since the early Miocene. Differential rotation between northern Marlborough and eastern North Island is likely to have occurred about the northern end of the Wairau Fault in Cook Strait. Rapid rotation throughout the Hikurangi margin since the early Pliocene may be linked to a change in the motion of the Pacific plate relative to the Australian plate.