Zealand Plate-boundary Zone Research Articles

Reconstructing a dismembered Neogene basin along the active Hikurangi subduction margin, New Zealand

AbstractThe East Coast Basin (ECB), New Zealand, preserves the most complete onshore stratigraphic record of the Cretaceous–Neogene development of Zealandia from Gondwana breakup, through rift-drift, to evolution of the modern Hikurangi subduction margin and Pacific-Australia plate boundary. As such, it provides important constraints for southwest Pacific plate tectonic reconstructions. The basin is, however, deformed and variably dismembered, and in previous tectonic models it has been treated as a zone of poorly constrained deformation. Here, multiple geological and geophysical data sets are integrated with a tectonic reconstruction to provide a synthesis of regional and intrabasin-scale structural evolution of the ECB during the Neogene, producing a new geologically constrained approach toward qualitative and quantitative assessment of deformation across the New Zealand plate boundary zone. We produce the first palinspastic reconstructions for the entire basin during key times in plate boundary development, within a well-constrained plate-tectonic framework, supported by independent deformation estimates. These reconstructions account for contraction, strike-slip, and vertical-axis rotation of crustal blocks. In the context of the reconstruction model, the ECB has dominantly experienced upper-plate shortening and vertical-axis rotations; strike-slip processes are considered comparatively minor. Comparison of the reconstruction model with independent data highlight several pronounced deformation intervals (26–23 Ma, 20–15 Ma, 11 Ma, and 7–4 Ma) in the structural and sedimentary evolution of the ECB, Hikurangi subduction margin, and New Zealand plate boundary zone, demonstrating that it is possible to integrate a wide range of geological data sets to develop meaningful reconstructions in highly tectonised regions.

Taking time to twist a continent—Multistage origin of the New Zealand orocline

Abstract In New Zealand, a giant coherent “Z” shape is defined by several curvilinear pre-Cenozoic basement terranes that extend across Zealandia for >1500 km along strike. It is widely assumed that this curvature was the result of bending during the Neogene, which together with ∼450 km of dextral displacement on the Alpine fault accommodated a total of ∼750 km of dextral shear through the New Zealand plate boundary zone between the Australian and Pacific plates. This would make it a very simple form of orocline. In fact, we show that its development was surprisingly complex and protracted, with a composite origin. Its western and southern parts were bent as much as 70° in the Mesozoic. In the Late Cretaceous, the already bent terranes were offset sinistrally by ∼250 km along the cross-cutting proto–Alpine fault, which acted as a transform to the rift between East and West Antarctica. Since the Eocene, and after Zealandia had completely separated from Antarctica, the two sides of the Alpine fault have undergone 45° of relative plate rotation, further bending the terranes. However, the eastern part of what appears today to be the same oroclinal structure has been created entirely since the Eocene, and mainly during the Neogene phase of dextral shear through the plate boundary, with large-scale internal bending and shortening. We suggest that multistage and composite evolutions may be typical features of oroclines, which would be difficult to unravel without a rich tectonic and plate motion database, such as that available for the New Zealand region.

Cretaceous to present-day tectonic reconstructions of Zealandia

Reconstructions of the past relative positions of northern and southern Zealandia provide important constraints on the orientation and amount of strain accumulated between rigid plates within the Australia–Pacific plate tectonic circuit. This configuration of plates ultimately determines how, where and when sedimentary basins formed during and since continental breakup along the eastern margin of Gondwana. Although the first-order geometry of Zealandia is well established, uncertainty remains regarding plate motions through the latest Cretaceous to Eocene. Recent reconstructions are, in some cases, inconsistent with geological observations at key time intervals, highlighting uncertainties inherent in plate reconstructions for the south-west Pacific. Building on previous tectonic reconstructions and incorporating published seafloor magnetic interpretations, paleomagnetic observations and geological constraints (e.g. terrane geometry and distribution), we developed a tectonic framework to reconstruct Zealandia back through to the latest Cretaceous. Using GPlates, we use a simple double-hinge slat concept to describe Neogene deformation within the New Zealand plate boundary zone, while the geometry of northern and southern Zealandia during the Eocene is modified from recently published models based on geologic considerations. This study ultimately highlights the need for integrated studies of the Zealandia plate circuit.

Stress Orientations in a Locked Subduction Zone at the Southern Hikurangi Margin, New Zealand

AbstractWe analyze the orientation of the stress field in the southern Hikurangi subduction zone, New Zealand, using focal mechanism inversions, S wave splitting fast directions, and gravitational stresses. Here the oceanic Pacific plate is being obliquely subducted beneath the continental Australian plate in the New Zealand plate boundary zone. The study makes use of 399 earthquakes for focal mechanism inversion and 425 earthquakes for shear wave splitting analysis, located with a network of seismic stations spanning southern North Island. We distinguish between stresses in the Pacific plate (from focal mechanism inversion) and Australian plate (from S wave fast directions) and gravitational stresses, in three regions: Western, Central Basin, and Eastern. In the Western region, the principal axis of horizontal compression (SHmax) is oriented NE‐SW, parallel to the margin, in the upper Australian and lower Pacific plate. In the Central Basin, SHmax in the Australian plate is oriented NW‐SE, perpendicular to the margin; in the lower subducting Pacific plate SHmax is oriented NE‐SW. In the Eastern region, SHmax is oriented NE‐SW in the upper plate, while in the lower plate there is a change in orientation to NNW‐SSE. We interpret the stress orientations of the lower plate in the Western and Central Basin regions as a consequence of bending of the subducting plate. Sources of upper Australian plate stresses are likely to be bending stresses, gravitational stresses, and tectonic loading, with differing relative magnitudes across the study area.

Kinematics to dynamics in the New Zealand Plate boundary zone: implications for the strength of the lithosphere

Kinematics to dynamics in the New Zealand Plate boundary zone: implications for the strength of the lithosphere

Large tectonic rotations in a wide zone of Neogene distributed dextral shear, northeastern South Island, New Zealand

The New Zealand plate-boundary zone in the northern part of South Island, New Zealand, comprises a series of active dextral strike-slip faults, referred to as the Marlborough Fault Zone (MFZ), which link oblique continental convergence farther south, along the Alpine Fault, to subduction of the Pacific plate along the Hikurangi margin. New paleomagnetic data are presented which, together with previous studies, suggest coherent Neogene clockwise rotation up to ~ 130° about a vertical axis of crustal blocks in the MFZ, starting at ~ 20 Ma. There is a striking correlation between the amount of observed rotation and the trend of Mesozoic basement structures, creating this part of the New Zealand orocline in a zone of distributed dextral shear, about 100 km wide, which has persisted for about 20 Ma. In the southern part of the MFZ, crustal blocks appear to be on a scale of 1–10 km, with boundaries that are subparallel to the basement strike. This way, deformation is accommodated by a combination of fault slip and rotation in a zone of more pervasive dextral shear, resulting in coherent bending of the basement structure. In the central part of the MFZ, this rotation appears to be that of elongate crustal blocks, ~ 50 km × 10 km in size, rotating ~ 80° clockwise at an average rotation rate ~ 4°/Ma. In detail, the rotation rate has decreased towards the present, from ~ 6°/Ma at ~ 20 Ma to < 2/Ma today. The overall effect of all this deformation is a ‘straightening-out’ of the major faults, so that the northern segments have evolved from ~ NW trending thrusts in the early Miocene to ~ NE dextral strike-slip faults today. A zone of higher shear strain farther north has resulted in an additional clockwise rotation 40–50°, with a total of ~ 130° clockwise rotation. The combination of rotation and fault displacement has acted as a hinge at the southern end of the Hikurangi margin, accommodating a drastic swing in trend from ~ NW in the early–middle Miocene to ~ NE trend.

Large tectonic rotations since the Early Miocene in a convergent plate-boundary zone, South Island, New Zealand

A palaeomagnetic study in part of the New Zealand plate-boundary zone provides new constraints on the temporal and spatial distribution of Neogene and Quaternary tectonic rotations. Thermal demagnetization of samples from Cretaceous basaltic dykes, Palaeocene-Oligocene micritic limestone, and Miocene and Pliocene siltstones in the Marlborough region, South Island, have defined stable, high-temperature magnetic components, which are interpreted as the primary magnetization. Declination anomalies, after tectonic corrections, are interpreted as rigid body rotations about a vertical axis of sample sites relative to the Pacific plate. All palaeomagnetic data from Marlborough cluster into three main groups. A 60–100° clockwise rotation affected Palaeocene to Middle Miocene sedimentary sequences across Marlborough between ∼ 18 Ma and ∼ 8 Ma, coeval with a phase of low-angle thrusting. The absence of this rotation in a Late Cretaceous dyke swarm defines the present western limit of the early rotating zone. A regional ∼ 20° clockwise rotation occurred in the last 4 Ma during the development of the Marlborough Fault System in a zone of dextral transpression, although locally clockwise rotations ≤ 40° may have occurred near some of the major dextral strike-slip faults. However, a negligible rotation is observed in the same period in the region to the southeast of the major Kekerengu dextral strike-slip fault, which appears to have acted as a hinge zone, accommodating relative rotation by dextral strike-slip on an arcuate fault, bending, and internal deformation. The observed tectonic rotations record the overall clockwise rotation of the trend of the southern end of the Hikurangi margin from W to NW in the Early Miocene to ∼ NE today, determined independently from the long-term relative plate motion data for the Pacific and Australian plates.

A simple method for estimating the horizontal velocity field in wide zones of active deformation-II. Examples from New Zealand, Central Asia and Chile

SUMMARY Lamb (1994) has described a simple method for estimating the horizontal velocity field in a wide zone of active deformation. This method is now used to investigate the horizontal velocity or displacement field in part of the New Zealand plate-boundary zone, Central Asia and southern central Chile. In each region, a range of weighted least-squares solutions have been found, depending on assumptions about aspects of the internal deformation. In the New Zealand example, observed tectonic rotations are successfully predicted. Solutions for Central Asia place constraints on the manner in which the northward convergence of India into Eurasia is accommodated. Horizontal surface displacement fields are determined for the elastic rebound during the 1960 Chilean great earthquake, based on available retriangulation data and assumptions about the slip vector on the fault. These solutions suggest, but do not prove, a nearly pure thrust event for this earthquake.

The last 25 Ma of rotational deformation in part of the New Zealand plate-boundary zone

The Marlborough fault system is a zone of dextral transpression in continental crust, and marks the southern end of the Hikurangi subduction system between the obliquely convergent Australian and Pacific plates. Detailed mapping over an area of 100 km 2 has revealed a complex history of deformation during the last 25 Ma. The development of a thrust belt in the Miocene, referred to as D 1 deformation, was synchronous with the deposition of a thick submarine fan complex. The beginning of D 1 deformation may mark the initiation of the subduction zone in this part of the plate-boundary zone. Subsequently, in the Plio-Pleistocene, the region formed a zone of dextral shear, marked by major strike-slip faults and zones of distributed deformation, referred to as D 2 deformation. Crustal blocks, less than 5 km across, may have rotated ca 60° clockwise about vertical axes relative to the Pacific plate in the last 4 Ma. The pattern of deformation during the last 10 Ka to 100 years is consistent with the long-term D 2 deformation, and suggests that large crustal blocks resting on the subducted Pacific plate have rotated clockwise about vertical axes relative to the Pacific plate, accommodated by dextral shear further to the SW in the Marlborough fault system. The overall effect of the deformation has been the rotation of the edge of the southern part of the Hikurangi margin from a W to NW trend in the early Miocene to a NE trend today.

Tectonic rotations about vertical axes during the last 4 Ma in part of the New Zealand plate-boundary zone

The declinations of the primary magnetization of sedimentary rocks in the northern part of the New Zealand plate-boundary zone, after thermal or alternating field cleaning, have been used to determine tectonic rotations about vertical axes of rigid crustal blocks. The pattern of rotations during the last 4 Ma, combined with structural style and continuity, defines seven structural domains each ca 100 × 100–200 km across, which contain crustal blocks up to 100 km across and 20 km thick. Large crustal blocks (tens of km across) in two of these domains have rotated clockwise more than 20° relative to one of the margins of the plate-boundary zone in the last 4 Ma. Their behaviour appears to be controlled by the nature of the plate boundaries, such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge. Small crustal blocks (< 10 km across) may have rotated clockwise through angles greater than 20° during the last 4 Ma, floating on an underlying zone of more distributed deformation. The tectonic rotations of the large crustal blocks, and the nature of the deformation at their boundaries, combined with an interpolation of the relative plate positions, can be used to reconstruct the plate-boundary zone at ca 4 Ma.

The last million years of deformation in part of the New Zealand plate-boundary zone : Lamb, S.H. and P. Vella, 1987. J. struct. Geol., 9(7):877–891

The last million years of deformation in part of the New Zealand plate-boundary zone : Lamb, S.H. and P. Vella, 1987. J. struct. Geol., 9(7):877–891

The last million years of deformation in part of the New Zealand plate-boundary zone

Tectonic deformation of Pleistocene sediments is occurring within the plate-boundary zone in the North Island of New Zealand. Folds, associated with an active fault, are forming mainly by flexural-slip, and contain comformable and unconformable relations which show that the folding started ca 1.0 Ma. Simple geometric models of fold growth indicate that the fold structure has shortened at an average rate of 0.76 ± 0.1 mm a −1 across 4.5 km, though with an increase in the last 100 Ka of about 1.8 times the average rate, which is entirely compatible with the rate of shortening observed in the last 50 years from retriangulation studies. Areas for 15 km to the southeast of the fold structures show significantly lower horizontal shortening rates. The strain rates across the plate-boundary zone vary both with position and time, with a negative correlation between the uplift pattern and the inferred geological shortening rates. This can be explained by the presence of large semi-rigid blocks within the plate-boundary zone which are back-tilting and overriding weaker more rapidly deforming areas. The available evidence suggests that a large proportion of the deformation is occurring seismically.