Australia-Pacific Plate Boundary Research Articles

Structural evolution, segmentation and activity of the onshore-offshore Waimea-Flaxmore Fault System in south-eastern Tasman Bay, South Island, New Zealand

ABSTRACT The Waimea-Flaxmore Fault System (W-FFS) from the Nelson-Richmond urban area to D’Urville Island is analysed through six regional transects that join depth-converted seismic lines in Tasman Bay to onshore cross sections. Reverse reactivation of basement faults of the W-FFS by mechanisms of fault-propagation folding at c. 19-10 Ma occurred along the entire length of the fault system. However, the W-FFS remained Quaternary active only from the vicinity of Cape Soucis southwards into onshore Nelson, in contrast with cessation of activity along eastern Tasman Bay since 7 Ma. The inactive western faults of the W-FFS are buried below Plio-Quaternary marine sediments and segmented by the newly identified, c. E-W, Croisilles Fault Zone, Cross Point and D’Urville faults, with cumulative dextral offset of c. 27 km. The c. E-W faults extend from the onshore into Tasman Bay and are interpreted as the northernmost Late Miocene strands of the Marlborough Fault System that have accommodated incipient distributed shear in the crustal block north of the Queen Charlotte Fault Zone. Segmentation and along-strike changes of the W-FFS occur at the transition from the contractional domain of onshore Nelson to the northern Marlborough strike-slip domain, driven by large-scale kinematics of the Australia-Pacific plate boundary.

Composition and Miocene deformation of the lithospheric mantle adjacent to the Marlborough Fault System in North Canterbury

ABSTRACT Mantle xenoliths in a nephelinite in the Little Lottery River provide insight into the Miocene mantle adjacent to the Australia-Pacific plate boundary in N Canterbury. The xenoliths comprise peridotite and pyroxenite extracted from depths of ∼ 40 to 60 km. The olivine Mg# < 89, a lack of spinel and occurrence of ilmenite, elevated bulk Cr and Al, light rare Earth element (REE)-enriched clinopyroxene, and 87Sr/86Sr15 Ma clinopyroxene populations (0.7029–0.7047) in most harzburgite and dunite samples, and in all lherzolite, wehrlite, olivine websterite and orthopyroxenites, indicate that most are the product of reaction of depleted peridotite with Fe, Ti, Al and light REE-bearing silicate melts of basaltic or similar composition. Available isotopic data indicate that the xenoliths could be derived from mantle lithosphere beneath the Hikurangi Plateau, in which case the reactive melts may have been associated with the Cretaceous plateau formation. Extensive recrystallisation of olivine indicates that portions of this lithosphere mantle were affected by the deformation close to the time of entrainment. Since the eruption occurred at ∼ 15 Ma, these textures require that the mantle lithosphere in this area was deforming before formation of today's expression of the proximal Hope Fault strand of the Australia-New Zealand plate boundary.

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.

Three-dimensional coseismic deformation of the 2016 MW7.8 Kaikuora, New Zealand earthquake obtained by InSAR and offset measurements

The 2016 MW7.8 Kaikoura earthquake struck the northern part of south Island, New Zealand, within the active and complex Australia–Pacific plate boundary system. Firstly, we used the InSAR method to obtain coseismic LOS deformation fields based on SAR images and applied offset tracking methods to obtain offset measurements based on optical satellite images. The maximum displacement of about 6 m is detected in the direction away from the satellite on the south-west side and also towards the satellite on the north-east side. The 3D deformation field is then resolved by the combination of these measurements with a least-square solve method, and comparisons with 3 components of GPS stations show good consistency. Despite complex features demonstrated in the 3D deformation field, there are still clear spatial correlations between surface deformation and faults distribution. It reveals that more than ten faults were ruptured during the earthquake, including some faults were previously understudies for their tectonic activities. The maximum horizontal deformation of about 10 m occurs along the Kekerengu fault with the vertical deformation up to 2 m. The 3D deformation shows that the mainshock is a multi-segments faulting with a rupture process of strike-slip, compression, transpressional rupture and strike-slip in space along the NE direction.

Comparative tomography of reverse-slip and strike-slip seismotectonic provinces in the northern South Island, New Zealand

Actively deforming crust and upper mantle around the obliquely convergent Pacific-Australia plate boundary in the northern South Island of New Zealand have been investigated by seismic tomography using data from a temporary c. 50 station network plus GEONET. The Alpine-Wairau Fault (principal component of the plate boundary at the surface) transects the study area, separating the Buller-Nelson (BN) province of active compressional inversion involving steep reverse faulting to the northwest, from the Marlborough fault system (MA) dominated by dextral strike-slip faulting to the southeast. In the course of this study, MA hosted the 2013 Seddon earthquake sequence (M5.9, 6.6) and the 2016 M7.8 Kaikoura earthquake. Active fault structures and present seismicity are associated with a heterogeneous distribution of low-velocity and Vp/Vs anomalies. Elsewhere, there is a general association between crustal seismicity and low-velocity zones along major fault structures within both the MA and BN seismotectonic provinces along which high Vp/Vs anomalies are locally conspicuous. Areas of active crustal seismicity are also generally characterized by high Vp/Vs, for instance the hypocentre and aftershocks of the 2016 M7.8 Kaikoura earthquake overlie a low-velocity region with high Vp/Vs. The coincidence of anomalously low Vp and Vs and anomalously high Vp/Vs with zones of high electrical conductivity defined by a previous MT transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone, weakening the overlying crust and promoting seismic rupture along major fault systems. It seems possible that the Association of mid-crustal low-velocity zones with anomalously high Vp/Vs may diagnose rupture preparation zones where frictional strength is being lowered by the build-up of fluid overpressure concurrent with accumulating shear stress, so that eventual fault failure is ‘dual-driven’.

Aftershock Sequences of the Recent Major Earthquakes in New Zealand

The three clusters of the epicenters of the nine recent (1993–2018) earthquakes of magnitude 7.0 or larger in New Zealand are located in three different tectonic environments of the Australia–Pacific Plate boundary, including the southern part of the Kermadec Trench (showing rapid westward subduction), the oblique collision zone between the Pacific Plate and Indo-Australian Plate with the dominant Alpine Fault (showing right-lateral strike-slip movement), and the Puysegur Trench (showing eastward oblique subduction). From the viewpoint of the unified scaling law for earthquakes (USLE), these regions are characterized by different levels of seismic rate (A), earthquake magnitude exponent (B), and fractal dimension of epicenter loci (C). The recent major earthquakes exemplify different scenarios of aftershock sequences in terms of either the dynamics of interevent time (τ) or the USLE control parameter (η = τ × 10B×(5−M) × LC), where τ is the time interval between two successive earthquakes, M is the magnitude of the second one, and L is the distance between them. We find the existence, in the long term, of different, intermittent levels of rather steady seismic activity characterized by near-constant values of mean η (〈η〉), which, in the mid-term, switch between one another at times of critical transitions, including those associated with all but one magnitude 7.0 or larger earthquake. At such a transition, seismic activity may follow different scenarios with interevent time scaling of different kinds. Evidently, although these results based on analysis of an individual series do not support the presence of universality in seismic energy release, they provide constraints on modeling realistic seismic sequences for earthquake physicists and supply decision-makers with information for improving local seismic hazard assessments.

Tectonic evolution and copper-gold metallogenesis of the Papua New Guinea and Solomon Islands region

Papua New Guinea and the Solomon Islands are in one of the most prospective regions for intrusion-related mineral deposits. However, because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets, the link between mineralization and the regional-scale geodynamic framework has not been understood. Here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region, combined with the spatio-temporal distribution of intrusion-related mineral deposits, and six new deposit ages. Convergence at the Pacific-Australia plate boundary was accommodated, from at least 45 Ma, by subduction at the Melanesian trench, with related Melanesian arc magmatism. The arrival of the Ontong Java Plateau at the trench at ca. 26 Ma resulted in cessation of subduction, immediately followed by formation of Cu-Au porphyry-epithermal deposits (at 24–20 Ma) throughout the Melanesian arc. Late Oligocene to early Miocene tectonic reorganization led to initiation of subduction at the Pocklington trough, and onset of magmatism in the Maramuni arc. The arrival of the Australian continent at the Pocklington trough by 12 Ma resulted in continental collision and ore deposit formation (from 12 to 6 Ma). This is represented by Cu-Au porphyry deposits in the New Guinea Orogen, and epithermal Au systems in the Papuan Peninsula. From 6 Ma, crustal delamination in Papua New Guinea, related to the prior Pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse Cu and Au porphyry and epithermal deposits within the Papuan Fold and Thrust Belt and Papuan Peninsula from 6 Ma to the present day. Subduction at the New Britain and San Cristobal trenches from ca. 10 Ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern Papua New Guinea and the Solomon Islands. This is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting, primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab.

Lateral H2O variation in the Zealandia lithospheric mantle controls orogen width

Mantle xenoliths exhumed from beneath the South Island of New Zealand offer an opportunity to investigate the role of lithospheric mantle water in influencing deep lithosphere deformation and formation of the narrow Southern Alps mountain range during orogeny along the Australia–Pacific plate boundary. Fourier transform infrared spectrometry (FTIR) water measurements on clinopyroxene (cpx), orthopyroxene (opx) and olivine of peridotite xenoliths from seven localities across the Zealandia continent reveal a lateral variation of H2O within the Oligocene–Miocene lithospheric mantle. There is a “wet” domain beneath what is now the Southern Alps mountain range with average measured H2O concentrations of 610 ppm for cpx and 298 ppm for opx. In contrast, dominantly “dry” domains with average H2O concentrations of 110 ppm for cpx and 49 ppm for opx reside beneath East Otago, Chatham Island and Auckland Islands. Contrasting H2O concentrations within olivine (calculated to be 60 to 100 ppm for the wet domain versus <10 ppm for the dry domains) indicate a lateral gradient in mantle effective viscosity of up to one order of magnitude beneath the Southern Alps and the surrounding regions. The narrow width of the Southern Alps orogen, ∼80 km, contrasts with most of its broad counterparts worldwide, such as the North American Cordillera where the mantle was hydrated and weakened by the subduction of the Farallon plate. However, those developed orogens were often complicated by subsequent tectonic overprinting. Our findings show that the pre-existing rheologically weak zone of the mantle lithosphere, most likely resulting from hydration by the subduction of the oceanic lithosphere attached to the Australian plate beneath the South Island of New Zealand at ∼25 Ma, affects the continental deformation at or near the surface and controls the width of the developing Southern Alps orogen.

Deformed Neogene basins, active faulting and topography in Westland: Distributed crustal mobility west of the Alpine Fault transpressive plate boundary (South Island, New Zealand)

Tectonic activity in the South Island of New Zealand is dominated by the Alpine Fault component of the Australia–Pacific plate boundary. West of the Alpine Fault deformation is recorded by Paleogene–Neogene basins coeval with the evolution of the right-lateral/transpressive plate margin. Initial tectonic setting was controlled by N–S normal faults developed during Late Cretaceous and Eocene–early Miocene rifting. Following inception of the Alpine Fault (c. 25Ma) reverse reactivation of the normal faults controlled tectonic segmentation that became apparent in the cover sequences at c. 22Ma. Based on restored transects tied to stratigraphic sections, seismic lines and wells, we reconstruct the vertical mobility of the Top Basement Unconformity west of Alpine Fault. From c. 37–35Ma to 22Ma subsidence was controlled by extensional faulting. After 22Ma the region was affected by differential subsidence, resulting from eastward crustal flexure towards the Alpine Fault boundary and/or components of transtension. Transition from subsidence to uplift started at c. 17Ma within a belt of basement pop-ups, separated by subsiding basins localised in the common footwall of oppositely-dipping reverse faults. From 17 to 7–3Ma reverse fault reactivation and uplift migrated to the WSW. Persistent reverse reactivation of the inherited faults in the present stress field is reflected by the close match between tectonic block segmentation and topography filtered at a wavelength of 25km, i.e. at a scale comparable to crustal thickness in the region. However, topography filtered at wavelength of 75km shows marked contrasts between the elevated Tasman Ranges region relative to regions to the south. Variations in thickness and rigidity of the Australian lithosphere possibly control N–S longitudinal changes, consistent with our estimates of increase in linear shortening from the Tasman Ranges to the regions located west of the Alpine Fault bend.

New Zealand GPS velocity field: 1995–2013

ABSTRACTWe collate nearly two decades of campaign GPS data gathered at over 900 sites throughout New Zealand to release a New Zealand nationwide GPS velocity field. The data span the entire North and South islands of New Zealand with a typical spacing of 10–20 km and a denser network (c. 2–8 km spacing) in the Wellington region, central Taupo Volcanic Zone and parts of the Arthur's Pass area. The dataset provides the most comprehensive-to-date view of crustal deformation within the Australia–Pacific plate boundary zone in the New Zealand region. We discuss the data acquisition, processing and derivation of the velocities and uncertainties. We also undertake corrections for earthquake displacements to obtain a velocity field that is largely representative of interseismic deformation between 1995 and 2013.

Low-temperature thermochronology of the northern Thomson Orogen: Implications for exhumation of basement rocks in NE Australia

The Tasmanides of eastern Australia record much of the Phanerozoic tectonic development of the retreating Pacific-Australia plate boundary and are an oft-cited example of an orogen that has undergone “tectonic mode switching.” To begin to constrain the timing of exhumation of basement rocks that are now exposed in portions of the NE Tasmanides, we measured apatite and zircon (U–Th)/He ages from the Thomson Orogen and overlying Paleozoic strata in the back-arc of the New England Orogen in NE Australia. Zircon (U–Th)/He ages from basement samples (including those recovered from boreholes at depths of up to 1.1km) are characterized by large inter- and intra-sample variability and range from approximately 180Ma (Early Jurassic) to 375Ma (Late Devonian). (U–Th)/He zircon ages from several individual samples are negatively correlated with effective uranium (eU), a pattern that is also true of the dataset as a whole, suggesting that variations in U and Th zoning and radiation damage are partially responsible for the age variability. The oldest zircon (U–Th)/He cooling ages coincide with the formation of regionally extensive Late Devonian–early Carboniferous back-arc basins, suggesting that Late Devonian extension played a significant role in exhumation of parts of the northern Thomson Orogen. Apatite (U–Th)/He ages from a basement sample and a late Permian sandstone in the overlying Bowen Basin, which are also marked by intra-sample variability and age–eU correlations, span from the Early Cretaceous through Oligocene, in general agreement with previous apatite fission track data. In conjunction with observations of key geologic relationships and prior K–Ar and 40Ar/39Ar data, our results suggest four overall phases in the thermal history of the northern Thomson Orogen: (1) Cambrian–early Silurian metamorphism during the Delamerian and Benambran Orogenies; (2) protracted cooling during the Late Devonian through mid-Permian that likely resulted from extensional exhumation; (3) Permian–Triassic reheating during burial beneath thick sedimentary basins; and (4) Cretaceous and Paleogene cooling during uplift and erosion.

Scenario-based numerical modelling and the palaeo-historic record of tsunamis in Wallis and Futuna, Southwest Pacific

Abstract. We investigated the tsunami hazard in the remote French territory of Wallis and Futuna, Southwest Pacific, using the Gerris flow solver to produce numerical models of tsunami generation, propagation and inundation. Wallis consists of the inhabited volcanic island of Uvéa that is surrounded by a lagoon delimited by a barrier reef. Futuna and the island of Alofi form the Horn Archipelago located ca. 240 km east of Wallis. They are surrounded by a narrow fringing reef. Futuna and Alofi emerge from the North Fiji Transform Fault that marks the seismically active Pacific-Australia plate boundary. We generated 15 tsunami scenarios. For each, we calculated maximum wave elevation (MWE), inundation distance and expected time of arrival (ETA). The tsunami sources were local, regional and distant earthquake faults located along the Pacific Rim. In Wallis, the outer reef may experience 6.8 m-high MWE. Uvéa is protected by the barrier reef and the lagoon, but inundation depths of 2–3 m occur in several coastal areas. In Futuna, flow depths exceeding 2 m are modelled in several populated areas, and have been confirmed by a post-September 2009 South Pacific tsunami survey. The channel between the islands of Futuna and Alofi amplified the 2009 tsunami, which resulted in inundation distance of almost 100 m and MWE of 4.4 m. This first ever tsunami hazard modelling study of Wallis and Futuna compares well with palaeotsunamis recognised on both islands and observation of the impact of the 2009 South Pacific tsunami. The study provides evidence for the mitigating effect of barrier and fringing reefs from tsunamis.

Wairau Basin and fault connections across Cook Strait, New Zealand: seismic and geological evidence

Cook Strait separates North Island and South Island of New Zealand. It contains the Wairau and other sedimentary basins, which have sedimentary thicknesses of up to 4 km. The Strait overlies part of the Australia–Pacific plate boundary where subduction below the North Island changes to strike-slip motion through the South Island. Strike-slip faults are well documented in the South Island and less so in the North Island. Cook Strait has been surveyed by a variety of shallow penetration seismic in the past, and this has resulted in various models and uncertainty about how major faults join across the strait. The shallow-penetration data image numerous fault traces close to the sea bed, most of which can be traced for only a short distance. Gaps between individual fault traces are common. In recent years, a growing body of research, government and oil company seismic data has imaged structures much more deeply and is the subject of this paper. The Wairau Basin structural style is one dominated by normal faulting, with expansion of basin area and progressive eastwards shift in depocentres with time. A steep structural gradient or faulted margin to basement subcrop and outcrop on the North Island margin indicates a significant north-northwest to northwest component of fault control in this area of Neogene development. The seismic interpretation of this deeper data suggests there are continuous seismogenic zones underlying the discontinuous young seafloor fault scarps. Such discontinuities have led to a perception that Cook Strait faults are not connecting through, that fault rupturing across Cook Strait is improbable. Fault connections across Cook Strait along seismogenic zones have recently been interpreted from historical records. These indicate that the October 1848 earthquakes involved mainshock (Mw 7.4–7.7) displacement on the Awatere Fault in the South Island and probable aftershock displacement on the Ohariu and Wellington faults in the North Island. The January–February 1855 earthquakes (mainshock Mw ∼8.2–8.4) occurred on the Wairarapa Fault in southern North Island, Wharekauhau, Nicholson Bank faults in Cook Strait, Vernon and Awatere faults in the South Island, and Needles Fault off the NE Marlborough coast. These interpretations are supported by recent seismic data that indicate underlying basin-defining seismogenic zones connect across Cook Strait, with movements that date back at least to the top Paleogene.

Geology and Age of Solander Volcano, Fiordland, New Zealand

AbstractThe Solander Islands are the eroded remnants of a once much larger volcanic complex, the only known subduction-related magmatism on the Pacific-Australia plate boundary south of New Zealand. The largest island, Hautere, preserves an eruptive sequence of subaerial adakitic andesite domes and related block and ash flows, followed by a phase of phreatomagmatic deposits. Ar-Ar plagioclase ages, detrital U-Pb zircon ages, and magnetostratigraphic data from Hautere indicate that magmatism probably occurred in the interval 100–350 ka. In contrast, an andesite dome on nearby Little Solander Island gives an age of 20–50 ka. Local hydrothermal activity is manifested as orange-colored alteration and tuffisite dikes. There is no evidence, at least in the small preserved remnants of the Solander Islands, for tectonic uplift or subsidence; most of Solander Volcano was eroded away during Late Pleistocene marine planation.

Sub-crustal earthquakes within the Australia–Pacific plate boundary zone beneath the Southern Alps, New Zealand

Sub-crustal earthquakes have been observed sporadically for ∼40 years in the central South Island of New Zealand. We report on 20 events recorded between December 2008 and February 2012 near the Alpine Fault in the continental collision zone between the Australian and Pacific plates. A subset of 18 events at depths of 47–74 km occurs south of Mt. Cook and together with recently reported tremor locations indicates along-strike variations in deformation behaviour along the plate boundary. The sub-crustal earthquakes south of Mt. Cook increase in depth, frequency and size southwards towards the Puysegur subduction zone. Focal mechanisms could be determined for 14 earthquakes and exhibit predominantly strike-slip and reverse faulting solutions. Stress inversion analysis of the focal mechanisms yields a stress field favouring oblique-reverse faulting. We interpret the geographic and vertical distributions of these sub-crustal events in relation to a previously proposed tectonic model of a remnant passive margin that formed south of New Zealand in the Eocene and was overridden when dextral strike-slip motion initiated on the Alpine Fault. We infer that sub-crustal earthquakes occur along the leading edge of this structure, which is attached to the continental Australian crust.

Three-dimensional variations in present-day tectonic stress along the Australia–Pacific plate boundary in New Zealand

Using a newly compiled data set of 3424 focal mechanisms, we have estimated tectonic stress parameters at 100 locations throughout central New Zealand in the largest study to date of the tectonic stress field along the Australia–Pacific plate boundary. The results reveal pronounced changes in the azimuth of maximum horizontal compressive stress SHmax along the shallow portion (<50km depths) of the Hikurangi subduction margin, with a marked change from margin-parallel SHmax north of Hawke's Bay (latitude c. 40°S) to a more oblique SHmax orientation further south. This change appears to coincide with the along-strike variations in subduction thrust coupling inferred from geodetic and seismological observations. In contrast, the orientation of SHmax is highly uniform across most of the South Island (averaging c. 115°) and collinear with the axis of relative contractional strain rate. Analysis of focal mechanisms recorded before and after the damaging MW6.2 Christchurch earthquake of 22 February 2010 reveals no significant change in SHmax orientation or the overall stress regime: this suggests that even the high-stress drop Christchurch earthquake was incapable of substantially modifying the ambient stress field, at least on the scales at which focal mechanism stress estimation can be performed.

The geological setting of the Darfield and Christchurch earthquakes

The 2010–2011 Canterbury earthquake sequence occurred near the southeastern margin of Neogene deformation associated with the Australia–Pacific plate boundary. Basement comprises indurated rocks of the Torlesse Composite Terrane, of Permian to Early Cretaceous age, overlain by 1–2 km of less-indurated Cretaceous–Neogene rocks and unconsolidated Quaternary sediments. Proximity to the subduction interface between Gondwana and the paleo-Pacific Ocean produced a Mesozoic-age structural grain in the basement rocks, aligned broadly east–west in the Canterbury to Chatham Rise areas. These structures provided an inherited weakness that was likely reactivated by present-day stress. Mid- to Late Cretaceous extension, marked by localised fault-bounded grabens, was followed by deposition of a Late Cretaceous to Paleogene passive-margin transgressive sedimentary sheet and minor intraplate basaltic volcanics. Mid-Cenozoic inception of the modern Australia–Pacific plate boundary heralded deposition of a regressive succession of Neogene sediments and further episodes of volcanism, most notably constructing the Late Miocene Banks Peninsula intraplate volcanoes. The east- to northeast-striking faults associated with the Darfield and Christchurch earthquakes are probably aligned with the Mesozoic structural grain within the Torlesse basement rocks.

The Moho in Australia and New Zealand

Australia and New Zealand share in part a history in the Gondwana supercontinent. Australia has a long and complex tectonic history with the last major accretion in the early Paleozoic, whereas New Zealand is still undergoing major plate boundary processes. The Australian continent is relatively well covered with both active and passive seismic techniques. Multiple sources of information are therefore available for building a model of Moho depth. Results from on-shore and off-shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations. Moho picks from more than 10 500 km of full-crustal reflection profiles provide valuable additional constraints. The composite data set provides good sampling of much of Australia, though coverage remains low in some remote desert areas. The various datasets provide multiple estimates of the depth to Moho in many regions, and the consistency between the different techniques is high. Some of the thinnest crust lies beneath the Archean craton in the Pilbara, and in the neighbourhood of the Simpson desert. Thick crust is encountered beneath parts of the Proterozoic in Central Australia, and beneath the Paleozoic Lachlan fold belt in southeastern Australia. There are a number of zones of sharp contrast in depth to Moho, notably in the southern part of Central Australia. Despite most of the continental material around New Zealand being submerged, Moho data for this region is mainly onshore concentrating on the Australia–Pacific plate boundary. Two major wide-angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore. The plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks. Onshore the combined coverage is as dense as that of Australia, although it could be argued that a higher spatial resolution is required to capture the nature of the Moho of tectonically active New Zealand. Three regions of thickened crust can be identified, one beneath the Southern Alps, another beneath Fiordland, and below the Wanganui Basin between the North and South Island. Thin crust is identified west of the volcanic arc, with extensive underplating below the back-arc region.

Evolution of Macquarie Ridge Complex seamounts: Implications for volcanic and tectonic processes at the Australia–Pacific plate boundary south of New Zealand

The Macquarie Ridge Complex (MRC) coincides with the Australia–Pacific plate boundary south of New Zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length. Tectonic plate reconstructions show that the Australia–Pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca. 40 to 6Ma. However, limited sampling and high-resolution mapping of the region have meant that the composition and morphology of the MRC remain relatively unknown. Here we present results from multibeam mapping, photographic investigation and petrologic and geochemical analyses from 10 seamounts along the MRC. Dredge samples collected from the seamounts are alkaline to sub-alkaline basalts, containing 45.1–50.8wt.% SiO2, 3.5–12.0wt.% MgO and 1.9–4.9wt.% total alkalis. All samples are enriched mid-ocean ridge basalts that have K/Ti>0.15, which originated as low degree partial melts during the late stages of mid-ocean ridge volcanism. This oceanic crust has been sheared and accreted along the plate boundary since ca. 6Ma by cumulative transpressional relative plate motion, such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge. MRC seamounts have formed as discrete elevations as a result of dip-slip and strike-slip faulting of the ridge axis. Three guyot-type seamounts have flat-top summit plateaux that were presumably formed by wave erosion via relative sea-level lowering due to uplift of the ridge. These seamounts have since subsided due to extension associated with the step-over fault geometry of the transform plate boundary along the McDougall and Macquarie ridge segments. While the seamounts are of volcanic origin, they are manifestations of tectonic and geomorphic processes. Seamounts distal from the ridge axis, however, are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes.

Lithosphere delamination with foundering of lower crust and mantle caused permanent subsidence of New Caledonia Trough and transient uplift of Lord Howe Rise during Eocene and Oligocene initiation of Tonga-Kermadec subduction, western Pacific

[1] We use seismic reflection and rock sample data to propose that the first-order physiography of New Caledonia Trough and Norfolk Ridge formed in Eocene and Oligocene time and was associated with the onset of subduction and back-arc spreading at the Australia-Pacific plate boundary. Our tectonic model involves an initial Cretaceous rift that is strongly modified by Cenozoic subduction initiation. Hence, we are able to explain (1) complex sedimentary basins of inferred Mesozoic age; (2) a prominent unconformity and onlap surface of middle Eocene to early Miocene age at the base of flat-lying sediments beneath the axis of New Caledonia Trough; (3) gently dipping, variable thickness, and locally deformed Late Cretaceous strata along the margins of the trough; (4) platform morphology and unconformities on either side of the trough that indicate a phase of late Eocene to early Miocene uplift to near sea level, followed by rapid Oligocene and Miocene subsidence of ∼1100–1800 m; and (5) seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of New Caledonia Trough by 1800–2200 m since Eocene time. The Cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab. This created a deep (>2 km) enclosed oceanic trough, ∼2000 km long and 200–300 km across, in Eocene and Oligocene time as the lower crust detached, with simultaneous uplift and local land development along basin flanks. Disruption of Late Cretaceous and Paleogene strata was minimal during this Cenozoic phase and involved only subtle tilting and local reverse faulting or folding. Basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into Norfolk Basin. We suggest that delamination of the lithosphere, with possible mixing of the lower crust back into the mantle, is more widespread than previously thought and may be commonly associated with subduction initiation, such as Cenozoic events in the Mediterranean and western Pacific.