Hikurangi Plateau Research Articles

Evidence From Intermediate‐Depth Earthquakes of Slab‐Derived Fluids Beneath the Taupō Volcanic Zone

AbstractAhi Tupua, the central section of the Taupō Volcanic Zone in the central North Island of New Zealand, encompasses Taupō and Ōkataina calderas and has been the most frequently active and productive silicic magma system worldwide during the Quaternary. The entire Taupō Volcanic Zone is underlain by the Hikurangi Subduction Zone where a Large Igneous Province, the Hikurangi Plateau, is being subducted, but Ahi Tupua exhibits much higher rates of magma output than either the northern or the southern sections. Trace element signatures of Ahi Tupua eruptive products suggest that both decompression melting and flux melting play a role in this rifted arc setting, with fluid flux signatures being more prevalent in regions of active caldera volcanism. Intermediate‐depth (50–300 km) earthquakes provide a means of studying faulting and fluid flow processes within and around the subducting slab beneath Ahi Tupua. Using data from the national seismic network and a 13‐station temporary network (“ECLIPSE”), we repick and relocate 397 intermediate‐depth earthquakes of magnitudes M2.5+ that occurred in a 29‐month interval, and compute focal mechanisms for a subset of 47 earthquakes. We observe some seismicity that may be within the mantle wedge but most of the relocated earthquakes occur within the crust and uppermost mantle of the subducted slab and exhibit a weak transition from predominantly normal‐faulting in the slab crust to predominantly reverse‐faulting in the slab mantle. No double seismic zone is observed beneath Ahi Tupua. These observations are consistent with dehydration of the slab and flow of slab‐derived fluids along existing faults into the mantle wedge, that drives flux melting and accounts for the distinctive geochemical signatures and voluminous output of Ahi Tupua calderas.

Volcanic crustal structure of the western Hikurangi Plateau (New Zealand) from marine seismic reflection imaging

Seamounts and basaltic basement can influence deformation and mass fluxes within subduction zones. We examined seamounts and volcanic units across the western Hikurangi Plateau, near the Hikurangi subduction margin, New Zealand, with seismic reflection images. Volcanism at the Hikurangi Plateau occurred in at least three phases that we attribute to (1) Early Cretaceous large igneous province formation, the top of which is marked by laterally continuous and dipping wedges of reflections that we interpret as lava flows; (2) Late Cretaceous seamounts and volcaniclastics that erupted onto the crust of the Hikurangi Plateau and make up the majority of seamount volume and basement relief; and (3) late-stage, Pliocene volcanics that erupted through and adjacent to Cretaceous seamounts and younger sediments of the north-central Hikurangi Plateau. The Pliocene volcanoes do not appear to be strongly welded to the plateau basement and may be petit spot volcanoes that are related to the displacement and accumulation of hydrous transition zone melts. Large seamounts and volcaniclastic units are evenly distributed across most of the Hikurangi Plateau near the Hikurangi margin but are absent from the Pegasus Basin. Although faults are imaged throughout the basement of the Pegasus Basin, contemporary normal faulting of the Hikurangi Plateau is uncommon, except for a zone of Quaternary normal faults near the Pliocene volcanics. These trends indicate that the Hikurangi megathrust may be more influenced by volcanic structures in the north and central Hikurangi margin, where plateau rifting and voluminous seamount eruptions have more substantially overprinted the original Early Cretaceous basement.

A new family of Asellota (Crustacea, Isopoda) from the deep sea of Zealandia

A new family-level taxon of deep-sea isopods, Basoniscus hikurangi gen. et sp. nov., was recovered from the Hikurangi Plateau in the deep sea off eastern New Zealand. The broad-bodied, eyeless, seemingly unremarkable isopod was unusual in its possession of features that characterize two different families: the shallow water Joeropsididae Nordenstam, 1933 and the deep-sea Haploniscidae Hansen, 1916. An analysis of superfamily Janiroidea G.O. Sars, 1897 was conducted to establish the affinities of the species. Multiple analyses were done using unweighted and implied weighted characters. Existing families were well supported, with B. hikurangi intermediate between Joeropsididae and Haploniscidae. The new species, however, cannot be placed in either family owing to its lack of important defining synapomorphies of each family. As a result, Basoniscidae fam. nov. was created to contain this new species. That rocky hard substrates are undersampled is another implication. Our understanding of deep-sea species richness will not be accurate until more efforts are made to survey these habitats, especially more sites in the southern hemisphere. These gaps in our knowledge of the deep sea impairs any general claims about the distribution of biodiversity on a global scale. This find demonstrates that museums hold underused but valuable resources for understanding and describing the biodiversity of the deep sea.

Heterogeneous Crustal Structure of the Hikurangi Plateau Revealed by SHIRE Seismic Data: Origin and Implications for Plate Boundary Tectonics

AbstractMarine multichannel and wide‐angle seismic data constrain the distribution of seamounts, sediment cover sequence and crustal structure along a 460 km margin‐parallel transect of the Hikurangi Plateau. Seismic reflection data reveals five seamount up‐to 4.5 km high and 35–75 km wide, with heterogeneous internal velocity structure. Sediment cover decreases south‐to‐north from ∼4.5 km to ∼1–2 km. The Hikurangi Plateau crust (VP 5.5–7.5 km/s) is 11 ± 1 km thick in the south, but thins by 3–4 km further north (∼7–8 km). Gravity models constructed along two seismic lines show the reduction in crustal thickness persists further east, coinciding with a bathymetric scarp. Gravity data suggest the transition in crustal thickness may reflect spatial variability in deformation and lithospheric extension associated with plateau breakup. Variability in the thickness of subducting crust may contribute to differences in megathrust geometry, upper‐plate stress state and high‐rates of contraction and uplift along the southern Hikurangi margin.

Subducting volcaniclastic-rich upper crust supplies fluids for shallow megathrust and slow slip.

Recurring slow slip along near-trench megathrust faults occurs at many subduction zones, but for unknown reasons, this process is not universal. Fluid overpressures are implicated in encouraging slow slip; however, links between slow slip, fluid content, and hydrogeology remain poorly known in natural systems. Three-dimensional seismic imaging and ocean drilling at the Hikurangi margin reveal a widespread and previously unknown fluid reservoir within the extensively hydrated (up to 47 vol % H2O) volcanic upper crust of the subducting Hikurangi Plateau large igneous province. This ~1.5 km thick volcaniclastic upper crust readily dewaters with subduction but retains half of its fluid content upon reaching regions with well-characterized slow slip. We suggest that volcaniclastic-rich upper crust at volcanic plateaus and seamounts is a major source of water that contributes to the fluid budget in subduction zones and may drive fluid overpressures along the megathrust that give rise to frequent shallow slow slip.

Rapid Formation of the Ellice and Osbourn Basins and Ontong Java Nui Breakup Kinematics

AbstractBreakup of the proposed greater Ontong Java Nui large igneous province during the Cretaceous Normal Superchron can be constrained by the opening of the Ellice Basin (EB) separating the Ontong Java and Manihiki Plateaus and the Osbourn Basin separating the Manihiki and Hikurangi Plateaus. Dating of recovered dredged samples using plagioclase 40Ar/39Ar and zircon U/Pb geochronology methods indicates that spreading was well underway in the EB by 118 Ma with full spreading rates up to 3X faster than any observed today of 30–45 cm/yr and spreading likely continued until 112‐108 Ma. Ellice Basin samples show diverse geochemical affinities ranging from mid‐ocean ridge basalt (MORB) or Ontong Java‐like to more enriched OIB‐like. Pb and Nd isotopes from six samples contain varying influences from Pacific MORB and possibly Ontong Java. The geochemistry shows a lack of a clear mantle plume influence despite EB's close temporal and spatial relationship to Ontong Java, while some data resemble the Louisville Seamounts. This compositional diversity complements morphological differences among dredge sites and shows that both in situ MORB and younger overprinted features related to the nearby Tuvalu Seamounts were sampled. 40Ar/39Ar geochronology confirms the age of International Ocean Discovery Program Site U1365 near the Osbourn Trough (OT) to be 102.60 ± 0.26 Ma (2σ, n = 18). This age constrains the timing of a spreading reorientation event observed in the OB to coincide with a global plate reorganization event around 105 Ma and estimates the cessation of spreading at the OT to 96 Ma.

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.

New evidence for the Ontong Java Nui hypothesis

The formation of the Ontong Java Nui super oceanic plateau (OJN), which is based on the model that the submarine Ontong Java Plateau (OJP), Manihiki Plateau (MP), and Hikurangi Plateau (HP) were once its contiguous fragments, could have been the largest globally consequential volcanic event in Earth’s history. This OJN hypothesis has been debated given the paucity of evidence, for example, the differences in crustal thickness, the compositional gap between MP and OJP basalts and the apparent older age of both plateaus relative to HP remain unresolved. Here we investigate the geochemical and 40Ar-39Ar ages of dredged rocks recovered from the OJP’s eastern margin. Volcanic rocks having compositions that match the low-Ti MP basalts are reported for the first time on the OJP and new ~ 96–116 Ma and 67–68 Ma 40Ar-39Ar age data bridge the temporal gap between OJP and HP. These results provide new evidence for the Ontong Java Nui hypothesis and a framework for an integrated tectonomagmatic evolution of the OJP, MP, and HP. The isotopic data imply four mantle components in the source of OJN that are also expressed in present-day Pacific hotspots sources, indicating origin from (and longevity of) the Pacific Large Low Shear-wave Velocity Province.

Cretaceous tectonostratigraphy of ‘the Great Coverham section’ and adjacent areas, northeastern Waiau Toa/Clarence valley, New Zealand

ABSTRACT Late Early–Late Cretaceous strata preserved at the northeastern end of the Waiau Toa/Clarence valley, northeastern South Island, New Zealand, have been central to ongoing debates concerning the end of Mesozoic subduction on the Zealandian margin of Gondwana and the initiation of extension. New geological mapping within this area at Coverham, Kekerengu and Wharekiri, has resolved complex relationships between Cretaceous stratigraphic units and faults. Results indicate that, within the interval 110–100 Ma, Champagne and lower Split Rock formations record syn-tectonic deposition by marine mass-transport processes within confined sub-basins with steep seafloor gradients. Upper Split Rock Formation and overlying Nidd Formation record progressive burial of seafloor topography between c. 100–86 Ma. Although uncertainties remain, three or four faults appear to have been low-angle structures with normal displacements that were active within the interval c. 105–89 Ma; one of these faults experienced brief reversal of offset between c. 91 and 87 Ma. The new data are most consistent with a model of subduction shutdown between c. 105–100 Ma, following impact of the Hikurangi Plateau large igneous province at the subduction margin. Widespread uplift along the margin between 97 and 95 Ma may reflect subsequent rebound of the subducted plate.

Causes of permanent vertical deformation at subduction margins: Evidence from late Pleistocene marine terraces of the southern Hikurangi margin, Aotearoa New Zealand

Theoretical studies of the seismic cycle at convergent plate boundaries anticipate that most coseismic deformation is recovered, yet significant permanent vertical displacement of the overriding plate is observed at many subduction margins. To understand the mechanisms driving permanent vertical displacement, we investigate tectonic uplift across the southern Hikurangi subduction margin, Aotearoa New Zealand, in the last ∼200 ka. Marine terraces preserved along the Wellington south coast have recently been dated as Marine Isotope Stage (MIS) 5a (∼82 ka), 5c (∼96 ka), 5e (∼123 ka) and 7a (∼196 ka) in age. We use these ages, together with new reconstructions of shoreline angle elevations, to calculate uplift rates across the margin and to examine the processes responsible for their elevation. The highest uplift rate—1.7 ± 0.1 mm/yr–and maximum tilting—2.9° to the west–are observed near Cape Palliser, the closest site to (∼50 km from) the Hikurangi Trough. Uplift rates decrease monotonically westward along the Palliser Bay coast, to 0.2 ± 0.1 mm/yr at Wharekauhau (∼70 km from the trough), defining a gently west-tilted subaerial forearc domain. Locally, active oblique-slip upper-plate faults cause obvious vertical offsets of the marine terraces in the axial ranges (&amp;gt;70 km from the trough). Uplift rates at Baring Head, on the upthrown side of the Wairarapa-Wharekauhau fault system, are ∼0.7–1.6 mm/yr. At Tongue Point, uplift on the upthrown side of the Ōhāriu Fault is 0.6 ± 0.1 mm/yr. Dislocation and flexural-isostatic modelling shows that slip on faults within the overriding plate—specifically the Palliser-Kaiwhata Fault and the Wairarapa-Wharekauhau fault system—may dominate uplift in their immediate hanging walls. Depending on their slip rate and geometry, slip on these two upper-plate fault systems could plausibly cause &amp;gt;80% of late Pleistocene uplift everywhere along the south coast of North Island. Our modelling suggests that subduction of the buoyant Hikurangi Plateau contributes uplift of 0.1–0.2 mm/yr and uplift due to sediment underplating at Tongue Point and Wharekauhau is likely ≤0.6 mm/yr but could be significantly lower. Earthquakes on the subduction interface probably contribute ≤0.4 mm/yr of late Pleistocene uplift, with ≤10% of uplift due to each earthquake being stored permanently, similar to other subduction zones. These results indicate a significant contribution of slip on upper-plate faults to permanent uplift and tilting across the subduction margin and suggest that in regions where upper-plate faults are prevalent, strong constraints on fault geometry and slip rate are necessary to disentangle contributions of deeper-seated processes to uplift.

Temporal Relationship of Slow Slip Events and Microearthquake Seismicity: Insights From Earthquake Automatic Detections in the Northern Hikurangi Margin, Aotearoa New Zealand

AbstractSlow slip events in the northern Hikurangi margin of Aotearoa New Zealand occur every 18–24 months and last for several weeks before returning to average convergence rates of around 38 mm/yr. Along this plate boundary, the Hikurangi plateau subducts beneath the overlying Australian plate and slow slip events occur along their plate interface at depths between 2 and 15 km. To explore whether there is a temporal relationship between slow slip events and earthquake occurrence, the Regressive ESTimator automated phase arrival detection and onset estimation algorithm was applied to a data set of continuous waveform data collected by both land and ocean bottom seismometers. This detector uses an autoregressive algorithm with iterative refinement to first detect seismic events and then create a catalog of hypocenters and P and S wave arrival times. Results are compared with an available catalog of manually detected seismic events. The auto‐detector was able to find more than three times the number of events detected by analysts. With our newly assembled data set of automatically detected earthquakes, we were able to determine that there was an increase in the rate of earthquake occurrence during the 2014 slow slip event.

Effects of variable slab components and tectonics on magma composition in the intra-oceanic Kermadec Arc-Backarc system

The Kermadec Arc-Backarc system (25–39°S) in the southwest Pacific is an important natural laboratory to examine the controls on arc magma compositions, because the physical parameters that are thought to influence subduction-related magmatism (the thickness and composition of the upper plate, mantle depletion and partial melting, and the slab components) change along the arc. Furthermore, the presence of back-arc and rear-arc volcanism allow insight into how mantle composition varies with distance to the trench. We present new major and trace element and Pb–Sr–Nd–Hf isotope data for 101 lava samples from 10 volcanoes along the Kermadec Arc, and combine these with published data for volcanic arc front, rear-arc, and back-arc lavas. The Kermadec Arc-Backarc magmas experienced similar fractional crystallization paths, and those erupted onto oceanic crust are not affected by crustal assimilation. The degree of partial melting varies along and across the arc, partly due to variations in the thickness of the lithosphere of the upper plate, but also due to variations in slab fluid input. We distinguish four different arc segments varying from 100 to 600 km wide, which erupt lavas with distinct compositions due to differences in along-arc slab input. All Kermadec arc and rear-arc lavas were derived from mantle sources to which the sediment melt and a fluid from subducted Pacific oceanic crust or the Hikurangi Seamounts had been added. Increasing sediment melt input into the mantle wedge southwards is reflected in increasing 87Sr/86Sr and Th/Nd, and decreasing 143Nd/144Nd southwards towards New Zealand. Some volcanoes from the central Kermadec Arc show with unusually radiogenic Pb isotope compositions that are probably derived from subducted alkaline rocks from seamounts built on the Hikurangi Plateau. The geochemical segmentation of the Kermadec Arc indicates that slab-derived components can be relatively homogeneous on large scales, but small features like seamounts cause complex additions to the slab component.

The 2021 Mw 7.3 East Cape Earthquake: Triggered Rupture in Complex Faulting Revealed by Multi‐Array Back‐Projections

AbstractOn 04 March 2021 the Mw 7.3 East Cape, New Zealand earthquake occurred at the southern end of the Kermadec Trench. The United States Geological Survey W‐phase solution shows an unexpected focal mechanism compared to historical earthquakes. We combine earthquake back‐projections of 4 arrays to image the kinematic process of the Mw 7.3 earthquake. Our results reveal a bilateral rupture predominantly propagating upward at a slow speed of 1 km/s and a triggered rupture at ∼10 km, corresponding to the deep reverse faulting event at 70 km and a triggered shallow normal faulting event (∼20 km) resolved by previous subevent and finite fault inversions. The epicenter and a group of aftershocks are deeper than the bottom of the slab according to a regional tomography model. Such deep failure is possibly enabled by a delaminating lower crust of the Hikurangi plateau.

The Origin of Magmas and Metals at the Submarine Brothers Volcano, Kermadec Arc, New Zealand

Abstract International Ocean Discovery Program (IODP) Expedition 376 cored the submarine Brothers volcano of the Kermadec arc to provide insights into the third dimension and the evolution of the volcano and its associated ore-forming systems. We present new petrological and geochemical data on dacitic rocks drilled from Brothers as well as mafic rocks collected at two adjacent ridges. These data include major and trace element compositions of whole rocks, including many economically important metals and metalloids such as Cu, Ag, Pt, Au, Mo, As, Sb, Tl, and Bi, plus Sr-Nd-Pb isotope compositions as well as in situ analyses of glasses and minerals. We show that the basalts and basaltic andesites erupted at the volcanic ridges near Brothers represent potential mafic analogues to the dacites that make up Brothers volcano. Mantle melting and ore potential of the associated magmas are locally enhanced by raised mantle potential temperatures and a high flux of subducted components originating from the partially subducted Hikurangi Plateau. As a result, the parental melts at Brothers are enriched in ore metals and metalloids relative to mid-ocean ridge basalts (MORBs), and a high melt oxidation state (Δ log fO2 of +1.5 fayalite-magnetite-quartz [FMQ]) suppresses early sulfide saturation. However, solid sulfide crystallization occurs late during magma differentiation, with the result that the dacitic lavas at Brothers volcano are strongly depleted in Cu but only moderately depleted in Ag and Au. The dacites at Brothers thus have a high fertility for many metals and metalloids (e.g., As, Sb, Bi), and fluids exsolving from the cooling magma have a high ore-forming potential.

Crustal Structure of the Hikurangi Margin From SHIRE Seismic Data and the Relationship Between Forearc Structure and Shallow Megathrust Slip Behavior

AbstractMarine multichannel and wide‐angle seismic data constrain crustal structure along a 530 km margin‐parallel transect of the Hikurangi subduction zone. The subducting Hikurangi Plateau crust (VP 5.0–7.4 km/s) is ∼1 km thicker (11 ± 1 km) and mantle velocities are ∼0.2 km/s higher (VP 8.3–8.5 km/s) beneath south/central Hikurangi relative to north Hikurangi. In the overthrusting plate, an abrupt 0.5 km/s south‐to‐north reduction in forearc wavespeeds occurs in concert with a change in seismic reflection character. We analyze legacy seismic data to show that the forearc transition likely reflects lateral variability in the updip extent of the Torlesse Backstop. Furthermore, we map this unit along‐strike and note a broad correlation between the backstop and down‐dip extent of shallow slow‐slip. We propose that the geological architecture of the overthrusting plate contributes to spatial variability in the location of shallow frictional transitions along the Hikurangi margin, impacting both seismic and tsunami hazard.

Illuminating a Contorted Slab With a Complex Intraslab Rupture Evolution During the 2021 Mw 7.3 East Cape, New Zealand Earthquake

AbstractThe state‐of‐stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large‐scale plate geometry and the stress regime relate to the physical nature of the deep intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike‐slip, and normal faulting. Reverse faulting due to the trench‐parallel compression is unexpected given the apparent subduction direction, so we require a differential buoyancy‐driven stress rotation, which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs.

A Forearc Stratigraphic Response to Cretaceous Plateau Collision and Slab Detachment, South Island, New Zealand

AbstractSedimentary successions deposited within arc‐trench gaps (forearcs) record topographic oscillations related to subduction tectonism. The collision of oceanic plateaus can inhibit normal subduction and induce detachment of the subducted oceanic lithosphere (slab), leading to subduction translation or shut‐down. Such second‐order subduction processes complicate the stratigraphic record making discrete events difficult to resolve, as preserved examples are rare and typically overprinted by deformation. We compiled and averaged (stacked) top subduction wedge (Pahau Terrane) subsidence and paleo‐water depth curves from 30 locations containing mid‐Cretaceous forearc successions in Marlborough, New Zealand. Subsidence curve stacking proved effective in removing deformation due to normal subduction, allowing for resolution of higher order plate‐margin events. The stacked curves reveal a synchronous topographic oscillation affecting over 100 km of the paleo‐coastline between 110 and 95 Ma, which penetrated almost 250 km into the forearc hinterland and created up to 3 km of accommodation space. Topographic drawdown and highstand deposition from 110 Ma were followed by rebound and volcanism by 98 Ma, a response to regional tectonism that was compared to theoretical predictions for ridge and plateau collision. We conclude that plate flexure, and slab extension and tearing reasonably account for the synchronous topographic oscillation observed, which we attribute to Hikurangi Plateau collision. We postulate that collision began east of Marlborough along the present day Chatham Rise from 110 Ma, producing topographic upheaval and disruption of adjacent sedimentary systems. Sediments deposited contemporaneously along the margin preserve a record of these events and provide a rare insight into evolving mid‐Cretaceous plate dynamics.

Origin and diagenetic priming of a potential slow-slip trigger zone in volcaniclastic deposits flanking a seamount on the subducting plate, Hikurangi margin, New Zealand

ABSTRACT Slow-slip megathrust events in the Hikurangi forearc region may be related to seamount subduction. IODP Expedition 375 drilled the flank of Tūranganui Knoll, a Cretaceous seamount east of the subduction zone containing units analogous to intervals associated with subducted seamounts. Detailed investigations of the volcaniclastic Unit V at Site U1520, based on scanned core images, petrographic observations, point-counting, and μXRF mapping, reveal that it is a mafic volcaniclastic succession rich in clay minerals which are products of a multiphased history of diagenetic alteration. This unit is interpreted as resulting from downslope transport of volcaniclastic debris from submarine eruptions. In contrast, most volcanic clasts recovered at the nearby Site U1526, near the top of Tūranganui Knoll, are interpreted as products of subaerial eruptions reworked by wave action. This shallow water sedimentary package experienced minimal clay-mineral alteration and more extensive early carbonate cementation. Differences in the clast origins, depositional settings and diagenetic histories of these volcaniclastic units highlight the heterogeneity of volcanic systems on the Hikurangi Plateau. However, the presence of voluminous hydrous clays within thick, altered flanking volcaniclastic units at Site U1520 provides a ready source for excess pore fluids that may enable slow-slip events along the Hikurangi subduction zone.

Heterogeneous material properties—as inferred from seismic attenuation—influenced multiple fault rupture and ductile creep of the KaikouraMw 7.8 earthquake, New Zealand

SUMMARYThe 2016 Mw 7.8 Kaikoura, New Zealand, earthquake occurred along the eastern margin of the transition region between active subduction in the North Island and oblique collision in the South Island. To infer crustal properties, we imaged Q (1/seismic attenuation) by combining selected M &amp;gt; 3.5 aftershocks with data from previous Q models. For 158 distributed aftershocks, we fit spectral decay on temporary stations and all Geonet stations, providing 6194 t*p and 19 497 t*s. Considering the varied rheology and faults, we also used 2.5-D numerical models to study ductile strain development. The complex earthquake ruptured an ∼180-km-long zone of multiple faults, which involved jumping around the complicated eastern end of the Hope fault, without significant slip on the Hope fault. The Qs and Qp results show features in the upper and lower crust which correlate to the distribution and types of fault rupture. This earthquake involved numerous faults over a region of greywacke crust, where the underlying high Q Cretaceous slab is about 30-km depth. It initiated with ∼5-m slip on the Humps fault in a region of background seismicity and low Q lower crust, adjacent to the Hope fault. The central region near Kaikoura shows a high Q crustal block, which appears to have inhibited rupture; as the rupture progressed over several small faults to jump offshore of the apparently strong block. Underlying the Kaikoura greywacke crust, below 20-km depth, there is a 40-km-long region of increased Vp, Qs and gravity, which likely represents an intraplate plutonic complex emplaced into the Hikurangi Plateau, forming an elevated section which influences deformation. In the northern section, in a region with relatively uniform moderate to low Q, the earthquake evolved into the relatively continuous ∼80-km-long major rupture along the Jordan, Kekerengu and Needles faults, with ∼6–20 m dextral slip at depth and surface displacements of ∼10 m dextral and ∼2 m vertical. The northern progression of the rupture stopped when it approached an abrupt change to high Q crust across Cook Strait. At 20–30-km depth northwest of the rupture, deeper zones with low Q are consistent with regions of distributed ductile shear and creep where the observed afterslip may have occurred, where the underlying slab is 25–40 km deep. The numerical model shows that ductile deformation localizes in this area of lower crust above the relatively strong slab, connecting outer faults (Kekerengu) to inland faults (Clarence, Awatere, Wairau), and demonstrates that no subduction thrust is required under the Marlborough region.

Crustal Structure of the Northern Hikurangi Margin, New Zealand: Variable Accretion and Overthrusting Plate Strength Influenced by Rough Subduction

AbstractExploring the structure of convergent margins is key to understanding megathrust slip behavior and tsunami generation. We present new wide‐angle and marine multichannel seismic data that constrain the crustal structure and accretion dynamics of the northern Hikurangi margin. The top of the basement of the Hikurangi Plateau is overlain by a rough, 2–3 km thick layer of volcanic cover with P‐wave velocities (VP) between 3 and 5 km/s. This volcanic cover contributes significantly to seismic reflectivity beneath the shallow subduction plate boundary. The frontal prism structure varies along‐strike from ∼25 km wide with imbricate thrust faults where accretion of trench sediments is undisrupted, to narrower (∼14 km) with slumps and branching, irregular thrust fault geometries, which may reflect lower sediment supply or past seamount collisions. A large thrust fault network in the inner prism with a seismically fast hanging wall indicates a mechanical boundary between a seismically faster deforming backstop and the seismically slower frontal prism. Near the coastline, VP increases between 2.8 and 4 km/s at 2–8 km depth and is 0.5–1.7 km/s slower than the southern Hikurangi margin. Low seismic wavespeeds and low vertical velocity gradients in the inner prism support the hypothesis that a weak overthrusting plate contributes to historic tsunami‐earthquakes and long duration seismic ground motion.