Academic literature on the topic 'Hikurangi subduction zone'

Continuously operating global positioning system sites in the North Island of New Zealand have revealed a diverse range of slow motion earthquakes on the Hikurangi subduction zone. These slow slip events (SSEs) exhibit diverse characteristics, from shallow (<15 km), short (<1 month), frequent (every 1–2 years) events in the northern part of the subduction zone to deep (>30 km), long (>1 year), less frequent (approximately every 5 years) SSEs in the southern part of the subduction zone. Hikurangi SSEs show intriguing relationships to interseismic coupling, seismicity, and tectonic tremor, and they exhibit a diversity of interactions with large, regional earthquakes. Due to the marked along-strike variations in Hikurangi SSE characteristics, which coincide with changes in physical characteristics of the subduction margin, the Hikurangi subduction zone presents a globally unique natural laboratory to resolve outstanding questions regarding the origin of episodic, slow fault slip behavior. ▪ New Zealand's Hikurangi subduction zone hosts slow slip events with a diverse range of depth, size, duration, and recurrence characteristics. ▪ Hikurangi slow slip events show intriguing relationships with seismicity ranging from small earthquakes and tremor to larger earthquakes. ▪ Slow slip events play a major role in the accommodation of plate motion at the Hikurangi subduction zone. ▪ Many aspects of the Hikurangi subduction zone make it an ideal natural laboratory to resolve the physical processes controlling slow slip.

Abstract Geological records of subduction earthquakes, essential for seismic and tsunami hazard assessment, are difficult to obtain at transitional plate boundaries, because upper-plate fault earthquake deformation can mask the subduction zone signal. Here, we examine unusual shell layers within a paleolagoon at Lake Grassmere, at the transition zone between the Hikurangi subduction zone and the Marlborough fault system. Based on biostratigraphic and sedimentological analyses, we interpret the shell layers as tsunami deposits. These are dated at 2145–1837 and 1505–1283 yr B.P., and the most likely source of these tsunamis was ruptures of the southern Hikurangi subduction interface. Identification of these two large earthquakes brings the total record of southern Hikurangi subduction earthquakes to four in the past 2000 yr. For the first time, it is possible to obtain a geologically constrained recurrence interval for the southern Hikurangi subduction zone. We calculate a recurrence interval of 500 yr (335–655 yr, 95% confidence interval) and a coefficient of variation of 0.27 (0.0–0.47, 95% confidence interval). The probability of a large subduction earthquake on the southern Hikurangi subduction zone is 26% within the next 50 yr. We find no consistent temporal relationship between subduction earthquakes and large earthquakes on upper-plate faults.

This article summarises work that has been undertaken to compile the New Zealand Strong Motion Database, which is intended to be a significant resource for both researchers and practitioners. The database contains 276 New Zealand earthquakes that were recorded by strong motion instruments from GeoNet and earlier network operators. The events have moment magnitudes ranging from 3.5 to 7.8. A total of 134 of these events (49%) have been classified as occurring in the overlying crust, with 33 events (12%) located on the Fiordland subduction interface and 7 on the Hikurangi subduction interface (3%). 8 events (3%) are deemed to have occurred within the subducting Australian Plate at the Fiordland subduction zone, and 94 events (34%) within the subducting Pacific Plate on the Hikurangi subduction zone. There are a total of 4,148 uniformly-processed recordings associated with these earthquakes, from which acceleration, velocity and displacement time-series, Fourier amplitude spectra of acceleration, and acceleration response spectra have been computed. 598 recordings from the New Zealand database are identified as being suitable for future use in time-domain analyses of structural response. All data are publicly available at http://info.geonet.org.nz/x/TQAdAQ.

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

The study focused on the comparative analysis of the submarine geomorphology of three oceanic trenches: Hikurangi Trench (HkT), Puysegur Trench (PT) and Hjort Trench (HjT), New Zealand region, Pacific Ocean. HjT is characterized by an oblique subduction zone. Unique regional tectonic setting consist in two subduction zones: northern (Hikurangi margin) and southern (Puysegur margin), connected by oblique continental collision along the Alpine Fault, South Island. This cause variations in the geomorphic structure of the trenches. PT/HjT subduction is highly oblique (dextral) and directed southwards. Hikurangi subduction is directed northwestwards. South Island is caught in between by the “subduction scissor”. Methodology is based on GMT (The Generic Mapping Tools) for mapping, plotting and modelling. Mapping includes visualized geophysical, tectonic and geological settings of the trenches, based on sequential use of GMT modules. Data include GEBCO, ETOPO1, EGM96. Comparative histogram equalization of topographic grids (equalized, normalized, quadratic) was done by module ’grdhisteq’, automated cross-sectioning – by ’grdtrack’. Results shown that HjT has a symmetric shape form with comparative gradients on both western and eastern slopes. HkT has a trough-like flat wide bottom, steeper gradient slope on the North Island flank. PT has an asymmetric V-form with steep gradient on the eastern slopes and gentler western slope corresponding to the relatively gentle slope of a subducting plate and steeper slope of an upper one. HkT has shallower depths < 2,500 m, PT is <-6,000 m. The deepest values > 6,000 m for HjT. The surrounding relief of the HjT presents the most uneven terrain with gentle slope oceanward, and a steep slope on the eastern flank for PT, surrounded by complex submarine relief along the Macquarie Arc. Data distribution for the HkT demonstrates almost equal pattern for the depths from -600 m to ₋2,600 m. PT has a bimodal data distribution with 2 peaks: 1) -4,250 to -4,500 m (18%); 2) -2,250 to -3,000 m, < 7,5%. The second peak corresponds to the Macquarie Arc. Data distribution for HjT is classic bell-shaped with a clear peak at -3,250 to -3,500 m. The asymmetry of the trenches resulted in geomorphic shape of HkT, PT and HjT affected by geologic processes.

Abstract The Hikurangi subduction zone (HSZ) is the collisional boundary between the Pacific and Australian tectonic plates along the eastern coast of the North Island of New Zealand. The region is believed to be capable of hosting large megathrust earthquakes and associated tsunamis. Recent studies observe a range of slip behavior along the plate interface, with a sharp contrast between locked and creeping parts of the megathrust along the margin. This work uses teleseismic scattering data (receiver functions [RFs]) recorded at 53 long-running seismograph stations on the North Island of New Zealand to constrain the structure and mechanical properties of the forearc in the HSZ. We observe directional variations in RF phases at P–S converted delay times (i.e., depths) associated with the overlying forearc crust and note a general correlation with spatial variations in plate coupling as well as other geophysical properties. Our results suggest differences in the nature of crustal deformation (and stress state) along the Hikurangi margin, with evidence of clockwise rotation and/or extension in the northern HSZ, where the overriding forearc crust is uncoupled from the subducting Pacific slab.

SUMMARY Seismic and geodetic examinations of the Hikurangi subduction zone (HSZ) indicate a remarkably diverse and complex system. Here, we investigate the 3-D P-wave velocity structure of the HSZ by applying an iterative, nested regional-global tomographic algorithm. The new model reveals enhanced details of seismic variations along the HSZ. We also relocate over 57 000 earthquakes using this newly developed 3-D model and then further improve the relative locations for 75 per cent of the seismicity using waveform cross-correlation. Double seismic zone characteristics, including occurrence, depth distribution and thickness change along the strike of the HSZ. An aseismic but fast Vp zone separates the upper and lower planes of seismicity in the southern and northern North Island. The upper plane of seismicity correlates with low Vp zones below the slab interface, indicating fluid-rich channels formed on top and/or within a dehydrated crust. A broad low Vp zone is resolved in the lower part of the subducting slab that could indicate hydrous mineral breakdown in the slab mantle. In the northern North Island and southern North Island, the lower plane of seismicity mostly correlates with the top of these low Vp zones. The comparison between the thermal model and the lower plane of seismicity in the northern North Island supports dehydration in the lower part of the slab. The mantle wedge of the Taupo volcanic zone (TVZ) is characterized by a low velocity zone underlying the volcanic front (fluid-driven partial melting), a fast velocity anomaly in the forearc mantle (a stagnant cold nose) and an underlying low velocity zone within the slab (fluids from dehydration). These arc-related anomalies are the strongest beneath the central TVZ with known extensive volcanism. The shallow seismicity (&lt;40 km depth) correlates with geological terranes in the overlying plate. The aseismic impermeable terranes, such as the Rakaia terrane, may affect the fluid transport at the plate interface and seismicity in the overlying plate, which is consistent with previous studies. The deep slow slip events (25–60 km depths) mapped in the Kaimanawa, Manawatu and Kapiti regions coincide with low Vp anomalies. These new insights on the structure along the HSZ highlight the change in the locus of seismicity and dehydration at depth that is governed by significant variations in spatial and probably temporal attributes of subduction zone processes.

We present a precis of our reconnaissance trip to the area of the magnitude 8.3 June 23 2001 southern Peru earthquake and tsunami. The trip was undertaken because of the relevance of the event to hazard assessment in New Zealand. It is the best example in nearly 40 years of the maximum-size earthquake that might occur on the Hikurangi subduction zone, an event that is absent from the historical record of New Zealand (since 1840) and therefore of unknown potential in terms of hazard. Despite the great magnitude of this subduction interface earthquake, it produced only "moderately strong" levels of earthquake shaking (peak ground acceleration of 0.3g on alluvium from the one strong motion accelerograph in the earthquake area, and Modified Mercalli Intensity 8 in the epicentral area), and relatively minor ground damage (liquefaction and landslides). It did however produce a large and devastating tsunami. Our comparison of the one accelerograph record and attenuation curves for subduction interface earthquakes shows that the strength of shaking was typical for subduction interface earthquakes. If we apply our observations to New Zealand, they imply that a Hikurangi subduction interface earthquake may be less damaging to built-up areas in the southeastern part of the North Island (e.g. Wellington and Napier/Hastings) than earthquakes on major active faults in the shallow crust. However, the lateral extent of the strongest shaking in a subduction earthquake (300 km for the southern Peru event) and the associated tsunami generation will make the earthquake very significant in the national context.

The seismic attenuation structure of a subduction zone can constrain variations in temperature, composition and fluids. Amplitude spectra of 2206 local earthquakes, recorded by a dense network of 116 seismic stations in central North Island (NI), New Zealand, are modelled to image the three-dimensional (3D) shear attenuation structure of the Hikurangi subduction zone, down to 100km. Shear attenuation images are obtained by inverting 22260 t* observations for Qs (the quality factor of S waves) using a previously determined seismic velocity model. A frequency dependence of Q s is applied below 10 Hz, by parameterising t* as tol-ex where to is t* at 1 Hz, I is frequency and a = 0.3. Qs is frequency-independent above 10Hz. The 3D Qs images are interpreted alongside previously determined 3D Qp and Vp/Vs images, providing further constraints on features associated with subduction and magmatism in the Hikurangi subduction zone. The subducted slab is a prominent feature, exhibiting Qs = 1000, consistent with a 120Myr old slab. The upper surface of the slab is lined with hydrous fluids from 10km to > 100 km depth, derived from the dehydration of subducted sediments and hydrated oceanic crust. Between 50 and 75 km depth, the hydrous blanket lining the slab extends 20 km into the mantle wedge, and is imaged as having moderately low Qs ~ 300. The mantle wedge beneath the rhyolite-dominated, central segment of the Taupo Volcanic Zone (TVZ) is highly attenuating, with Qs < Qp and Qp < 100. This region is anomalously hot (> 1400øC) and is responsible for the large extent of melting and high rate of melt production observed in the TVZ. Along-strike to the southwest, the hydrous blanket lining the slab ceases abruptly at a trench-perpendicular plane coincident with the southernmost point of the TVZ, as does a very low Qs (~ 50) region in the forearc crust (0-25 km depth). The cessation correlates with geodetic data. These fluid-rich regions reduce friction at the interface of the two plates and permit low-resistance subduction. Southwest of the trench-perpendicular plane, hydrous fluids are absent and the plates are locked. The seismic anisotropic structure of a subduction zone is a consequence of strain and flow in the mantle, and stress and deformation in the crust. In order to refine interpretations of crustal and upper mantle structure in the Hikurangi subduction zone, anisotropy is mapped by analysing shear wave splitting (SWS) of 773 local earthquakes recorded at 29 seismic stations (3371 station-event pairs). SWS results are interpreted in the context of previous anisotropy studies that used local, regional and teleseismic events, alongside 3D Qs, Qp and Vp/Vs structures. In eastern NI, fast directions are trench-parallel, following the trends of major faults in the axial ranges. The observed 3.7% anisotropy is crustal and results from the alignment of cracks and rock fabric in NI crust, 'perpendicular to the direction of maximum horizontal stress. In southern TVZ, fast directions are NE-SW, NW-SE and N-S, consistent with the alignment of highly-fractured crustal structures associated with rifting in discrete rift segments and intruding dikes. Anisotropy beneath the TVZ is larger (>5%) and is entirely crustal. In western NI, fast directions are predominantly N-S, following the trend of major tectonic features west of the TVZ (e.g., the Hauraki Rift and Coromandel Peninsula). There, anisotropy is small ®2%), crustal and results from the alignment of crustal fabric with trends in regional deformation.

In the northern region of South Island, New Zealand, a major tectonic transition occurs in the obliquely convergent Australia-Pacific plate boundary. The southern end of the Hikurangi subduction zone terminates against the Chatham Rise, a submerged continental plateau on the Pacific Plate, which is too buoyant to be subducted. Relative plate motion that is accommodated along the Hikurangi margin is transferred by a complex arrangement of faults, to a zone of transpressive, continental collision across the Southern Alps. A detailed study of offshore seismic-reflection profiles, sediment cores and bathymetry from the north Canterbury continental margin and north-western Chatham Rise reveals the complex interactions between late Cenozoic sedimentation and tectonics at the southern termination of the Hikurangi subduction zone. The north Canterbury shelf and the NW Chatham Rise slope are separated by major submarine canyons that link the shelf with the 3000 m-deep Hikurangi Trough. The sedimentary succession beneath the shelf and slope attains a maximum thickness of about 2 km and is inferred to be underlain by Torlesse terrane basement of Mesozoic age. The late Cenozoic stratigraphy of both regions has been established by correlating unconformity-bounded sedimentary units between seismic-reflection profiles, sampling the units in cores from exposures at the seabed, and dating the sediments by foraminifera and nannoflora biostratigraphy. Tectonic structures have been mapped from seismic profiles and the stratigraphy has been used to constrain the structural and sedimentary evolution of each area. The north Canterbury shelf and the NW Chatham slope exhibit contrasting tectonic and sedimentation styles, which reflect differences in proximity to sediment sources, bathymetry, physical oceanography, sedimentation response to global climate cycles and relative sea-level changes, and different stresses imposed on the basement rocks within the plate-boundary zone. Late Quaternary sedimentation patterns on the NW Chatham slope and in the southern Hikurangi Trough have been studied using 3.5 kHz echo-character mapping. The slope is dominated by current-controlled sedimentary processes, whereas turbidite processes characterise the adjacent part of the southern Hikurangi Trough. On the slope north of Mernoo Saddle (a 580 m-deep depression between the South Island shelf and the Chatham Rise) a 160 x 30 km zone of current erosion occurs between 700 m and 2300 m water depths. Within this region are several northeast trending channels, 5-20 km wide and up to 105 km long, scoured obliquely down-slope. These scours are inferred to have been formed by a northward flowing current of Antarctic Intermediate Water passing through the Mernoo Saddle, then braiding as it cascaded down and across the mid-slope before merging again further east into a contour current on the unstable lower slope of the northern Chatham Rise. The lower slope between and below the scours comprises a complex of coalescing sediment drifts. The adjacent Hikurangi Trough is characterised by a canyon and levee-channel system that guide turbidites from the eastern South Island margin and Cook Strait. On the trough floor is a meandering axial channel up to 10 km wide, with a left-bank dominated levee off Cook Strait where the trough widens. Within the down-slope thickening, late Cenozoic succession on the NW Chatham slope there is a stratigraphic change in acoustic impedance that is inferred to mark a change from predominantly carbonate to terrigenous sedimentation in the Late Miocene (c. 9-10 Ma). This change might reflect an increase in uplift and erosion of the Southern Alps at this time. Analysis of 13 unconformity-bounded seismic units of Pliocene-Recent age indicates an episodic history of mid-bathyal (c. 700-2300 m) current erosion and deposition on the NW Chatham slope. Erosion began in the mid-Pliocene and was most widespread in the Late Pleistocene, when several regional scale erosion surfaces developed. The regional extent of the older surfaces differ from the pattern of oblique-to-slope, en echelon, scour channels and associated sediment drifts which are related only to the five youngest depositional units(< 0.25 Ma). All erosional or non-depositional unconformities between the 13 Plio-Pleistocene seismic units resulted from major velocity changes in the northward, mid-bathyal flow over the Mernoo Saddle. Therefore, the sedimentary units and their intervening unconformities have a different origin to sea-level-controlled sequences in the Vail/Exxon stratigraphic model. The eight youngest seismic units are Late Pleistocene and have a cyclicity of about 57-75 ka, which is similar to high-order (40 and 100 ka) glacio-eustatic sea-level cycles. The older units, deposited between Early Pliocene and Late Pleistocene, have a longer frequency of about 750 ka. The similarity of the Late Pleistocene sequence cyclicity to that of high-order glacio-eustatic cycles, together with consideration of the physical oceanography, a recent phase of reduced erosion during the Holocene, and the inferred subsidence history of the region collectively suggest that the paleoceanographic fluctuations causing the sequences are related to high-amplitude Plio-Pleistocene glacial-interglacial climatic oscillations superimposed on the late Cenozoic subsidence of Mernoo Saddle. The north Canterbury inner-middle shelf is underlain by twelve unconformity bounded seismic units of Late Pliocene-Early Pleistocene to Recent age. The units consist predominantly of terrigenous silty mud and thin layers of gravel, which are inferred to have been deposited in c. < 70-80 m water depth predominantly during transgressions and relative highstands of high amplitude, glacio-eustatic sea-level cycles. Erosional unconformities of middle Pleistocene to Recent age have been progressively tilted seaward as a result of contemporaneous coastal uplift and outer shelf subsidence. The north-western corner of the Chatham Rise has been extending by normal faulting since the Late Miocene (c. 8-6 Ma). The North Mernoo Fault Zone (NMFZ) is a 100 x 300 km extensional province that evolved contemporaneously with offshore sedimentation and with the plate-boundary zone in northern South Island. Growth faults are characteristic, but the distribution of faulting has varied temporally; The fault zone is seismically active and consists of a domino-style array of overlapping, southward dipping normal faults which are typically 2-5 km apart and trend roughly east-west at a high angle to the plate-boundary zone. Late Quaternary surface traces are widely distributed on the mid-upper continental slope but many surface scarps are poorly preserved due to extensive erosion of the seafloor. Despite the wide distribution of faulting, late Cenozoic extensional strain is < 2%. The geometry of the NMFZ is partially inherited from older basement structures. Many of the late Cenozoic faults are reactivated Late Cretaceous and Eocene normal faults which developed during periods of widespread extension of the New Zealand region, in tectonic settings different from now. Two possible models for extension of the edge of continental Pacific Plate are considered: (1) lateral buckling of the upper continental crust across the southern termination of the Hikurangi subduction zone; and (2) flexure of the NW Chatham Rise as the region is bent downward into the southern end of the Hikurangi subduction zone. The extensional NMFZ is one of three offshore fault systems that almost merge together over the southern end of the Hikurangi subduction zone. The western end of the NMFZ crosses submarine canyons at the southern end of the Hikurangi Trough and extends to within 20 km of two opposite-verging, NE-trending fold and thrust fault systems on the north-eastern South Island continental margin. One fold and thrust system verges eastward and represents the southern part of the Hikurangi margin imbricated frontal wedge that is deforming the Marlborough continental slope above the southern part of the Hikurangi subduction zone. The other fold and thrust fault system verges north-westward and is deforming the north Canterbury shelf to the west of the NMFZ. In addition to tilting of the north Canterbury shelf, the inner edges of the Plio-Pleistocene units have been progressively deformed since the middle Pleistocene. Gentle, asymmetric folds up to 35 km long are inferred to be developing above the propagating tips of SE-dipping thrust faults. Some structural elements of the fold and thrust system may be reactivated Late Cretaceous extensional faults. The fold and thrust region extends 20 km offshore between central Pegasus Bay and Kaikoura. The north-eastern end of the zone extends to within 20 km of the extensional NMFZ, but these two fault systems are not linked kinematically, Two possible tectonic models for the north Canterbury coastal region are considered. The preferred model involves NW-SE oriented, upper-crustal shortening of much of the north Canterbury region, which is required to accommodate a component of the relative plate motion in northern South Island. A comparison with other obliquely convergent plate boundaries and with other tectonic settings where continental extensional faulting is occurring today, suggests that the style of tectonic interactions at the southern termination of the Hikurangi subduction zone is rare in the world.

The steep forearc slope along the northern sector of the obliquely convergent Hikurangi subduction zone is characteristic of non-accretionary and tectonically eroding continental margins, with reduced sediment supply in the trench relative to further south, and the presence of seamount relief on the Hikurangi Plateau. These seamounts influence the subduction process and the structurally-driven geomorphic development of the over-riding margin of the Australian Plate frontal wedge. The Poverty Indentation represents an unusual, especially challenging and therefore exciting location to investigate the tectonic and eustatic effects on this sedimentary system because of: (i) the geometry and obliquity of the subducting seamounts; (ii) the influence of multiple repeated seamount impacts; (iii) the effects of structurally-driven over-steeping and associated widespread occurrence of gravitational collapse and mass movements; and (iv) the development of a large canyon system down the axis of the indentation. High quality bathymetric and backscatter images of the Poverty Indentation submarine re-entrant across the northern part of the Hikurangi margin were obtained by scientists from the National Institute of Water and Atmospheric Research (NIWA) (Lewis, 2001) using a SIMRAD EM300 multibeam swath-mapping system, hull-mounted on NIWA’s research vessel Tangaroa. The entire accretionary slope of the re-entrant was mapped, at depths ranging from 100 to 3500 metres. The level of seafloor morphologic resolution is comparable with some of the most detailed Digital Elevation Maps (DEM) onshore. The detailed digital swath images are complemented by the availability of excellent high-quality processed multi-channel seismic reflection data, single channel high-resolution 3.5 kHz seismic reflection data, as well as core samples. Combined, these data support this study of the complex interactions of tectonic deformation with slope sedimentary processes and slope submarine geomorphic evolution at a convergent margin. The origin of the Poverty Indentation, on the inboard trench-slope at the transition from the northern to central sectors of the Hikurangi margin, is attributed to multiple seamount impacts over the last c. 2 Myr period. This has been accompanied by canyon incision, thrust fault propagation into the trench fill, and numerous large-scale gravitational collapse structures with multiple debris flow and avalanche deposits ranging in down-slope length from a few hundred metres to more than 40 km. The indentation is directly offshore of the Waipaoa River which is currently estimated to have a high sediment yield into the marine system. The indentation is recognised as the “Sink” for sediments derived from the Waipaoa River catchment, one of two target river systems chosen for the US National Science Foundation (NSF)-funded MARGINS “Source-to-Sink” initiative. The Poverty Canyon stretches 70 km from the continental shelf edge directly offshore from the Waipaoa to the trench floor, incising into the axis of the indentation. The sediment delivered to the margin from the Waipaoa catchment and elsewhere during sea-level high-stands, including the Holocene, has remained largely trapped in a large depocentre on the Poverty shelf, while during low-stand cycles, sediment bypassed the shelf to develop a prograding clinoform sequence out onto the upper slope. The formation of the indentation and the development of the upper branches of the Poverty Canyon system have led to the progressive removal of a substantial part of this prograding wedge by mass movements and gully incision. Sediment has also accumulated in the head of the Poverty Canyon and episodic mass flows contribute significantly to continued modification of the indentation by driving canyon incision and triggering instability in the adjacent slopes. Prograding clinoforms lying seaward of active faults beneath the shelf, and overlying a buried inactive thrust system beneath the upper slope, reveal a history of deformation accompanied by the creation of accommodation space. There is some more recent activity on shelf faults (i.e. Lachlan Fault) and at the transition into the lower margin, but reduced (~2 %) or no evidence of recent deformation for the majority of the upper to mid-slope. This is in contrast to current activity (approximately 24 to 47% shortening) across the lower slope and frontal wedge regions of the indentation. The middle to lower Poverty Canyon represents a structural transition zone within the indentation coincident with the indentation axis. The lower to mid-slope south of the canyon conforms more closely to a classic accretionary slope deformation style with a series of east-facing thrust-propagated asymmetric anticlines separated by early-stage slope basins. North of the canyon system, sediment starvation and seamount impact has resulted in frontal tectonic erosion associated with the development of an over-steepened lower to mid-slope margin, fault reactivation and structural inversion and over-printing. Evidence points to at least three main seamount subduction events within the Poverty Indentation, each with different margin responses: i) older substantial seamount impact that drove the first-order perturbation in the margin, since approximately ~1-2 Ma ii) subducted seamount(s) now beneath Pantin and Paritu Ridge complexes, initially impacting on the margin approximately ~0.5 Ma, and iii) incipient seamount subduction of the Puke Seamount at the current deformation front. The overall geometry and geomorphology of the wider indentation appears to conform to the geometry accompanying the structure observed in sandbox models after the seamount has passed completely through the deformation front. The main morphological features correlating with sandbox models include: i) the axial re-entrant down which the Poverty Canyon now incises; ii) the re-establishment of an accretionary wedge to the south of the indentation axis, accompanied by out-stepping, deformation front propagation into the trench fill sequence, particularly towards the mouth of the canyon; iii) the linear north margin of the indentation with respect to the more arcuate shape of the southern accretionary wedge; and, iv) the set of faults cutting obliquely across the deformation front near the mouth of the canyon. Many of the observed structural and geomorphic features of the Poverty Indentation also correlate well both with other sediment-rich convergent margins where seamount subduction is prevalent particularly the Nankai and Sumatra margins, and the sediment-starved Costa Rican margin. While submarine canyon systems are certainly present on other convergent margins undergoing seamount subduction there appears to be no other documented shelf to trench extending canyon system developing in the axis of such a re-entrant, as is dominating the Poverty Indentation. Ongoing modification of the Indentation appears to be driven by: i) continued smaller seamount impacts at the deformation front, and currently subducting beneath the mid-lower slope, ii) low and high sea-level stands accompanied by variations on sediment flux from the continental shelf, iii) over-steepening of the deformation front and mass movement, particularly from the shelf edge and upper slope.

This thesis examined how deformation structures hosted in sedimentary rocks of the Wairarapa, New Zealand, record subduction initiation and the development and evolution of subduction wedges. Field and laboratory techniques are employed to assess the control of tectonic setting and lithology on the nature and spatial distribution of small-scale deformation structures in the shallow parts of subduction wedges. The results provide insights into the failure modes of, stress and strain state within, and mechanisms of fluid movement through subduction wedges, which represent key tectonic settings for geochemical exchange and material recycling between the Earth’s surface, crust, and mantle.

Topography growth and sediment fluxes in active subduction margin settings are poorly understood. Geological record is often scarce or hardly accessible as a result of intensive deformation. The Hawke Bay forearc basin of the Hikurangi margin in New Zealand is well suited for studying morphstructural evolution. It is well preserved, partly emerged and affected by active tectonic deformation during Pleistocene stage for which we have well dated series and well-known climate and eustasy. The multidisciplinary approach, integrating offshore and onshore seismic interpretations, well and core data, geological mapping and sedimentological sections, results in the establishment of a detailed stratigraphic scheme for the last 1.1 Ma forearc basin fill. The stratigraphy shows a complex stack of 11 eustasy-driven depositional sequences of 20, 40 and 100 ka periodicity. These sequences are preserved in sub-basins that are bounded by active thrust structures. Each sequence is characterized by important changes of the paleoenvironment that evolves between the two extremes of the glacial maximum and the interglacial optimum. Thus, the Hawke Bay forearc domain shows segmentation in sub-basins separated by tectonic ridges during sea level lows that become submerged during sea level highs. Over 100 ka timescale, deformation along active structures together with isostasy are responsible of a progressive migration of sequence depocenters towards the arc within the sub-basins. Calculation of sediment volumes preserved for each of the 11 sequences allows the estimation of the sediment fluxes that transit throughout the forearc domain during the last 1.1 Ma. Fluxes vary from c. 3 to c. 6 Mt.a⁻¹. These long-term variations with 100 ka to 1 Ma timescale ranges are attributed to changes in the forearc domain tectonic configuration (strain rates and active structure distribution). They reflect the ability of sub-basin to retain sediments. Short-term variations of fluxes (<100 ka) observed within the last 150 ka are correlated to drastic Pleistocene climate changes that modified erosion rates in the drainage area. This implies a high sensitiveness and reactivity of the upstream area to environmental changes in terms of erosion and sediment transport. Such behaviour of the drainage basin is also illustrated by the important increase of sediment fluxes since the European settlement during the 18th century and the following deforestation.