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Late Pleistocene and Holocene sedimentation records changes in glacio-eustatic cycles, oceanography, active tectonics, and sediment supply. Here, we use high-resolution seismic-reflection profiles in Te Tai-o-Aorere Tasman Bay to investigate sedimentary deposit distributions associated with sea-level cyclicity over ∼140 ka. We identify nine seismic stratigraphic units below the inner continental shelf, interpret their character based on stratal relationships and acoustic properties, and infer ages using global sea level. The sequence includes stacked deposits and unconformities associated with the shifting shoreline over glacial cycles. Prominent erosional features include a lowstand ravinement surface and two transgressive surfaces formed during sea-level rise following the last two glacial maxima of ∼20 ka and ∼140 ka. The youngest surface marks the base of post-glacial sediment accumulated since ∼12 ka in the study area. Post-glacial sediment includes five seismically mappable units, covering an area of ∼1400 km2 and deposit volume of ∼9 km3. Time-averaged sediment accumulation rates are estimated to range from a maximum of ∼1.6–2.8 m/kyr to ∼0.5–0.9 m/kyr. Sequence stratigraphy interpreted in Te Tai-o-Aorere Tasman Bay is representative of accumulation in a relatively wide-shelf, sheltered inner bay setting supplied by mountainous catchments, and provides stratigraphic context necessary for future stratigraphic and tectonics studies. [ABSTRACT FROM AUTHOR]

Turbidites, deposited by sub‐aqueous gravity flows, are common in sedimentary archives worldwide and present a unique challenge and opportunity when reconstructing past vegetation through pollen analysis. When sampling pollen from a sediment core for palaeovegetation records, it is common practice to target background sediments (i.e. pelagic sediment) and avoid sampling turbidites, as they are presumed to portray a misleading picture of past vegetation. This assumption stems from our limited understanding of pollen abundance and distribution through turbidites, meaning that palynologists overlook deposits that could potentially be used to reconstruct past vegetation and climate. We present pollen assemblage and sedimentological data from four recent (<150 years) deep marine turbidite deposits from the Hikurangi Subduction Margin, Aotearoa‐New Zealand, with the aim of understanding the abundance and distribution of pollen in fine‐grained turbidites. We find that pollen is diluted in the bases of turbidites, but despite this dilution, the proportions of different pollen taxa remain consistent through each turbidite. These results confirm that pollen can be sampled from turbidites for palaeovegetation reconstructions and that sampling the fine‐grained upper parts of turbidites will provide the best pollen recovery. [ABSTRACT FROM AUTHOR]

The New Zealand Community Fault Model (NZ CFM) is a publicly available representation of New Zealand fault zones that have the potential to produce damaging earthquakes. Compiled through collaborative engagement between New Zealand earthquake-science experts, this first edition (version 1.0) of the NZ CFM builds upon previous compilations of earthquake-source active fault models with the addition of new and modified information. Developed primarily to support an update of the New Zealand National Seismic Hazard Model, the NZ CFM comprises two principal components. The first dataset is a two-dimensional map representation of the surface traces of 880 generalised fault zones. Each fault zone is assigned specific geometric and kinematic attributes, including uncertainties, supplemented with a subjective quality ranking focused primarily on the confidence in assigned slip rates. The second component is a three-dimensional representation of the fault zones as triangulated mesh surfaces that are projected down-dip from the two-dimensional mapped traces to a geophysically-defined maximum fault rupture depth. This article summarises the compilation and parameterisation of the NZ CFM, along with background on its relation to predecessor datasets, and forward applications to probabilistic seismic hazard assessment and physics-based earthquake models currently being developed for Aotearoa New Zealand. [ABSTRACT FROM AUTHOR]

Geological displacement rates for major plate boundary strike-slip faults have seldom been determined with precision from the marine environment. In this paper I present high-quality slip rates derived from the southern Alpine fault using multibeam bathymetric data and dated samples from the Fiordland continental margin. These rates are derived from dextral displacements of relict but well-preserved glacial geomorphology (moraines and outwash fans), interpreted to 17 (+2/-1, calendar) ka. The weighted mean slip rate is 27.2 (-3.0/+1.8) mm/yr on the shelf between Milford and George Sounds, increasing southward to 31.4 (-3.5/+2.1) mm/yr between Caswell and Doubtful Sounds, with uncertainties at the 95% confidence level. These offshore rates are higher than those from an adjacent 80-km-length section of the fault on land in south Westland, and are in good agreement with global positioning system data. The southern slip rates represent some of the highest strike-slip rates on Earth, and ~90% of the total plate motion in southern New Zealand. The southward increase in strike-slip rate on the Alpine fault occurs despite a southward reduction in Pacific-Australian relative plate motion rate and despite the prediction of southward-decreasing plate-parallel motion and southward-increasing normal convergence. The Fiordland region is thus an excellent example of geological strain being highly partitioned within an obliquely convergent plate boundary zone.

[1] Oblique rifting in the offshore Taupo Volcanic Zone, New Zealand, is expressed in widely distributed active normal faulting in the 20-km-wide Whakatane Graben. Active faults are identified along seafloor scarps and displacements of the post-last glacial transgressive ravinement surface (400 faults. We derive an extension rate at seismogenic depths (6-10 km) across the graben of 13 [+ or -] 6 mm [yr.sup.-1] , by summing surface measurements, assuming an average crustal fault dip of 45 [+ or -] 15[degrees], and correcting for the discrepancies between surface and deep crustal extension estimates. Structural and kinematic data implies an extension direction 20[degrees] oblique to the rift axis, resulting in up to 4.6 [+ or -] 2.1 mm [yr.sup.-1] of dextral motion parallel to the rift axis. The strike-slip motion is accommodated by dip-slip displacements on oblique faults in the center of the graben, and oblique-slip faulting along the rift margins. Pure dip-slip in the graben center represents >50% of the total slip, with the Rangitaiki Fault accommodating 25% of the total extension in the graben. Citation: Lamarche, G., P. M. Barnes, and J. M. Bull (2006), Faulting and extension rate over the last 20,000 years in the offshore Whakatane Graben, New Zealand continental shelf, Tectonics, 25, TC4005, doi: 10.1029/2005TC001886.

The Alpine Fault is an 850-km-long, continental dextral strike-slip fault that accommodates some 60%-90% (~20-30 mm/yr) of the obliquely convergent motion between the Pacific and Australian Plates in South Island, New Zealand. The southern 230 km of the fault traverses the continental margin off Fiordland and intersects the subduction thrust at the northern end of the Puysegur Trench. Marine seismic reflection profiles and bathymetric data are used to evaluate the late Quaternary structure and 3 m.y. evolution of the fault and five sedimentary basins. The fault offshore is more complexly segmented than its onshore counterpart, and at sedimentary basins has substantial (>1 km) bathymetric relief. Three right-stepping and overlapping active sections are identified on the basis of structural continuity and geomorphic expression. The northern, Milford-Caswell section, is 90 km long and continuous with the southern Westland section on land. The southern, Resolution section is 150 km long and developed from initially discontinuous, right-stepping segments. These sections overlap by 35 km at the large Secretary-Nancy Basin, which is currently being dissected longitudinally by the Nancy section, resulting in a straighter, almost fully linked principal displacement zone. The average strike of the fault differs from the azimuth of Pacific-Australian Plate motion by 11[degrees]-25[degrees]. Nevertheless, on the whole, the structure, tectonic geomorphology, and lateral displacements indicate predominantly dextral strike-slip displacement. The basins have the structural characteristics of pull-apart and releasing-bend basins. They commonly initiated at step-overs in the fault and evolved by development of very oblique cross- and along-basin faults that linked the principal displacement zone, resulting in longer, through-going surface traces. Localized transpressional features are developing contemporaneously with the basins and may relate to constrictional fault geometries and/or a 10[degrees] rotation of part of the fault within the last 3 m.y., rather than to changes in plate motions. The absence of significant regional contraction across the fault indicates that obliquely convergent plate motion is strongly partitioned. The convergence normal to the boundary is accommodated largely on thrust and reverse faults offshore and onshore, to the northwest and southeast of the Alpine Fault, respectively. The position of the Alpine Fault is clearly associated with the position of inherited Eocene rift structures carried in the subducting Australian Plate. Our observations and kinematic model imply a spatial-temporal evolution where the position of the southwestern end of the fault trace is transitory on a time scale of [10.sup.5]-[10.sup.6] years, and the fault matures toward the northeast. The data are consistent with models invoking dextral reactivation of the subducting rift structures and tearing of the Australian Plate in the approximate direction of plate convergence, and/or the loading of shear stresses and strike-slip deformation on the Alpine Fault in response to spatial differences in surface roughness and interplate coupling. Several potential earthquake scenarios on the Alpine Fault are considered, ranging from rupture of individual structural sections associated with earthquakes of magnitude M 7.0-7.8, to larger, onshore-offshore composite ruptures of up to M 8.1. Keywords: Alpine Fault, New Zealand, strike slip, strike-slip basin, pull-apart, structure, bathymetry, subduction, Puysegur Trench, tectonic deformation, plate boundary, seismic reflection, transpression, earthquake potential.

[1] We analyze a thrust triangle zone, which underlies the continental shelf of Hawke Bay, eastern New Zealand, within the Hikurangi subduction margin. This triangle zone differs from many other examples in that it is active, 90 km from the leading edge of the overriding plate, and formed due to polyphase deformation involving opposed dipping thrust duplex and backthrust, with the later structure forming in response to inversion of an extensional graben. The component structures of the zone mainly developed sequentially rather than synchronously. High-quality marine seismic reflection lines, tied to well and seabed samples, reveal the three-dimensional structure of the zone, together with its 25 Myr evolution and late Quaternary activity. The triangle zone occurs in the lateral overlap between a stack of NW dipping blind thrusts, and a principal backthrust, the Kidnappers fault. The NW dipping thrusts initiated in the early-middle Miocene during the early stages of subduction, with subsequent thrust duplex formation producing major uplift and erosion in the late Miocene-early Pliocene. The active backthrust formed during the late Miocene to early Pliocene as a thin-skinned listric extensional fault confined to the cover sequence. Structural inversion of the extensional fault commenced in the early-middle Pliocene, produced the backthrust and marks the formation of the thrust triangle zone. The thrust duplex and backthrust accrued strain following inversion; however, the later structure accommodated most of the surface deformation in the Quaternary. Section balancing of the triangle zone together with a detailed analysis of reverse displacements along the backthrust reveal spatial and temporal variations of strain accumulation on the two principal components of the zone. Although the formation of the triangle zone is strongly influenced by regional tectonics of the subduction system, these variations may also, in part, reflect local fault interaction. For example, high Quaternary displacement rates on the backthrust accounts for ~70% of the displacement loss that occurs on the southern segments of the overlapping, Lachlan fault. Understanding the tectonic evolution of such complex, polyphase thrust triangle zones requires the preservation of growth strata that record sequential deformation history. In the absence of such data, synchroneity of opposed dipping thrusts in triangle zones cannot be assumed. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8107 Tectonophysics: Continental neotectonics; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 3025 Marine Geology and Geophysics: Marine seismics (0935); 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: thrust faults, inversion, subduction, tectonic deformation, structure, New Zealand. Citation: Barnes, P. M., and A. Nicol (2004), Formation of an active thrust triangle zone associated with structural inversion in a subduction setting, eastern New Zealand.

In the center of the frontal wedge of the Hikurangi subduction zone, New Zealand, Mahia Peninsula and its submarine continuation, Lachlan Ridge, are being uplifted and folded above an active landward-dipping thrust-fault complex that is 80 km long. High-quality marine seismic reflection profiles reveal complex deformation of a Cretaceous to Holocene sedimentary section and enable a detailed analysis of the stratigraphy, structural evolution, deformation rates, and future earthquake potential. The structural analysis is facilitated by uplifted marine terraces on Mahia Peninsula and by 14 submarine unconformities in the hanging-wall sequence, five of which are correlated across the eroded crest of Lachlan Ridge and into the footwall basin. The ages of the unconformities are determined by seismic ties to an offshore exploration well, onshore outcrops on the peninsula, and seabed samples dated by pollen, coccolith nannoflora, and foraminifera biostratigraphy. Nine regional Quaternary unconformities, which developed in response to eustatic fluctuations in sea level and are not older than ca. 1 Ma, are correlated with oxygen isotope stages in equatorial Pacific Ocean Drilling Project core 677. The ages of fault-growth strata and progressive restorations of deformed stratigraphy indicate that Lachlan Ridge developed during three phases of deformation since subduction of the Pacific plate commenced beneath the Australian plate in the early Miocene. These include an initial phase of thrust faulting, a subsequent phase dominated by extensional faulting, and the current, mainly Pleistocene to Holocene phase of structural inversion, reactivated listric thrust faulting, and folding. Early to middle Miocene thrusts in the deeper core of the complex developed out of sequence, by sequentially stepping up into the hanging-wall section, creating an imbricate fan with emerging thrust tips now buried within the forelimb basin. In the middle Miocene-early Pliocene, listric extensional faults developed in the active thrust wedge--possibly as a result of substantial relief on the subducted Pacific plate--and controlled the development of Lachlan Basin to the west of the ridge. The principal active thrust is the Lachlan fault, a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right-stepping segments. Depth-converted seismic profiles indicate that the fault dips westward at 15[degrees]-20[degrees], 6-8 km beneath the western flank of Lachlan Ridge, and steepens to 55[degrees]-70[degrees] or even steeper in the upper 1-2 km of section beneath the eastern flank. Syninversion Pleistocene fault-growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold-limb rotation resulting from the listric-fault geometry. A geometric analysis of the fault-growth strata and deformed terraces was used to derive a maximum dip-slip displacement rate of as fast as 3.0-6.5 mm/yr. The implied shortening rate of 2.6-6.3 mm/ yr represents ~8%-20% of the total 31 mm/yr of orthogonal plate convergence across this part of the upper plate of the Hikurangi subduction zone. The top of the pre-fault-growth Paleogene section reveals as much as 5.8 [+ or -] 1.5 km of vertical separation in the north, decreasing to ~30%-50% of this value in the south. Temporal ([10.sup.3]-[10.sup.6] yr) and spatial ([10.sup.3]-[10.sup.4] m of strike length) variations in vertical-deformation rate have occurred during the past 1 m.y.; maximum rates occurred in the Holocene and middle Quaternary. A long-term increase in vertical-separation rate on all segments during the Pleistocene largely reflects a change in thrust kinematics associated with structural inversion. The relatively greater increase in uplift rate on the northern part of the fault during the past 1 m.y. could be related to the possibility that a subducted seamount lies > 10 km beneath the peninsula. Estimates of earthquake source parameters, incorporating paleoseismic uplift data from Mahia Peninsula, indicate a potential moment magnitude of up to [M.sub.w] 7.6-8.0 for an earthquake that ruptures all three segments of the Lachlan fault. The average recurrence interval for such events is estimated to be 615-2333 yr, which is consistent with a mean recurrence interval of 1062 yr for four late Holocene earthquakes. Thus, the uplift and folding of Mahia Peninsula and Lachlan Ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large-magnitude earthquakes. Keywords: earthquake potential, forearc, New Zealand, structure, subduction, tectonic, formation, thrust faulting.

Major strike-slip faults within the eastern part of the Marlborough fault system of New Zealand extend offshore beneath the continental shelf, into the southern end of the Hikurangi margin. Six major submarine faults, each tens of kilometers in length and exhibiting the three-dimensional structural characteristics typical of strike-slip deformations zones, are mapped in detail using closely spaced high-resolution seismic-reflection profiles. Each fault displaces late Quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established history of glacio-eutatic sea-level cyclicity. Some of the faults are associated with strike-slip microearthquake focal mechanisms. The faults are within a complex structural high that underlies much of the outer continental shelf, and within the Flaxbourne basin between the structural high and the coast. The plan-view pattern of faulting recorded in the Quaternary sediments reflects only the major surface traces that have propagated upsection as contemporaneous sedimentation has blanketed most of the evolving structures. Regionally, the late Quaternany faults compose two groups that bound blocks with rhumboid surface areas on a scale of tens of square kilometers to hundreds of square kilometers. One group strikes typically between 023 degrees and 057 degrees, i.e., 22 degrees-56 degrees from the plate motion vector (079 degrees), and includes dextral, oblique-slip thrust faults as well as inherited, steeply dipping, pre-Pliocene strike-slip faults that bound thick sedimentary basins. The second group comprises possibly young (

A complex pattern of strain partitioning was discovered in the area between oblique continental collision in northern South Island and the oblique subduction under southern North Island in New Zealand. This can be observed from the deformation rates and the spatial differences in late Quaternary structural style. Sediment samples, seismic reflection profiles, sonar swath images and backscatter images were used for the study.

The marine seismic reflection data and Quaternary stratigraphic study outline the seismic potential of actively growing folds that overlie a system of blind thrusts, offshore North Canterbury, New Zealand. Eleven sedimentary units of Pleistocene to Recent age have low fold amplitude growth rates. The blind thrust faults have slip rates of the order of 0.1-0.9 mm/year. The coseismic uplift, occurring during thrust fault earthquakes of moderately large magnitude, causes fold growth. Probable recurrence intervals of large magnitude thrust earthquakes are of the order of tens of thousands of years.

The active Aotea and Evans Bay faults underlie Wellington Harbour/Te Whanganui a Tara, and are inferred to extend south beneath Wellington City and eastern suburbs. The Aotea Fault dips east, has reverse displacement, and projects onshore beneath the western flank of Mount Victoria/Matairangi and Te Aro, where a basement fault has been inferred previously. East of Mount Victoria, the Evans Bay Fault may dip west and extend southward beneath the Rongotai isthmus. Marine seismic-reflection profiles tied to dated sediment cores indicate vertical separation rates of ∼0.6 ± 0.3 mm/yr on both the Aotea and Evans Bay faults, and evidence of at least two, and one, seafloor-rupturing earthquakes during the last 10,000 years on each fault, respectively. Previously, neither fault has been recognised specifically in seismic hazard assessments of New Zealand. We infer that earthquakes activating these faults have magnitude Mw ≥ 7, potential rupture lengths exceeding 30 km, and possibly involve multiple faults. [ABSTRACT FROM AUTHOR]