Peter R. King

Tectonic reconstructions of New Zealand: 40 Ma to the Present

Abstract Reconstructions of the New Zealand subcontinent from 40 Ma to the Present are presented. Assumptions that have constrained the model include: semi‐straight initial alignment of basement terranes and markers; Australian plate fixed; onset of Emerald Basin spreading at c. 45 Ma; and Pacific plate subduction north of New Zealand from c. 30 Ma. Five independent, rigid, crustal blocks are employed, including: Northland‐Taranaki‐western South Island (combined), East Coast (North Island), east Nelson (Marlborough Sounds), eastern South Island, and Fiordland. At 40 Ma the Pacific/Australian rotation pole was located close to or within the Wanganui region. A proto‐plate boundary zone was propagating through western New Zealand, as an incipient link between Emerald Basin spreading in the south and subduction in the northwest. Lateral offset on the Alpine Fault was initiated by c. 23–22 Ma, mainly as an effect of changing subduction kinematics and increasing relative motion of the Australian and Pacific plates in the central New Zealand region. Extension in the south became increasingly oblique, whereas areas north of the Alpine Fault were affected by near‐orthogonal compression. From c. 23 Ma to the Present, the foci of contraction, extension, and arc volcanism in central‐western New Zealand have migrated southwards, in response to progressive changes in the location, orientation, inclination, and kinematics of the subducted Pacific plate.

High-level stratigraphic scheme for New Zealand rocks

We formally introduce 14 new high-level stratigraphic names to augment existing names and to hierarchically organise all of New Zealands onland and offshore Cambrian–Holocene rocks and unconsolidated deposits. The two highest-level units are Austral Superprovince (new) and Zealandia Megasequence (new). These encompass all stratigraphic units of the countrys Cambrian–Early Cretaceous basement rocks and Late Cretaceous–Holocene cover rocks and sediments, respectively. Most high-level constituents of the Austral Superprovince are in current and common usage: Eastern and Western Provinces consist of 12 tectonostratigraphic terranes, 10 igneous suites, 5 batholiths and Haast Schist. Ferrar, Tarpaulin and Jaquiery suites (new) have been added to existing plutonic suites to describe all known compositional variation in the Tuhua Intrusives. Zealandia Megasequence consists of five predominantly sedimentary, partly unconformity-bounded units and one igneous unit. Momotu and Haerenga supergroups (new) comprise lowermost rift to passive margin (terrestrial to marine transgressive) rock units. Waka Supergroup (new) includes rocks related to maximum marine flooding linked to passive margin culmination in the east and onset of new tectonic subsidence in the west. Māui and Pākihi supergroups (new) comprise marine to terrestrial regressive rock and sediment units deposited during Neogene plate convergence. Rūaumoko Volcanic Region (new) is introduced to include all igneous rocks of the Zealandia Megasequence and contains the geochemically differentiated Whakaari, Horomaka and Te Raupua supersuites (new). Our new scheme, Litho2014, provides a complete, high-level stratigraphic classification for the continental crust of the New Zealand region.

An integrated sequence stratigraphic, palaeoenvironmental, and chronostratigraphic analysis of the Tangahoe Formation, southern Taranaki coast, with implications for mid-Pliocene (c. 3.4-3.0 Ma) glacio-eustatic sea-level changes

Abstract Sediments of the mid‐Pliocene (c. 3.4–3.0 Ma) Tangahoe Formation exposed in cliffs along the South Taranaki coastline of New Zealand comprise a 270 m thick, cyclothemic shallow‐marine succession that has been gently warped into a north to south trending, low angle anticline. This study examines the sedimentologic, faunal, and petrographic characteristics of 10 Milankovitch‐scale (6th order), shallow‐marine depositional sequences exposed on the western limb of the anticline. The sequences are recognised on the basis of the cyclic vertical stacking of their constituent lithofacies, which are bound by sharp wave cut surfaces produced during transgressive shoreface erosion. Each sequence comprises three parts: (1) a 0.2–2 m thick, deepening upwards, basal suite of reworked bioclastic lag deposits (onlap shellbed) and/or an overlying matrix supported, molluscan shellbed of offshore shelf affinity (backlap shellbed); (2) a 5–20 m thick, gradually shoaling, aggradational siltstone succession; and (3) a 5–10 m thick, strongly progradational, well sorted “forced regressive” shoreline sandstone. The three‐fold subdivision corresponds to transgressive, highstand, and regressive systems tracts (TSTs, HSTs, and RSTs) respectively, and represents deposition during a glacio‐eustatic sea‐level cycle. Lowstand systems tract sediments are not recorded because the outcrop is situated c. 100 km east of the contemporary shelf edge and was subaerially exposed at that time. Well developed, sharp‐ and gradational‐based forced regressive sandstones contain a variety of storm‐emplaced sedimentary structures, and represent the rapid and abrupt basinward translation of the shoreline on to a storm dominated, shallow shelf during eustatic sea‐level fall. Increased supply of sediment from north‐west South Island during “forced regression” is indicated from petrographic analyses of the heavy mineralogy of the sandstones. A chronology based on biostratigraphy and the correlation of a new magnetostratigraphy to the magnetic polarity timescale allows: (1) identification of the Mammoth (C2An.2r) and Kaena (C2An. 1r) subchrons; (2) correlation of the coastal section to the Waipipian Stage; and (3) estimation of the age of the coastal section as 3.36–3.06 Ma. Qualitative assessment of foraminiferal census data and molluscan palaeoecology reveals cyclic changes in water depth from shelf to shoreline environments during the deposition of each sequence. Seven major cycles in water depth of between 20 and 50 m have been correlated to individual 40 ka glacio‐eustatic sea‐level cycles on the marine oxygen isotope timescale. The coastal Tangahoe Formation provides a shallow‐marine record of global glacio‐eustasy prior to the development of significant ice sheets on Northern Hemisphere continents, and supports evidence from marine δ18O archives that changes in Antarctic ice volume were occurring during the Pliocene.

Paleogeography of the Taranaki Basin region during the latest Eocene–Early Miocene and implications for the ‘total drowning’ of Zealandia

Latest Eocene–earliest Miocene strata in the subsurface of the Taranaki Basin provide important new regional paleogeographic and tectonic constraints not available from outcrop. Six paleogeographic maps of the Taranaki Basin region have been produced utilising extensive well, seismic reflection and outcrop data. These record three broad periods of sedimentation characterised by (1) variable transgression and initial deformation (c. 40–30 Ma); (2) maximum transgression with moderate deformation (c. 30–21 Ma); and (3) regression with accelerated deformation (< 21 Ma). Local sedimentation patterns were influenced by reverse faulting, producing depocentres and topographic highs adjacent to the Taranaki Fault System. Reverse faulting commenced as early as c. 40 Ma and may signify the onset of subduction beneath the North Island. In common with other parts of New Zealand, the region reached maximum marine inundation in the Waitakian (c. 23 Ma). However, the deposition of thick clastic sediments in eastern parts of Taranaki Basin, coupled with ongoing tectonism, suggests the presence of land throughout the Oligocene and Early Miocene, and is inconsistent with total Oligocene drowning of Zealandia.

Grain‐size characteristics for distinguishing basin floor fan and slope fan depositional settings: Outcrop and subsurface examples from the late Miocene Mount Messenger Formation, New Zealand

Abstract An outcrop section of late Miocene deep‐water sediments of the Mount Messenger Formation in Taranaki, New Zealand, displays distinctive physical sedimentary features that allow differentiation of basin floor and slope fan depositional units. Sandstone grain‐size characteristics have been examined in this study to differentiate these two types of deep‐water deposits. Outcrop data indicate that basin floor fan sandstones are relatively sand rich in comparison to silt‐rich slope fan sandstones. Both basin floor and slope fan sandstones show better sorting with increasing grain size, though cross‐plots show the nature of this relationship differs slightly for basin floor and slope fan samples. These relationships appear to hold for both outcrop and subsurface sandstone samples from the formation. This finding is unexpected given the c. 600 m stratigraphic thickness of the formation, representing several million years of depositional history, and implies a uniform sediment texture was supplied to the basin through time. The differentiation of basin floor fan and slope fan deposits is significant especially in subsurface settings involving petroleum well data. Hydrocarbon exploration strategies will vary markedly for basin floor fan versus slope fan reservoirs, making such differentiation of lithofacies types important to optimise hydrocarbon discovery. With subsurface data, the interpretation of these two reservoir sandstone lithofacies is often difficult to make. The grain‐size changes appear to mimic the contrasting depositional mechanisms operative in these two deep‐water settings.

Spatial patterns of deformation and paleoslope estimation within the marginal and central portions of a basin-floor mass-transport deposit, Taranaki Basin, New Zealand

This study describes the character of submarine mass movement and associated deformation as revealed by an exceptionally well-exposed portion of a seismic-scale mass-transport deposit (MTD) within the upper Miocene Mohakatino Formation (Taranaki Basin, New Zealand). The North Awakino MTD is at least 55 m thick and crops out along the northern Taranaki coastline for ∼11 km in wave-cut platforms and in cliffs as much as 100 m high. Spectacular soft-sediment deformation features are developed in remobilized sediment gravity flow deposits that initially accumulated within a low-gradient intraslope basin. Sedimentary facies within the North Awakino MTD comprise laterally extensive, thick- to thin-bedded volcaniclastic sandstone and mudstone. Distinct postdepositional deformation styles are associated with bedding type: folds developed in thick-bedded sandstone are larger (fold heights to tens of meters) and more laterally continuous (to 1 km) than those developed in thinner bedded facies. Regional geologic relationships suggest that nearly the full width of the North Awakino MTD is exposed in outcrop, providing a rare opportunity to observe lateral relationships between the marginal and central portions of the MTD. We conducted a rigorous paleoslope analysis of slump fold, fault, and bedding orientations using both existing and newly proposed methodologies. Separate analysis of seven subregions within the North Awakino MTD reveals that the predicted MTD transport direction varies widely along the outcrop extent. Most notably, slump folds and faults within the inferred margins have mean orientations that are suborthogonal to those within the central portions of the MTD. This relationship is hypothesized to be a consequence of edge effects that may be related to lateral compression along the margins of the MTD. Our analysis demonstrates the importance of accounting for spatial heterogeneity in slump structure orientations when determining the paleoslope orientation through kinematic analysis. Our inference of west-directed translation suggests that the North Awakino MTD formed in response to a local change in the bathymetric slope orientation that was likely the result of tectonically induced basin deformation.

Use of ancient wave-ravinement surfaces to determine palaeogeography and vertical crustal movements around New Zealand

Wave-ravinement (shallow marine erosion) surfaces are formed during landwards migration of the shoreline due to rising relative sea level. They can be preserved if subsequently buried by sediment, and are easily identifiable on seismic reflection data. We present two examples from New Zealands north-western offshore frontier region where these surfaces, despite a diachronous nature, represent sequence stratigraphic and geomorphic markers that are used to constrain regional palaeogeography and tectonic history. Thermal subsidence that followed the break-up of Gondwana led to the landwards migration of shorelines across the then-emergent Challenger Plateau and formation of a prominent Late Cretaceous–Eocene wave-ravinement surface. In the Reinga Basin, subsidence of uplifted land areas that had previously emerged during Cenozoic initiation of Tonga–Kermadec subduction was similarly accompanied by formation of transgressive wave-ravinement surfaces. Wave-ravinement surfaces serve as useful proxies for palaeo-sea level and, as the above two examples show, they are powerful tools for characterising regional deformation, uplift and subsidence histories and tectonic influence on relative base level fluctuations (changes in shoreline position). The study of these and other remarkable palaeo-sea-level markers around New Zealand and in the circum-Pacific region provides a different perspective on constraining vertical crustal movements associated with major tectonic events.

Two-phase Cretaceous–Paleocene rifting in the Taranaki Basin region, New Zealand; implications for Gondwana break-up

The break-up of Gondwana resulted in extension of New Zealand continental crust during the Cretaceous–Paleocene. Offshore the geometry and rift history are well imaged by new regional mapping of a large seismic reflection dataset, tied to wells, used here to document the Cretaceous–Paleocene (c. 105 – 55 Ma) evolution of the greater Taranaki Basin region. Two temporally distinct phases of rifting have been recognized in the region, and record Gondwana break-up. The first (Zealandia rift phase) produced half-grabens trending NW to WNW during the mid-Cretaceous (c. 105 – 83 Ma). These rift basins predate, and are parallel to, Tasman Sea spreading centres. They record distributed stretching of northern Zealandia prior to the onset of seafloor spreading in the Tasman Sea. A short period (c. 83 – 80 Ma) of uplift and erosion followed, possibly representing a break-up unconformity, with erosion in southern Taranaki Basin and deposition of the ‘Taranaki Delta’ sequence in Deepwater Taranaki. The second, West Coast–Taranaki rift phase produced north- to NE-trending extensional half-grabens in the shelfal Taranaki Basin during the latest Cretaceous–Paleocene (c. 80 – 55 Ma). This rift was narrow (<150 km wide), orthogonal to Zealandia phase rifting, affected mainly western Zealandia and did not progress to full break-up. Supplementary material: A full set of eight palaeogeographical maps as well as expanded versions of the seismic figures, with both uninterpreted and interpreted versions, are available at https://doi.org/10.6084/m9.figshare.c.3772175

Refined depositional history and dating of the Tongaporutuan reference section, north Taranaki, New Zealand: new volcanic ash U–Pb zircon ages, biostratigraphy and sedimentation rates

ABSTRACT This study presents new radiometric ages from volcanic ash beds within a c. 1900 m thick, progradational, deep-water clastic slope succession of late Miocene age exposed along the north Taranaki coast of the North Island, New Zealand. The ash beds yield U–Pb zircon ages ranging from 10.63 ± 0.65 Ma to 8.97 ± 0.22 Ma. The new ages are compatible with and provide corroboration of New Zealand Tongaporutuan Stage planktic foraminiferal and bolboformid biostratigraphic events identified in the same section. The close accord between these two age datasets provides a stratigraphically consistent and coherent basis for examining margin evolution. The arrival of a prograding clastic wedge and ensuing upward shoaling is recorded by sedimentation rates c. 2000 m/Ma–1 that are an order of magnitude higher than sedimentation rates on the precursor deep basin floor. This outcrop study provides new constraints for interpreting analogous subsurface deposits in Taranaki Basin and complements the regional late Miocene biostratigraphic dating framework.

Cretaceous to present-day tectonic reconstructions of Zealandia

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