Brent V. Alloway

A 28 000-6600 cal yr record of local and distal volcanism preserved in a paleolake, Auckland, New Zealand

Abstract A 52.5 m core was extracted from Pukaki Crater, an infilled basaltic explosion crater in the Auckland Volcanic Field, for detailed tephra and palynological analysis. The core consists of a lower 6 m of finely laminated lacustrine sediments representing the interval c. 28 000–6600 cal yr overlain by 46.5 m of homogeneous marine silts deposited between c. 7600 and 6600 cal yr. Favourable conditions have preserved at least 40 tephra layers in the sediments. These have been derived from one local and five distal sources and were deposited within the crater lake between c. 28 000 and c. 7600 cal yr. The tephra beds were identified by stratigraphic position, geochemical analyses, and ferro‐magnesian mineral assemblage. This tephrostratigraphic framework is underpinned by three distinctive tephra beds, namely Tuhua (c. 6950 cal yr), Rotoma (c. 9500 cal yr), and Kawakawa (c. 26 500 cal yr). Of the 40 tephra beds, 7 are sourced from the rhyolitic Okataina Volcanic Centre (Mamaku c. 8200 cal yr; Rotoma c. 9500 cal yr; Waiohau c. 13 800 cal yr; Rotorua c. 15 800 cal yr; Rerewhakaaitu c. 17 700 cal yr; Okareka c. 21 400 cal yr; Te Rere c. 25 000 cal yr), 3 from the rhyolitic Taupo Volcanic Centre (Opepe c. 10 200 cal yr; Kawakawa c. 26 500 cal yr; Poihipi c. 27 500 cal yr), 5 from the andesitic Tongariro Volcanic Centre, 14 from the andesitic Taranaki Volcano, 1 from Mayor Island (Tuhua c. 6950 cal yr), and 8 from the basaltic Auckland Volcanic Field. In addition, two previously unidentified rhyolitic tephra (c. 17 100 cal yr and c. 20 720 cal yr) are recorded. The occurrence of numerous andesitic and rhyolitic tephra beds in the Auckland region extends the known dispersal of the units and has implications for the assessment of volcanic hazards from distal sources. Many of the Taranaki‐derived tephra beds do not stratigraphically match those recorded in the Waikato lakes region and this suggests that Taranaki Volcano produced more ash than previously estimated. The distal tephra record preserved at Pukaki provides age constraints for Auckland Volcanic Field basaltic tephra that are otherwise poorly dated. Basaltic fall events are recorded at c. 14 450 cal yr, 15 750 cal yr, 19 380 cal yr, 19 420 cal yr, 23 825 cal yr, 24 175 cal yr, 25 200 cal yr, and 25 700 cal yr. Fresh glass in the basaltic tephra allows them to be chemically fingerprinted and discriminated, and this will open a new avenue to development of a regional basaltic tephrostratigraphy.

Revision of the marine chronology in the Wanganui Basin, New Zealand, based on the isothermal plateau fission-track dating of tephra horizons

Abstract The occurrence of tephra horizons in basins adjacent to volcanic arcs provide an excellent opportunity for establishing a reliable chronostratigraphic framework for detailed sedimentological studies. In this study, three widespread and stratigraphically important rhyolitic tephra horizons interbedded in Plio/Pleistocene strata of the Wanganui Basin, New Zealand, are dated by application of the isothermal plateau fission-track (ITPFT) technique to hydrated glass shards. All glass samples were corrected for annealing and consequently yield reliable ages. Rangitawa Tephra yielded statistically indistinguishable ages from three localities that are in excellent agreement with recently determined zircon fission-track age estimates of ca. 0.35 Ma. ITPFT ages of 1.05 ± 0.05 and 1.63 ± 0.15 Ma for Potaka Pumice and Pakihikura Pumice, respectively, are considerably older than previous FT estimates but consistent with new magnetostratigraphic data that places the Potaka within the Jaramillo Subchron, and Pakihikura within the Matuyama Chron between the Cobb Mountain and Olduvai Subchrons. Combining our fission-track ages with the magnetostratigraphy, the true age of sediments within the Wanganui Basin is found to be significantly underestimated. Sedimentation rates of between ca. 680-630 m/Ma from 1.63 Ma to 0.35 Ma are calculated in the eastern part of the basin and are much lower than those calculated using the previous FT chronology. This new ITPFT-age data demonstrates that the existing Plio/Pleistocene marine chronology in New Zealand will require age revision and has important implications when considering the evolution of several other sedimentary basins in southern North Island that contain the same ITPFT-dated tephra horizons.

Early to middle Pleistocene tephrochronology of North Island, New Zealand: Implications for volcanism, tectonism, and paleoenvironments

Thick volcaniclastic sequences of early to middle Pleistocene age in southern North Island, New Zealand, contain rhyolitic tephra beds that record the early history of the Taupo volcanic zone (TVZ). At least 54 different tephra beds are recorded, and their chronology is defined by glass fission-track ages and paleomagnetism. The tephra beds span the interval ca. 2.0−0.6 Ma and provide an event frequency of 1/19 k.y., significantly higher than the frequency of sheet-forming ignimbrites preserved in the TVZ at this time (≈1/100 k.y.). The distal tephra beds thus provide a record of volcanism not revealed in the proximal volcanic region and suggest a major period of explosive activity at ca. 1.79−1.60 Ma. Several important marker horizons are identified: Pakihikura tephra (1.63 Ma), Mangatewaiiti tephra (1.24 Ma), Potaka tephra (1.00 Ma), Kidnappers B tephra (1.00 Ma), and Kaukatea tephra (0.88 Ma). These tephra beds allow direct correlation between (1) marine and nonmarine facies and (2) the fore-arc and back-arc regions of New Zealand. The tephra beds provide a framework for a paleoenvironmental reconstruction of the southern North Island. Volcaniclastic transport routes from the TVZ to basins in the south and southeast, and through the site of present mountain ranges, supplied material to a terrestrial lowland fore-arc area in the interval 1.64−0.7 Ma. Uplift and deformation since 0.7 Ma have disrupted paleodrainage routes, diverting them to the north and southwest.

Climate of the last glaciation in New Zealand, based on aerosolic quartz influx in an andesitic terrain

Abstract On western North Island, New Zealand, a record of climatic change during the last glaciation is preserved in a terrestrial coverbed sequence of dominantly andesitic provenance. Here, a succession of five loess-like Andisol units postdates the global high sea-level stand of oxygen isotope substage 5e (

Paleoecological insights into subduction zone earthquake occurrence, eastern North Island, New Zealand

Paleoecological investigations of three Holocene marginal-marine sedimentary sequences provide information on vertical tectonic deformation in a transect across the forearc basin adjacent to the Hikurangi subduction zone, New Zealand. The elevation of maximum postglacial sea level indicators at Te Paeroa Lagoon and Opoho is between 4 and 6 m below present mean sea level, indicating net subsidence since 7200 yr B.P. Opoutama is closer to the Hikurangi Trench and appears to lie near the edge of the zone of subsidence, as evidence for vertical movement there is equivocal. Some of the subsidence at Te Paeroa Lagoon and Opoho is likely to be a result of compaction. However, a component of subsidence probably happened coseismically in two events at ca. 7100 and 5550 yr B.P. Event signatures consist of tsunami deposits overlain by chaotically mixed, reworked sediment that appears to have filled rapidly created accommodation space at marine inlet sites 10 km apart. Large offshore earthquakes are suggested by the coincidence of tsunami inundation with sudden subsidence. Forward elastic-dislocation models indicate that the observed subsidence could be achieved in ∼M w 7.9 earthquakes on either the subduction interface or the Lachlan Fault, which would involve synchronous uplift of Mahia Peninsula. Combined rupture of the interface and the Lachlan Fault, either simultaneously in a ∼M w 8.1 earthquake, or consecutively, could explain larger amounts (>1.5 m) of coastal subsidence.

Late Quaternary (post 28,000 year B.P.) tephrostratigraphy of northeast and central Taranaki, New Zealand

Mt. Egmont/Taranaki in western North Island, New Zealand, has been inactive during historic time and thus its pattern of eruptive history must be reconstructed by inference from the stratigraphy and chronology of associated volcanic and pyroclastic deposits. The most complete record of eruptive activity from Egmont Volcano is found on the surrounding ring plain rather than on the volcanic cone, where surficial deposits are readily removed by erosion or deeply buried by the products of more recent eruptions. In this study, a comprehensive post‐28 ka record of the volcanos eruptive history is presented, and the relationship of andesitic tephra beds to andic soil material, Egmont‐sourced volcaniclastic detritus, and two silicic tephra beds from Taupo Volcano are discussed, along with implications for inter‐regional correlation. The post‐28 ka tephra succession is recorded in sixteen andesitic tephra formations. These formations and their approximate ages are as follows: Manganui tephra (4 beds; c. 3.1 ka), ...

Magnetostratigraphic, lithostratigraphic and tephrostratigraphic constraints on Lower and Middle Pleistocene sea-level changes, Wanganui Basin, New Zealand

Wanganui Basin, North Island, New Zealand, contains a complex sedimentary record of Lower and Middle Pleistocene sea-level changes. Palaeomagnetic results allow identification of the Matuyama/Brunhes transition, the Jaramillo Subchron and the Cobb Mountain Subchron. Correlations of rhyolitic tuff horizons across the basin are consistent with magnetostratigraphic correlations between sections. Isothermal plateau fission-track (ITPFT) ages of 1.05 ± 0.05 Ma and 1.63 ± 0.15 Ma on two tuffs (Potaka Pumice and Pakihikura Pumice, respectively) are consistent with the interpreted magnetostratigraphy and also with the astronomically tuned timescale of ODP Site 677. Magnetostratigraphy and ITPFT ages allow correlation of sedimentary cycles at Wanganui with odd-numbered oxygen isotope stages 17–31 in deep-sea cores. The stratotype section for the New Zealand Castlecliffian Stage is shown to be incomplete relative to other studied sections in the basin. Below stage 31, the character of the cyclothems changes at Wanganui, from marine dominated, to a greater representation of non-marine and estuarine strata. This change may be partly related to a change in amplitude and frequency of climatic cycles identified in oxygen isotope records, but the primary cause is likely to be increased basinal uplift.

Multi-method dating comparison for mid-pleistocene Rangitawa Tephra, New Zealand

Abstract The Rangitawa Tephra, a widespread mid-Pleistocene marker in the New Zealand region, is dated by fission-track, thermoluminescence (TL), infrared stimulated luminescence (IRSL), K Ar and single crystal laser fusion (SCLF) 40 Ar 39 Ar methods, as well as by matching to the astronomical timescale via oxygen isotope stratigraphy in deep sea cores. Fifty one independent numerical age estimates for Rangitawa Tephra range from 173 to 456 ka. However, only 15 of these results are considered to be reliable: five zircon fission-track results, four isothermal plateau fission-track (ITPFT) age estimates on glass shards, four SCLF 40 Ar 39 Ar results, and two astronomically calibrated age estimates from deep sea cores. The weighted mean of the nine reliable fission-track age determinations is 345 ± 12 ka, in excellent agreement with the weighted mean of 340 ± 7 ka for the two reliable astronomically calibrated age estimates. The weighted mean of the four reliable 40 Ar 39 Ar results (302 ± 8 ka) is significantly younger, and further Ar Ar dating is recommended to investigate this difference. Since the TL and IRSL dating methods are experimental for sediments beyond 200 ka, we have excluded these from the group of reliable age results. TL and IRSL results on dunesand appear to be systematically younger than TL results on loess, and loess results depend upon analytical procedures.

Demise of one volcanic zone and birth of another - A 12 m.y. marine record of major rhyolitic eruptions from New Zealand

Ocean Drilling Program Sites 1123 and 1124 provide an unprecedented 12 m.y. record of major rhyolitic eruptions from the Coromandel and Taupo volcanic zones of New Zealand. Macroscopic tephras (n = 197) were dated using magnetostratigraphy, supplemented by geochemical correlation with subaerial tephra, isothermal plateau fission-track ages, and orbitally tuned stable isotope data. Eruptions began in the Coromandel volcanic zone ca. 12 Ma, ∼1.6–1 m.y. earlier than previously known. Thereafter, volcanism was fairly continuous with a tempo and intensity that increased through the late Miocene–Pliocene and into the Quaternary, when the Taupo volcanic zone formed. The transition from the Coromandel to the Taupo zone, previously placed as ca. 4–2 Ma, was seamless, without obvious breaks or changes in ash composition. This well-dated history of long-lived and productive volcanism allows for more confident correlation with other circum-Pacific tephra records, thus helping confirm the occurrence of widespread coeval eruptions throughout the region.

Growth of contractional structures during the last 10 m.y. at the southern end of the emergent Hikurangi forearc basin, New Zealand

Abstract Growth histories of contractional structures at the southern end of New Zealands Hikurangi forearc basin have been analysed for the last c. 10 m.y. Growth data are from outcrop and seismic‐reflection profiles that contain syntectonic strata and angular unconformities, and from deformed fluvial terrace surfaces. Deformation is described for up to eight intervals of time, spanning c. 12 000 yr to 5 m.y., the ages of which were determined by biostratigraphy and tephrochronology. Reverse faults and related asymmetric folds, which strike parallel to the subduction margin and verge troughwards, experienced variable rates of shortening through time. The current period of deformation commenced at c. 1.8 Ma with displacement rates of c. 0.1–0.7 mm/yr on the main faults (i.e., Martinborough, Huangarua, and Mangaopari Faults). Before this time there were periods of accelerated deformation during the mid Pliocene (c. 3.4–2.4 Ma) and latest Miocene (c. 8.0–6.0 Ma). Therefore, shortening since 10 Ma accumulated mainly during three periods of 1–2 m.y., with structures active in the Quaternary forming in the late Miocene or earlier. Local intervals of accelerated deformation are coincident with the timing of intervals of uplift and faulting along much of the emergent forearc and cannot be attributed to local transfer of displacements between faults. Instead, these intervals of deformation appear to reflect regional changes in the kinematics of the upper plate. These changes could arise due to margin‐normal migration of strain to regions outside the forearc basin or may indicate temporal variations in the dynamics of subduction.