Christian Timm

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.

Louisville seamount subduction and its implication on mantle flow beneath the central Tonga–Kermadec arc

Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga-Kermadec arc, west of the contemporary Louisville-Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated (206)Pb/(204)Pb, (208)Pb/(204)Pb, (86)Sr/(87)Sr in lavas from the central Tonga-Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga-Kermadec arc system.

Geochemical evolution of Monowai volcanic center: New insights into the northern Kermadec arc subduction system, SW Pacific

We present trace element and Sr-Nd-Pb isotope data on volcanic rocks recovered from the submarine Monowai volcanic center, which marks the midpoint of the ∼2500 km long Tonga-Kermadec arc. The center consists of a large (12 × 9 km) partly hydrothermally active caldera and a 12 km diameter ∼1500 m high volcanically and hydrothermally active stratovolcano. The stratovolcano lavas are tholeiitic basalts, which show variable evidence for plagioclase (±pyroxene) accumulation. The caldera lavas range from basalt to andesite, with the compositional variation being consistent with fractional crystallization as the dominant process. The mafic Monowai magmas were generated by relatively high degrees (12%–20%) of partial melting of a previously depleted MORB-type spinel-peridotitic mantle, metasomatized by slab-derived fluids. Strongly fluid mobile 87Sr/86Sr and 207Pb/204Pb ratios of the Monowai basaltic lavas are similar to those from the Putoto, Raoul, and Macauley volcanic centers 200–400 km to the south, suggesting derivation largely from subducted sediment. In contrast, variably fluid immobile 143Nd/144Nd ratios suggest an isotopically heterogeneous mantle along this segment of the arc. Higher 206Pb/204Pb in Monowai lavas imply some influence from the nearby subducting Louisville seamounts in melt generation. The formation of one of the Earths largest submarine mafic calderas can best be explained through drainage of a single magma reservoir and subsequent collapse caused by trench-perpendicular extension, probably via southward progressive rifting of the northern Havre Trough.

Crustal structure of the Kermadec arc from MANGO seismic refraction profiles

Three active-source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the Kermadec subduction zone. These observations are used to test contrasting tectonic models for an along-strike transition in margin structure previously known as the 32°S boundary. We use residual bathymetry to constrain the geometry of this boundary and propose the name Central Kermadec Discontinuity (CKD). North of the CKD, the buried Tonga Ridge occupies the forearc with VP 6.5–7.3 km s-1 and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110 ± 20 km) and strike (~005° south of 25°S). South of the CKD the forearc is structurally homogeneous down-dip with VP 5.7–7.3 km s-1. In the Havre Trough backarc, crustal thickness south of the CKD is 8-9 km, which is up-to 4 km thinner than the northern Havre Trough and at least 1 km thinner than the southern Havre Trough. We suggest that the Eocene arc did not extend along the current length of the Tonga-Kermadec trench. The Eocene arc was originally connected to the Three Kings Ridge and the CKD was likely formed during separation and easterly translation of an Eocene arc substrate during the early Oligocene. We suggest that the first-order crustal thickness variations along the Kermadec arc were inherited from before the Neogene and reflect Mesozoic crustal structure, the Cenozoic evolution of the Tonga-Kermadec-Hikurangi margin and along-strike variations in the duration of arc volcanism.

The geochemistry and petrogenesis of Carnley Volcano, Auckland Islands, SW Pacific

ABSTRACT Intraplate volcanism across Zealandia, South Eastern Australia, the Ross Sea Embayment and Marie Byrd Land in Antarctica define a magmatic province characterised by basalts with elevated 206Pb/204Pb (18.9–22.5), 87Sr/86Sr = ∼0.7035, Light Rare Earth enrichment [(Ce/Yb)n > 10], and convex-up mantle normalised incompatible multi-element patterns, peaking at Nb-Ta, with negative K and Pb anomalies. Trace element abundances and ratios (e.g. Zr/Nb, Y/Zr) resemble Ocean Island Basalts (OIB), distinct from Mid-Ocean Ridge Basalt (MORB), suggesting derivation from OIB-like reservoirs. Our preferred scenario envisages partial melting across the garnet-spinel stability fields involving asthenospheric and lithospheric mantle components. Melts accumulate in a column where the deep (asthenospheric) source is PM and the shallower source a melange of PM and subcontinental lithospheric mantle (DMM+1) enriched by carbonatite. Evolution of primary and near-primary magmas is controlled by olivine + clinopyroxene fractionation. Trachybasalts, trachytes and rhyolites show isotopic evidence for interaction with continental crust.

Colville III '16 (TAN1611) cruise report Wellington-Auckland, September 26-October 16, 2016

This research voyage successfully acquired geophysical data and rock samples from the northern part of the Colville Ridge, within New Zealand’s Exclusive Economic Zone (EEZ). The main output of this voyage is providing data and fundamental base maps for future regional prospectivity surveys, tectonic plate reconstructions, and general understanding of the architecture of the Kermadec backarc. The Colville III ‘16 voyage builds on two previous voyages to the southern and mid Colville ridge in 2013 and 2015 respectively and completes a comprehensive and dataset that extends unbroken for over 1000 km along the Colville Ridge. In 2013 and 2015 voyages acquired a suite of geophysical (gravity, magnetic, bathymetry and backscatter) data and rock samples (de Ronde et al 2016a & de Ronde et al 2016b). The 21 day Colville III ’16 cruise was successful in acquiring gravity and magnetic measurements, along with bathymetry and backscatter data, during 18 days of surveying, over an area of ~21,815 km2. Dredge sites were selected from features and anomalies shown in the newly acquired geophysical data, 47 rock samples were obtained from 11 successful dredge deployments. Combined with Colville I and Colville II voyage data, the data collected from Colville III ’16 are crucial for our ability to understand the evolution of New Zealand’s on and offshore territory. These data are required to identify prospective sites to carry out more focused, follow-up surveys by AUVs, ROVs, and possibly manned submersibles along the Colville Ridge. The results from this survey will be published as maps and papers in international journals and will become publicly accessible online at GNS. (auth)

Colville II '15 Cruise report. Wellington to Auckland September 5 to 25 2015

This research cruise was successful in swath mapping the mid-part of the Colville Ridge and collecting underway-geophysical measurements as part of an ongoing project to survey the Colville Ridge within New Zealand’s Exclusive Economic Zone (EEZ). The main outcome of this expedition is providing fundamental base maps for future mineral exploration. The Colville Ridge is a prominent tectonic feature that marks the western boundary of the Kermadec Arc-Havre Trough backarc system. Very little is known about this large feature on the seafloor, which extends over 1000 km within NZ’s EEZ. It is thought to be at least 2 Myr old (and likely much older) and is possibly composed of various volcanic centres. EM 300 standard multi-beam data was largely missing for the Colville Ridge, with only single ship’s tracks crossing the ridge in a few areas. In 2013 a comprehensive survey of the southern portion of the Colville Ridge was completed during the 15 day OS2020 Colville ’13 cruise that included 12 days on station, with 11 days used for swath mapping and 1 day for dredging. This cruise was highly successful, having produced the first complete map of detailed bathymetry, backscatter, magnetics and gravity over a large part of the southern Colville Ridge. Rocks were also collected from 13 separate stations. The 21 day Colville II ’15 cruise was successful in acquiring gravity and magnetic measurements, along with bathymetry and backscatter data, during 17 days on station, over an area of ~21,320 km2. The datasets have 100% coverage of these parameters, enabling us to ‘stack’ the gravity and magnetic maps on top of the bathymetry and backscatter, enabling a multi-disciplinary approach to interpretation. Rock samples were collected from 20 dredge sites that covered the length of the ridge mapped during the survey, and which included a variety of bathymetric features. Together, the information collected are crucial in our ability to identify prospective sites to carry out more focused, follow-up surveys by AUVs, ROVs, and possibly manned submersibles along the Colville Ridge. The results from this survey will be published as maps and papers in international journals and will be publicly accessible online at GNS. (auth)