Ian C. Wright

Intra-oceanic subduction-related hydrothermal venting, Kermadec volcanic arc, New Zealand

Intra-oceanic volcanic arcs mark the boundaries between converging lithospheric plates where subduction produces volcanic and tectonic activity that ensures a steady supply of magmatic heat and hydrothermal fluids to the seafloor. Here we report on the first broad and systematic survey of hydrothermal emissions generated along a submarine arc front. More than half (seven of 13) of the volcanoes surveyed along 260 km of the southern Kermadec arc, NE of New Zealand, are hydrothermally active. Our results indicate that volcanic arcs represent a previously unheeded but potentially extensive source of shallow (<2 km water depth) vent fields expelling fluids of a unique and heterogeneous composition into the oceans.

SOUTHERN KERMADEC SUBMARINE CALDERA ARC VOLCANOES (SW PACIFIC) : CALDERA FORMATION BY EFFUSIVE AND PYROCLASTIC ERUPTION

Abstract Three newly identified submarine caldera volcanoes (Brothers, Healy, and Rumble II West), of the southern Kermadec arc (within water depths 80%) and low density (0.3–0.5 g cm−3) erupted pumiceous clasts. These observations are interpreted as evidence of subaqueously quenched, pyroclastic eruption(s), sheathed by ambient water flashing to steam, discharging some 5 km3 of pyroclastic material from Healy volcano. Caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption(s), although it is possible formation as an explosive crater occurred, but we consider the latter unlikely given the volume of erupted material and water depth. Extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas, and syn-eruptive caldera collapse, is possible within water depths of 500–1000 m.

Pre-spread rifting and heterogeneous volcanism in the southern Havre Trough back-arc basin

Abstract The Havre Trough is an active back-arc basin associated with Pacific—Australian plate convergence. Two segments within the southern Havre Trough (SHT), surveyed with gloria , Sea beam and SYS09 swath data, now provide the first detailed data to reliably document the structure and morphology of this complex back-arc region. At the southern segment (35°40′–37°S), along a strike length of some 120 km, the SHT structure comprises a series of segmented, en echelon, generally flat floored, sediment filled, axial rift grabens, flanked by ridge and basin rift block topography. Prominent fault escarpments, with relief of 500–1000 m and a marked zig-zag arrangement, mark the outer margins of the rift system. Within the rift grabens extensional tectonism is pervasive. Extensive shallow seismicity within the SHT can only partly be associated with the rift graben system at present. Youthful, constructional volcanic terrains within the rift grabens are not common, and invariably have a restricted areal extent and a relief An identical rift morphology has been identified at the northern (33°–34°S) site. Data from both sites show individual rifts are 15–35 km in length, 6–10 km wide, and have axial depths of 3100–4000 m. Apart from at least one 15–20 km wide, cross trough ridge, available data indicate the rift graben system is contiguous between 33°S and 37°S. This rift graben morphology, by analogy with other known rifting back-arc basins, is considered here not to be consistent with true oceanic back-arc spreading. Rather it is argued that the SHT, possibly indeed all of the Havre Trough, is undergoing back-arc rifting prior to true spreading. Further, the rifting tectonic fabric within the SHT, and in the onshore Taupo to the south, extension is consistently dextrally oblique relative to the bounding Colville and Kermadec Ridges. The degree of obliquity increases from south to north. If accepted that the SHT is rifting rather than spreading, magnetic anomaly data within the trough do not record the accretion of new oceanic crust, and consequently can no longer be used to establish the “spreading” history of the SHT. The SHT magnetic anomalies are here interpreted as “pseudo-linear” magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older, low magnetisation arc basement rift blocks flanking the axial rift. Proxy data from onshore North Island, New Zealand, and offshore SHT suggest that the rate of extension and age of initiation of back-arc rifting are, between 35° and 37°S, respectively, 15–20 mm yr −1 and 5 Ma.

Mantle dynamics, element recycling, and magma genesis beneath the Kermadec Arc‐Havre Trough

New geochemical and isotopic data are presented for lavas from three sites in the Havre Trough-Lau Basin back arc and six volcanoes along the Kermadec arc. The back arc basalts range from MORB-like to arc-like in composition and contain a variable contribution from the underlying slab. The least contaminated MORB-like back arc lavas from 24°–29°S are low degree partial melts of a source with Pacific MORB isotopic characteristics. A transition occurs at 30°S between the strongly depleted northern Kermadec (and Tonga) arc lavas and the mildly depleted southern Kermadec arc lavas. This transition does not correlate with changes in the back arc extension rate or width but may reflect inhibited mantle wedge replenishment behind the shallower-dipping northern Kermadec-Tonga slab. Northern Kermadec lavas require mixing between two components: (1) depleted Havre Trough mantle and (2) fluid derived from altered MORB crust with a slight input of sediment lead. Inter-volcano differences in fluid compositions probably reflect local variations on the subducting slab rather than mineralogical variation in the mantle wedge. Southern Kermadec lavas require an additional component: (3) Pacific sediment melt. This sediment melt is only detected where the subduction rate is 650°C before passing through the sub-arc melt generation zone.

Elemental mercury at submarine hydrothermal vents in the Bay of Plenty, Taupo volcanic zone, New Zealand

Hot springs in active geothermal areas such as Yellowstone National Park, the Geysers geothermal field in California, and the Taupo volcanic zone in New Zealand are notably enriched in the trace metals Au, Ag, As, Sb, and Hg. Such near-surface hot springs have formed many of the worlds important deposits of gold and silver and some of the largest deposits of mercury. The majority of these are associated with continental geothermal systems in subaerial environments. Here we report the discovery of active mercury-depositing hot springs in a submarine setting, at nearly 200 m water depth, within the offshore extension of the Taupo volcanic zone of New Zealand. These vents contain the first documented occurrence of elemental mercury on the sea floor and provide an important link between offshore hydrothermal activity and mercury-depositing geothermal systems on land. The discovery has implications for mercury transport in sea-floor hydrothermal systems and underscores the importance of submarine volcanic and geothermal activity as a source of mercury in the oceans.

Arc and back-arc geochemistry in the southern Kermadec arc-Ngatoro Basin and offshore Taupo Volcanic Zone, SW Pacific

Abstract Back-arc basin basalts from the Ngatoro Basin (the southern end of the Havre Trough) are similar geochemically to, yet subtly distinct from, basalts of the Havre Trough to the north. Whole rock and glass chemistry are consistent with derivation from a fertile mantle source with subsequent evolution by fractionation of olivine (+ Cr-spinel) + plagioclase, and then clinopyroxene. Basalts from the vicinity of Rumble IV seamount at the southern end of the Kermadec island arc, and the eastern Ngatoro rift escarpment, are strongly porphyritic relative to the back-arc basin basalts and show trace element (high LIL abundances and highly depleted HFS abundances) and isotopic signatures of subduction zone basalts. At its southern end, the Ngatoro Basin penetrates the continental crust of New Zealand creating a major, 3000 m deep bathymetric re-entrant in the slope-break; the slope-break marks the transition from oceanic to continental crust. Basalts from the floor of the Ngatoro Basin re-entrant are isotopically distinct from the basalts of the oceanic sector in that they have higher Sr and correspondingly lower Nd isotope ratios and are comparable to basalts of the Taupo Volcanic Zone (TVZ) to the south. In contrast to the basalt-dominated oceanic sector, basalts from the offshore TVZ, a 100 km long area extending roughly NNE from White Island to the submarine Whakatane arc volcano at the edge of the continental slope-break, occur in association with andesites, dacites and rhyolites. These basalts are generally strongly porphyritic (olivine + plagioclase + clinopyroxene) and show trace element abundances typical of suprasubduction zone rocks. However, offshore TVZ basalts show subtle distinctions from onshore TVZ basalts to the south; the former have more radiogenic Sr isotopes. Furthermore, their high field strength element and transition element systematics appear to overlap with those of basalts from the Kermadec arc to the north. The authors attribute these lateral (along arc) and transverse (across arc) variations to source heterogeneity and variable fertility in the sources of the arc and back-arc basin magmas. Sources of the arc-front magmas are more refractory and also more susceptible to contamination by slab-derived fluids than sources for back-arc basin magmatism, reflecting the dynamic nature of flow from the back-arc into the mantle wedge beneath the volcanic front.

Seafloor geology and petrology in the oceanic to continental transition zone of the Kermadec‐Havre‐Taupo Volcanic Zone arc system, New Zealand

Abstract To the north of New Zealand, the oceanic Tonga‐Kermadec island arc system strikes roughly NNE‐SSW and intersects the continental shelf of New Zealand in the vicinity of the Whakatane arc volcano, a submarine edifice which rises around 1000 m from the surrounding seafloor. A major bathymetric slope‐break here coincides with the proposed oceanic‐continental transition zone. Northwest of the Whakatane volcano, dredging on Colville Knolls recovered samples of deformed Mesozoic(?) metasedimentary rocks, whose petrography and geochemistry resemble Waipapa Terrane rocks of onshore New Zealand, thereby confirming the extent and “continental” status of the offshore region. The transition zone from continental crust to oceanic crust is delineated by a marked slope‐break at the 2000 m isobath, oriented roughly parallel to the Bay of Plenty coastline. The linearity of this boundary is broken by a well‐defined reentrant structure, the Ngatoro Basin, which here is recognised as a southern extension of the ocea...

From rifting to active spreading in the Lau Basin – Havre Trough backarc system (SW Pacific): Locking/unlocking induced by seamount chain subduction

Associated with Pacific-Australia plate convergence, the Lau Basin – Havre Trough is an active back-arc basin that has been opened since ?5.5 Ma by rifting and southward propagating oceanic spreading. Current back-arc opening rates decrease from 159 mm yr?1 in the northern Lau Basin to 15 mm y?1 in the southern Havre Trough. Major tectonic changes occur at the transition between Havre Trough rifting and full oceanic spreading of the Eastern Lau Spreading Center (ELSC), where the oblique-to-trench, westward subducting Louisville Seamount Chain (LSC) sweeps southwards along the Tonga trench. New swath bathymetry, seismic reflection data, and limited rock sampling in this area constrain a tectonic and kinematic back-arc model that incorporates the effects of LSC subduction. The ELSC, which extends southward to 24°55?S, forms a deep rift valley propagating southward through older, rifted arc basement. Present-day seismicity and fresh and fractured pillow lavas at 23°42?S are consistent with rift valley neovolcanism. Conversely, the northern Havre Trough has low seismicity and rifted volcanic basement ridges trending 25–45° oblique to the basin axis consistent with low levels of extensional tectonism and volcanism. This latter structural fabric is interpreted as an early stage of rifting that is now “locked” due to compression on the arc exerted by LSC subduction, while in the Lau Basin such effects have passed as the LSC swept along the Tonga Trench. It is proposed that the Lau-Havre back-arc opening is controlled by tectonic constraints exerted at the limits of the system by the LSC subduction, which determines the southward migration of the Tonga Arc pole of rotation and associated Lau Basin opening. A discrete three-stage back-arc opening evolution is proposed, comprising: (1) an initial phase of back-arc rifting along the whole length of the plate boundary, beginning at ?6–5 Ma; (2) a subsequent phase, mostly present in the southern part of the back-arc domain and still active in the Havre Trough, of transpression and transtension, starting at ?4 Ma in the north, as the LSC starts to subduct and sweeps southward along the Tonga trench; and (3) a renewed opening phase in the northern segment of the back-arc domain, with rifting and spreading, starting at ?3.5 Ma, as subduction of the LSC along the northern Tonga trench is progressively completed.

Physical characterisation and a biologically focused classification of “seamounts” in the New Zealand region

Abstract The physical, biological, and oceano‐graphic characteristics of seamounts of the New Zealand region of the South Pacific Ocean are poorly known. The aim of this study was to present a synopsis of the physical characteristics of seamounts within the region, and to present a preliminary classification using biologically meaningful variables. Data for up to 16 environmental variables were collated and used to describe the distribution and characteristics of the c. 800 known seamounts in the New Zealand region. Seamounts span a wide range of sizes, depths, elevation, geological associations and origins, and occur over the latitudinal range of the region, lying in different water masses of varying productivity, and both near shore and off shore. As such, it was difficult to generally describe New Zealand seamounts, as there is no “typical” feature. Thirteen environmental variables were included in a multivariate cluster analysis to identify 12 seamount similarity groupings, for a subset of over half the known seamounts. The groupings generally displayed an appreciable geographic distribution throughout the region, and were largely characterised by a combination of four variables (depth at peak, depth at base, elevation, and distance from continental shelf). In the future, the findings of the present study can be tested to determine the validity and usefulness of the approach for directing future biodiversity research and informing management of seamount habitat.

Discovery of hydrothermal sulfide mineralization from southern Kermadec arc volcanoes (SW Pacific)

Abstract We report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern Kermadec frontal arc volcanoes (Brothers and Rumble II West) of the ∼1200 km long, Kermadec–Havre arc–back-arc system. The Brothers and Rumble II (West) calderas, both with re-surgent domes and comprising mostly dacite and basalt–andesite host rocks, respectively, rise to water depths of ≤1500 m. These calderas comprise mostly effusive lavas and volcaniclastic deposits, including talus breccias along their inner caldera walls. These two southern Kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites. Two principle ore assemblages are identified, comprising: (i) chalcopyrite–pyrite–barite, and (ii) sphalerite–marcasite–barite ± pyrite. The most prevalent ore texture consists of massive chalcopyrite + pyrite ± sphalerite, commonly overprinting barite. Early marcasite cores within large, euhedral pyrites, and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization. The ore mineralogy is consistent with postulated venting temperatures of ≥300°C. Sulfide geochemistry is consistent with the ore mineralogy with concentrations of Cu, Fe, and Zn up to 15.3, 19.1, and 18.8 wt%, respectively. The sulfide geochemistry is distinct, however, from comparable western Pacific vent sites, and may reflect source heterogeneity due to proximity to continental New Zealand and sediment recycling along the southern Kermadec trench–arc system. The recovery of two partial caridean vent shrimps suggests that present-day hydrothermal venting is occurring within the Brothers caldera.