Early evolution of a young back-arc basin in the Havre Trough

Back-arc basins are found at convergent plate boundaries. Nevertheless, they are zones of significant crustal extension that show volcanic and hydrothermal processes somewhat similar to those of mid-ocean ridges. Accepted models imply the initial rifting and thinning of a pre-existing volcanic arc until seafloor spreading gradually develops over timescales of a few million years. The Havre Trough northeast of New Zealand is a unique place on Earth where the early stages of back-arc basin formation are well displayed in the recent geological record. Here we present evidence that, in this region, rifting of the original volcanic arc occurred in a very narrow area about 10–15 km wide, which could only accommodate minimal stretching for a very short time before mass balance required oceanic crustal accretion. An initial burst of seafloor spreading started around 5.5–5.0 million years ago and concluded abruptly about 3.0–2.5 million years ago, after which arc magmatism dominated the crustal accretion. The sudden transition between these different tectonomagmatic regimes is linked to trench rollback promoted by gradual sinking of the subducting lithosphere, which could have diverted the arc flux outside the region of seafloor spreading and induced the vertical realignment of surface volcanism with the source of arc melts at depth

High-resolution magnetics reveal the deep structure of a volcanic-arc-related basalt-hosted hydrothermal site (Palinuro, Tyrrhenian Sea)

High-resolution magnetic surveys have been acquired over the partially sedimented Palinuro massive sulfide deposits in the Aeolian volcanic arc, Tyrrhenian Sea. Surveys flown close to the seafloor using an autonomous underwater vehicle (AUV) show that the volcanic-arc-related basalt-hosted hydrothermal site is associated with zones of lower magnetization. This observation reflects the alteration of basalt affected by hydrothermal circulation and/or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and/or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids. To discriminate among these inferences, estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area—the stockwork—we use high-resolution vector magnetic data acquired by the AUV Abyss (GEOMAR) above a crater-shaped depression hosting a weakly active hydrothermal site. Our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones. Thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit, forming lobe-shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones

The largest deep-ocean silicic volcanic eruption of the past century

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 4 (2018): e1701121, doi:10.1126/sciadv.1701121.The 2012 submarine eruption of Havre volcano in the Kermadec arc, New Zealand, is the largest deep-ocean eruption in history and one of very few recorded submarine eruptions involving rhyolite magma. It was recognized from a gigantic 400-km2 pumice raft seen in satellite imagery, but the complexity of this event was concealed beneath the sea surface. Mapping, observations, and sampling by submersibles have provided an exceptionally high fidelity record of the seafloor products, which included lava sourced from 14 vents at water depths of 900 to 1220 m, and fragmental deposits including giant pumice clasts up to 9 m in diameter. Most (>75%) of the total erupted volume was partitioned into the pumice raft and transported far from the volcano. The geological record on submarine volcanic edifices in volcanic arcs does not faithfully archive eruption size or magma production.This research was funded by Australian Research Council Postdoctoral fellowships (DP110102196 and DE150101190 to R. Carey), a short-term postdoctoral fellowship grant from the Japan Society for the Promotion of Science (to R. Carey), National Science Foundation grants (OCE1357443 to B.H., OCE1357216 to S.A.S., and EAR1447559 to J.D.L.W.), and a New Zealand Marsden grant (U001616 to J.D.L.W.). J.D.L.W. and A.M. were supported by a research grant and PhD scholarship from the University of Otago. R.W. was supported by NIWA grant COPR1802. J.D.L.W. and F.C.-T. were supported by GNS Science grants CSA-GHZ and CSA-EEZ. M.J. was supported by the U.S. Department of Defense (DoD) through the National Defense Science and Engineering Graduate Fellowship (NDSEG) Program

Subduction of the oceanic Hikurangi Plateau and its impact on the Kermadec arc

Large igneous province subduction is a rare process on Earth. A modern example is the subduction of the oceanic Hikurangi Plateau beneath the southern Kermadec arc, offshore New Zealand. This segment of the arc has the largest total lava volume erupted and the highest volcano density of the entire Kermadec arc. Here we show that Kermadec arc lavas south of B32°S have elevated Pb and Sr and low Nd isotope ratios, which argues, together with increasing seafloor depth, forearc retreat and crustal thinning, for initial Hikurangi Plateau—Kermadec arc collision B250 km north of its present position. The combined data set indicates that a much larger portion of the Hikurangi Plateau (the missing Ontong Java Nui piece) than previously believed has already been subducted. Oblique plate convergence caused southward migration of the thickened and buoyant oceanic plateau crust, creating a buoyant ‘Hikurangi’ me´lange beneath the Moho that interacts with ascending arc melts

Semi-groupe de Lie associé à un cône symétrique

Volcanic arcs are the surface expression of magmatic systems that result from the subduction of mostly oceanic lithosphere at convergent plate boundaries. Arcs with a submarine component include intraoceanic arcs and island arcs that span almost 22,000 km on Earth\u27s surface, the vast majority of which are located in the Pacific region. Hydrothermal systems hosted by submarine arc volcanoes commonly contain a large component of magmatic fluid. This magmatic-hydrothermal signature, coupled with the shallow water depths of arc volcanoes and their high volatile contents, strongly influences the chemistry of the fluids and resulting mineralization and likely has important consequences for the biota associated with these systems. The high metal contents and very acidic fluids in these hydrothermal systems are thought to be important analogs to numerous porphyry copper and epithermal gold deposits mined today on land

Volcanism in slab tear faults is larger than in island-arcs and back-arcs

The volcanism of subduction settings concentrates in island-arcs and back-arc basins. Here, the authors show that the lithospheric tear faults bounding roll-backing slabs may focus huge volcanism with a volume of the erupted products exceeding that of the island-arcs edifices and back-arcs spreading centres

Slides: Addressing bathymetry uncertainty beneath the Ross Ice Shelf

These slides were presented at the 2023 New Zealand Australia Antarctic Science Conference in Christchurch, NZ.Abstract:The bathymetry underlying Antarctica’s Ross Ice Shelf exterts a strong control on it’s stability. The bathymetry guides the circulations of melt-inducing water masses and defines the geometry of pinning points. Collecting sub-ice shelf bathymetry data using typical polar surveying methods (e.g. seismic surveying or direct observations) can be inefficient, expensive or unfeasible. Gravity inversions provide a more practical alternative, in which observed variations in Earth's gravitational field are used to predict the bathymetry. Here we present a gravity inversion algorithm designed specificy to model sub-ice shelf bathymetry. Features include several methods to separate the regional gravity field, various options to impose model regularization, and the ability to quantify spatially variable model uncertainties. Here, we use this inversion with airborne gravity data from the Ross Ice Shelf and model the underlying bathymetry. Our results build upon the Tinto et al. 2019 model by using the new algorithm as well as incorporating additional gravity data and bathymetric constraints, collected since 2019.Gravity Inversion Github Repository: https://github.com/mdtanker/RIS_gravity_inversion</p

Gaussian envelope for 3D geomagnetic data inversion

We describe an inversion method for 3D geomagnetic data based on approximation of the source distribution by means of positive constrained Gaussian functions. In this way, smoothness and positivity are automatically imposed on the source without any subjective input from the user apart from selecting the number of functions to use. The algorithm has been tested with synthetic data in order to resolve sources at very different depths, using data from one measurement plane only. The forward modeling is based on prismatic cell parameterization, but the algebraic nonuniqueness is reduced because a relationship among the cells, expressed by the Gaussian envelope, is assumed to describe the spatial variation of the source distribution. We assume that there is no remanent magnetization and that the magnetic data are produced by induced magnetization only, neglecting any demagnetization effects. The algorithm proceeds by minimization of a (chi)2 misfit function between real and predicted data using a nonlinear Levenberg-Marquardt iteration scheme, easily implemented on a desktop PC, without any additional regularization. We demonstrate the robustness and utility of the method using synthetic data corrupted by pseudorandom generated noise and a real field data set.ENI Division of AGIP for allowing us to use the data of the Tyrrhenian Sea.Published996–10072.6. TTC - Laboratorio di gravimetria, magnetismo ed elettromagnetismo in aree attiveJCR Journalreserve

Gsolve, a Python computer program with a graphical user interface to transform relative gravity survey measurements to absolute gravity values and gravity anomalies

A Python program (Gsolve) with a graphical user interface has been developed to assist with routine data processing of relative gravity measurements. Gsolve calculates the gravity at each measurement site of a relative gravity survey, which is referenced to at least one known gravity value. The tidal effects of the sun and moon, gravimeter drift and tares in the data are all accounted for during the processing of the survey measurements.The calculation is based on a least squares formulation where the difference between the absolute gravity at each surveyed location and parameters relating to the dynamics of the gravimeter are minimized with respect to the relative gravity observations, and some supplied gravity reference site values. The program additionally allows the user to compute free air gravity anomalies, with respect to the GRS80 and GRS67 reference ellipsoids, from the determined gravity values and calculate terrain corrections at each of the surveyed sites using a prism formula and a user supplied digital elevation model.This paper reviews the mathematical framework used to reduce relative gravimeter survey observations to gravity values. It then goes on to detail how the processing steps can be implemented using the software. Keywords: Relative gravity, Python, Least squares, Free air anomalies, Terrain correction