Solid as a rock: Tectonic control of graben extension and dike propagation

The 2014–2015 CE rift event associated with the Bárðarbunga eruption at Holuhraun, Iceland, offers a unique opportunity to study the spatial and temporal evolution of a rift graben. We present the first four-dimensional (three-dimensional plus time) monitoring of the formation and evolution of a graben during active magma transport using a suite of digital elevation models spanning from shortly before the eruption throughout 6 months of magma transport and up to 4.5 years after the eruption. This multiscale data set enables investigations of how magma supply and eruption dynamics affect tectonic structures that feed eruptions. After formation (time scale of a few days), the graben is remarkably stable throughout the eruption and for years beyond. It is unaffected by large changes in eruptive activity and effusion and seismicity rates within the plumbing system. These data document that (1) there was no direct feedback between eruptive dynamics and graben topography, and (2) graben formation is near instantaneous on tectonic time scales. These results challenge the overarching role ascribed to magma transport in recent studies of tectonomagmatic relationships in rift events, favoring regional tectonics as the fundamental driving force.

Transport of mafic magma through the crust and sedimentary basins: Jameson Land, East Greenland

Igneous sheet-complexes transport magma through the crust, but most studies have focused on single segments of the magma-transport-system or have low resolution. In the Jameson Land Basin in East Greenland, reflection-seismic data and extensive outcrops give unparalleled constraints on mafic intrusions down to 15 km. This dataset shows how sill-complexes develop and how magma is transported from the mantle through sedimentary basins. The feeder zone of the sill-complex is a narrow zone below basin, where a magmatic underplate body impinges on thinned crust. Magma was transported through the crystalline crust through dykes. Seismic data and published geochemistry indicate magma was supplied from a magmatic underplate, without perceptible storage in crustal magma-chambers and crustal assimilation. As magma entered the sedimentary basin, it formed distributed, bowl-shaped sill-complexes throughout the basin. Large magma volumes in sills (4-20 times larger than the Skaergaard Intrusion), and few dykes highlight the importance of sills in crustal magma-transport. On scales smaller than 0.2 km, host-rock lithology, and particularly mudstone tensile strength-anisotropy, controls sill-architecture in the upper 10km of the basin, whereas sills are bowl-shaped below the brittle-ductile transition zone. On scales of kilometres and towards basin margins, tectonic stresses and lateral lithological changes dominate architecture of sills.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5670470

Magma transport beneath mid-ocean ridges

<p>Melt transport beneath the lithosphere is elusive. With a distinct viscosity and density from the surrounding mantle, magmatic melt moves on a different time scale as the surrounding mantle. To resolve the temporal scale necessary to accurately capture melt transport in the mantle, the model simulations become numerically expensive quickly. Recent computational advances make possible two-phase numerical explorations to understand magma transport in the mantle. We review results from a suite of two-phase models applied to the mid-ocean ridges, where we varied half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the lithosphere-asthenosphere boundary (LAB). Here, we highlight the importance of viscosities for the melt focusing mechanisms. In addition, magmatic porosity waves that are a natural consequence of these two-phase flow formulations. We show that these waves could explain long-period temporal variations in the seafloor bathymetry at the Southeast Indian Ridge.</p>

Long-term gas observations track the early unrest phases of open-vent basaltic volcanoes

<p align="justify"><span>At open-vent basaltic volcanoes, resolving the activity escalation that heralds larger, potentially harmful eruptions is challenged by the persistent mild ordinary activity, which often masks the precursory unrest signals related to heightened magma transport from depth. Gas (SO</span><sub><span>2</span></sub><span> and CO</span><sub><span>2</span></sub><span>) fluxes at surface are controlled by rate of magma transport and degassing within the magma plumbing system, and thus constitute key parameters to infer deep magma budget and dynamics. </span></p><p align="justify"><span>Here, we use several year-long (2014-present) gas observations at Etna and Stromboli volcanoes, in Sicily, to provide new evidence for the utility of long-term instrumental gas monitoring in real-time detecting the early phase of unrest prior eruption, and for characterizing syn-eruptive dynamics. To this aim, we use information from a gas monitoring network </span>of<span> permanent ultraviolet (UV) cameras and automatic Multi-Gas instruments that, combined with geophysical observations, allow characterizing changes in degassing and eruptive dynamics at high temporal/spatial resolution. </span></p><p align="justify"><span>Our results show that the </span><span>paroxysmal (lava fountaining) explosions that periodically</span> <span>interrupted </span><span>persistent</span><span> open-vent activity on Etna (during 2014-2020) were accompanied by systematic, repetitive SO</span><sub><span>2</span></sub><span> emission patterns prior, during, and after eruptions. These allow us identifying the characteristic pre- syn- and post- eruptive degassing regimes, and to establish thresholds in the SO</span><sub><span>2</span></sub><span> flux record that mark phases of unrest. </span></p><p align="justify"><span>On Stromboli, the much improved temporal/spatial resolution of UV cameras allows resolving the escalation of regular strombolian activity, and its concentration toward its North-east crater, that heralds onset of effusive eruptions. During effusive eruption, although magma level drops in the conduit and explosive summit activity ceases, UV camera observations can still detect explosive gas bursts deep in the conduit while no infrasonic activity is detected. </span>Combining the<span> UV camera-derived SO</span><sub><span>2</span></sub><span> fluxes with CO</span><sub><span>2</span></sub><span>/SO</span><sub><span>2</span></sub><span> ratio records measured by the Multi-Gas, the CO</span><sub><span>2</span></sub><span> flux can be inferred. We find that such CO</span><sub><span>2</span></sub><span> flux time-series can allow tracking degassing of deeply stored mafic magma months before Stromboli’s eruptions. We finally show that remotely sensed gas emission and thermal activity can be combined together to characterize the dynamics of shallow magmatic system prior to and during unrest, ultimately helping to define timing of magma re-charging events driving the eruptions. </span></p>

Using rock magnetics to resolve composite magmatic state fabrics: a case study from the Younger Giant Dyke Complex, SW Greenland

<p>Understanding the geometry of magma chambers plays a critical role in determining the igneous petrogenic processes that occur as intrusions cool. Quantitative fabric analysis methods, such as anisotropy of magnetic susceptibility (AMS), are routinely used to measure magma flow dynamics and determine the mechanism of magma transport and emplacement. However, magma mushes typically experience multiple flow events; e.g. emplacement, convection, and interstitial melt percolation. There is thus a need to develop a more a sophisticated approach to unravelling complex rock fabrics that record more than one magmatic state process. This study uses novel rock magnetic datasets to untangle the evolution of the 1163 Ma Younger Giant Dyke Complex (YGDC) of SW Greenland, a multi-sheeted troctolite dyke system that attains widths up to 800 m and encloses several evolved and/or modally layered ovoid pods.</p><p>Field results identify that ovoid pods occur in the thickest dyke segments. Several pods are defined by gently inward dipping modal layers and/or a parallel mineral foliations, and in-phase AMS magnetic foliations lie parallel to the observed field fabrics. Critically, imbricated plagioclase crystals record a magma transport direction toward the center of each pod, and this observation is substantiated by in-phase AMS lineations that plunge down dip of the foliation and shallow toward the center of each pod. These observations are interpreted to show gravitational settling under a convective flow regime.</p><p>In addition, 66% of out-of-phase AMS fabrics are non-parallel with in-phase AMS results. Out-of-phase AMS is a relatively new technique and is thought to reflect anisotropy controlled by a restrictive group of ferromagnetic minerals such as single domain magnetite and pyrrhotite. Out-of-phase lineations in layered pods are relatively steeply inclined and do not shallow towards the center, we therefore hypothesize that these lineations record a late stage filter-pressing process within the crystal mush. To test this hypothesis, anisotropy of anhysteretic remanent magnetism (AARM) data were collected from 15 samples. Results show that the AARM and out-of-phase AMS tensor axes are parallel, indicating that the sub-fabric detected by out-of-phase AMS is normal and most likely controlled by single domain magnetite.</p><p>Our results show that the application of rock magnetic techniques is effective in unravelling magma convection fabrics from later melt migration fabrics in mushy magmas.</p>

Emplacement of the Little Minch Sill Complex, Sea of Hebrides Basin, NW Scotland

The Little Minch Sill Complex is comprised of a series of stacked, multi-leaved Paleocene aged dolerite sills, which have been primarily intruded into Mesozoic sedimentary rocks and Paleocene tuffs/?hyaloclastites within the Sea of Hebrides Basin, situated on the NE Atlantic margin. Two previously proposed models for the emplacement of the sill complex have opposing ideas for the location of magma input and emplacement mechanisms. Both models have been constructed using data primarily from onshore outcrops, located on the Isle of Skye, Raasay and the Shiant Isles. However, onshore outcrops only represent a quarter (1040 km2) of the entire extent of the sill complex, which is largely situated offshore. In order to understand how the sill complex as a whole was emplaced within the basin, both onshore and offshore magma transport needs to be considered. Using high resolution multibeam bathymetry data (up to 2m resolution) obtained between 2008 and 2011 along with supporting seismic reflection, sparker and pinger data, a new assessment of the offshore extent and character of the sill complex has been constructed. Mapping of large-scale relationships between intrusions and the host rock, along with morphological features such as magma lobes, magma fingers, transgressive wings, en-echelon feeder dykes and the axis of saucer/half-saucer shaped intrusions, has indicated magma flow directions within the intrusive network. Assessing the flow kinematics of the sills has provided insights into magma transport and emplacement processes offshore. Combining data from previously mapped onshore sills with data from our newly constructed model for magma emplacement offshore has allowed us to construct a new model for the emplacement of the Little Minch Sill Complex. This model demonstrates that major basin bounding faults may play a lesser role in channelling magma through sedimentary basins than previously thought. Applying the knowledge obtained from this study could further progress understanding of the effect of sill emplacement on fluid flow within volcanic rift basin worldwide, with direct impacts on the exploitation of petroleum and geothermal systems.

Dyke apertures record stress accumulation during sustained volcanism

The feedback between dyke and sill intrusions and the evolution of stresses within volcanic systems is poorly understood, despite its importance for magma transport and volcano instability. Long-lived ocean island volcanoes are crosscut by thousands of dykes, which must be accommodated through a combination of flank slip and visco-elastic deformation. Flank slip is dominant in some volcanoes (e.g., Kilauea), but how intrusions are accommodated in other volcanic systems remains unknown. Here we apply digital mapping techniques to collect > 400,000 orientation and aperture measurements from 519 sheet intrusions within Volcán Taburiente (La Palma, Canary Islands, Spain) and investigate their emplacement and accommodation. We show that vertically ascending dykes were deflected to propagate laterally as they approached the surface of the volcano, forming a radial dyke swarm, and propose a visco-elastic model for their accommodation. Our model reproduces the measured dyke-aperture distribution and predicts that stress accumulates within densely intruded regions of the volcano, blocking subsequent dykes and causing eruptive activity to migrate. These results have significant implications for the organisation of magma transport within volcanic edifices, and the evolution and stability of long-lived volcanic systems.