Dyke Propagation Research Articles

Emplacement and flow dynamics in a small volcanic dyke swarm: The example of Mount Calanna (Etna, Italy)

The emplacement mechanism and flow dynamics of small dyke swarms associated with monogenetic eruptive centres are less well studied than those of large swarms associated with shield volcanoes and flood basalts. This is due to their limited exposure and ease of reworking by later events. However, the study of small dykes helps to better constrain the forces controlling dyke propagation and thus reconstruct the emplacement history of the volcanic system. This study presents a multidisciplinary analysis of Mount Calanna (Mount Etna, Italy) where we sampled a total of 11 dykes among the two groups: 1) sub-vertical dykes from the lower part of the system, and 2) sub-horizontal dykes from the upper part. To infer the magma flow direction, we used Anisotropy of Magnetic Susceptibility (AMS) and compared the results to the Shape Preferred Orientation (SPO) of the plagioclase crystal fabric. To quantify the relative timing of dyke emplacement, and to characterise the magma plumbing system, we also used the palaeomagnetic and geochemical signature of the sampled dykes.The fabric analysis highlights a NE-SW flow direction in the sub-vertical dykes, and a NE-SW and NW-SE direction in the sub-horizontal dykes. Dykes emplaced at the lowest levels in the swarm show sub-vertical to inclined magma flow, whereas those towards the top of the Mount Calanna show inclined to sub-horizontal flow. The combined structural and geochemical analyses suggest the Mount Calanna swarm is a coherent intrusive complex associated with a phase of activity of Ancient Alkaline centre (AAC), with Mount Calanna representing an example of the uppermost portion of a shallow plumbing system in which intrusion orientations are controlled by regional tectonics.

Banding in the Margins of Basaltic Dykes Indicates Pulsatory Propagation During Emplacement

AbstractBasaltic fissure eruptions, which are the most common type of eruption on Earth, are fed by dykes which mediate magma transport through the crust. Dyke propagation processes are important because they determine the geometry of the transport pathway and the nature of any geophysical signals associated with magma ascent. Here, we investigate small‐scale (mm–cm wide) banding features at the margins of dykes in the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (CRBP) (USA). Similar marginal bands have been reported for dykes in numerous localities around the world. Dyke margins record valuable information about propagation because they are the first material to solidify against the host rock at the propagating dyke tip. We find that the marginal bands are defined by cyclic variations in phenocryst concentration and vesicularity, and we infer that these cyclic variations in texture are a product of cyclic variations in magma flow rates and pressures within the dyke tip. This indicates that dyke emplacement occurs in pulses, with propagation repeatedly hindered by the rapid cooling and solidification of magma in the narrow dyke tip. Using a 1D conduction model, we estimate the time taken for each band to cool and solidify, which provides a timescale of several minutes to tens of minutes for the pulses. The occurrence of similar bands in various volcanic settings suggests that pulsatory propagation is a common, if not ubiquitous, process associated with dyke emplacement.

Lithological influence on sill geometry in sedimentary basins: Controls and recognition in reflection seismic data

AbstractSills play a leading role in the transport of magma in sedimentary basins. The contact between sills and host rocks reflects the acting emplacement processes during sill propagation and evolution. Recent studies have shown that the propagation of sills and dykes is strongly influenced by the lithology of the host rocks, but none have detailed documentation of marginal features in large‐scale intrusive complexes. Three‐dimensional seismic data is the primary method of mapping and investigating such complexes, but it is difficult to accurately image sills due to their low thickness compared to seismic resolution. By understanding the relationship between local lithology and marginal sill features, we can better understand the imaging of sills in seismic datasets and their resulting geometry. In this study, we present a seismic‐scale sill analogue through multiple high‐resolution three‐dimensional models, with corresponding logs and field observations from Cedar Mountains, San Rafael Swell, US. This model was further used to develop a synthetic seismic dataset, providing us with a strong control on which marginal sill features fall beneath seismic resolution. We found that lithology plays a critical control in sill geometry and morphology. In Cedar Mountains, sills emplaced within massive sandstones frequently exhibit strata‐discordant base contact with the host rock. Conversely, sills found within heterolithic intervals and mudstones typically display strata‐concordant base contact with the host rocks. Sills within heterolithic intervals also tend to exhibit a more complex segmentation with multiple broken bridges. Furthermore, our findings show that sills are more than 3.7 times more likely to intrude in mudstone compared to sandstone and heterolithic intervals. These results suggest how sill geometries can be adapted to interpret lithology in seismic datasets from sedimentary basins with little to no well control. We anticipate that our findings may provide better knowledge for interpreting sills in sedimentary basins and contribute to developing more sophisticated geomechanical emplacement models for igneous intrusions.

A step forward to understanding the development of volcanotectonic rifts: the structure of the Fremrinamar Fissure Swarm (Iceland)

We analysed all the Holocene structures defining the Fremrinamar Fissure Swarm (FFS), in the Northern Volcanic Zone of Iceland, through the interpretation of aerial photos, orthomosaics and Digital Surface Models (DSMs), and field surveys. We measured the strike, dip, length and kinematics of 761 normal faults and reconstructed the slip profile of 76 main faults (length >2 km), with the purpose of evaluating the overall direction of along-axis rift propagation. We also measured the strike of 146 eruptive fissures and 1,128 extension fractures. A total of 421 faults dip towards the east and 340 dip towards the west, mainly striking N0°-10°E. Maximum fault length is 14.2 km, and W-dipping faults are longer than E-dipping faults. The majority of eruptive fissures strike N10°-20°E, and are concentrated in the southern part of the FFS, around the Fremrinamar central volcano. Extension fractures mainly strike N0°-10°E, with a maximum length of 2,508 m. We evaluated the variation of strike, fracture density and spacing along the FFS, and observed a change of its trend from NNE-SSW in the central-southern part, to NNW-SSE in the northern part. We interpret this evidence as the effect of the intersection with the Grimsey Lineament. The tapering of fault slip profiles indicates a main northward propagation of the rift, and thus of the deformation, interpreted as the effect of lateral propagation of dykes from the magma chamber below the central volcano towards the north. Such interpretation is also supported by the distribution of normal faults, vertical offset and dilation values, and also by the rift width, which tend to decrease towards the north.

Dyke to sill deflection in the shallow heterogeneous crust during glacier retreat: part I

Dykes and sills occupy Mode I (extension), Mode II (shear), or hybrid mode fractures and most of the time transport and store magma from deep reservoirs to the surface. Subject to their successful propagation, they feed volcanic eruptions. Yet, dykes and sills can also stall and become arrested as a result of the crust’s heterogeneous and anisotropic characteristics. Dykes can become deflected at mechanical discontinuities to form sills, and vice versa. Although several studies have examined dyke propagation in heterogeneous and anisotropic crustal segments before, the conditions under which dykes propagate in glacial-volcanotectonic regimes remain unclear. Here, we coupled field observations with 2D FEM numerical modelling to explore the mechanical conditions that encourage (or not) dyke-sill transitions in volcanotectonic or glacial settings. We used as a field example the Stardalur cone sheet-laccolith system, which lies on the Esja peninsula, close to the western rift zone, NW of the southern part of the Icelandic rift. The laccolith is composed of several vertical dykes that transition into sills and form a unique stacked sill ‘flower’ structure. Here, we investigate whether the Stardalur laccolith was formed under the influence of stresses caused by glacial retreat due to thickness variations (0–1 km) in addition to regional and local tectonic stresses (1–3 MPa extension or compression) and varied magma overpressure (1–30 MPa), as well as the influence of the mechanical properties of the lava/hyaloclastite contact. Our results show that the observed field structure in non-glacial regimes was formed as a result of either the mechanical (Young’s modulus) contrast of the lava/hyaloclastite contact or a compressional regime due to pre-existing dykes or faulting. In the glacial domain, the extensional stress field below the ice cap encouraged the formation of the laccolith as the glacier became thinner (subject to a lower vertical load). In all cases, the local stress field influenced dyke to sill deflection in both volcanotectonic regimes.

Modeling the Shape and Velocity of Magmatic Intrusions, a New Numerical Approach

AbstractDykes are magma‐filled fractures propagating through the brittle crust. Understanding the physics of dyking process is essential to mitigate the volcanic hazard associated with the opening of new eruptive fissures at the surface. Often, physics‐based models view either fracturing of the host rock or viscous flow of the magma as the dominating energy sink during dyke propagation. Here, we provide a numerical model that captures the coupling of fracturing at the crack tip and the transport of a viscous fluid. Built with the boundary element technique, our model allows for computation of the shape and velocity of a growing fluid‐filled crack accounting for the viscosity of the fluid: the fluid flow induces a viscous pressure drop acting at the crack walls, and modifies the shape of the crack. The energy conservation equation provides the constraints to solve for the crack growth velocity, assuming that brittle fracturing and viscous flow are the main processes that dissipate energy. Using a parameter range that represents typical magmatic intrusions, we obtain crack shapes displaying some typical characteristics, including a tear‐drop head and an open tail that depend on rock rigidity, magma viscosity, and buoyancy. We show that viscous forces significantly contribute to the energy dissipated during the propagation of magmatic dykes. Applied to the 1998 intrusion at Piton de la Fournaise (La Réunion Island), we provide ranges of dyke lengths and openings by adjusting the numerical velocity to the one deduced from the migration of volcano‐tectonic events.

Joint inversion of tectonic stress and magma pressures using dyke trajectories

AbstractIn volcano-tectonic regions, dyke propagation from shallow magmatic chambers is often controlled by the interaction of the local and regional stress fields. The variations of the stress fields result from a combination of factors including the regional tectonic stress, the geometry of pressurized magma chambers, the layering and the pre-existing discontinuities (e.g. fractures). In this contribution, we describe and apply a new multiparametric inversion technique based on geomechanics that can invert for both the far field stress attributes and the internal pressure of magma chambers or stocks, constrained by observed dyke or eruptive fissure orientations. This technique is based on the superposition principle and uses linear elastic models that can be solved using many types of numerical methods. For practical reasons, we chose a 3D boundary element method (BEM) for a heterogeneous elastic half-space, where magma chambers are modelled as pressurized cavities. To verify this approach, the BEM solution has been validated against the known 3D analytical solution of a pressurized cylindrical cavity. Then the effectiveness of this technique and its practical use is demonstrated through application to natural examples of dyke network development around two different volcanic systems, the Spanish Peaks (USA) and the Galapagos Islands (Ecuador). Results demonstrate that regional stress characteristics as well as the internal pressure of magma chambers can be estimated from observed radial and circumferential dyke patterns and some knowledge of magma chamber geometry.

Swarm seismicity illuminates stress transfer prior to the 2021 Fagradalsfjall eruption in Iceland

The 2021 Fagradalsfjall volcanic eruption in the Reykjanes Peninsula, Iceland, was followed by effusive lava outflow lasting six months. It was preceded by an intensive earthquake swarm lasting one month with the largest earthquake exceeding ML 5. We analyze seismic data recorded by the Reykjanet local seismic network to trace the processes leading to the eruption to understand the relation between seismic activity and magma accumulation.Precise relocations show two hypocenter clusters of the 2021 swarm in the depth range of 1-6 km; a NE-SW trending cluster that maps the dyke propagation, and a WSW-ENE trending cluster that follows the axis of the oblique plate boundary. Additionally, we relocated the preceding earthquake swarms of 2017, 2019 and 2020 and found that they form two branches along the oblique plate boundary, which coincide with the WSW-ENE trending cluster of the 2021 swarm. These branches form a stepover of ∼1 km offset, forming a pull-apart basin structure at the intersection with the dyke. This is the place where the eruption occurred, suggesting that magma erupted at the place of crustal weakening.The strong seismic activity started with a ML 5.3 earthquake of 24 February 2021, which triggered the aftershocks on the oblique plate boundary and in the area of magmatic dyke, both in an area of elevated Coulomb stress. The seismicity shows a complex propagation of the dyke, which started at its northern end, migrated southwestward and then jumped back to the central part where the effusive eruption took place. The observed N-S striking focal mechanisms are interpreted as right-lateral antithetic Riedel shears that accommodate the left lateral slip along the oblique plate boundary. The co-existence of seismic and magmatic activity suggests that the past seismic activity weakened the crust in the eruption site area, where magma accumulated. The following ML 5.3 earthquake of 24 February 2021 triggered the seismic swarm and likely perturbed the magma pocket which led to the six-months lasting eruption that started on 19 March.

Reactivation of Magma Pathways: Insights From Field Observations, Geochronology, Geomechanical Tests, and Numerical Models

AbstractField observations and unmanned aerial vehicle surveys from Caldera Taburiente (La Palma, Canary Islands, Spain) show that pre‐existing dykes can capture and re‐direct younger ones to form multiple dyke composites. Chill margins suggest that the older dykes were solidified and cooled when this occurred. In one multiple dyke example, an 40Ar/39Ar age difference of 200 kyr was determined between co‐located dykes. Petrography and geomechanical measurements (ultrasonic pulse and Brazilian disc tests) show that a microscopic preferred alignment of plagioclase laths and sheet‐like structures formed by non‐randomly distributed vesicles give the solidified dykes anisotropic elastic moduli and fracture toughness. We hypothesize that this anisotropy led to the development of margin‐parallel joints within the dykes, during subsequent volcanic loading. Finite element models also suggest that the elastic contrast between solidified dykes and their host rock elevated and re‐oriented the stresses that governed subsequent dyke propagation. Thus, the margin‐parallel joints, combined with local concentration and rotation of stresses, favored the deflection of subsequent magma‐filled fractures by up to 60° to form the multiple dykes. At the edifice scale, the capture and deflection of active intrusions by older ones could change the organization of volcanic magma plumbing systems and cause unexpected propagation paths relative to the regional stress. We suggest that reactivation of older dykes by this mechanism gives the volcanic edifice a structural memory of past stress states, potentially encouraging the re‐use of older vents and deflecting intrusions along volcanic rift zones or toward shallow magma reservoirs.

Beyond elasticity: Are Coulomb properties of the Earth's crust important for volcano geodesy?

Geodetic modelling has become an established procedure to interpret the dynamics of active volcanic plumbing systems and magma transfer within the crust. Most established geodetic models implemented for inverting geodetic data share similar physical assumptions: (1) the Earth's crust is modelled as an infinite, homogeneous elastic half-space with a flat surface, (2) there is no anisotropic horizontal stress to simulate tectonic stresses, (3) the source boundary conditions are kinematic, i.e. they account for an instantaneous inflation or deflation of the source. Field and geophysical observations, however, provide evidence that significant inelastic shear deformation of the host rock can accommodate the propagation of dykes and sills. We show that inelastic processes accommodating the emplacement of dykes in the brittle crust have large implications for dyke-induced surface deformation patterns. We present two quantitative laboratory experiments of dyke emplacement, during which the syn-emplacement surface deformation is monitored. In one experiment, the host material is elastic gelatine, whereas in the other experiment the host material is cohesive Coulomb, plastic silica flour. Even if both experiments produce sub-vertical dykes of similar shapes, their emplacement mechanisms and their associated surface deformation strongly differ. In the gelatine experiment, the dyke propagates as a tensile fracture in a dominantly elastic host, and the surface deformation exhibits two uplifting bulges separated by a trough parallel to, and above the apex of, the underlying dyke. Conversely, in the silica flour experiment, the dyke propagates as viscous indenter through a dominantly plastic host, and the surface deformation exhibits a single uplifting area that narrows through time. The comparison of our experiments shows that (1) plastic deformation (e.g., shear failure, compaction) of the host has large effects on dyke-induced surface deformation patterns and needs to be considered in geodetic models, and (2) dyke emplacement mechanisms matter in geodetic modelling, strongly suggesting that commonly used kinematic geodetic models such as the opening rectangular dislocation model (Okada 1985) are limited for revealing the physics and dynamics of volcano plumbing systems. Finally, our silica flour experiment shows that pure uplift geodetic signals can result from the emplacement of a dyke emplaced as viscous indenter, whereas such signals are commonly modelled using geodetic models of inflating spherical/elliptical or horizontal planar source. Our experiments call for the design of new geodetic models that account, even partly, for the plastic deformation component of the Earth's brittle crust.

Along-rift propagation of Pleistocene-Holocene faults from a central volcano

The mechanisms of rift propagation are still not fully understood, especially at mid-oceanic ridges, owing to the inherent difficulty in collecting submarine data. Here, we investigate the fault slip profiles of the 60-km-long Theistareykir rift (northern Iceland) that may suggest the direction of along-axis rift propagation. This is one of the few places on Earth where rifting processes and mid-oceanic ridge formation can be studied directly. Moreover, this rift hosts an active central volcano, and this enables to fully understand the relations between rift propagation and magma systems. We reconstructed the slip profiles of all the 281 main Pleistocene-Holocene faults that compose this N-S rift, by merging measurements performed in the field, collected by Unmanned Aerial Vehicle surveys, and derived from Digital Surface Models. Results indicate that north of the volcano, 75% of the asymmetric faults propagated northward; the value increases to 82% if the cumulated fault length is considered. South of the volcano, 47% of the asymmetric faults propagated southward, 54% if the cumulated fault length is considered. These data point to a dominant mechanism of along-axis propagation of the rift outward from the volcano, suggesting a genetic link with the underlying magma chamber. Two possible processes are suggested, which can also occur in combination: i) faults develop following lateral dyke propagation outward from the magma chamber, and ii) faults nucleate near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux. The rift architecture is complicated by the presence of tectonic zones with different dominant fault dips, separated by transversal accommodation zones. The latter also play the role of barriers guiding local fault propagation.

Crystals reveal magma convection and melt transport in dyke-fed eruptions

The processes and ranges of intensive variables that control magma transport and dyke propagation through the crust are poorly understood. Here we show that textural and compositional data of olivine crystals (Mg/Fe, Ni and P) from the tephra of the first months of Paricutin volcano monogenetic eruption (Mexico, 1943–1952) record fast growth and large temperature and oxygen fugacity gradients. We interpret that these gradients are due to convective magma transport in a propagating dyke to the Earth’s surface in less than a few days. The shortest time we have obtained is 0.1 day, and more than 50% of the calculated timescales are < 2 days for the earliest erupted tephra, which implies magma ascent rates of about 0.1 and 1 m s−1. The olivine zoning patterns change with the eruptive stratigraphy, and record a transition towards a more steady magma flow before the transition from explosive to effusive dynamics. Our results can inform numerical and experimental analogue models of dyke propagation, and thus facilitate a better understanding of the seismicity and other precursors of dyke-fed eruptions.

Preliminary anisotropy of magnetic susceptibility studies on 2367 Ma Bangalore-Karimnagar giant dyke swarm, southern India: Implications for magma flow

Anisotropy of magnetic susceptibility (AMS) fabrics have been studied for the first time on a well constrained Paleoproterozoic Bangalore-Karimnagar giant mafic dyke swarm at ca. 2367 Ma in the Dharwar craton, south India. A total number of 43 samples from nine sites representing five N-E trending dykes have been subjected to AMS and conventional rock magnetic studies. The magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are >10−3 SI, which is further supported by Isothermal Remanent magnetization (IRM) studies. Hysteresis loops, Isothermal remanent magnetisation (IRM), thermomagnetic properties and optical observations indicate that pseudo single domain (PSD) low-titanium magnetite particles dominate the low field susceptibility of samples. The magnetic fabrics results correspond well with the comprehensive field studies. However, the degree of anisotropy results ranges from 0.4% to 12.8%, with a mean total anisotropy of 4.7%. The principal AMS axes for samples from all the sites are tightly clustered on equal area plots, and two main fabrics were revealed in the Bangalore-Karimnagar giant dyke swarm. In the first, the maximum susceptibility axes is sub-horizontal and parallel to the dyke walls, minimum susceptibility axes plot near the pole of the dyke. It is interpreted as the primary magma flow direction. A second fabric with maximum susceptibility axis is sub-horizontal and perpendicular to the dyke walls, and is interpreted as a result of vertical compaction at a late stage of crystallization. Preliminary results suggest that dyke propagation in the Bangalore-Karimnagar giant dyke swarm preserve a flow-induced fabric slightly upwards to the NE and magma flow is sub-horizontal.

The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption

SUMMARYOn 10 May 2018, an unprecedented long and intense seismic crisis started offshore, east of Mayotte, the easternmost of the Comoros volcanic islands. The population felt hundreds of events. Over the course of 1 yr, 32 earthquakes with magnitude greater than 5 occurred, including the largest event ever recorded in the Comoros (Mw = 5.9 on 15 May 2018). Earthquakes are clustered in space and time. Unusual intense long lasting monochromatic very long period events were also registered. From early July 2018, Global Navigation Satellite System (GNSS) stations and Interferometric Synthetic Aperture Radar (InSAR) registered a large drift, testimony of a large offshore deflation. We describe the onset and the evolution of a large magmatic event thanks to the analysis of the seismicity from the initiation of the crisis through its first year, compared to the ground deformation observation (GNSS and InSAR) and modelling. We discriminate and characterize the initial fracturing phase, the phase of magma intrusion and dyke propagation from depth to the subsurface, and the eruptive phase that starts on 3 July 2018, around 50 d after the first seismic events. The eruption is not terminated 2 yr after its initiation, with the persistence of an unusual seismicity, whose pattern has been similar since summer 2018, including episodic very low frequency events presenting a harmonic oscillation with a period of ∼16 s. From July 2018, the whole Mayotte Island drifted eastward and downward at a slightly increasing rate until reaching a peak in late 2018. At the apex, the mean deformation rate was 224 mm yr−1 eastward and 186 mm yr−1 downward. During 2019, the deformation smoothly decreased and in January 2020, it was less than 20 per cent of its peak value. A deflation model of a magma reservoir buried in a homogenous half space fits well the data. The modelled reservoir is located 45 ± 5 km east of Mayotte, at a depth of 28 ± 3 km and the inferred magma extraction at the apex was ∼94 m3 s−1. The introduction of a small secondary source located beneath Mayotte Island at the same depth as the main one improves the fit by 20 per cent. While the rate of the main source drops by a factor of 5 during 2019, the rate of the secondary source remains stable. This might be a clue of the occurrence of relaxation at depth that may continue for some time after the end of the eruption. According to our model, the total volume extracted from the deep reservoir was ∼2.65 km3 in January 2020. This is the largest offshore volcanic event ever quantitatively documented. This seismo-volcanic crisis is consistent with the trans-tensional regime along Comoros archipelago.

Rifting Kinematics Produced by Magmatic and Tectonic Stresses in the North Volcanic Zone of Iceland

In the North Volcanic Zone of Iceland, we studied with the greatest possible detail the complete structural architecture and kinematics of the whole Theistareykir Fissure Swarm (ThFS), an N-S-trending, 70-km-long active rift. We made about 7500 measurements along 6124 post-Late Glacial Maximum (LGM) extension fractures and faults, and 685 pre-LGM structures. We have collected the data over the last six years, through extensive field surveys and with the aid of drone mapping with centimetric resolution. In the southern sector of the study area, extension fractures and faults strike mainly N10°-20°, the opening direction is about N110°, and the dilation amount is in the range 0.1-10 m. In the central sector, faults and extension fractures strike mainly N00-10°, the opening direction is N90-100°, and the dilation amount is 0.1-9 m. In the northern sector, extension fractures and faults strike N30-40°, the opening direction is about N125°, and the dilation amount is 0.1-8 m. The variations in strike are attributable to two processes: the interaction with the WNW-ESE-striking Husavik-Flatey transform fault and Grímsey Oblique Rift (Grímsey lineament), and the structural inheritance of older NNE- to NE-striking normal faults. Most extension fractures show a minor strike-slip component: a systematic right-lateral component can be accounted for by the interaction with the WNW-ESE-striking fault zones and the regional, oblique opening of the rift. We regard dyke propagation as a possible cause for the more complex strike-slip components measured at several other fractures. Cumulated dilation and fracture frequency decrease along the rift with distance away from the Theistareykir volcano, situated in the central sector of the ThFS. This is interpreted as a decrease in the number of dykes that are capable of reaching great distances after being injected from the magma chamber.

Magma flow pattern of the 1.78 Ga dyke swarm of the North China Craton during the initial assembly of the Supercontinent Nuna/Columbia: Constraints from rock magnetic and anisotropy of magnetic susceptibility studies

The geodynamic origin of the 1.78 Ga dyke swarm in the North China Craton (NCC) is long debated. It is considered either a post-collision process or paleoplume-related. Constraining its origin is essential for the reconstruction of the NCC position within the supercontinent Nuna (also referred to as Columbia). Here, we report on the rock-magnetism and anisotropy of magnetic susceptibility (AMS) of thirty-three 1.78 Ga dykes, to constrain the magma center of the igneous event. The samples were collected from the entire swarm across the NCC. Magnetite appears to be the dominant magnetic mineral in all the dykes, occasionally with minor maghemite, while the magnetite grain size within the dykes is generally increasing from south to north. AMS results of the dykes portray a magma flow-related fabric. Over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow, while a few have been formed by vertical intrusion. These observations imply that the 1.78 Ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the NCC. During dyke propagation, some localized magma subchambers may have been formed; they could be responsible for the less common vertically intruded dykes and the EW-trending dykes. Therefore, a plume-related tectonic setting for the 1.78 Ga dyke swarm is more compelling. With the eruption center along the margin of the NCC, it may serve as an essential criterion to search for possible counterparts in other tectonic units within Nuna/Columbia to evaluate the proximity of the NCC with them.

Dyke-arrest scenarios in extensional regimes: Insights from field observations and numerical models, Santorini, Greece

During a volcanic unrest period with magma-chamber rupture, fluid-driven fractures (dykes) are injected either from deep reservoirs or shallow magma chambers. Subsequently, the dykes follow propagation paths towards the surface, some eventually reaching the surface to erupt while others become arrested. Here we study dyke paths resulting in eruption or arrest in an excellent 5-km wide exposure of the northern caldera wall of the Santorini volcano in Greece. Mapping of >90 dyke segments shows that they were emplaced in a host rock consisting of layers (of breccia, tuff, scoria, and lava) with a wide variety of mechanical properties. At the contacts, some dykes are arrested or deflected and hence change their propagation paths. Here we combine the field data with numerical models to explore dyke paths resulting in (1) arrest and (2) eruption. We investigate the effect of different host-rock mechanical properties, magmatic overpressures, and tectonic loading on dyke paths. We find that layers with unfavorable local stresses for dyke propagation, namely stress barriers, result from layer stiffness (Young's modulus) contrast and thickness variations and are a common cause of dyke arrest. The study also shows how the details of the dyke path, and eventually dyke-fed eruptions, depend on the mechanical layering and local stresses in volcanoes. The results are of great importance for understanding dyke-propagation paths, and the likelihood of eruption, during unrest periods, particularly in stratovolcanoes fed by shallow chambers, such as Santorini.

Thermal mapping of the 2009 dyke emplacement at Harrat Lunayyir, Saudi Arabia

The 2009 seismic episode at Harrat Lunayyir signalled a renewed geohazard and resulted in a regional dyke that propagated to a very shallow depth (a few hundred metres) below the surface. Since then, there has been an extensive research debate over the potential links between the volcanic/intrusive activity and tectonic processes, particularly because the earthquake swarm and dyke propagation did not eventually result in an eruption. The current study seeks to estimate the relative changes in surface temperatures as a means for detecting an impending dyke-fed eruption or, alternatively, dyke arrest at a shallow depth. An analysis of thermal (infrared) data with a focus on the spatial distribution of land surface temperatures over a longer period of observation may help reveal the link between volcanic activity and dyke propagation. Here, the land surface temperature changes in the centre of Harrat Lunayyir were recorded when the 2009 dyke was propagating toward the surface. The spatial distribution of the land surface temperatures in the area indicated the segmentation of the dyke and suggested the segments were arrested at somewhat different depths below the surface.

Mechanical energy balance and apparent fracture toughness for dykes in elastoplastic host rock with large-scale yielding

SUMMARYThe dynamics of dyke emplacement are typically modelled by assuming an elastic rheology for the host rock. However, the resulting stress field predicts significant shear failure in the region surrounding the dyke tip. Here, we model the dyking process in an elastic-perfectly plastic host rock in order to simulate distributed shear fracturing and subsequent frictional slip on the fracture surfaces. The fluid mechanical aspects of the magma are neglected as we are interested only in the fracture mechanics of the process. Magma overpressure in dykes is typically of the same order of magnitude as the yield stress of the host rock in shear, especially when the pressure effect of volatiles exsolving from the magma is taken into account. Under these conditions, the plastic deformation zone has spatial dimensions that approach the length of the dyke itself, and concepts based on linear elastic fracture mechanics (LEFM) no longer apply. As incremental plasticity is path dependent, we describe two geologically meaningful endmember cases, namely dyke propagation at constant driving pressure, and gradual inflation of a pre-existing crack. For both models, we find that plastic deformation surrounding the fracture tip enhances dyke opening, and thus increases the energy input into the system due to pressure work integrated over the fracture wall. At the same time, energy is dissipated by plastic deformation. Dissipation in the propagation model is greater by about an order of magnitude than it is in the inflation model because the propagating dyke tip leaves behind it a broad halo of deformation due to plastic bending and unbending in the relict process zone. The net effect is that plastic deformation impedes dyke growth in the propagation model, while it enhances dyke growth in the inflation model. The results show that, when the plastic failure zone is large, a single parameter such as fracture toughness is unable to capture the physics that underpin the resistance of a fracture or dyke against propagation. In these cases, plastic failure has to be modelled explicitly for the given conditions. We provide analytical approximations for the propagation forces and the maximum dyke aperture for the two endmember cases, that is, the propagating dyke and the dyke formed by inflation of a crack. Furthermore, we show that the effect of plasticity on dyke energetics, together with an overestimate of magma pressure when interpreting dyke aspect ratios using elastic host rock models, offers a possible explanation for the long-standing paradox that laboratory measurements of fracture toughness of rocks consistently indicate values about two orders of magnitude lower than those derived from dyke observations.

Space- and Ground-Based Geophysical Data Tracking of Magma Migration in Shallow Feeding System of Mount Etna Volcano

After a month-long increase in activity at the summit craters, on 24 December 2018, the Etna volcano experienced a short-lived lateral effusive event followed by a rapid resumption of low-level explosive and degassing activity at the summit vents. By combining space (Moderate Resolution Imaging Spectroradiometer; MODIS and SENTINEL-2 images) and ground-based geophysical data, we track, in near real-time, the thermal, seismic and infrasonic changes associated with Etna’s activity during the September–December 2018 period. Satellite thermal data reveal that the fissural eruption was preceded by a persistent increase of summit activity, as reflected by overflow episodes in New SouthEast Crater (NSE) sector. This behavior is supported by infrasonic data, which recorded a constant increase both in the occurrence and in the energy of the strombolian activity at the same crater sectors mapped by satellite. The explosive activity trend is poorly constrained by the seismic tremor, which shows instead a sudden increase only since the 08:24 GMT on the 24 December 2018, almost concurrently with the end of the infrasonic detections occurred at 06:00 GMT. The arrays detected the resumption of infrasonic activity at 11:13 GMT of 24 December, when tremors almost reached the maximum amplitude. Infrasound indicates that the explosive activity was shifting from the summit crater along the flank of the Etna volcano, reflecting, with the seismic tremor, the intrusion of a gas-rich magma batch along a ~2.0 km long dyke, which reached the surface generating an intense explosive phase. The dyke propagation lasted for almost 3 h, during which magma migrated from the central conduit system to the lateral vent, at a mean speed of 0.15–0.20 m s−1. Based on MODIS and SENTINEL 2 images, we estimated that the summit outflows erupted a volume of lava of 1.4 Mm3 (±0.5 Mm3), and that the lateral effusive episode erupted a minimum volume of 0.85 Mm3 (±0.3 Mm3). The results presented here outline the support of satellite data on tracking the evolution of volcanic activity and the importance to integrate satellite with ground-based geophysical data in improving assessments of volcanic hazard during eruptive crises.