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Field 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.
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1.  Introduction
Dykes are a common means of magma transport in magma plumbing systems (Rivalta etal.,2015). Individ-
ual dyking events can transport magma laterally up to tens to hundreds of kilometers from its point of origin
(e.g., Neal etal.,2018; Sigmundsson etal.,2014), and vertical magma transport through dykes is often in-
ferred to explain recharge of shallow magma chambers (Karlstrom etal.,2010). Understanding likely prop-
agation paths of future dykes is therefore an important component of hazard assessment in volcanic areas.
Dykes propagate as fluid-driven fractures (Gudmundsson,2011). Their propagation and or arrest is con-
trolled by: (1) the magma pressure and buoyancy; (2) the mechanical properties and structure of the host
rock; (3) the stress field into which they intrude; and (4) the temperature and rheology of the magma (Ri-
valta etal.,2015). Theoretical and field studies of dyke propagation have highlighted the importance of
pre-existing structures in the host rock. For example, it is well established that contacts between units
of different elastic stiffness (e.g., lava flows vs. pyroclastic layers) promote dyke arrest and the formation
of sills (Gudmundsson,2005a, 2011; Kavanagh et al.,2006). Similarly, many authors have suggested that
dykes tend to propagate along regional and volcanic faults (e.g., Browning & Gudmundsson,2015; Gaffney
etal.,2007; Valentine & Krogh,2006; van den Hove etal.,2017).
Dykes formed from two or more parallel and co-located intrusions are also commonly observed in volcanic
regions. Extreme examples of these include sheeted dyke complexes exposed in ophiolites (Gass, 1968),
mid-ocean ridges (Stewart etal.,2002), and ocean-island volcanoes (Walker,1992), in which dykes intrude
so densely that they comprise >90% of the rock mass, often making it impossible to distinguish individual
Abstract Field 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.
© 2021. The Authors.
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Reactivation of Magma Pathways: Insights From Field 
Observations, Geochronology, Geomechanical Tests, and 
Numerical Models
Samuel T. Thiele1  , Alexander R. Cruden2  , Xi Zhang3  , Steven Micklethwaite4  , and 
Erin L. Matchan5
1Helmholtz Institute Freiberg for Resource Technology, Helmholtz-Zentrum Dresden-Rossendorf, Freiberg, Germany,
2School of Earth, Atmosphere and Environment, Monash University, Melbourne, VIC, Australia, 3SCT Operations Pty
Ltd, Wollongong, NSW, Australia, 4Sustainable Minerals Institute, University of Queensland, Brisbane, QLD, Australia,
5School of Earth Sciences, The University of Melbourne, Parkville, VIC, Australia
Key Points:
Dykes form significant and highly
oblique mechanical discontinuities
in volcanic edifices
Local stresses and mechanical
anisotropy within solidified dykes
can capture and deflect subsequent
intrusions to form a multiple-dyke
Reactivation of magma pathways by
these mechanisms influences the
emergent organization of magma
plumbing systems
Supporting Information:
Supporting Information may be found
in the online version of this article.
Correspondence to:
S. T. Thiele,
Thiele, S. T., Cruden, A. R., Zhang,
X., Micklethwaite, S., & Matchan,
E. L. (2021). Reactivation of
magma pathways: Insights from
field observations, geochronology,
geomechanical tests, and
numerical models. Journal of
Geophysical Research: Solid Earth,
126, e2020JB021477. https://doi.
Received 4 DEC 2020
Accepted 19 APR 2021
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Journal of Geophysical Research: Solid Earth
dykes. In less extreme cases, internal chilled margins or compositional variations are most simply explained
by multiple co-located magma injections. These dykes are commonly referred to as “multiple dykes”
(known as “composite dykes” when the different injections have contrasting compositions), although for
brevity and to avoid grammatical confusion we prefer to use the term “multi-dyke.
While geochemical variations within multi-dykes have been widely studied (e.g., Ehlers & Ehlers,1977;
Esteve etal.,2014; Gibson & Walker,1963; Kanaris-Sotiriou & Gibb,1985; Sun et al.,2013), literature on
the mechanics of their formation is lacking. Indeed, it is commonly assumed that multi-dykes form either
because the initial dyke did not have time to solidify completely before the subsequent injection, or because
the solidified dyke or its chilled margin were weaker than the host rock it intruded (Ehlers & Ehlers,1977;
Esteve etal.,2014; Gudmundsson,1984; Guppy & Hawkes,1925; Marinoni & Gudmundsson,2000; Plat-
ten,2000; Walker,1992).
Here we present observations of exceptionally well-exposed multi-dykes on the island of La Palma (Canary
Islands, Spain) that appear not to have formed by either of these mechanisms.
2.  The Taburiente Dyke Swarm
The Taburiente dyke swarm is found within the 2–0.5Ma Volcán Taburiente (Carracedo etal.,1999), a
large basaltic edifice that forms the northern portion of the island of La Palma (Canary Islands, Spain; Fig-
ure1a). At the heart of this edifice a deeply incised and eroded collapse-scarp forms a bowl-like depression,
Caldera Taburiente, bounded by spectacular cliffs up to 1km high. This highly eroded landscape provides
exceptional exposure of the volcano's shallow plumbing system, revealing a complete stratigraphic section
through the most recent eruptive products to the edifice basement.
The Taburiente dykes radiate from a focal point in the southern part of the caldera, as described in detail in
Thiele etal.(2020). Along the northern side of the caldera these dykes crosscut an earlier NE striking and
shallower-dipping (45–60°) dyke set interpreted to have formed relatively early in the growth of Volcán
Taburiente (Thiele etal.,2020). Flow lineations indicate sub-horizontal flow both toward and away from the
focal point, suggesting that ascending dykes became radially oriented as they interacted with topographic
loading below the Taburiente edifice and then began to flow laterally to form blade-shaped dykes (Thiele
3.  Field Observations and Mapping
To gain access to the steep and unstable exposures within Caldera Taburiente, images collected via un-
manned aerial vehicle (UAV) were used to construct three-dimensional (3-D) digital outcrop models using
a structure-from-motion multi-view-stereo photogrammetric workflow (SfM-MvS; cf. Bemis et al.,2014;
Dering et al.,2019). These digital outcrop models and details of the methods used to construct them are
described in (Thiele etal.,2019,2020). For this study, we focus on three surveys from a site known locally
as Hoyo Verde (Figure1a), where dykes of different orientations intersect to form multi-dykes (Figure1b).
The dykes at Hoyo Verde intrude volcaniclastic breccia deposits (Figure1c), welded scoria (Figure1d) and
finely laminated palagonite tuffs (Figure1e). Multi-dykes were observed in all of these lithologies. The
volcaniclastic breccias occur in >10m thick, polylithic, poorly sorted, and matrix-rich beds toward the
bottom of the section, but become finer-grained and more well-sorted toward the middle of the section. The
breccias are overlain by thick-bedded scoria deposits. Beds of laminated palagonite tuff of 1–20m thickness
occur in both the breccia and scoria units, and dip 20–30° north-west.
Mapping of the dyke network (Figure2) shows that older, more shallowly dipping (45°) dykes appear to
capture younger intrusions and re-orient them by up to 60°, forming thick multi-dyke bundles typically
characterized by complex cross-cutting relationships. While some of the captured dykes propagate along the
contact of the older dyke, many also propagate along dyke cores. Although the resolution of the models is
not sufficient to accurately track individual dykes through these bundles, dykes are observed to re-emerge
from the tops of the bundles after tens of meters, suggesting that in some cases the capture is transient.
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Journal of Geophysical Research: Solid Earth
The dykes are basaltic, variably vesiculated and sometimes crosscut by 5–15cm spaced cooling joints (Fig-
ure3). Some dykes, typically those with fewer cooling joints, are also crosscut by well-developed sets of
internal margin-parallel joints (MPJs). These MPJs are orthogonal to the cooling joints, and can be observed
in dykes throughout La Palma, though their length and spacing varies greatly: some form shale-like fracture
cleavages (Figure3a) while others persist laterally over many meters and are closely (Figures3b and3c) to
widely spaced (Figure3d). Similar sets of MPJs have also been observed in basaltic dykes from other volcan-
ic islands (Delcamp etal.,2012; Porreca etal.,2006) and the Troodos ophiolite (Kidd & Cann,1974), and are
probably common; our fieldwork suggests they are present in 40% of the dykes on La Palma.
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Figure 1.  Location of La Palma, Hoyo Verde, and sites where dykes were sampled for microstructural and geomechanical testing (red triangles) (a). Cliffs at
Hoyo Verde are crosscut by moderately to steeply dipping dykes (b) that intrude volcaniclastic breccia (c), bedded scoria (d) and finely laminated palagonite
tuff (e). A 30m thick sill crosscuts dipping scoria layers near the top of the section, and breccias at the base of the section contain several 1m thick basaltic
units that could be sills or the cores of lava flows. A lower hemisphere stereographic projection (stereonet) with density contours of poles to the older, shallower
dipping dykes (blue) and steeper cross cutting dykes (red) is provided in (b) for reference, along with a stratigraphic log interpreted from UAV images. A total of
46,715 dyke orientation estimates were extracted from the UAV data using the method of Thiele, Grose, etal.(2019). The average orientation of the cliff is given
by the black triangle.
Journal of Geophysical Research: Solid Earth
The margin-parallel orientation of these joints suggests they are not related to the thermal stresses that
form cooling joints, which should propagate inwards and parallel to the thermal gradient to form joints per-
pendicular to the dyke margin (Budkewitsch & Robin,1994). Hence, their formation remains unexplained,
although Porreca etal.(2006) tentatively suggested that similar joints observed on Mount Somma-Vesuvio
could result from elevated stresses within the dykes during volcanic loading.
Many of the dykes contain non-randomly distributed vesicles, which form margin-parallel sheets at regu-
larly spaced intervals near the dyke margins (Figures4a and4b) or as a single sheet in the dyke core (Fig-
ure4c). MPJs commonly link vesicles within these sheets (Figure4d); similar structures in dykes on Tener-
ife have been described by Delcamp etal.(2012). These authors attribute these vesicle sheets to preferential
4 of 17
Figure 2.  Anastomosing dyke network exposed in a cliff face above Hoyo Verde. Mapping of these using the digital
outcrop model (a) shows a set of shallow-dipping (45° NW) dykes capturing and re-orienting a later generation of
steeply dipping dykes (b, c), The acute intersection angles between the older intrusions and captured dykes have been
measured using structure-normal estimates (cf. Thiele, Grose, etal.,2019) created during the digital outcrop mapping
and are shown in (a).
Journal of Geophysical Research: Solid Earth
degassing pathways that form along straight bands parallel to the dyke margins, although we speculate that
such sheets could form when vesicles adhere to solidifying dyke margins during periods of lower magma
pressure or flux, and in the case of sheets within dyke cores, during the final stages of dyke activity. Regard-
less of the mechanism of their formation, sheet-like concentrations of vesicles would significantly weaken
these parts of the dyke and promote fracture growth, especially if they have been stretched into non-spher-
ical shapes during cooling (Bubeck etal.,2017; Delcamp etal.,2012).
Finally, internal contacts within multi-dykes at many locations have distinct glassy chilled margins up to
5mm thick (Figures5a–5c), suggesting that the external dyke cooled prior to the injection of the subse-
quent intrusion. Some of the external dykes at Hoyo Verde also show peperitic margins (Figure5d), indicat-
ing that the host rocks were water-saturated during the emplacement of the earliest intrusions.
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Figure 3.  A selection of different margin-parallel joint styles observed in dykes throughout La Palma, ranging from
shale-like fracture cleavages (a) to persistent (b) and sometimes imbricated joints (c) and more widely spaced but highly
persistent (tens of meters) “tram-track” joints (d). High resolution versions of these images can be downloaded from
FigShare (Thiele,2020) for closer inspection.
Journal of Geophysical Research: Solid Earth
3.1.  40Ar/39Ar Geochronology
To test our interpretation that the external dykes cooled completely prior to intrusion of the internal ones,
two samples were collected from an accessible multi-dyke at Hoyo Verde for 40Ar/39Ar geochronology. This
location was chosen because (1) neither dyke showed any sign of alteration and (2) both the internal and ex-
ternal dykes could be sampled several meters from the point where they intersect, minimizing the potential
for thermal overprinting of the external dyke by the internal one. Studies of dyke cooling (e.g., Bonneville &
Capolsini,1999) suggest that temperatures at distances of greater than a few meters from a 1m thick dyke
should not exceed the closure temperature of the potassium-argon system, especially if host rock pore-flu-
ids rapidly advect heat away from the dyke.
Following petrographic inspection (Figures 6a and 6b), groundmass concentrates were separated from
crushed samples of HV4 (external dyke) and HV6 (internal dyke) and irradiated according to standard
40Ar/39Ar techniques (see Supporting information for details). Aliquots of irradiated groundmass were ana-
lyzed via the 40Ar/39Ar step-heating method following procedures described by Matchan and Phillips(2014).
Samples from HV4 and HV6 yielded plateau ages of 796±4 ka (MSWD=0.75; P=0.67) and 596±8 ka
(MSWD= 0.53; P=0.71) respectively (Figures6c–6f ). These values are interpreted as the emplacement
ages of the dykes. The 200 kyr difference indicates that the external dyke had completely solidified and
cooled before it was intruded by the younger dyke. A detailed analysis of the robustness and significance of
these Ar-Ar plateau ages is included in the Supporting Information.
3.2.  Dyke Microstructure and Mechanical Properties
To gain further insight into the formation of the MPJs and multi-dykes, oriented samples from the cores and
margins of 10 different dykes were collected at four locations (Figure1a). Thin sections (n=26) oriented
perpendicular to the dyke margins were prepared, along with 22mm3 ultrasonic pulse specimens (n=25)
and pairs of 25mm thick and 45mm diameter Brazilian discs (n=17 pairs). The ultrasonic pulse and Bra-
zilian disc specimens were selected to represent intact rock, taking care to avoid cooling joints and MPJs.
Inspection of the thin sections reveal that most of the dykes contain preferentially aligned and 0.5mm
long plagioclase laths (Figure7a). Near the margins of some dykes, rotation of plagioclase lathes relative
to this preferential alignment suggest the presence of small shear-bands (Figure7b). Imbrication of (and
shearing between) domains of aligned minerals is commonly observed at the margins of igneous intrusions,
and generally attributed to shearing during magma flow (Holness & Humphreys,2003).
MPJs are generally parallel to the plagioclase alignment, although in some cases they appear to have formed
along the (older) shear-bands instead (e.g., Figures7a and7b). The MPJs show no evidence for significant
shear offset which, combined with the irregular fracture surfaces, suggests that they formed as predom-
inantly Mode I fractures. Where vesicle bands are present, MPJs link closely spaced vesicles (Figure 7c).
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Figure 4.  Alternating vesicle-rich and -poor layers (a) and distinct vesicle sheets (b) commonly observed near dyke margins. Dykes with vesiculated cores
(c) were also observed, although less frequently. Margin-parallel joints appear to sometimes form preferentially in the vesicle-rich layers by linking individual
vesicles together (d). High resolution versions of these images can be downloaded from FigShare (Thiele,2020) for closer inspection.
Journal of Geophysical Research: Solid Earth
These plagioclase alignments and MPJs are similar to those observed in basaltic dykes in the Azores (Morei-
ra etal.,2015), Tenerife (Delcamp etal.,2012) and sills on the Isle of Mull (Holness & Humphreys,2003).
Delcamp etal.(2012) interpret that these alignments give basaltic dykes on Tenerife a preferential parting
direction, which they also relate to MPJs. Our observations corroborate this hypothesis.
Results from the ultrasonic pulse tests indicate that the dykes have anisotropic elastic properties (Figure8),
which we attribute to the plagioclase alignment and vesicle distribution, although other structures such
as microfractures could also play a role. P- and S-wave velocities were calculated in three perpendicular
directions by measuring the travel time of 50kHz ultrasonic pulses over distances of 20mm. Proper cou-
pling between the ultrasonic pulse transmitter and each sample was ensured using a coupling agent and by
applying a small force to the transmitter and receiver pads. As the flow direction of the dykes are unknown
7 of 17
Figure 5.  Black, glassy chill margins (a, b, c) suggesting the exterior dyke was cooled prior to intrusion of the interior
one. Black arrows point toward glassy chill margins. The external dyke in (c) contains abundant vesicles, unlike the
interior one. External dyke margins are also glassy (d), and locally develop peperitic textures. The pen used for scale in
(c) and (d) is 14cm long.
Journal of Geophysical Research: Solid Earth
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Figure 6.  Cross-polarised (XP) photomicrographs of the two dykes sampled for geochronology, HV4 (a) and HV6 (b). The resulting heating spectra show that
emplacement (plateau) ages of the external (c) and internal (d) dykes are separated by 200 ka. Plateau steps are green, rejected steps are cyan. Inverse isochron
diagrams (e, f) are also shown for each dyke. Uncertainties are all 2σ.
Journal of Geophysical Research: Solid Earth
and likely variable, measurements in the dyke-parallel direction (which we refer to as L and X) represent an
arbitrary coordinate system within the flow-plane, while the dyke-perpendicular measurements (P-direc-
tion) is consistently perpendicular to the dyke margin (and flow direction).
The ultrasonic pressure- and shear-wave velocities VP and VS were used to estimate the Young's (E) and
shear (G) moduli in each of these directions, using Equations 1 and 2. Bulk-density (p) values for this
calculation were obtained by calculating the buoyancy of each sample in water, as described in detail by
Houghton and Wilson(1989).
22 2
G pV
Paired t-tests found no significant difference between the L- and X-directions, but comparisons with the
P-direction suggest significant differences in both E and, to a lesser extent, G (Table1). This indicates that
the dykes are an average of 8% stiffer under dyke-parallel strains than dyke-perpendicular ones, with
anisotropies of up to 25% measured on individual samples. Anisotropy of Poisson's ratio, which has been
shown to control stress rotation in anisotropic materials (Faulkner etal.,2006; Healy,2008), is also 8% on
average and up to 30% for individual samples.
The Brazilian tests also show significant differences between dyke-perpendicular and dyke-parallel direc-
tions (Table1), with lower tensile strength (unconfined tensile strength, UTS) and fracture toughness (K)
generally observed for failure parallel to the dyke margins (Figure8). Each pair of Brazilian discs were
loaded at 0.02mm/sec in an Instron 5982 100kN testing machine fitted with Brazilian frames such that one
sample failed parallel to the dyke margin (in the L–X plane) and the other perpendicular to it (X–P plane).
Three pairs of samples were discarded as invalid, as one or both samples did not fail along the disc axis but
instead underwent a mixture of shear and tensile fracturing. Peak stress σpeak was recorded and used to esti-
mate the ultimate tensile strength (UTS) in the direction of loading using Equation3 and measurements of
the samples' diameter D (45mm) and thickness t (±25mm).
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Figure 7.  Plane-polarized (PPL) photomicrographs showing plagioclase alignments (a) and shear bands (b) observed due to the alignment of plagioclase laths
within dykes from various locations on La Palma. The open fractures in each of these (MPJs) are parallel to dyke strike, and are unfilled (except by epoxy during
sample preparation). These appear to have been influenced by the plagioclase alignment, which would give the dykes a preferential parting direction (Delcamp
etal.,2012), and sheet-like domains of high vesicularity (c). High resolution versions of these images can be downloaded from FigShare (Thiele,2020) for closer
Journal of Geophysical Research: Solid Earth
Post-failure residual stress was also recorded and, using the methodology and calibration curves presented
by Guo etal.(1993), the magnitude of the stress-drop at failure of the Brazilian disc was used to estimate
K. While this method does not give as accurate a measure of K as more conventional tests (e.g., three-point
bend; Kuruppu etal.,2014), it is sufficient to provide a qualitative comparison of K in dyke-perpendicular
and dyke-parallel directions.
Specimens with MPJs were not tested due to the practical difficulty of preparing samples containing frac-
tures, however other studies have shown that fracturing induces significant elastic anisotropy (e.g., Heap
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Figure 8.  Geomechanical tests conducted using ultrasonic pulse velocity measurements (a) and Brazilian disc tests (b).
Young's modulus (a), tensile strength (b), shear modulus (c) and fracture toughness (d) were measured in directions
perpendicular and parallel to dyke orientation and associated flow fabrics. The results show that the dykes have larger
tensile strengths and fracture toughness in perpendicular directions and higher elastic moduli in parallel directions.
These differences make it easier to propagate fractures parallel to the dyke margins and increase the energy required for
cross-cutting fractures. Outliers (gray) were excluded when calculating the trend (dashed blue line). Equal values for
the parallel and perpendicular directions fall on the solid gray line.
Journal of Geophysical Research: Solid Earth
etal.,2009; Heap etal.,2010), and it seems reasonable to assume that a similar or even more pronounced
effect may be expected for fracture toughness and tensile strength.
4.  Discussion
Based on our observations at Hoyo Verde, we hypothesize that (1) solidified dykes form important mechani-
cal discontinuities in basaltic volcanoes, and (2) the mechanical contrast between these dykes and the rocks
they intrude can explain the formation of the MPJs and multi-dykes. Mechanical layering and anisotropy
in volcanic rocks has previously been recognised as a significant control on dyke propagation and volcano
deformation (Gudmundsson, 2005b, 2012; Gudmundsson & Brenner, 2001; Kavanagh et al.,2006). Giv-
en this context, we use our observations to evaluate hypotheses (1) and (2) in the following sections, and
present some preliminary modeling results that explore their physical plausibility. Finally, we suggest that
multi-dykes represent reactivated magma pathways, and explore implications of this reactivation for the
organization of volcanic plumbing systems and the distribution of volcanic risks.
4.1.  Stress-Concentration, Rotation, and the Formation of MPJs
The dykes at Hoyo Verde are much stiffer than the generally compliant breccia and pyroclastic tuff they
cross-cut. These volcanogenic rocks were not directly tested, but similar tuffs and scoria from Gran Canaria
and Tenerife have Young's moduli of <1–20GPa and shear moduli of <1–8GPa (de Vallejo etal.,2008;
Rodríguez-Losada etal.,2009), making them 3–15 times more compliant than the dykes (E35–70GPa,
G15–25GPa; Section3.2). This elastic contrast will cause stress concentration within the dykes during
volcanic inflation-deflation cycles and progressive gravitational loading, and, due to their oblique orienta-
tion, rotation of the principal compressive stress (σ1) toward parallelism with the dyke margins (Figure9).
The tendency for the surrounding, poorly cemented, granular rocks such as tuff and volcanic breccia to un-
dergo inelastic deformation (de Vallejo etal.,2008; Heap etal.,2014; J. S. Lee etal.,2012) would exaggerate
this effect, as would the dykes' elastic anisotropy (Section3.2). The distribution of stress within a stiff, inter-
connected network of solidified dykes embedded in 3-D within more compliant host rock is thus expected
to result in a complex and heterogeneous distribution of stress, akin to engineered composite materials.
As previously suggested by Porreca etal.(2006), we hypothesize that the stress concentration is sufficient
to initiate and drive joint propagation parallel to the dyke margins, facilitated by internal margin-parallel
structures such as plagioclase alignments and vesicle sheets. The formation of these initial fractures would
have enhanced the dyke's elastic anisotropy, further rotating the maximum compressive stress (Faulkner
etal.,2006; Heap etal.,2009,2010) toward parallelism with the dyke margins.
Interestingly, dykes with well-developed cooling joints were rarely observed to have MPJs. This suggests
that the presence of cooling joints inhibits the rotation of the principal compressive stress by reducing the
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Mean difference Mean anisotropy Standard deviation p-value
Young's modulus (E)
X 4.6GPa 9% 4.9GPa 2.3×10−4
L 4.4GPa 8% 3.75GPa 1.4×10−5
Shear modulus (G)
X 1.6GPa 9% 2.2GPa 0.002
L 1.35GPa 7% 1.69GPa 0.003
Tensile strength (UTS) 3.5MPa 26% 2.4MPa 6.6×10−5
Fracture toughness (K) 0.35MPa.m0.5 17% 0.44MPa.m0.5 0.02
Note. Differences are all significant at a 0.05 level based on p-values produced from the paired t-tests.
Table 1 
Paired t-Tests Comparing Dyke-Parallel Versus Dyke-Perpendicular Elastic Moduli, Tensile Strength, and Fracture
Journal of Geophysical Research: Solid Earth
bulk stiffness of the dyke, while blunting of MPJ fracture tips along margin-perpendicular cooling joints
would further suppress their growth.
4.2.  Anisotropy and the Formation of Multi-Dykes
Regardless of their mechanism of formation, the MPJs reduce the dyke-parallel fracture toughness while
encouraging the arrest of dyke-cross-cutting fractures. Similar anisotropy of fracture toughness has been
well documented in shales, and is known to divert fractures along the fabric (Chandler etal.,2016; Forbes
Inskip etal.,2018; H. P. Lee etal.,2015). Microstructural layering has also been shown to increase the frac-
ture toughness and strength of mollusc shells by several orders of magnitude, due to the arrest or deflection
of cross-cutting fractures (Kamat etal.,2000).
This anisotropy, combined with the rotation and increase of σ1 described in Section4.1, can explain the
observed capture and re-orientation of new dykes along older ones. Where dyke margins were weak, in-
creased stress within the older dyke would have encouraged the arrest of the cross-cutting dyke followed
by propagation along the older dyke's margin (e.g., the left-hand dyke in Figure2c). Where dyke margins
were well-bonded, the favored propagation path may instead have followed MPJs within the dyke, to form a
multi-dyke with internal chill-margins (e.g., Figure5), perhaps encouraged by delamination and opening of
the MPJs (Cook & Gordon,1964). The abundance of multi-dykes on La Palma (and elsewhere in the Canary
Islands) suggest that intrusions can remain captured over long distances (102–103m).
4.3.  Numerical Modeling
As a preliminary investigation of these hypotheses, we have used Irazu (Lisjak etal.,2018) to construct 2-D
plane-strain finite element (FE) models that evaluate the stresses within and around 2 m thick solidified
dykes within a 100×50m domain. The dykes dip at angles of 30–75° (Figure9) and were given a Young's
modulus of 25GPa, 2.5 times stiffer than the surrounding host rocks (10GPa). A Poisson's ratio of 0.25 was
used for both the dyke and host rock. Boundary conditions correspond to the lithostatic stress at 1km
depth, producing lateral confinement and 25MPa overburden pressure). The corresponding stress field
under the given boundary conditions is obtained through the FE models.
Next, the propagation of a new fracture through this initial stress field was simulated using the bounda-
ry-element linear-elastic fracture mechanics code developed by Zhang etal.(2014) and initial stress fields
obtained by FE models. New magma was injected at 0.01m3/sec per unit of dyke thickness along an initially
10m long vertical fracture at the base of the model. The distribution of fluid pressure within the new dyke
and corresponding elastic stress induced in the surrounding host rock was calculated at each timestep, and
12 of 17
Figure 9.  Finite element (Irazu) model of the initial-stress within and around a 2m thick dyke dipping at 30–75° within compliant host rock (a–d) under an
applied overburden stress of 25MPa and lateral confinement, equivalent to 1km depth. Each model extends 100m horizontally and 50m vertically, although
(a–d) only show a region of 20×20m to clearly show the rotation (thin black lines show the orientation of σ1) and increase in stress-magnitude caused
by stress-concentration within the solidified dyke. The stress concentration is maximized when the dyke is vertical (parallel to the loading direction), and
minimized when it is horizontal (perpendicular to the loading direction). This initial stress rotation causes the deflection of a subsequent intrusion (thick black
line) modeled using the linear-elastic fracture mechanics code described by Zhang etal.(2014). Note that these models do not include the effects of anisotropic
elastic moduli or fracture toughness, which would increase the amount of deflection (see text for details). Colors show the magnitude of the maximum
principal compressive stress σ1 (compressive stresses are positive).
Journal of Geophysical Research: Solid Earth
fracture growth triggered when the maximum stress intensity factor in one orientation reaches a critical
value equal to the mode I fracture toughness, which we set at 10MPa·m0.5 for both the dyke and the host
rock. Due to numerical limitations, we use relatively low elastic contrasts (2.5 times) and magma viscosity
(1Pa·sec). These models also do not include the previously described anisotropic elastic properties or frac-
ture toughness, so give minimum estimates for the amount of deflection that might be expected. In addi-
tion, actual magma injection conditions are likely to vary in rate and overpressure with time, but a constant
injection rate is used in this study.
Despite these simplifications, the models clearly show that (1) stress is concentrated and rotated in the
solidified dyke, and (2) this causes deflection of the cross-cutting fracture (Figure9). The amount of deflec-
tion varies from 1 to >10m depending on the dip of the solidified dyke, with smaller intersection angles
causing longer offsets. Browning and Gudmundsson(2015) developed a similar mechanical model to show
that stiff dykes within compliant fault damage zones can also capture intrusions by this mechanism.
The modeled geometries generally match those observed in the field, although with an order of magnitude
less offset (Figure2). We attribute this difference to the influence of (1) larger elastic contrasts, (2) elastic
anisotropy enhancing rotation of the principal stress, and (3) blunting and deflection of the cross-cutting
dyke tip along MPJs. The 2-D plane-strain geometry used in our model is also a significant simplification,
as most of the dykes we observed showed evidence of lateral propagation (Thiele etal.,2020). Nevertheless,
the results suggest that a mechanical explanation for the multi-dyke formation (Section4.2) is reasonable.
4.4.  Implications of Reactivation for Magma Transport and Associated Hazards
By influencing the propagation path of intrusions and other fractures, solidified dykes give volcanic edifices
a “structural memory” of past stress states. Deflection of active dykes along older ones (i.e., reactivation)
will cause them to become misoriented with respect to regional or topographic far-field stress, and hence
lead to unexpected propagation paths. This reactivation will also encourage the self-organization of the
magma plumbing system, promoting re-use of older vents and directing dykes along rift zones or toward
shallow magma reservoirs.
The importance of dykes as structural discontinuities that favor strain localization onto volcanic rift zones
has been proposed by several authors. While discussing volcanic rift zones in the Canary Islands, Carrace-
do(1994) suggested that solidified dykes in the core of rift zones force active ones into parallelism with the
rift. Similarly, Walker(1992) suggested that dyke margins form structural weaknesses that provide prefer-
ential propagation pathways in the densely packed dyke-swarms beneath rift zones in the Hawaiian islands.
While the mechanics of our model for multi-dyke formation are somewhat different, treating dykes as
mechanically competent units rather than structural weaknesses, the overall effect will be the same: dykes
will tend to be re-directed along earlier volcanic rift zone dykes.
Similarly, multi-dykes formed by the mechanisms proposed above could focus dykes into shallow magma
chambers. Due to their greater stiffness, solidified dykes will carry stresses induced by a pressurized magma
chamber to a greater distance than more-compliant host rock, guiding dykes toward the chamber (Karl-
strom etal.,2009). Reactivation of older dykes to form multi-dykes would further enhance these processes,
assuming the older dykes also intersect the chamber. By increasing the probability of magma recharge in
this manner, a volcano might support smaller shallow magma chambers than otherwise expected. Similarly,
the chance of a batch of magma interacting with or assimilating older magma would also increase, allowing
for greater geochemical diversity of erupted products.
We speculate that it is also possible that dykes reactivating older intrusions to form multi-dykes are less
likely to be arrested at bedding interfaces, because the stiff pre-existing intrusion (and local stress field
within it) reduces the stress change experienced by the younger dyke as it passes between stratigraphic units
with different mechanical properties. A partially arrested dyke was observed at one location (Figure 10),
corroborating this hypothesis. Many studies have demonstrated that an anisotropy oriented at a high an-
gle to the dyke propagation direction (e.g., bedding) increases the chance of dyke arrest (e.g., Gudmunds-
son,2005a,2005b). However, to our knowledge, the effect of stiff mechanical discontinuities with similar
orientation to the propagation direction (e.g., older, solidified dykes) has received much less attention. If
the presence of these older intrusions reduces the chance of dyke arrest, then eruption (rather than arrest)
13 of 17
Journal of Geophysical Research: Solid Earth
becomes more likely in areas that have already been extensively intruded, such as along rift zones, promot-
ing localization of eruptive activity. Conversely, if the accommodation of previous intrusions increases the
stress contrast between stiff and compliant stratigraphic units, then dyke arrest becomes likely and eruption
from previously intruded regions is inhibited due to the formation of a “stress plug” (cf., Thiele etal.,2020).
We suggest that the interactions between concordant (bedding parallel) and discordant (highly oblique)
mechanical discontinuities, and their influence on dyke propagation, is a fertile avenue for future research.
Finally, an intrusion propagating along an older, solidified feeder-dyke might be expected to erupt in close
proximity to old vents. Vents and fissures formed during the Holuhraun eruption (Iceland) sometime be-
tween 1794 and 1864 were re-used by the 2014 Bárðarbunga eruption (Sigmundsson etal.,2014), which
Ruch et al. (2016) argue is evidence that the dyke that fed the eruption intersected and was captured by
a pre-existing fissure. While indistinguishable based on the available evidence, these observations could
equally be explained by the formation of a multi-dyke. Similarly, Carracedo etal.(1996) describe the 1677
Fuencaliente eruption, during which the main Strombolian vent formed on the much older (>3.3 ka) San
Antonio scoria cone. Although plausibly a coincidence, the location of this vent could also be elegantly
explained by the presence of a multi-dyke.
5.  Conclusions
We propose that solidified dykes form highly discordant mechanical discontinuities in volcanic edifices.
Where dykes are stiffer than the material they intrude, local stress concentration and re-orientation can
favor the formation of dyke-parallel fractures, including MPJs and multi-dykes. Plagioclase alignments and
other internal structures in the solidified dykes, including MPJs that form during volcanic inflation/defla-
tion cycles, result in significant mechanical anisotropy that further encourages reactivation to form mul-
ti-dykes. Field observations of dykes on La Palma suggest that these processes can result in the capture and
re-orientation of dykes by up to 60°. Multi-dykes formed in this way are thus a type of mechanical memory,
as dykes intruded during previous volcanic activity and potentially different stress fields can influence fu-
ture dyke-propagation paths. Over longer timescales these processes influence the emergent structure of
shallow volcanic plumbing systems by capturing and redirecting dykes along rift zones, toward shallow
magma chambers, and possibly even old vents.
14 of 17
Figure 10.  Photograph (a) and interpretation (b) of a partially arrested multi-dyke. The first dyke was arrested after
propagating through compliant bedded scoria into a stiff lava flow. A younger intrusion that propagated along the older
dyke (to form a multi-dyke) was not arrested, possibly because the older dyke reduced the stiffness contrast as it crossed
the interface. For a 3-D view of this outcrop the reviewer is referred to the digital outcrop models described by Thiele
etal.(2020), and available for download on Figshare (Thiele, Cruden, & Micklethwaite,2019).
Journal of Geophysical Research: Solid Earth
Data Availability Statement
The geomechanical and geochronology datasets presented in this work are available in the Supplemen-
tary Material. UAV surveys and the derived digital outcrop models can be downloaded from https://doi.
org/10.26180/5d688c17f2ed2. Geomechanical and geochronology data can be downloaded from https://doi.
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The authors gratefully acknowledge
the staff at Parque Nacional Caldera de
Taburiente for their generous support
and hospitality during collection of
the field data presented in this study.
ST was supported by a Westpac Future
Leaders Scholarship and Australian
Postgraduate Award. ARC's research on
magma plumbing systems is supported
by Australian Research Council Discov-
ery Grant DP190102422. The University
of Melbourne Ar-Ar Laboratory receives
support under the AuScope program of
the National Collaborative Research In-
frastructure Strategy (NCRIS). Finally,
we would like to thank John Browning,
Dave Healy, Hans Jørgen Kjøll and
an anonymous reviewer for their con-
structive and helpful comments. Open
Access funding enabled and organized
by Projekt DEAL.
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... Another hypothesis for the pore pressure abnormal regime against hot shale section is that the study area is situated on mega-shear zones (Deramchi et al., 2020;Araïbia et al., 2022), and thus constitutes metamorphic fluid circulation zones. The fluid circulation results in an increase in permeable porosity and high-stress concentration (Cas et al., 2017;Thiele et al., 2021). ...
This work aims to determine the present-day in-situ stress and pore pressure in the Ahnet Basin, Algeria, through a 1D geomechanical approach. We investigated the drilling-induced fracture (DIF) from FMI log data to ascertain the direction of maximum horizontal stress (SHmax) from the Ahnet Field. A mean orientation of N140° ( ± 10°) has been interpreted, which is NW-SE (N140°-N320°), with a local variation of ( ± 20°) compared to fields such as Illizi, Hassi Messaoud, and North Algeria, which can be explained by depth variation and intrinsic rock properties. A 1.05 psi/ft gradient of overburden stress (Sv) has been obtained from density. Pore pressure has been estimated from the sonic log by a normal compaction trend, indicates a hydrostatic regime from the surface to the top of the Silurian unit with an average pore pressure gradient of 0.43 psi/ft and an overpressure regime against the hot shale unit with a gradient of 0.58 psi/ft caused by the high in situ temperature in the study area and possible activity of the mega-shear zone. The poroelastic approach under transverse isotropic vertical conditions (VTI) has been used to calculate the minimum and maximum horizontal stress magnitudes. The outlines indicate a high-stress gradient close to 0.82 psi/ft, for Shmin calibrated with MDT stress points and 1.10 psi/ft for SHmax. The stress magnitudes results, suggest a present-day normal to strike-slip stress regime in the Ahnet Basin. Fault reactivation potential at two Silurian units has been inferred from the frictional theory analysis. The results indicate that increased pore pressure in hot shale formations due to by fluid injection and hydraulic fracturing causing shear slippage of the pre-existing faults, resulting in induced seismicity. Our study has contributed to the understanding of stress state origin in Ahnet Basin, the relationship between in situ temperature and pore pressure, and the fault stability analysis in such unconventional reservoir development.
... Dykes normally make their own paths, but some become deflected into pre-existing fractures such as faults or joints (e.g. Delaney et al. 1986;Gudmundsson 1999Gudmundsson , 2011Gudmundsson , 2020Ebinger et al. 2008;Le Corvec et al. 2018;Thiele et al. 2021;Drymoni et al. 2021). Hence, field observations of dyke-fault relationships are essential for understanding dyke-path formation and the likelihood of a volcanic eruption (Gudmundsson 2020). ...
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Volcanic and tectonic activities in the Aegean region have controlled the evolution of Santorini volcano, including changes in the shape and size of the island through time. Previous studies associate much of the island’s volcanic activity with the presence of regional faults, but a comprehensive volcanotectonic study that clarifies the relationship between dyking and faulting in the island has not been made. Here we present a detailed structural analysis focused on the northern caldera wall of Santorini, where numerous dykes and faults outcrop and can be studied in the mesoscale. To augment our discussion of dyke and fault interactions, we combine previous volcanological and geophysical observations with our structural analysis to report the volcanotectonic evolution of the northern part of the island and design a conceptual spatial-temporal model. We mapped 91 dyke segments and 15 faults and classified the latter, where possible, with respect to their observed or recorded kinematics, their size, and the active stress field under which they were formed based on prior geophysical data. We relate our observations to a mechanical unconformity within the northern caldera wall. Our field observations, coupled with previous numerical, geophysical, and volcanological studies, offer insights on the interaction between dykes and faults and indicate the conditions under which the faults facilitated magma emplacement, or not, during the volcano’s activity. Our analysis attempts to answer an essential question: under what conditions do crustal faults facilitate or inhibit magma propagation to the surface, with application to the island of Santorini.
... This advance allows for studying aspects of plumbing systems, such as the growth of (crypto)domes, the quantification of magmainduced deformation, and the detailed study of intrusion geometries (e.g. Belousov et al. 2005;Thiele et al. 2021;Rhodes et al. 2021). ...
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Prior to and during eruptions, magma is stored and transported within volcanic and igneous plumbing systems (VIPS) that comprise a network of magma reservoirs and sheet intrusions. The study of these VIPS requires the combination of knowledge from the fields of igneous petrology, geochemistry, thermodynamic modelling, structural geology, volcano geodesy, and geophysics, which express the physical, chemical, and thermal complexity of the processes involved, and how these processes change spatially and temporally. In this contribution, we review the development of the discipline of plumbing system studies in the past two decades considering three angles: (1) the conceptual models of VIPS and paradigm changes, (2) methodological advances, and (3) the diversity of the scientific community involved in VIPS research. We also discuss future opportunities and challenges related to these three topics.
... Results of melting experiments and thermodynamic modeling by Meade et al. (2014) revealed that, although crustal sections not previously affected by magma intrusion are highly fusible, repeated heating events will make the crust more refractory over time. Crustal pre-conditioning by previous trachytic injections would be capable of shielding crustal lithologies and inhibiting magma-country rock interaction, as the magmatic intrusions tend to follow pre-established crustal pathways (Thiele et al., 2021). ...
Phonolite-trachyte associations are a common feature of alkaline volcanoes in intraplate settings, and their coexistence challenges closed-system magmatic differentiation scenarios. Here we have investigated the mineralogical and petrochemical features of dikes, lavas, pyroclastic deposits, and comagmatic crystal-rich enclaves outcropping at Dunedin Volcano (Otago region, southern New Zealand). These alkaline magmatic products show both highly and mildly alkaline affinities, trending towards phonolitic and trachytic end-members, respectively. Intermediate rocks are phonotephrites + tephriphonolites (highly alkaline series) and mugearites + benmoreites (mildly alkaline series) with a phenocryst assemblage of clinopyroxene + plagioclase ± amphibole formed at low to mid-crustal levels (i.e., ~29–16 km). Phonolites are porphyritic rocks characterized by alkali feldspar ± amphibole ± clinopyroxene. Their whole-rock compositions are highly enriched in incompatible elements, with variable Ba + Sr contents. A weak negative to slightly positive Eu anomaly is also associated with 87Sr/86Sr ratios of 0.7028–0.7031, which are comparable to those of parental magmas. Geochemical models indicate that phonolites originate as interstitial melts that are generated via abundant alkali feldspar crystallization from a shallow crystalline mush (i.e., ~14–5 km). Strong melt differentiation and extraction is testified by crystal-rich enclaves, as remnants of the mush region. On the other hand, trachytes are phenocryst-poor products strongly depleted in Ba + Sr and with a marked negative Eu anomaly. Trachytes are characterized by 87Sr/86Sr ratios of 0.7040–0.7060, which are different from intermediate rocks and phonolites, and trend towards crustal isotopic compositions. Integrated mass balance, trace element, and energy-constrained modeling confirm that trachytes originate from mildly alkaline magmas interacting with the country rock during feldspar fractionation. We interpret the transition from trachyte to phonolite formation and eruption resulting from the maturation of the plumbing system through accumulation, cooling, and degassing of both highly and mildly alkaline magmas.
... If multiple injections occurred (e.g. Thiele et al., 2021) they do not need to intrude along the dike centerline, so such non-monotonic thermal histories are one way to explain the slight asymmetries in D c observed in our data. Alternatively, these asymmetries could be explained by mechanical erosion of the wall-rock contact during magma transport. ...
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Flood basalts represent major events in Earth History, in part because they are linked to large climate perturbations and mass extinctions. However, the durations of individual flood basalt eruptions, which directly impact potential environmental crises, are poorly constrained. Here we use a combination of paleomagnetic data and thermal modeling to create a magnetic geothermometer (MGT) that can constrain the active transport lifetime of magmatic conduits and intrusions. We apply the MGT technique to eight feeder dike segments of the Columbia River basalts (CRB), demonstrating that some dike segments were actively heating host rocks for less than one month, while other segments may have been active for several years. Results suggest that eruption rates, localized spatially along-strike of dike segments, were as high as 1–8 km³ day⁻¹. These results help contextualize field evidence for contrasting CRB eruption durations and suggest a pathway for constraining the tempo of global flood basalt magmatism that is beyond the resolution of geochronology.
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La Palma, Canary Islands, underwent volcanic unrest which culminated in its largest historical eruption. We study this unrest along 2021 using Interferometric Synthetic Aperture Radar (InSAR) and a new improved interpretation methodology, comparing achieved results with the crustal structure. We reproduce the final phase of La Palma volcanic unrest, highligthing a shallow magma accumulation which begins about 3.5 months before the eruption in a crustal volume charactherized by low density and fractured rocks. Our modeling, together with our improved pictures of the crustal structure, allows us to explain the location and characteristics of the eruption and to detect failed eruption paths. These can be used to explain post-eruptive phenomena and hazards to the local population, such as detected gases anomalies in La Bombilla and Puerto Naos. Our results have implications for understanding volcanic activity in the Canaries and volcano monitoring elsewhere, helping to support decision-making and providing significant insights into urban and infrastructure planning in volcanic areas.
The 1928 CE volcanic activity on eastern Etna, Italy, produced wide surface deformation and high effusion rates along fissures, with excess volumes of about 50 million m³ of lavas. This, in conjunction with the low elevation of the main eruptive vents (1150 m a.s.l.), caused the destruction of the Mascali town. Our research focuses on a multidisciplinary study from field observations and Finite Element Method modelling through COMSOL Multiphysics®, with the aim of reconstructing the geometry, kinematics and origin of the system of faults and fissures formed during the 1928 event. We collected quantitative measurements from 438 sites of azimuth values, opening direction and aperture amount of dry fissures, and attitude and vertical offsets of faults. From west to east, four volcanotectonic settings have been identified, related to dike propagation in the same direction: 1) a sequence of 8 eruptive vents, surrounded by a 385-m wide graben, 2) a 2.5-km long single eruptive fissure, 3) a half-graben as wide as 74 m and a symmetric, 39-m-wide graben without evidence of eruption, 4) alignment of lower vents along the pre-existing Ripe della Naca faults. Field data, along with historical aerial photos, became inputs to FEM numerical models. The latter allowed us to investigate the connection between diking and surface deformation during the 1928 event, subject to a range of overpressure values (1–20 MPa), host rock properties (1–30 GPa) and geometrical complexity (stratigraphic sequence, layer thickness). In addition, we studied the distribution of tensile and shear stresses above the dike tip and gained insights into dike-induced graben scenarios. Our multidisciplinary study reports that soft (e.g. tuff) layers can act as temporary stress barriers and control the surface deformation scenarios (dike-induced graben, single fracture or eruptive fissures) above a propagating dike by suppressing the distribution of shear stresses towards the surface.
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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.
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Measurement of structure orientations is a key part of structural geology. Digital outcrop methods provide a unique opportunity to collect such measurements in unprecedented numbers, and are becoming widely applied. However, orientation estimates produced by plane fitting can be highly uncertain, especially when observed data are approximately collinear or the structures of interest comprise differently oriented segments. Here we present a Bayesian approach to plane fittingthat can use data extracted from digital outcrop models to constrain the orientation of structures and their associated uncertainty. We also describe a moving-window search algorithm that exploits this Bayesian formulation to estimate local structure orientations for segmented structures. These methods are validated on synthetic datasets for which both the structure orientation and associated uncertainty is known. Finally, we implement the method in the point cloud analysis package CloudCompare and use it to estimate the orientation and thickness of dykes exposed in cliffs on the island of La Palma (Spain).The results highlight the potential of this method to generate structural data at unprecedented spatial resolution, while simultaneously characterising the associated uncertainties.
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Understanding how fracture networks develop in shale formations is important when exploiting unconventional hydrocarbon reservoirs and analyzing the integrity of the seals of conventional and carbon capture and storage reservoirs. Despite this importance, experimentally derived fracture data for shale remains sparse. Here we characterize shale from Nash Point in South Wales, United Kingdom, in terms of ultrasonic wave velocities, tensile strength, and fracture toughness (KIc). We measure these properties in multiple orientations, including angles oblique to the three principal fracture orientations—Short-transverse, Arrester, and Divider. We find that the Nash Point shale is mechanically highly anisotropic, with tensile strength and KIc values lowest in the Short-transverse orientation and highest in the Arrester and Divider orientations. Fractures that propagate in a direction oblique or normal to bedding commonly deflect toward the weaker Short-transverse orientation. Such deflected fractures can no longer be considered to propagate in pure mode-I. We therefore present a method to correct measured KIc values to account for deflection by calculating mode-I and mode-II deflection stress intensities (KId and KIId, respectively). Because of the mixed-mode nature of deflected fractures, we adopt a fracture (Gc) energy-based approach that allows analysis of critical fracture propagation conditions for both deflected and undeflected fractures in all orientations. We find that Gc increases as the angle from the Short-transverse plane increases. We conclude that a modified elliptical function, previously applied to tensile strength and KIc, can be used to estimate values of Gc at angles between the Short-transverse and Arrester orientations.
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The strength of rocks in the subsurface is critically important across the geosciences, with implications for fluid flow, mineralisation, seismicity, and the deep biosphere. Most studies of porous rock strength consider the scalar quantity of porosity, in which strength shows a broadly inverse relationship with total porosity, but pore shape is not explicitly defined. Here we use a combination of uniaxial compressive strength measurements of isotropic and anisotropic porous lava samples, and numerical modelling to consider the influence of pore shape on rock strength. Micro computed tomography (CT) shows that pores range from sub-spherical to elongate and flat ellipsoids. Samples that contain flat pores are weaker if compression is applied parallel to the short axis (i.e. across the minimum curvature), compared to compression applied parallel to the long axis (i.e. across the maximum curvature). Numerical models for elliptical pores show that compression applied across the minimum curvature results in relatively broad amplification of stress, compared to compression applied across the maximum curvature. Certain pore shapes may be relatively stable and remain open in the upper crust under a given remote stress field, while others are inherently weak. Quantifying the shape, orientations, and statistical distributions of pores is therefore a critical step in strength testing of rocks.
Connecting caldera collapse The Kīlauea Volcano on the island of Hawai‘i erupted for 3 months in 2018. Neal et al. present a summary of the eruption sequence along with a variety of geophysical observations collected by the Hawaiian Volcano Observatory. The cyclic inflation, deflation, and eventual collapse of the summit was tied to lava eruption from lower East Rift Zone fissures. A total volume of 0.8 cubic kilometers of magma erupted, roughly the equivalent of 320,000 swimming pools, which matched the change in volume at the summit. Science , this issue p. 367
This paper presents a novel implementation of a hydro-mechanically coupled, finite-discrete element method (FDEM) optimized to exploit the computing parallelism of graphics processing units (GPUs). A co-processing approach is adopted with the control loop of FDEM executed serially on the CPU and compute-intensive tasks off-loaded to the GPU. A benchmarking study indicates speedups of up to 100× compared to sequential CPU execution. The implementation is validated by comparing 3D laboratory-scale rock fracturing simulations with experimental results. The effectiveness of the approach for practical rock engineering applications is demonstrated through the back analysis of a slope in a fractured rock mass.
The Newer Volcanics Province (NVP) is a Pliocene to Recent intra-plate basaltic volcanic field (BVF) that has formed in a compressive tectonic setting (σv < σhmin < σHmax) and is not readily attributed to a single geodynamic process. A comprehensive spatial analysis of both monogenetic eruption centres and coeval vents of the NVP constrain factors that control the distribution and emplacement of volcanoes. A point-set of 434 eruption centres totalling 726 vents are divided into three geographical sub-provinces for analysis. Kernel density estimation and Poisson nearest neighbour analysis are used to scrutinize the distribution of eruption centres. A simple and novel fitted regression line method is used to determine the orientation of coeval vents, and Hough transform and two-point azimuth methods are used to identify alignments and alignment trends between eruption centres. The distribution of eruption centres and their relative spatial density corresponds with the extent of thinner lithosphere. Eruption centres of the NVP have a clustered distribution whilst smaller sub-sets of eruption centres are distributed more uniformly. The main alignment trends between coeval vents related to individual dikes and between eruption centres related to successive temporally discrete dikes are primarily oriented nearly parallel with pre-existing crustal fault trends. The frequency of volcanic alignments shows faults oriented nearly parallel to the orientation of the regional maximum horizontal compressive stress (σ1) are favourably utilised by intrusions over other fault trends. The depth from which pre-existing faults facilitate dike propagation is not constrained. We interpret they are likely important in preventing dikes from stalling and forming sills under a compressive stress field in the case of the NVP. It is also observed that coeval vent alignments are more strongly aligned in areas overlying consolidated basement relative to areas of poorly consolidated basin sediments. This could be explained by poorly aligned groups of vents being fed by shallow sills, preferentially forming in layered basin sediments.
The new generation of multi-collector mass spectrometers (e.g. ARGUSVI) permit ultra-high precision (<0.1%) 40Ar/39Ar geochronology of rocks and minerals. At the same time, the 40Ar/39Ar method is limited by relatively large uncertainties (>1%) in 40K decay constants and the ages of natural reference minerals that form the basis of the technique. For example, reported ages for widely used 40Ar/39Ar reference materials, such as the ca. 28 Ma Fish Canyon Tuff sanidine (FCTs) and the ca. 1.2 Ma Alder Creek Rhyolite sanidine (ACRs), vary by >1%. Recent attempts to independently calibrate these reference minerals have focused on K–Ar analyses of the same minerals and inter-comparisons with astronomically tuned tephras in sedimentary sequences and U–Pb zircon ages from volcanic rocks. Most of these studies used older generation (effectively single-collector) mass spectrometers that employed peak-jumping analytical methods to acquire 40Ar/39Ar data. In this study, we reassess the inter-calibration and ages of commonly used 40Ar/39Ar reference minerals Fish Canyon Tuff sanidine (FCTs), Alder Creek Rhyolite sanidine (ACRs) and Mount Dromedary biotite (MD2b; equivalent to GA-1550 biotite), relative to the astronomically tuned age of A1 Tephra sanidine (A1Ts), Faneromeni section, Crete (Rivera et al., 2011), using a multi-collector ARGUSVI mass spectrometer. These analyses confirm the exceptional precision capability (<0.1%) of this system, compared to most previous studies. All sanidine samples (FCTs, ACRs and A1Ts) exhibit discordant 40Ar/39Ar step-heating spectra, with generally monotonically increasing ages (∼1% gradients). The similarity in these patterns, mass-dependent fractionation modeling, and results from step-crushing experiments on FCTs, which yield younger apparent ages, suggest that the discordance may be due to a combination of recoil loss and redistribution of 39ArK and isotope mass fractionation. In contrast to our previous inferences, these results imply that the sanidine samples are suitable 40Ar/39Ar reference materials, provided appropriate corrections are included for differential recoil loss of 39ArK and contributions from xenocrysts/antecrysts can be resolved. Relative to an age of 6.943 ± 0.005 Ma for A1Ts, we calculate astronomically tuned ages for FCTs, ACRs and MD2b of 28.126 ± 0.019 (0.066%) Ma, 1.18144 ± 0.00068 (0.058%) Ma and 99.125 ± 0.076 (0.077%) Ma, respectively (95% internal errors). These results are consistent with recent 238U/206Pb age data from these localities, but are marginally younger (∼0.2%) than previous 40Ar/39Ar ages inter-calibrated with astronomically tuned tephra from the Mediterranean, and distinctly younger (0.6%) than results optimized against a broad array of 238U/206Pb zircon ages. Consideration of published and assumed recoil loss 39ArK proportions (0.18–0.40%), yields recoil-corrected age estimates of 28.187 ± 0.019 Ma, 1.18404 ± 0.00068 Ma and 99.204 ± 0.076 Ma, respectively. This comparison indicates inherent uncertainties of >0.1% in the 40Ar/39Ar ages of reference minerals without consideration of recoil artefacts, thus limiting the benefits of high precision multi-collector analyses. Significant improvement to the accuracy of the 40Ar/39Ar method (<0.1%) will require further inter-laboratory 40Ar/39Ar studies utilizing multi-collector mass spectrometry, additional constraints on recoil 39ArK loss from reference minerals, further resolution of discrepancies between astronomically tuned sedimentary successions and refinement of the 238U/206Pb zircon age cross-calibration approach.