Frictional Melting in Hydrothermal Fluid‐Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile)

Abstract

Tectonic pseudotachylytes are thought to be unique to certain water–deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid–rich faults hosted in the continental crust. Pseudotachylytes were found in the > 40 km–long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m–thick (ultra)cataclastic strands hosted in a damage–zone made of chlorite–epidote–rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite–, epidote– and calcite–bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s-1) resulted in the production of vesiculated pseudotachylytes both at dry and water–pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid–deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co–seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust.

Full text

1. Introduction
Tectonic pseudotachylytes are solidified frictional melts, formed within faults during co-seismic slip
(Maddock, 1983; Sibson, 1975), and are considered to be unambiguous evidence of past earthquakes
(Cowan,1999; Rowe & Griffith, 2015). Despite the clear seismic origin of pseudotachylytes, there has been
a long debate regarding the environmental conditions during their formation within fault zones. While some
authors argued in favor of a water-deficient environment condition hypothesis for pseudotachylyte formation
(Sibson,1975; Sibson & Toy,2006), there is a growing body of research pointing toward pseudotachylyte forma-
tion in “wet” environments (Allen,1979; Bjørnerud,2010; Boullier etal.,2001; Famin etal.,2008; Maglough-
lin,2011; Rowe etal.,2005; Williams etal.,2017). We define a wet environment as a rock volume with fluids
available in the form of (a) free pore water (i.e., hydrothermal fluids), or (b) volatile-rich minerals (e.g., hydrous
minerals). Pirajno(2009) defined a hydrothermal fluid as a hot (∼50 to >500°C) aqueous solution (with H2O as
solvent), containing solutes (including CO2, CH4 and diverse anions and cations) that are commonly precipitated
as the solution changes its properties in space and time.
According to several authors (e.g., Bjørnerud, 2010; Maddock, 1992; Magloughlin, 1989; 1992; Meneghini
etal.,2010), the formation of pseudotachylytes could be enhanced by the presence of cataclasites. Indeed, this
fine-grained and often water-rich fault material (e.g., presence of hydrous minerals as epidote, chlorite, etc., free
Abstract Tectonic pseudotachylytes are thought to be unique to certain water-deficient seismogenic
environments and their presence is considered to be rare in the geological record. Here, we present field
and experimental evidence that frictional melting can occur in hydrothermal fluid-rich faults hosted in the
continental crust. Pseudotachylytes were found in the >40 km-long Bolfín Fault Zone of the Atacama Fault
System, within two ca. 1 m-thick (ultra)cataclastic strands hosted in a damage-zone made of chlorite-epidote-
rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during
the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite-,
epidote- and calcite-bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates
pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on
bare surfaces of hydrothermally altered rocks at seismic slip velocities (3m s
−1) resulted in the production of
vesiculated pseudotachylytes both at dry and water-pressurized conditions, with melt lubrication as the primary
mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that
pseudotachylytes are limited to fluid-deficient environments, and gives insights into the ancient seismic activity
of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be
produced in hydrothermal environments, and could be a common co-seismic fault product. Consequently, melt
lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline
basement rocks of the continental crust.
GOMILA ET AL.
© 2021. The Authors.
This is an open access article under
the terms of the Creative Commons
Attribution-NonCommercial License,
which permits use, distribution and
reproduction in any medium, provided the
original work is properly cited and is not
used for commercial purposes.
Frictional Melting in Hydrothermal Fluid-Rich Faults: Field
and Experimental Evidence From the Bolfín Fault Zone
(Chile)
R. Gomila1 , M. Fondriest1,2 , E. Jensen3 , E. Spagnuolo4 , S. Masoch1 ,
T. M. Mitchell5 , G. Magnarini5 , A. Bistacchi6 , S. Mittempergher7 ,
D. Faulkner8 , J. Cembrano9 , and G. Di Toro1,4
1Dipartimento di Geoscienze, Università degli Studi di Padova, Padova, Italy, 2Institut des Sciences de la Terre (ISTerre),
Université Grenoble Alpes, Grenoble, France, 3Departamento Ingeniería y Ciencias Geológicas, Universidad Católica del
Norte, Antofagasta, Chile, 4Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy, 5UCL Earth Sciences, University
College of London, London, UK, 6Dipartimento di Scienze dell'Ambiente e della Terra, Università di Milano-Bicocca,
Milano, Italy, 7Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Modena, Italy,
8School of Environmental Sciences, University of Liverpool, Liverpool, UK, 9Escuela de Ingeniería, Pontificia Universidad
Católica de Chile, Santiago de Chile, Chile
Key Points:
• The Bolfín Fault Zone shows the first
pseudotachylytes described in the
Atacama Fault System giving insights
of past seismic activity
• Rotary shear experiments at seismic
slip velocities resulted in formation
of vesiculated pseudotachylytes in all
tested conditions
• Natural pseudotachylytes formed in a
hydrothermal fluid-rich environment
where vesiculation is related to CO2
degassing from calcite veins
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
R. Gomila,
r.gomilaolmosdeaguilera@unipd.it
Citation:
Gomila, R., Fondriest, M., Jensen, E.,
Spagnuolo, E., Masoch, S., Mitchell,
T. M., etal. (2021). Frictional melting
in hydrothermal fluid-rich faults:
Field and experimental evidence
from the Bolfín Fault Zone (Chile).
Geochemistry, Geophysics, Geosystems,
22, e2021GC009743. https://doi.
org/10.1029/2021GC009743
Received 26 FEB 2021
Accepted 26 JUN 2021
10.1029/2021GC009743
RESEARCH ARTICLE
1 of 17

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water within pores and fluid inclusions), would lower the melting point of the fault rock assemblage and promote
frictional melting during seismic slip (Lee etal.,2017; Magloughlin & Spray,1992; Spray,1992). Hence, fault
rocks enriched in hydrous minerals would favor pseudotachylyte formation. However, during seismic slip, pore
water may absorb heat and expand, and trigger fault weakening by thermal pressurization if the fluids are trapped
within the fault slipping zone (Lachenbruch,1980; Sibson, 1973). Thermal pressurization, if efficient, would
buffer the temperature increase in the slipping zone and prevent frictional melting (Rice,2006).
Vesicles in pseudotachylytes have been first described in the early work of Nockolds(1940) and Philpotts(1964)
who pointed out the idea that the abundance of amygdales (mineral-filled vesicles) in pseudotachylytes suggests
that the liquids from which these rocks were formed must have been highly charged with gases, giving the first
hints that some sort of fluid or vapor phase must be present at the time of pseudotachylyte generation. Whilst
Maddock etal.(1987,1989), argued for the use of vesicle abundance in pseudotachylytes as a proxy for palae-
oseismic depth, Dixon and Dixon(1987) suggested that a small amount of free water in the fault zone could be
responsible for their formation. Later work performed on subduction complexes (e.g. Meneghini etal., 2010;
Phillips etal.,2019; Rowe etal.,2005; Ujiie, Yamaguchi, etal.,2007) showed that the presence of vesicles in
pseudotachylytes formed in this fluid-rich environment is also possible.
Previous experimental evidence suggests that friction melting in silicate rocks is possible under water-satu-
rated conditions (Violay, Di Toro, etal.,2015; Violay, Nielsen, etal.,2014). The experiments showed that both
microgabbro and basalt melt under effective normal stress (

n
eff
) of 20MPa and pore fluid pressure (Pf) of 5MPa,
resulting in a slipping zone with glassy matrix full of vesicles, although no further attention was given to this
texture in these studies.
In this paper we describe the first pseudotachylytes recognized in the Atacama Fault System (AFS) in North-
ern Chile, characterized by the presence of amygdales both, spatially and temporally associated with foliated
hydrothermally altered fault-core rocks in a strong fluid-rock interaction environment. We then investigate the
conditions controlling the formation of vesicles in frictional melts originated from volatile-enriched cataclasites,
through a series of high velocity friction experiments at different fluid pore pressures on the same cataclasites
hosting the natural pseudotachylytes, with the aim to understand at which conditions vesiculated pseudotachy-
lytes can be produced. Vesicles were produced at all tested conditions, but we demonstrate that while they can be
generated by mineral dehydration (chlorite, etc.) in experimental samples, in nature they are most likely derived
from CO2 (derived from calcite in the fault rock assemblage) exsolved from the melt.
2. Geological Setting
The main structural feature of the Coastal Cordillera, in northern Chile, corresponds to the N–S trending,
trench-parallel, Atacama Fault System (AFS; Arabasz,1971; Scheuber & González,1999). The AFS is ca. 1,000
km-long and records left-lateral finite shear, interpreted as a response of the oblique subduction of the Aluk
(Phoenix) plate beneath the South American plate during the Mesozoic (Figure1).
Deformation along the AFS is both spatially and temporally related to the magmatism of the Coastal Cordillera,
with pluton cooling and crystallization ages between 150 and 110Ma (Olivares etal.,2010; Ruthven etal.,2020;
Scheuber etal.,1995; Seymour etal.,2020). The central segment of the AFS is ca. 150 km-long and it consists
of (a) steeply dipping sinistral strike-slip duplexes with a hierarchical pattern of master and subsidiary faults,
fault-veins and veins (i.e., mode I+II/III and mode I filled fractures, respectively), developed in a transtensional
regime near Antofagasta, at 24°S (Figure1a) (Cembrano etal.,2005; Jensen etal.,2011; Olivares etal.,2010;
Veloso etal.,2015) and (b) steeply dipping sinistral mylonitic fabrics overprinted by brittle faults developed in
a transpressional regime at ca. 25°S (Ruthven etal., 2020). The hydrothermal nature of fluid transport in the
northern part of the central segment has been largely documented by several authors (Arancibia etal., 2014;
Gomila etal.,2016; Herrera etal.,2005; Veloso etal.,2015) as a pervasive fault-related chloritic and propylitic
alteration evidenced by a widespread chlorite-epidote-calcite precipitation. Studies on chlorite geothermometry
of Arancibia etal.(2014) established an average Tchlorite values of 323°C based on the tetrahedral Al occupancy
in chlorite, obtained from the Caleta Coloso Fault (Figure1a), implying that the maximum depth constrain of
hydrothermal alteration and faulting is around 6–8km, assuming a geothermal gradient between 40 and 50°C/km.

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The Bolfín Fault Zone, located in the northern part of the central segment of the AFS, is a NNW- to NW-trending
left-lateral strike-slip fault zone, which is exposed over a minimum length of ca. 45km (since it runs beneath the
ocean in its northern sector; Figure1). The fault zone is exposed almost continuously from the northern border of
the Playa Escondida sector (Figures1b and1c) until it merges with the Caleta Coloso Fault Zone at an angle of
20° (González,1999; González & Niemeyer,2005). The Bolfín Fault Zone cuts through Late Jurassic – Early
Figure 1. The Playa Escondida Outcrop.(a) Geological setting and simplified structural map of the northern part of the Bolfín Fault Zone, located in the Atacama
Fault System, in the western border of northern Chile (modified from Cembrano etal.[2005]). (b) Aerophotograph of the Playa Escondida Outcrop of the Bolfín Fault
Zone. Note the green-colored western side of the outcrop due to pervasive chlorite precipitation. (c) Structural map showing the presence of two main sub-parallel
fault-cores ca. 1m-thick and the cross-cutting relations of the fault and fracture network. (d) Structural data of the different stages according to their timing and main
mineral phase. Poles to planes (n: number of planes) of the different structural elements (magmatic foliations, faults, fault-veins and veins). Early Elements consist
of early formed structural elements (i.e., before brittle faulting occurred) such as magmatic foliations and basaltic to andesitic dykes; chlorite-rich stage consists of
faults and fault-veins temporally and spatially associated with chlorite infills, chlorite-epidote-rich foliated cataclasites with minor amounts of calcite, chlorite-epidote-
calcite veins and pseudotachylyte fault-veins. The epidote-rich stage is represented by faults and fault-veins marked by hydrothermal epidote, quartz and minor calcite
precipitation, whereas the calcite-rich stage consists of faults filled with calcite (±gypsum±palygorskite±halite); Pst: pseudotachylyte, Chl: chlorite, Ep: epidote, Cal:
calcite, Gp: gypsum, Qz: quartz, Act: actinolite.

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Cretaceous crystalline rocks belonging to the meta-diorites and meta-gabbros of the Bolfín Complex and the
tonalitic to granodioritic unit of the Cerro Cristales Pluton (Figure1a).
3. Methods
We integrated Unmanned Aerial Vehicle (UAV) photo surveys with detailed field mapping of ductile shear zones,
brittle faults and fault-veins of the Bolfín Fault Zone at the Playa Escondida outcrop to produce a high-resolution
structural map (Figure1c). We used a DJI Phantom 4 Pro drone, from which 48 nadir-directed aerophotographs
were taken at a mean flight elevation of 10m over the ground level. The images were processed in Agisoft Photo-
scan Pro to generate a high-resolution georeferenced orthomosaic image with a spatial resolution of ca. 0.6cm/
pix (Figure1c).
Twenty oriented fault rock samples were collected from the Playa Escondida outcrop.Most were used for micro-
structural, geochemical and mineralogical investigations, while two large block samples were used for rotary
shear experiments (samples B21 and B23). The blocks were big enough to drill 50mm in diameter and ca. 60
mm-long cores to be used in rotary-shear experiments (see FigureS1).
The experiments were performed with the Slow to HIgh Velocity Apparatus (SHIVA) installed at the High
Pressure-High Temperature (HPHT) Laboratory of the Istituto Nazionale di Geofisica e Vulcanologia (INGV)
in Rome (for details about the experimental machine, its calibration, acquisition and sample preparation, see Di
Toro, Niemeijer, etal.,2010; Nielsen etal.,2008; Niemeijer etal.,2011). We performed six experiments under
different environmental conditions: two at room humidity, two in the presence of pressurized distilled water
(Pf=5MPa), one under vacuum (10
−4mbar) and one using steam on dry surfaces. The rock specimens were slid
following a trapezoidal velocity function with target slip rates (velocity, V) of 3m s
−1, acceleration of 25m s
−2
and total slip (displacement) of 1m under an effective normal stress (

n
eff
) of 20MPa (see TableS1). Exceptions
to these general conditions are experiments s1688 (which had 6.3m s
−2), s1691 (3m of slip) and s1690 (with an

n
eff
of 15MPa).
The mineral chemistry of the pseudotachylyte matrix and of the rock-forming minerals (plagioclase, amphibole,
epidote, chlorite, etc.) were obtained by several methods. First, wavelength-dispersive electron microprobe analy-
ses (EMPA), were carried out on polished thin sections at INGV, Rome, with Joel-JXA8200 microprobe equipped
with EDS-WDS. Analytical conditions were 15kV as accelerating voltage, a beam current of 7.5nA and a spot
size of 3μm. Second, semi-quantitative X-ray powder diffraction (XRPD) analyses were carried out at UNIPD,
Padova, through the Reference Intensity Ratio method with a PANalytical X’Pert Pro diffractometer equipped
with a Co radiation source, operating at 40mA and 40kV in the angular range 3°<2θ<85°. Lastly, a Zeiss
FEG Gemini 500 Field Emission Scanning Electron Microscope (FE-SEM), equipped with a Brucker QUAN-
TAX EBSD, was used to analyze mineral chemistry of the experimentally produced pseudotachylyte matrix at
the University of Milano-Bicocca, Italy. Analytical conditions were 20kV as accelerating voltage and a spot size
of 0.7μm.
4. The Playa Escondida Outcrop
The Playa Escondida outcrop covers an area of ca. 20×40m
2 and is located where the Bolfín Fault Zone enters
in the Pacific Ocean, at the northern end of the fault (Figure1b). At the outcrop (Figure1c), the Bolfín Fault Zone
corresponds of a N30W-striking and SW-subvertical dipping fault-core composed of two strands of ca. 1m-wide
foliated cataclasites. The damage zone consists of poorly to intensively altered tonalites showing magmatic folia-
tions and up to a few centimeters thick layers of cataclasites along subsidiary faults (Figure2).
Based on the structural features and mineral assemblage, and on their crosscutting relationships observed in the
field (Figure2), four main deformation stages were recognized (Figure1d): (a) an “early” stage, consisting of
NW- to N-striking early structural elements consisting of magmatic foliations and non-foliated basaltic to ande-
sitic dykes; (b) a second chlorite-rich stage which consists of NNW- to WNW-striking sinistral strike-slip faults
and fault-veins temporally and spatially associated with chlorite infills, chlorite-epidote-rich foliated cataclasites
with minor amounts of calcite, chlorite-epidote-calcite veins and pseudotachylytes, dominating the core of the
Bolfín Fault Zone. This stage corresponds to the first evidence of brittle deformation in the fault zone (Bolfín
Fault Zone sensu-strictu); (c) an intermediate epidote-rich brittle deformation stage represented by a series of

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N-, NW- and minor NE-striking faults and fault-veins marked by hydrothermal epidote, quartz and minor calcite
precipitation; (4) a later stage, representing the youngest deformation event along the Bolfín Fault Zone, and
characterized by NE-striking dextral-normal and N- and WNW-striking sinistral-normal faults filled with calcite
(±gypsum±palygorskite±halite). The field evidence suggests the formation of the pseudotachylytes in a fluid-
rich environment as discussed below.
5. The Natural Pseudotachylytes of the Bolfín Fault Zone
At Playa Escondida, the exposed Bolfín Fault Zone includes two 1–2 m-thick subvertical fault-core strands
spaced apart by 4m (Figure1c), containing pseudotachylyte-bearing foliated cataclasites (Figure2), which merge
to the south of the outcrop.The typical occurrence of pseudotachylyte is in several (ca. 20) short and narrow
fault-veins, which are generally brown to red while a few are dark gray, ranging in thickness from a few mm to
Figure 2. Fault zone rock assemblage of the Playa Escondida Outcrop.(a) Foliated chlorite-epidote-rich foliated cataclasites
in hydrothermally altered fault-core. (b) Multiple veins cutting the hydrothermally altered fault rocks hosted in tonalite
to quartz-diorite, suggesting several episodes of fluid infiltration. Note epidote-quartz vein cut by calcite-gypsum vein.
(c and d) Multiple generations of pseudotachylytes cutting altered cataclasites and poorly to intensively altered tonalites
to quartz-diorites. Note dismembered red-altered pseudotachylytes within the chlorite-rich fault-core. Alt. PST: altered
pseudotachylytes; iv: injection vein; fv: fault-vein; Qz: quartz; Ep: epidote; Cal: calcite; Gp: gypsum. For location of the
photos refer to Figure1c.

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a ca. 3cm, mostly distributed within the fault-core. In addition to the typical occurrence as fault-veins there are
some off-fault injection veins. Almost all the pseudotachylyte fault-veins follow pre-existing structures, such as
the magmatic foliation or the “brittle” chlorite-rich faults (Figure1d). Also, pseudotachylytes veins are altered
and dismembered within the fault-core, and are cutting the foliated chlorite-rich cataclasites (Figures2c and2d).
At the microscale, two types of pseudotachylytes were recognized: (a) non-vesiculated (Figure3) and (b) vesicu-
lated pseudotachylytes (Figure4), both showing flow structures (Figures3a and4a), albite microlites (Figures3b
and3c), chilled margins (Figure4b) and embayed lithic clasts (Figure3d), pointing toward their formation from
a frictional melt. Vesiculated pseudotachylytes present amygdales (Figure4) with different shapes and degrees
of elongation. In cross section, the amygdales are elliptical (Figure4d) or have sinuous and irregular contacts
with the matrix (Figure4e). In the latter case, the amygdales are interpreted to be collapsed (Magloughlin,2011).
The matrix of the pseudotachylytes is composed by albite+epidote+Fe-actinolite+chlorite (EMPA analysis,
TableS2); whereas the amygdales are filled by calcite+epidote+chlorite+quartz (Figure4).
Pseudotachylytes, both vesiculated and not vesiculated, cut chlorite and calcite+epidote+chlorite bearing veins
(Figure3e) and are cut by late K-feldspar and calcite veins (Figure3f). These crosscutting relations indicate that
hydrous- and carbonate-rich minerals, associated with hydrothermal alteration were precipitated in the fault zone
before, after and possibly during seismic faulting.
Figure 3. Natural non-vesiculated pseudotachylyte of the Bolfín Fault Zone: evidence for solidification from melt and of
seismic faulting in a fluid rich environment. (a) Injection vein (scan of thin section from sample B15-19, parallel nicols)
cutting through a chlorite-rich cataclasite. (b) Albite microlites in the pseudotachylyte matrix (sample AT0717). (c) Albite
microlites and quartz clasts in a chlorite-rich matrix (presumably devitrified and recrystallized glass; sample B15-19).
(d) Calcite and quartz clasts in a microlite-rich pseudotachylyte matrix. (e and f) cross-cutting relations of the natural
pseudotachylyte with calcite (early and late) veins and K-feldspar vein. (b–f) BSE-FESEM images. Ab, albite; Qz, quartz; Ep,
epidote; Cal, calcite; Chl, chlorite; Kfs, K-feldspar.

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6. Experimental Pseudotachylytes From the Bolfín Fault Zone Rocks
To understand in which environmental conditions vesiculated pseudotachylytes can be produced starting from
the Bolfín Fault Zone fault rock assemblage, we performed six high velocity experiments on the two common
host-rocks of the natural pseudotachylytes (i.e., the chlorite- and epidote-rich cataclasites). The experiments
were performed under vacuum, room humidity, steam and pressurized fluid conditions (see Method section and
Figure5). The sheared fault zone rocks were chlorite- and epidote-rich cataclasites (for the detailed mineralogical
assemblage of the sheared rocks, see Figure8 and TableS3). In these experiments, the frictional evolution with
slip was relatively similar and independent of the environmental conditions, loading condition (e.g. Figure5).
In fact, the friction coefficient μ (ratio between the shear stress and the applied effective normal stress; τ/

n
eff
)
decayed with slip distance from a peak value of ca. 0.66 at slip initiation, to a dynamic value of ca. 0.25 after
ca. 10cm of slip and (Figure5). This frictional evolution with slip is typical of cohesive silicate-rich (tonal-
ite, gabbro, peridotite, etc.) experimental faults, also in the presence of pressurized pore fluids (Del Gaudio
etal.,2009; Di Toro, Hirose, etal.,2006; Violay, Di Toro, etal.,2015; Violay, Nielsen, etal.,2014). In the pres-
ence of pressurized fluids, the dynamic friction value was larger than in the case of vacuum and RH experiments,
Figure 4. Natural vesiculated pseudotachylyte of the Bolfín Fault Zone: evidence for melt flow and vesiculation. (a)
Pseudotachylyte with flow structures and up to 1mm in size vesicles (scan of polished specimen, sample 2003-01)
hosted in a chlorite-rich ultracataclasite. (b) Two generations of pseudotachylytes from sample AT07-2017 (thin section
in parallel nicols). The older (Pst1) is crosscut by the younger and darker (Pst2). (c) Inset of (b) shows a transmitted light
microphotograph of filled vesicles. (d and e) BSE-SEM images of filled vesicles. Qz, quartz; Ep, epidote; Cal, calcite; Chl,
chlorite.

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while on contrary, the peak in shear stress was lower. However, independently of the imposed and environmental
conditions, both dynamic and peak friction coefficient were similar (μd and μp, respectively; Figure5b). Notably,
frictional melts were produced in all the experiments, independent of the presence or absence of pressurized
water. The maximum total shortening of the specimens due to rock melting and expulsion of the melt from the
slipping zone under these deformation conditions was ca 0.2mm (FigureS2). The experimental pseudotachylytes
were ca. 0.2mm thick and consisted of plagioclase and quartz “survivor” clasts suspended in a glassy-like matrix
(Figure6). However, the pseudotachylyte at the end of slip (i.e., after 1m of slip and 3m of slip for experiment
s1691) may not correspond to the total melt produced in the experiment, as a part of this melt was expelled from
the slipping zone. The WDS-FESEM composition of the experimental pseudotachylyte matrix (the spot size of
the WDS-FESEM analysis was ca. 700nm in diameter) was homogenous and very similar to the one of a vola tile-
free basalt, regardless of the environmental conditions imposed in the experiments (TableS3).
Vesicles in experimental pseudotachylytes were found in all the samples, regardless of the imposed environmen-
tal conditions, with very similar shape to the amygdales found in the natural pseudotachylytes of Playa Escondida
(i.e., both circular to elliptical and collapsed). Vesicles are circular and elliptical in thin section (Figures7a
and7c), and also with sinuous and irregular contacts with the glassy matrix (Figure7b). All these shapes are
almost identical to those of the natural amygdales (see Figure4), being the natural ones slightly elongated when
compared with the experimental vesicles. The abundance of vesicles is independent of whether free water is
present before the experiment. Next to the pseudotachylytes, protolith minerals sometimes are vesiculated and
dismembered (Figure7d).
7. Mineralogy and Geochemistry
7.1. Mineralogy of Host-Rocks and Natural Pseudotachylytes
X-ray powder diffraction (XRPD) analyses of the fault zone rocks (both host-rock and fault-core), host-rocks
of the natural pseudotachylytes (marked with HR after the sample name in Figure8) and of the protolith of the
experimental pseudotachylytes, indicate that the dominant mineral phases are quartz and plagioclase with minor
K-feldspar, actinolite and biotite as the mafic phases and titanite as accessory mineral, whereas chlorite, epidote
Figure 5. Experimental pseudotachylytes (mechanical data) of SHIVA. (a) Example of the mechanical data (experiment
s1692 at vacuum conditions): trapezoidal velocity function (m/s), normal stress (MPa) and obtained shear stress (MPa) and
shortening (mm) versus slip distance (m). Peak and dynamic shear stress are shown (τp and τd, respectively). Dashed black
box represents the range of data from which dynamic friction coefficient was estimated. (b) Peak and dynamic friction
coefficients (μp and μd, respectively) graph for all the experiments at different environmental conditions (shown below each
experiment name).

n
eff
, Effective normal stress; Pf, Fluid pressure; Acc., Acceleration.

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and calcite are the most abundant alteration (hydrous and carbonate) minerals (see TableS4). In contrast, the
natural pseudotachylytes (marked with a PST after the sample name in Figure8), in general, are clearly enriched
in K-feldspar and slightly in calcite and biotite and depleted of actinolite when compared to their host-rock.
There is a slight broadening of the XRPD spectrum between 20°<2θ<36° in the natural pseudotachylyte when
compared with adjacent host rock (see FigureS3). Though broadening of the XRPD spectrum has been interpreted
as due to the presence of glass (e.g., Lin & Shimamoto,1998), the presence of glass in the Bolfín pseudotachylyte
is unlikely because of the intense chlorite-epidote alteration. Another possibility of the slight broadening could be
due to remnants of nanomaterial within the pseudotachylytes formed during melt solidification.
Figure 6. Experimental pseudotachylytes produced under different environmental conditions (a) vacuum, (b) room humidity, and (c) pressurized water. (a–c)
Experimental pseudotachylytes layers (Exp.PST layer) and their protolith (both chlorite- and epidote-rich cataclasites; parallel nicols). (a′, b′, c′) Detail of the
experimental pseudotachylytes (reflected light). (a″, b″, c″) Vesicles dispersed in the matrix (black dots within the experimental pseudotachylyte layers; for a zoom
view of vesicles see Figure7) were found in all the experimental pseudotachylytes, regardless of the presence or absence of pressurized water at the initiation of slip
(BSE-FESEM images). Fragments within the experimental pseudotachylyte layers are mainly quartz (Qz) and plagioclase (Pl). Ap, apatite; Chl, chlorite; Ep, epidote.

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7.2. Elemental Composition of Experimental Pseudotachylytes
EMPA and FESEM-WDS investigations of the glassy matrix of experimental pseudotachylytes gave very similar
results, despite their different spot size (ca. 3μm for EMPA, ca. 0.7μm for FESEM-WDS). Moreover, the compo-
sitions of glass in samples produced in different ambient conditions (vacuum for experiment s1692, room humid-
ity for experiment s1688 and pressurized pore water for experiment s1690) are indistinguishable (Figure9). The
composition of the experimental glass (ca. 49.9% SiO2, 17.0% Al2O3, 10.8% FeO, 8.2% CaO, 5.2% MgO, 2.7
Na2O, 1.5% TiO2, and 1.0% K2O) corresponds to the one of a basalt (e.g., Papale etal.,2006).
8. Discussion
8.1. Formation of Pseudotachylytes in Fluid-Rich Conditions
It is commonly considered that pseudotachylytes mainly form during seismic slip under dry conditions (Sibson
& Toy,2006), as the presence of free pore fluid would reduce the normal effective stress during thermal pressur-
ization in the slipping zone before friction melt is generated (Rice,2006; Sibson,1975). Recent work has shown
that the effectiveness of this melt-limiting pressurization also depends of the permeability and storage capacity
of the damaged wall rock adjacent to the slip zone (Brantut & Mitchell,2018). However, we document compel-
ling evidence of natural pseudotachylyte formed in a hydrothermal fluid-rich environment (i.e., the volume of
fluid-rich fault rocks containing (a) free pore water or (b) volatile-rich minerals) supported by the presence of
chlorite+epidote+calcite veins and K-feldspar and calcite veins that pre- and post-date the generation of the
natural pseudotachylytes.
The presence of amygdales has already been proposed as a key characteristic of fluid-rich pseudotachylyte melts
(e.g., Maddock etal.,1987; Magloughlin,2011) and inferred to be the result of the degassing of a vapor phase
from the silicate melt (Kirkpatrick & Rowe,2013). Moreover, rotary shear experiments provided evidence that
frictional melts can be easily produced during seismic slip in the presence of pressurized water (Spray,1995;
Violay, Di Toro, etal.,2015; Violay, Nielsen, etal.,2014). In turn, our experiments showed that vesicles, under
these unconfined experimental conditions, may form regardless of the presence of free pore water at slip initiation
Figure 7. Experimental pseudotachylyte microstructures (BSE-FESEM) at different imposed ambient conditions (a) vacuum,
(b) room humidity, (c) 5MPa fluid pore pressure, and (d) pseudotachylyte – protolith contact (white dashed line). Note the
presence of vesicles in the protolith minerals (in this case Chl: chlorite) next to the pseudotachylyte.

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(Figure7), as also reported by Violay, Nielsen, etal.(2014) and Violay, Di Toro, etal.(2015), for both condi-
tions, and by several authors for dry experiments (e.g. Fondriest etal.,2020; Han etal., 2014; Mittempergher
etal.,2014; Ujiie, Tsutsumi, etal.,2009). This experimental evidence points toward the possibility that volatiles
(and, as a consequence, their separation from the frictional melt and the formation of vesicles) could have origi-
nated by two (not mutually exclusive) mechanisms: (a) the breakdown and release of volatiles from hydrous- and
carbonate-bearing minerals—in the case of the Bolfín Fault Zone natural pseudotachylytes from Playa Escon-
dida: chlorite, calcite, epidote and actinolite—and (b) the presence of free pore water before seismic slip.Unfor-
tunately, based on the geochemical evidence presented here and the intense chlorite-epidote alteration of the
natural pseudotachylytes and both their host-rock and protolith, we are not able to discern between the two mech-
anisms. However, in this study we report the presence of vesicles in both natural rocks (as well as amygdales)
and laboratory samples, thus indicating that even in the presence of pressurized and free pore water (i.e., case for
the experimental pseudotachylytes) frictional melting can easily occur during seismic slip.The field and petro-
graphic evidences indicate that the pseudotachylytes in the Bolfin Fault Zone were formed by seismic slip during
a period of hydrothermal activity. Moreover, the subsequent mineralogical and experimental results indicate that
seismic melting under water saturated conditions is possible for the rock and mineral alteration observed in the
fault. It is still possible that the fault could have been dried out just before the initiation of the seismic activity and
that melting occurred in dry conditions. However, this is less probable and there is no evidence for such a change.
8.2. Composition of the Friction Melt at the Time of Ancient Seismic Faulting
Degassing of volatiles (and formation of vesicles) from silicate melts is controlled by melt temperature and
composition, amount of volatiles and confining pressure (Papale etal.,2006; Shishkina etal.,2010). Because the
vesiculated and non-vesiculated natural pseudotachylytes are intensively altered, it is not possible to determine
the glass (and infer the melt) composition at the time of seismic faulting. But the natural pseudotachylytes, based
on cross-cutting relationships, are the result of frictional melting of altered rocks similar, if not identical, in
Figure 8. Mineralogical compositions (XRD semi-quantitative analysis) of the Bolfín Fault Zone host-rock and fault-core,
natural pseudotachylytes (including their host rocks) and of the protolith sheared with SHIVA to produce experimental
pseudotachylytes (B21: chlorite-rich, for experiment s1692, and B23: epidote-rich for experiment s1688). Cal, calcite; Ep,
epidote; Chl, chlorite; Ttn, titanite; Bt, biotite; Act, actinolite; Kfs, K-feldspar; Pl, Plagioclase; Qz, Quartz.

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composition to those sheared with SHIVA in the laboratory. As a consequence, we assumed that the composition
of the solidified friction melt (=glass) before alteration was similar in composition to the glass (and to the melt)
produced in our experiments (Figure9). It is significant to note that the composition of the two experimental
glasses is very similar, if not identical, and is independent of the imposed environmental conditions (room humid-
ity, pressurized H2O or vacuum) and corresponds to that of a basalt (Figure9 and TableS3).
8.3. Water and Carbonate Vesiculation
In the experiments, the total amount of volatiles available during frictional melting can be estimated based on the
amount of H2O- and CO2-bearing minerals in the protolith (Equation1; e.g.,
Chl
HR



=15% wt. in sample; see
XRD analysis, Figure8 and TableS4) and in their respective volatile content (e.g.
HO
Chl
2



=12 wt.% H2O see
TablesS4 andS5, after Deer etal.[2013] and Di Toro & Pennacchioni[2004]):
  
 
  
min _
min _ HR
min _1
1
%wt. volatiles volatiles Min
100
i
i
(1)
%wtHO HO Chl HO Ep
HO
Chl HR Ep HR
.
22 2
2
1
100













FFe act HR Bt HR
Fe act H O Bt









2
(2)
  

  
22
Cc HR
1
%wt. CO CO Cc
100
(3)
Based on Equations2 and3, in the experimentally sheared samples, the %wt. H2O and %wt. CO2 can be calcu-
lated for the experimentally sheared protolith samples. %wt. H2O ranges from 1.68 (sample B23) to 2.11 (sample
B21) and the %wt. CO2 from 1.74 (sample B23) to 3.04 (sample B21). This estimation assumes that both hydrous-
Figure 9. Elemental analyses (EMPA and FESEM-WDS) of experimental pseudotachylytes formed at different conditions:
RH, Room Humidity; Vac, vacuum; Pf, pore pressure (H2O=5MPa). The elemental compositions of the experimental glass
are compared with those of a common basalt (black in color columns, from Papale etal.,2006). The reported wt.% are the
average of 4–6 points analyses (see TableS3). Beam spot size is marked in the inset.

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and carbonate-rich minerals where all precipitated prior to frictional melting,
even though there is evidence for late fluid circulation as attested by calcite
veins cross-cutting through the natural pseudotachylytes (Figure3f). Conse-
quently, they must be considered as maximum values of %wt. H2O and CO2.
What is the fate of these volatiles in a basaltic-in-composition friction melt
(EMPA and FESEM-WDS analysis)? The solubility of two component,
H2O-CO2, fluids in basaltic melts varies depending on molar fraction of the
fluid, confining pressure and melt temperature. Assuming total melting of
chlorite, epidote, Fe-actinolite and calcite, the mole fractions x for H2O and
CO2 are:
  

HO
2
H O CO H O
2 22
H O CO
22
and 1
n
x xx
nn
(4)
Experimental and theoretical studies showed that under equilibrium condi-
tions, the solubility of CO2 is up to one order of magnitude lower than for H2O
in basaltic melts (Papale etal., 2006; Shishkina etal., 2010). In particular,
under the experimental conditions investigated here (maximum fluid pres-
sure limited to 5MPa, and a maximum total normal stress of 20MPa), vola-
tiles (both H2O and CO2) are virtually immiscible in silicate melts (Wallace
etal.,2015) and it is not surprising to find empty vesicles in the experimental
pseudotachylytes (Figure6). However, in the case of the natural pseudotac-
hylytes produced between 4 and 8km depth, confining pressures can be up
to 200MPa and hydrostatic pore pressure will be 60–80MPa, and poten-
tially much higher. Under these effective confining pressures and assuming
thermodynamic equilibrium, constant fluid composition and temperature,
volatiles dissolve in the melt. In particular, at T= 1,200°C (a reasonable
temperature for these friction melts) a two component H2O-CO2 fluid with
xHO
2
=0.6–0.7 (as in the case of samples B21-B23) may dissolve in a basal-
tic melt up-to 2.3–4.1%wt. H2O and up-to 0.025–0.05%wt. CO2 from 100
to 200 MPa, respectively (Figure 10; combination of theoretical estimates
and experimental measures of H2O and CO2 solubility in basaltic melts, see
Figure 12b in Papale etal.[2006] and Figure6 in Shishkina etal. [2010]).
Because the maximum wt.% H2O content available in the melt might be up
to 2.11%, the presence of amygdales (i.e., filled vesicles, see Figure4) may
suggest either (a) that the pseudotachylytes were produced under very shal-
low conditions or (b) that there were free pore fluids in the slipping zone.
However, microstructural evidence from the calcite veins truncated by the pseudotachylytes (Figure3e), and
calcite clasts included in the pseudotachylyte (Figure3d), suggest that breakdown of calcite occurred during
seismic faulting. Given the very limited solubility of CO2 in basaltic melts, the formation of CO2 bubbles during
frictional melting was expected. A similar result was obtained by studying fluid inclusions in pseudotachylytes of
the Noijima fault (Famin etal.,2008). As a matter of fact, though the composition of the altered tonalite is quite
similar, and so is the H2O content, the pseudotachylytes found at Playa Escondida have a widely varying vesicle
content. We speculate that the presence of vesicles is related to the presence of nearby (i.e., few cm away) calcite
veins truncated by the pseudotachylyte. The implication is that H2O was completely dissolved in the melt and,
due to the highly altered nature of the rock, that there is no possibility to our knowledge to show whether or not
the frictional melt formed in the presence of H2O-rich pore fluids.
8.4. Brittle Overprinting and Alteration of Pseudotachylytes
The pseudotachylytes of the Playa Escondida outcrop are found principally in the fault-core, cutting foliated cata-
clasites (chlorite-rich) and some are dismembered and much more altered within the cataclastic rocks (Figures2c
and2d). The mineralogy of the pseudotachylytes consists mainly of quartz and plagioclase, which can be found
as un-melted clasts and recrystallized microlites within the matrix, respectively (Figure3). Also they are enriched
Figure 10. Solubility graph for H2O and CO2 in a basaltic in composition
melt at 1,200°C (after Shishkina etal.,2010). The diagram reports the
dissolved wt.% of H2O and CO2 in a basaltic melt. The continuous curves are
the confining pressures (in blue are reported the estimated depth of formation
of the pseudotachylytes from Playa Escondida) and the dashed curves are the
estimated mole fraction of xH2O at the time of seismic faulting (Equation4).
The red-dashed area is the admissible natural condition of solubility for
the pseudotachylytes from Playa Escondida, whereas the pink and green
colored areas represent the saturation conditions of dissolved H2O and
CO2, respectively, in a basaltic melt at 1,200°C. Importantly, CO2 is almost
immiscible in basaltic melts: the green area is located to the top left where the
confining pressures should be of the order of several GPa (note the change in
scale of the ordinate axis) to allow the dissolution of CO2 in the melt. Because
of the quite common presence of chlorite-epidote-calcite-bearing veins in the
host-rocks and their breakdown with release of CO2 during frictional heating
associated with seismic slip, the formation of vesicles was almost inevitable in
the friction melts (now pseudotachylytes) from Playa Escondida.

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in K-feldspar and slightly in calcite (both observed as late veins cutting the pseudotachylytes and as alteration
products of the matrix), and biotite and depleted of actinolite when compared to their contact host-rock (Figures3
and8), while chlorite is mostly found as an alteration product of the pseudotachylyte matrix (former glass in
Figure 3c). This alteration mineral assemblage is characteristic of a (sub-) greenschist facies condition. The
observed amygdales are mainly filled by calcite, epidote, chlorite and minor quartz (Figure 3), which clearly
imply post-seismic fluid percolation through the pseudotachylyte matrix (Figure4). Hence, the natural pseudo-
tachylytes were produced by the melting of hydrous- and carbonate-rich minerals during seismic faulting, most
likely under the presence of infiltrated hydrothermal fluids, giving to the pseudotachylytes (both natural and
experimental) a dark gray and brownish-red color (Figures2c, 2d, and6), where in the field the brownish-red
correspond to the oldest, most altered ones that resemble a cataclastic rock (either a cataclasite or an ultracatacla-
site), making them hard to distinguish.
Experimental evidence testing the preservation potential of pseudotachylyte’s primary microtextures in the
presence of hydrothermal fluids (Pp ≥150 MPa, T ≥300°C), documents their rapid (days to months) altera-
tion (Fondriest etal.,2020). Experimental pseudotachylytes were heavily altered with dissolution of the matrix,
neo-formation of clay aggregates and generation of a clastic microtexture (Fondriest etal.,2020). The alteration
of pseudotachylytes driven by hydrothermal fluids is consistent with earthquake faulting being responsible for the
rapid infiltration and redistribution of large volumes of fluids in the upper crust (Sibson,1981) and the generation
of hydrothermal mineralization; a condition which sensu lato can apply to the Bolfín Fault Zone case.
Pseudotachylytes can be intrinsically difficult to identify in fault zones, because they are ultrafine-grained dark
colored rocks, often very thin, and spatially associated or overprinted by other macroscopically similar fault
products such as ultracataclasites and ultramylonites (Kirkpatrick & Rowe,2013). As a result, the potential rapid
alteration of pseudotachylytes by hydrothermal fluids can further hamper the identification of pseudotachylyte
in natural fault zones. Pseudotachylytes, therefore, may be largely under-reported in the geological record, thus
leading to a biased evaluation of frictional melting as a relevant on-fault co-seismic process.
9. Conclusions
Tectonic pseudotachylytes are thought to be rare in the geological record because they are either rarely produced,
rarely preserved or both (Kirkpatrick & Rowe,2013; Sibson,1975). Moreover, some authors hold that pseudotac-
hylytes are thought not to be produced in fluid-rich (especially water-rich) environments (Sibson & Toy,2006).
In this view, the formation of frictional melts should be hampered by (a) the heat adsorbed by water which
would buffer the temperature increase in the fault-core and (b) the thermal expansion of water, which would
lead to fault dynamic weakening by thermal pressurization before bulk frictional melting may occur (Rice,2006;
Sibson,1973). However, from our observations of both natural and experimental pseudotachylytes we conclude
the following:
• In the Bolfín Fault Zone, a ca. 45 km-long strike-slip fault segment of the ca. 1,000 km-long Atacama Fault
System (Chile), and capable of having produced>M 6.0 earthquakes, we found pseudotachylytes produced
in a hydrothermal fluid-rich environment (Figures2 and3). These pseudotachylytes, which are hosted in the
continental crystalline basement of the Coastal Cordillera, are, to our knowledge, the first pseudotachylytes
described in the Chilean Andes, giving hints that they could be more common than previously thought even
in this tectonic setting.
• Although there is ample evidence for fluid-rock interaction during ancient seismic faulting along the Bolfín
Fault Zone (Figures2,3, and8), unfortunately, we could not demonstrate if pressurized water was present or
not at the exact moment of friction melting.
• Experiments reproducing seismic slip conditions and performed on the altered host-rocks of the natural pseu-
dotachylytes from Playa Escondida (Figure 8), confirm that frictional melts can form in the presence of
pressurized water (Figures6 and7). Moreover, the experiments suggest that the primary composition of the
matrix of the pseudotachylytes from Playa Escondida was similar to the composition of a basalt (Figure9).
This primary composition of natural pseudotachylytes was then altered as more fluids percolated through the
fault zone.
• Several pseudotachylyte fault-veins from Playa Escondida include amygdales in their matrix (Figure4). The
amygdales are interpreted as the result of degassing and vesiculation of H2O and CO2 in the friction melt

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(Figure10) and later post-seismic filling (quartz, epidote, chlorite, calcite) from percolating fluids during
post-seismic fault healing or in situ alteration (Figure4). The two main volatile species (H2O and CO2) can be
produced by the break-down of water- (chlorite, epidote, actinolite and biotite) and carbonate-bearing miner-
als (i.e., calcite) during frictional heating associated with seismic slip (this is confirmed by the experiments
conducted under vacuum conditions, Figure7) or may be already present in the pores of the fault-core before
seismic slip.
• CO2 has an extremely low miscibility in basaltic melts at any given effective confining pressure and temper-
ature with respect to H2O (Figure10). We suggest that the occurrence of vesicles (now amygdales) in the
matrix of the pseudotachylytes from Playa Escondida (amygdales were found only in some pseudotachylytes,
though the composition of the altered host-rocks is quite homogeneous and always includes epidote and
chlorite, Figure8) is probably related to the nearby presence of calcite-bearing veins cut by the pseudotachy-
lyte-bearing fault.
• In hydrothermal fluid-rich environments, as the one found in Playa Escondida evidenced by the presence of
volatile-rich minerals (hydrous and carbonate minerals) and veins, pseudotachylytes are prone to alteration
and are easily lost from the geological record.
Based on this evidence, we conclude that frictional melting can easily occur in fluid-rich fault zones as well as in
the presence of pressurized pore fluids according to what our experiments indicate. This implies that pseudotac-
hylytes might be more common than usually thought and, as a consequence, melt lubrication could be considered
one of the most efficient seismic dynamic weakening mechanisms in the basement rocks of the continental crust.
Data Availability Statement
The field structural data, the mechanical data of the experiments and EMPA, FESEM-WDS and XRD analy-
ses and volatile calculations are available in the Mendeley Data Repository under http://dx.doi.org/10.17632/
kkbkfwtgbd.1.
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Acknowledgments
The authors would like to acknowledge
the support of ERC CoG No 614705
NOFEAR. R. Gomila has received fund-
ing from the European Union’s Horizon
2020 research and innovation program
under the Marie Skłodowska-Curie grant
agreement No 896346 – FRICTION.
Leonardo Tauro is thank for thin-sections
preparation, while Federico Zorzi and
Marco Favero for XRD analyses and Raul
Carampin for the EMPA analyses at the
University of Padua. Andrea Cavallo for
FESEM-WDS analysis. E. Jensen thanks
to Fondecyt grant 1200170. The authors
thank Giorgio Pennacchioni and Ashley
Griffith for their help and constructive
discussions during fieldwork and Paolo
Gentile for FE-SEM analysis in Milano
Bicoccca University. The authors would
like to thank the work of the Editor, Clau-
dio Faccenna, and to the two reviewers,
Mark T. Swanson and Raehee Han. Their
comments and suggestions helped to
greatly improve the quality of the manu-
script. Open Access Funding provided by
Universita degli Studi di Padova within
the CRUI-CARE Agreement.

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