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\DOI{10.5802/crgeos.209}
\datereceived{2023-01-06}
\daterevised{2023-03-15}
\dateaccepted{2023-03-30}
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\dateposted{2023-05-15}
\begin{document}

\begin{noXML}

%\TopicEF{Geodynamics of continents and oceans - A tribute to Jean
%Aubouin}{G\'eodynamique des continents et des Oc\'eans - Hommage \`a
%Jean Aubouin}

\shortrunauthors

\title{$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar geochronology of crustal
deformation}

\author{\firstname{Patrick} \lastname{Moni\'{e}}\CDRorcid{0000-0003-2448-6335}\IsCorresp}
\address{G\'{e}osciences Montpellier, 
Universit\'{e} de Montpellier, CNRS, 
Universit\'{e} des Antilles, 34095 Montpellier, 
France}
\email[P. Moni\'{e}]{patrick.monie@umontpellier.fr}

\author{\firstname{Philippe} \lastname{M\"{u}nch}\CDRorcid{0000-0003-4616-8039}}
\addressSameAs{1}{G\'{e}osciences Montpellier, 
Universit\'{e} de Montpellier, CNRS, 
Universit\'{e} des Antilles, 34095 Montpellier, 
France}
\email[P. M\"{u}nch]{philippe.munch@umontpellier.fr}

\author{\firstname{Ga\'{e}tan} \lastname{Milesi}\CDRorcid{0000-0001-8658-9837}}
\address{GeoRessources, CNRS, 
Universit\'{e} de Lorraine, 
Labcom CREGU, 
54506 Vand\oe{}uvre-l\`{e}s-Nancy, 
France}
\email[G. Milesi]{gaetan.milesi@univ-lorraine.fr}

\author{\firstname{Michael} \lastname{Bonno}\CDRorcid{0009-0000-9362-2690}}
\addressSameAs{1}{G\'{e}osciences Montpellier, 
Universit\'{e} de Montpellier, CNRS, 
Universit\'{e} des Antilles, 34095 Montpellier, 
France}
\email[M. Bonno]{michael.bonno@umontpellier.fr}

\author{\firstname{Arthur} \lastname{Iemmolo}\CDRorcid{0009-0008-1488-4309}}
\addressSameAs{1}{G\'{e}osciences Montpellier, 
Universit\'{e} de Montpellier, CNRS, 
Universit\'{e} des Antilles, 34095 Montpellier, 
France}
\email[A. Iemmolo]{arthur.iemmolo@umontpellier.fr}

\keywords{\kwd{Geochronology}
\kwd{Argon}
\kwd{Fault}
\kwd{\textit{In situ} dating}
\kwd{\textit{In vacuo} encapsulation}
\kwd{Clays}}

\begin{abstract} 
Crustal deformation is characterized by brittle and ductile faults that
accommodate at different scales the strain imposed by plate tectonics.
The aim of this contribution is to show with the help of different
examples how the \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating
of synkinematic neocrystallized minerals in ductile shear zones and the
step-heating $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of
synkinematic authigenic clays in fault gouges can bring information on
the timing of fault activity. However, due to their complex evolution,
interpretation of the argon signature in fault zones requires
consideration of several effects among which re- or neocrystallization,
inheritance and fluid interaction processes are dominant.\vspace*{-2pt}
\end{abstract}

\maketitle

\vspace*{-14pt}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar geochronology is one of the most
efficient tools for determining the timing of deformation events at
different crustal levels, thus it is very useful for constraining the
entire development of an orogen from the first stages of its building
until its final erosion. Since the first application of this method to
terrestrial samples  \citep{Fitchetal1969}, the dating of deformation
in crustal rocks has been the focus of many studies, even if until the
end of the 80s, few papers report $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
ages directly on this topic 
\citep[e.g.][]{Dallmeyer1975,Dallmeyeretal1985,AllenStubbs1982,Monieetal1984,AveLallemantetal1985,Kligfieldetal1986,McCaigMiller1986,KeppieDallmeyer1987,CostaMaluski1988,HubbardHarrison1989}.
However, it is well known that in mountain belts, a large amount of the
strain imposed by plate tectonics is accommodated by multiscale
deformation zones formed under variable pressure, temperature and fluid
regimes. Giving absolute time constraints on deformation is fundamental
for determining the propagation of deformation through space and time
and unraveling the complex evolution of the different units of an
orogen and their juxtaposition  \citep{Orioloetal2018}. However, dating
of deformation zones remains challenging since most of them record
variable $P$--$T$-strain conditions, resulting in the coexistence of several
mineral generations which can be pre-, syn- or postkinematic with
respect to the last deformation  \citep[e.g.][]{Lanarietal2014}. In
addition, the interpretation of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages
of K-bearing minerals (most often phyllosilicates, more rarely
feldspars or amphiboles) is complicated by the fact that deformation
introduces microstructural defects in these minerals, favoring their
partial or complete fluid-induced recrystallization that have a great
effect on the distribution and the retention of argon in their
crystalline lattice at the nanometer scale
\citep{Naumenko-Dezesetal2021,Villa2022}. It should also be stressed that for
these deformed rocks the interpretation of the
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar data in terms of cooling age or
(re)crystallization age remains a subject of debate as well as the
mechanisms of age reset during deformation 
\citep{Villaetal2014,Villa2016}. Not to mention that there is a wealth
of evidence in the literature that the isotopic behaviour of argon is
very different for deep/middle crustal and near-surface
deformation.\looseness=-1

In large and hot orogens, the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dated
minerals of deformed rocks usually reached a state of complete
textural, chemical and isotopic equilibrium, and for very slow cooling
rate, measured ages record cooling at temperature below the closure
temperature of the isotopic systems (in the range  300--550~\textdegree
C for mica and amphibole), which can occur hundreds of myrs after their
crystallization due to slow heat dissipation. In this type of
environment with important time-lag between the crystallization of
K-bearing minerals and their isotopic closure for argon (e.g. old
cratons), the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method is considered
essentially as a method of thermochronology  [it dates when a mineral
passes through a specific closure temperature, 
\citealp{Dodson1973,McDougallHarrison1999}] that can be combined with
other methods (e.g. U--Pb, Sm--Nd, Rb/Sr) to unravel the complex
evolution of rocks during their progressive cooling/exhumation. In
contrast, for colder  (500--300~\textdegree C), and/or more localized
deformations, the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar age interpretation
in deformed zones is not so straightforward, in particular in the case
of polydeformed rocks. Different issues have to be considered then: the
partial or total resetting of inherited minerals through a combination
of (i)~thermally activated volume diffusion, deformation and
fluid-assisted dissolution--recrystallization processes, (ii)~the
crystallization, cooling and closure of newly-formed, synkinematic
and/or postkinematic minerals and (iii)~eventually the presence of
excess argon held by fluids. Considering the fulfilled assumptions of a
complete reset of the inherited minerals and the absence of excess
argon, $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages are most often regarded
as (re)-crystallization ages and the method is considered as a method
of geochronology  [it dates mineral (re)crystallization, not cooling; 
e.g.
\citealp{MulchCosca2004,Villaetal2014,DiVincenzoetal2016,Villa2016,Beaudoinetal2020} 
among many others]. Ideally, the method must be combined with other
dating methods, such as the Rb--Sr internal isochron method 
\citep[e.g.][]{FaureMensing2004,RoselZack2021} or the U--Pb method on
monazite or apatite 
\citep[e.g.][]{Parrish1990,Harrisonetal2002,Chewetal2011} which can add
additional constraints on the metamorphic and tectonic evolution of
rocks deformed at intermediate temperatures. In all these deformed
rocks, the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of K-bearing
phases is now conducted using mainly lasers with different approaches:
step-heating of small mineral populations or single grains, fusion of
individual grains, \textit{in situ} ablation within a single grain or
at the scale of a small rock section, with a spatial resolution of
several tens of microns. As discussed below, this spatial resolution is
still insufficient to get a very precise distribution of argon atoms in
minerals from sheared zones. However, the combination of these
approaches, which has benefited from an improvement in the analytical
performance of mass spectrometers over the last ten years, can help to
better understand the behavior of argon in deformed rocks and the
meaning of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages.

For more superficial deformation conditions, many
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar studies have successfully focused on
fault gouges using authigenic phyllosilicates that form during the
activity of these faults in a temperature range corresponding to the
muscovite--illite transition  (i.e. ductile--brittle transition,
${\sim}300~\text{\textdegree}\mathrm{C}$) to the illite--smectite
transition  [${\sim}$140--90~\textdegree C, 
\citealp{FreedPeacor1989}]. Authigenic phyllosilicates in fault gouges
are characterized by their small size  $({<}2~\rmmu\mathrm{m})$, thus
corresponding to clay minerals, and usually coexist with inherited
K-bearing phases from the host rocks. The dating of these superficial
deformations is often rather tricky for several reasons: (1)~the size of
the neoformed clays requires efficient mineral separation techniques;
(2)~the proportions of authigenic and inherited phases in the separated
grain-size fractions have to be quantified; (3)~due to their small size,
clay minerals are subject to recoil of $\mbox{}^{39}$Ar during
irradiation, resulting in an ageing of the samples. It is shown in the
second part of this contribution that these limitations can be overcome
and that the situation can sometimes be more complex due to the
polyphased activity of faults and various fluid interactions.\looseness=-1

Therefore, the present contribution is organized into two main parts
addressing specific aspects of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
dating of ductile and brittle deformation in deep and superficial
crustal domains through various examples illustrating the complexity of
age interpretation in deformed rocks. These examples highlight in
particular the benefit of coupling \textit{in situ} laser
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses with detailed
microstructural and petrological observations for the study of ductile
deformations and the reconstruction of $P$--$T$--$d$--$t$
(pressure--temperature--deformation--time) paths. The second section is
focused on brittle deformation and fault gouges and shows the
advantages of coupling geochronology and thermochronology in orogenic
belts. 

\section{$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar geochronology of ductile
deformation}\label{sec2}

\subsection{Some methodological aspects}\label{sec2.1}
It is beyond the scope of this contribution to present the technical
details of the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method that can be
found in the reference book of  \citet{McDougallHarrison1999} and more
recently in  \citet{Jourdanetal2014} or  \citet{Schaenetal2021}. In a
simplified way, the method is derived from the
$\mbox{}^{40}$K--$\mbox{}^{40}$Ar method, $\mbox{}^{39}$Ar being a proxy
of the quantity of $\mbox{}^{40}$K after fast neutron irradiation in a
nuclear reactor and the age directly dependent on the ratio between the
two argon isotopes and the fluence of neutrons calculated from argon
isotopes released by co-irradiated flux monitors. Technical aspects of
the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method, which concern the age of
the monitors, decay constants, argon recoil effects, or the current
$\mbox{}^{40}$Ar/$\mbox{}^{36}$Ar atmospheric ratio, are still under
debate but will not be addressed here \citep{Schaenetal2021}. Soon
after the inception of the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method 
\citep{MerrihueTurner1966}, laser microprobe
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses were first performed on
lunar breccias \citep{Megrue1973} and some years later on terrestrial
samples  \citep{Yorketal1981,MaluskiSchaeffer1981} using continuous or
pulsed lasers for a sampling of small material amount 
(${<}$milligram). From there on, many laboratories worldwide developed this
technique using different lasers that can be coupled now with very
sensitive mass spectrometers to produce very precise and reproducible
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages (from 3.5~Ga to some ka) on
K-bearing terrestrial samples. The method is under constant improvement
and addresses a wide range of geological processes such as
metamorphism, magmatism, hydrothermalism, diagenesis, weathering,
deformation\,{\ldots}. Its range of applications is then very large from
geodynamics, paleomagnetism, mineral and geothermal resources, seismic
risk but also paleontological or climate
evolution. \looseness=-1

As indicated in the introduction and schematized on  Figure~\ref{fig1},
different methods of argon extraction can be achieved with the
different types of laser. This includes first the step-heating
degassing of single grains or micro-populations of K-bearing samples by
increasing progressively the power of a continuous infrared  CO$_{2}$
laser beam applied to the grains. An age is calculated at each argon
release increment and the whole step data is portrayed in an age
spectrum and inverse isochron plot. In some instances, Ca/K and Cl/K vs.
age correlation plots can help to better differentiate the various
argon reservoirs that can coexist in samples with a complex history of
metamorphic-deformation processes and fluid interactions 
\citep[e.g.][]{Mulchetal2005,Simon-Labricetal2009,Allazetal2011}. 


\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}From left to right,  (1)~different modes of laser
microprobe step-heating of a  single grain or micro-population, 
(2)~age and K/Ca spectra and (3)~Cl/K spectrum and inverse isochron
plot for a homogeneous amphibole micro-population 
\citep{Fernandezetal2020}.}
\end{figure*}

However, it is well-known that hydrous phases such as micas and
amphiboles do not release argon by volume diffusion when heated under
vacuum because of their metastability and that age spectra do not
reflect the true argon distribution in the dated minerals 
\citep[e.g.][]{SlettenOnstott1998,Loetal2000,Villa2021}. This is well
supported by the different studies comparing step-heating and
\textit{in situ} laser probe experiments, for instance in the case of
slow cooling terranes, excess argon contamination, thermal or tectonic
overprint 
\citep[e.g.][]{Phillips1991,Hodgesetal1994,Monieetal1994,Kelletetal2016}.
Caution should therefore be exercised when interpreting step heating
results and, in case of doubt, a comparison with other modes of argon
extraction or other isotope chronometers may be useful.

Continuous laser beams can also be used for the total fusion of
individual grains 
\citep[e.g. ][]{Uunketal2018,Gemignanietal2018,Porkolabetal2022}. Ages
are usually reported in probability or \mbox{Kernel} density plots
\citep{Ludwig2012,Vermeesch2018,Schaenetal2021} in order to identify
the different age populations in a rock sample  (Figure~\ref{fig2}), a
method that requires a large number of analyses to get statistically
meaningful peak ages. This approach, for which the main criterion of
grain selection is their large size, can be used to better interpret
discordant age spectra from complex polymetamorphic tectonites. 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Distribution of total fusion ages for  (a)~a
homogeneous population of biotite with few grains showing inherited or
excess argon  and  (b)~a strongly heterogeneous population of muscovite
showing decreasing amount of argon resetting from left to right  [Cap
de Creus samples,  \citealp{Monieetal2018}].}
\vspace*{-2pt}
\end{figure*}

However, in deformation zones where phyllosilicates are the main target
of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses, micas of different
origin, size and chemistry can occupy different microstructures and the
distinction between not reset, partially reset, totally reset and newly
crystallized populations is not always straightforward, without
discarding the problem of excess argon contamination. The
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar laser fusion dating of single grains
does not always resolve these complex isotopic situations since, in
general, few attention is paid to the grain-size effect on the age
distribution or to the chemical heterogeneities within and between
micas. By contrast, the \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method has the advantage to preserve
the textural and chemical relationships between the different mineral
generations in polydeformed rocks and to establish correlations between
ages, mineral fabrics and petrology at the scale of several tens of
microns, corresponding to a volume large enough to measure accurately
the different argon isotopes on the mass spectrometer (typically a
square of  $150\times150~\rmmu\mathrm{m}$ and  20 to 
$50~\rmmu\mathrm{m}$ deep). Different pulsed lasers have been used
since the first dating of terrestrial samples 40 years ago, including
infrared ruby lasers, argon-ion visible lasers, ultra-violet Nd--YAG
lasers and more recently UV excimer lasers that provide the best
spatial resolution for \textit{in situ} analyses. These analyses that
can be realized at the scale of a single grain if it is large enough to
perform several ablations inside  (Figure~\ref{fig3}), have the
potential to resolve core to rim zonations or other kinds of age
distribution due to slow cooling or partial resetting by thermally
activated diffusion or chemical exchanges and recrystallization 
\citep[e.g.][for some of the oldest
references]{PhillipsOnstott1988,Scailletetal1990,HamesHodges1993,HamesBowring1994,Monieetal1994}. 

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}\textit{In situ} ages obtained inside single mica
grains using an argon-ion visible laser (a,b) and a quintupled UV laser
(c).  (a)~Homogeneous Triassic ages inside a high-pressure phengite
from Turkey  \citep{OkayMonie1997};  (b)~core-rim age zonation in a
thermally overprinted Variscan magmatic biotite from the Eastern
Pyr\'{e}n\'{e}es  \citep{Monieetal1994};  (c)~irregular age zonation in
a partially chloritized Variscan biotite from the C\'{e}vennes schists
\citep{Montmartin2021}.}
\end{figure*}

However, for most \textit{in situ} studies of deformed rocks, dating
experiments are not realized at the scale of a single mineral, but at
the scale of a portion of thin section  (Figure~\ref{fig4}) in order to
deconvolute the complete history of deformation and metamorphism.

\begin{figure*}
\includegraphics{fig04}
{\vspace*{-1pt}}
\caption{\label{fig4}Different modes of \textit{in situ} argon
extraction with a quadrupled UV laser.  (a)~Typical surface of gneiss
thick section after lasing. The micas aligned along the two
schistosities S1 and S2 have been dated.  (b)~Detailed view of the
surface of a mica after a fast laser rastering. The depth of the pits
is less than $10~\rmmu\mathrm{m}$.
(c)~Laser drilling on a muscovite to a depth
of about $50\ndash70~\rmmu\mathrm{m}$.
Note the white ejecta around the craters.}
{\vspace*{-1pt}}
\end{figure*}

In an orogen, ductile shear zones concentrate a large part of
deformation and, because of their efficient reactivity, phyllosilicates
are ideal candidates for recording the short- and long-term evolution
of faults through their chemistry (major elements, trace elements and
fluid composition), grain size and isotopic ages. Thus, microscope
observation of thin sections usually is a first step to identify the
different generations of pre-, syn- and postkinematic minerals suitable
for dating  (Figure~\ref{fig5}).

\begin{figure*}
\includegraphics{fig05}
\vspace*{3pt}
\caption{\label{fig5}Examples of textural relationships in metamorphic
tectonites, from left to right,  (a)~micaschist with prekinematic large
muscovite (Musc1) in a fine-grained matrix of quartz--feldspar--muscovite
(Musc2)  [Cap de Creus massif,  \citealp{Monieetal2018}]; (b)~partially
reworked garnet--sillimanite--quartz--biotite (Bio1) gneiss in a matrix of
quartz and fine-grained biotite (Bio2). Note the different habits of
prekinematic biotite as clasts in the new foliation, in a garnet
pressure-shadow and in a crack inside garnet  [Agly massif, 
\citealp{Aumaretal2022}];  (c)~albite--quartz--epidote--phengite (Ph1)
micaschist crosscut by a chlorite--phengite (Ph2) shear band [Andros
island,  \citealp{Huetetal2015}].}
\vspace*{3pt}
\end{figure*}

\pagebreak
The examples of  Figure~\ref{fig5} illustrate the importance of
identifying the different textural habits of the minerals that can be
dated by \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar laser
analyses. Two samples from Cap de Creus and Agly massifs 
(Figure~\ref{fig5}a,b) highlight the evolution of grain size in
tectonites, with prekinematic micas being usually larger than
synkinematic matrix micas. In most tectonites, the fine-grained matrix
consists of two mica types: a first type resulting from a strong
mechanical reduction in size of part of the inherited micas
(comminution), and a second type that is newly formed by a
fluid-enhanced dissolution--precipitation mechanism. Therefore, the
matrix micas can only give homogeneous ages if those of the first type
have undergone a complete isotopic resetting of their K--Ar chronometer
during comminution, which is closely dependent on the crustal
deformation conditions (temperature, pressure, duration, fluid
behaviour, open/closed system behaviour of the host rock\,{\ldots}). For the
third sample from Andros island  (Figure~\ref{fig5}c), a first
generation of synkinematic phengite Ph1 is aligned along the main
foliation while synkinematic phengite Ph2 is also present in
association with chlorite along a late shear band. However, it is
likely that together with the neocrystallization of phengite Ph2, some
early phengite Ph1 was caught by the shear band. Therefore,
\textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar mica dating in this
kind of microstructure can constrain the last deformation age only if
micas inherited from the main foliation have been fully reopened (or
recrystallized) along the shear band, which is also dependent on the
deformation conditions. Insofar as the ages of the different
generations of synkinematic micas can be interpreted as reflecting
crystallization ages (see below), they can provide an indication of how
long a shear zone remains active  \citep[e.g.][]{Schneideretal2012}.

After this first step and before dating, it is fundamental to
consolidate the microstructural observations by a petrological study of
the mineral assemblages to constrain the $P$--$T$ conditions of the
different deformation episodes. Detailed EPMA analyses coupled with SEM
imaging or EDS mapping are the main tools used to decipher the chemical
composition of datable minerals, and to reveal at the scale of a thin
section inter- and intra-grain variations related to more or less
complex $P$--$T$--$D$ paths recorded by the tectonites during the orogenic
cycle.  Figure~\ref{fig6} shows some examples of chemical data that can
be served as a guide to perform texturally and chemically-controlled
\textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses in
deformed crustal rocks.

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}Examples of mica chemical variations that can be
observed in metamorphic rocks:  (a)~EPMA analyses of phengites in a
retrogressed high-pressure (HP) micaschist fron Iran with two
populations that can correspond to the situation in  Figure~\ref{fig6}b
[core-rim zoning, \citealp{Rossettietal2017}] or that in 
Figure~\ref{fig5}c (HP foliation crosscut by lower pressure shear
band);  (b)~SEM imaging of a zoned phengite with a celadonite-rich core
and a poorer rim;  (c)~EPMA analyses of  TiO$_{2}$ content of muscovite
in an orthogneiss from the South Armorican Massif. Note the high Ti
content in the mica core compared to the rim and to the newly
crystallized muscovite along the foliation;  (d)~X-ray mapping of Ti
content in muscovites from  Figure~\ref{fig6}c
\citep{Turrillotetal2011}.} 
\vspace*{2pt}
\end{figure*}

Therefore, before \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
dating, a detailed petro-structural study of the samples that are the
most representative of the study area is highly recommended and
depending on the goals pursued, the study should focus on texturally
and chemically homogeneous samples dominated by a single K-bearing
mineral population (early or late) or on more complex samples that
record several phases of deformation--(re)crystallization 
(Figure~\ref{fig6}). As shown on  Figure~\ref{fig4}, the \textit{in
situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method has the ability to
provide ages with a spatial resolution of about 
$100\ndash150~\rmmu\mathrm{m}^2$
that is enough to get first-order data on the homogeneity or
heterogeneity of dates at the scale of a thick section. It is likely
that this spatial resolution will be improved in the future with the
advent of increasingly sensitive mass spectrometers. However, many
studies have shown that reliable textural--chemical--age relationships
can be already established with current
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar spatial resolution 
\citep[e.g.][]{MaluskiMonie1988,Kramaretal2001,Agardetal2002,MulchCosca2004,Mulchetal2002,Augieretal2005,Mulchetal2005,DiVincenzoetal2007,Schneideretal2012,Angiboustetal2015,Bellangeretal2015,DiVincenzoetal2016,Kelletetal2016,Scharfetal2016,Bergeretal2017,Laurentetal2017,Angiboustetal2018,Tanetal2019,Beaudoinetal2020,Ghignoneetal2021,DiVincenzoetal2022}.

\subsection{Age interpretation}\label{sec2.2}
From the earliest applications of the method to the dating of magmatic
and metamorphic rocks, a notion of isotopic system closure temperature
emerged and was formulated by  \citet{Dodson1973} on the basis of
volume diffusion laws. During cooling, a K-bearing mineral will start
to accumulate radiogenic argon at a certain temperature  $T_{1}$ and
will become a closed system at lower temperature  $T_{2}$, the
temperature interval $T_{1}$--$T_{2}$ defining what is called the
partial retention zone. The closure temperature  $T_{c}$ of this
mineral lies within this partial retention zone whose range is
dependent on the cooling rate. For fast cooling rates (e.g. volcanic
rocks),  $T_{1}$,  $T_{2}$ and $T_{c}$ are very close and the
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages can be interpreted to date
crystallization. This is the case of the sanidine standards from
volcanic rocks used to monitor the fast neutron flux during sample
irradiation whose $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages agree with
U--Pb zircon ages  \citep[e.g.][]{Flecketal2019}. It is also common to
observe this concordance of U--Pb and
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages for granites emplaced in the
upper crust, with micas revealing no core to rim age zonation due to
fast cooling \citep[e.g.][]{Brichauetal2007}. Note that in deformed
rocks from the mid-upper crust, synkinematic minerals formed at
temperatures close to  500--550~\textdegree C can also provide similar
monazite U--Pb and $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar mica ages
interpreted as crystallization ages  \citep[e.g.][]{Aumaretal2022}. For
very slow cooling low- to mid-crustal deformed rocks (e.g. large
Proterozoic cratons), the partial retention zone is usually very large
and the time elapsed between the crystallization of K-bearing minerals
and their complete closure for argon diffusion can reach several
hundred million years. This results in core to rim age zonations
reflecting a progressive closure of the K--Ar chronometer 
\citep[e.g.][for muscovite]{Hodgesetal1994} that can be modeled using
thermal volume diffusion laws 
\citep[e.g.][]{Warrenetal2012,Skiptonetal2018}. Indeed, by contrast
with the fast cooling setting, the core to rim age range represents a
segment on the cooling path which length is dependent on grain size,
cooling rate and residence time of the crustal rocks at given
temperature and pressure, diffusion parameters of the mineral species
under consideration and the open behavior of the host rock. Available
K--Ar systems permit to cover a continuous range of closure
temperatures from about  550~\textdegree C (amphibole) to 
150~\textdegree C (K-feldspar) that have been determined empirically,
theoretically and experimentally. To reconstruct the $T$--$t$ cooling
trajectory above and below this temperature range, complementary
geochronological and thermochronological studies using U--Pb, Sm--Nd,
Rb--Sr, fission tracks and U--Th--Sm/He methods are required. Therefore,
in very slowly cooled high-temperature rocks with a single episode of
deformation at peak temperature, the ages provided by the
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method can only be considered as 
minimum estimates of the deformation age. When different deformation
episodes occur during cooling, thermo-barometric analysis of
synkinematic mineral assemblages allows in principle to assign the
different deformation stages along the $P$--$T$ path. If these deformations
occur at temperatures close to the isotopic closure temperatures of
argon chronometers, \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
dating of the synkinematic K-bearing minerals can constrain the timing
and duration of tectonic events and the rate of deformation propagation
or localization, provided that the dated minerals do not contain
inherited or excess argon which can be resolved by \textit{in situ}
intra- and inter-grain analyses. However, at present, large
uncertainties remain on the closure temperature of the mineral species
used for $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating, due to the lack of
precision of the experimentally determined diffusion parameters, the
validity of diffusion experiments themselves as well as the
influence of the multiple parameters that can affect argon retentivity
in the minerals during cooling (chemical composition, pressure,
lithology, open/closed behavior of host rock, fluid composition and
behaviour\,{\ldots}). This is true in particular for the white mica
group for which a large range of closure temperature is suggested
by different studies, from 350 to  550~\textdegree C or more 
\citep[e.g.][]{HamesBowring1994,Harrisonetal2009,Ntemeetal2022}, a
range which remains an area for future exploration. As a consequence,
the reconstruction of $P$--$T$--$D$--$t$ path in polydeformed high-temperature
rocks using $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar data can suffer from
this indetermination of the retention properties of the dated minerals
and for this purpose the use of multiple geochronometers is highly
recommended. It is important also to mention that the mechanisms
involved during the closure of an isotopic system may be different from
those leading to its resetting. In particular, various
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar laser probe studies of polydeformed
high-temperature rocks showed that a mineral can survive
temperatures well in excess of its closure temperature without loss of
radiogenic argon 
\citep[e.g.][]{Kelleyetal1997,DiVincenzoetal2001,Maureletal2003,Schneideretal2008}. This is interpreted to reflect a closed system behaviour of the host rock with no or limited fluid-rock interaction due to intrinsic rock properties (mineralogy, porosity, deformation, interstitial fluid amount) and conditions of overprinting (temperature, pressure, duration, fluid behavior). This is to say that in a polyphased context, the interpretation of ages strictly in terms of crossing a given temperature barrier in one direction or the other can be debated without a proper understanding of the variable parameters that control the closure and opening of the chronometers being used. This is a highlight made several years ago by 
\citet{ChopinMaluski1980} in their studies of some high-pressure rocks
of the Western Alps and subsequently validated by many geochronological
works [see  \citealp{Agardetal2002,Ghignoneetal2021} for \textit{in
situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar studies in the same area, or 
\citealp{McDonaldetal2016} for the HP gneisses
of~Norway].

\begin{figure*}
\includegraphics{fig07}
\vspace*{1pt}
\caption{\label{fig7}\textit{In situ} and step-heating
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar data from the In Ouzzal shear zone.
(a)~\textit{In situ} dates on prekinematic and synkinematic amphibole
from a sheared granophyre;  (b)~age spectra of amphibole and biotite
from a syntectonic gabbro; (c)~\textit{in situ} date distribution
(Kernel density plot) on fine biotites from a narrow mylonitic band 
\citep{FerkousMonie2002}.}
\vspace*{2pt}
\end{figure*}

For deformations that occur at lower temperature, from  500~\textdegree
C to the ductile--brittle transition at  300~\textdegree C, it is
frequent that different generations of K-bearing minerals
(re-)crystallized along the $P$--$T$--$D$--$t$ path which renders the
interpretation of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages somewhat
difficult due to the interaction of several effects of grain
deformation during shearing, chemical exchanges enhanced by fluid
circulation, dissolution--precipitation reactions that can result in
isotopic heterogeneities at a very fine scale. However, as mentioned
above, there is now compelling evidence that texturally and
chemically-controlled \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses of such rocks can provide
significant data to decipher the succession of deformation episodes in
tectonites. This is also due to the fact that for rocks deformed below 
500~\textdegree C, it is now widely accepted that
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages of synkinematic\unskip\break 
minerals
represent (re)-crystallization ages rather than cooling ages 
\citep[e.g.][]{Agardetal2002,Augieretal2005,Rollandetal2008,Sanchezetal2011,DiVincenzoetal2016,Villaetal2014,BosseVilla2019}.
However, the interpretation of \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses in polydeformed rocks can be
faced with different problems: the isotopic inheritance in relict
phases that are not fully reset during shearing, the chemical
complexity of newly formed minerals (zonation, mica interlayering) and
their fine grain size, the presence of short circuit pathways for argon
migration, the presence of excess argon trapped at the time of mineral
crystallization or later, the late interaction with fluids. Below, we
present some examples that highlight the relevance of \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses compared to more
conventional argon degassing modes for dating mid-upper crustal
deformation in
orogens.

\subsection{Examples of \textit{in situ}\ 
$\mbox{}^{40}$Ar/\,$\mbox{}^{39}$Ar dating of shear zones}\label{sec2.3}

\subsubsection{\textit{In situ}\ $\mbox{}^{40}$Ar/\,$\mbox{}^{39}$Ar
dating of amphibole from shear zones}\label{sec2.3.1}
In crustal deformation zones, micas are the main target of \textit{in
situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar investigations because of their
ability to accommodate strain constraints by different deformation
mechanisms (kinking, gliding parallel to cleavage) and different
(re-)crystallization processes in the presence of fluids. On rare
occasions, shear zone amphiboles were dated by this technique and
an example from the upper \mbox{Neoproterozoic} shear zone of In Ouzzal 
in southern
\mbox{Algeria} is given below  \citep[from][]{FerkousMonie2002}. In
south Algeria, the Hoggar shield belongs to a Neoproterozoic
(800--600~Ma) orogenic belt that was juxtaposed to the Archean
West African craton by large-scale N--S striking subvertical
strike-slip shear zones under upper greenschist--amphibolite
metamorphic conditions. The studied shear zone is a 3~km wide corridor
made of protomylonites, mylonites and ultramylonites and contains
abundant pre- to synkinematic igneous rocks, as well as syn- to
postkinematic auriferous quartz veins. Amphibole from a prekinematic
granophyre is of two types, brown magmatic porphyroclastic pargasite
and blue-green ferrotschermakite in the pressure shadow of pargasite. 
\textit{In situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses
(Figure~\ref{fig7}a) provide dates between 749 and 849~Ma for the
magmatic amphibole and between 626 and 650~Ma for the metamorphic one,
consistent with the step-heating analyses of amphibole (623~Ma) and
biotite (613~Ma) from a synkinematic granodiorite in the shear zone 
(Figure~\ref{fig7}b). The estimated $P$--$T$ conditions of shearing are
about 0.4~GPa and  450--500~\textdegree C, suggesting that the dates of
synkinematic ferrotschermakite can be interpreted as crystallization
ages close to the peak of metamorphism related to the main episode of
fault activity along the In Ouzzal shear zone, in agremeent with the
muscovite age of synkinematic gold-rich quartz veins (611~Ma). In this
sample, the primary magmatic amphibole is partially reset and the date
of 849~Ma can be taken as a minimum estimate for the age of granophyre
emplacement and cooling. This large deformation corridor was
locally \mbox{reactivated} about 100~Ma later by a second episode of
strike-slip tectonics as attested by  \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of fine grained biotite in a
narrow ultramylonitic band  (Figure~\ref{fig7}c).

This study highlights (1)~the interest of the laser microprobe approach
to differentiate the ages of synkinematic amphibole from prekinematic
amphibole, which is not always possible with the dating of mineral
populations; (2)~the common characteristic of faults to be weakness
zones that can have a long term activity, and which can result in the
coexistence of multiple generations of newly formed minerals during
diachronous episodes of shearing. 

\subsubsection{\textit{In situ}\ $\mbox{}^{40}$Ar/\,$\mbox{}^{39}$Ar
dating of white mica from shear zones}\label{sec2.3.2}
Three examples are presented in this section, the Oisans external
massif (western Alps), and two massifs of the eastern Pyrenees, the
Agly massif (North Pyrenean Zone) and the Cap de Creus massif (Axial
Zone). All these massifs are made of a Variscan basement overprinted to
various extent by Alpine deformations under greenschist to upper
greenschist metamorphic conditions, resulting in a mixture of Variscan
and Alpine mineral assemblages dominated by Variscan minerals in the
weakly deformed rocks and sometimes vice-versa. For these massifs,
there were few geochronological studies of the deformation zones
themselves and debates persisted about the relative importance of
pre-Alpine or Alpine deformation on the present structuration of these
two belts. This is particularly true for the two Eastern Pyrenees
massifs for which in the absence of systematic geochronological studies
focused on shear zones, controversies still remain since several years
on the age of their activity, varying by authors from the late
Paleozoic to the Eocene. 

In the Oisans massif, \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar studies were carried on different
W-directed reverse shear zones that accommodate E--W shortening during
the Alpine collision. The main goal of this study was to document the
westward propagation of the deformation on variously strained samples
and to evaluate the effect of mixture between partially or totally
recrystallized Variscan muscovite and newly-formed synkinematic Alpine
phengite in protomylonites, mylonites and phyllonites formed at
temperature below  350~\textdegree C \citep{Bellangeretal2015}. EPMA
analyses of white-micas yield two distinct end-members with very few
intermediate values. The first end-member corresponds to prekinematic
muscovite clasts (Si $=$ 3.15; Na $=$ 0.06--0.08) and the second one to
synkinematic phengite (Si $=$ 3.2--3.4; Na $=$ 0--0.02).  \textit{In situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses were obtained on three
samples selected over a single strain gradient of a major shear zone.
The results are synthesized in  Figure~\ref{fig8} which shows for each
protomylonite, mylonite and phyllonite sample, the textural aspect of
the dated rock section, the distribution of \textit{in situ} dates on
this section and the Kernel density plot for these
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages.\looseness=-1

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}\textit{In situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
analyses on three samples of Variscan gneiss showing, from left to
right, an increase of Alpine deformation over a major shear zone of the
Oisans massif. The red line corresponds to the maximum age of Alpine
deformations imposed by stratigraphic constraints  
\citep[modified from][]{Bellangeretal2015}.}
\end{figure*}

In the less deformed sample containing discrete shear bands
(protomylonite), \textit{in situ} ages are scattered between 
$207\pm5$~Ma and  $24\pm2$~Ma (Triassic to Miocene). Of the 29 dates,
only 6 are younger than 34~Ma, the maximum age of Alpine deformations
imposed by stratigraphic constraints. Interestingly, these dates come
from small and low Cl/K (${<}$0.01) phengites from the core of
the shear bands. 12 analyses provide the oldest dates
(${>}$50~Ma) outside the shear bands and are correlated with
relatively high Cl/K ratio (0.01--0.02). They correspond to partially
recrystallized inherited muscovites with no peak age in the cumulative
correlation plot, which indicates that Variscan muscovite experienced
variable argon loss during Alpine thermotectonic overprint and
interaction with Alpine fluids.
The data do not enable to discount
the possibility that an initial argon resetting may have occurred as
early as the Permian or Triassic. The mylonite sample partially
preserved its Variscan fabrics (high-temperature foliation) and is cut
by a 3 mm-thick shear band where Alpine synkinematic recrystallizations
are almost complete. In the shear band, phengite yields a range of
\textit{in situ} dates between 30 and 40~Ma with a peak age at about
32~Ma for the micas from the core of the shear band (12 of 30 analyses)
while those from its inner edge are older than 34~Ma (red line on 
Figure~\ref{fig8}). Outside the shear band, muscovite clasts yield
older dates (48--129~Ma) with higher Cl/K ratios than phengite, as
observed in the protomylonite. These relationships, only detectable by
\textit{in situ} analyses, indicate that synkinematic crystallization
of phengite by dissolution--precipitation processes at the expense of
K-bearing Variscan minerals (muscovite and feldspar) was more complete
in the core of the shear band than at its edge and even more than in
the host rock. In the phyllonite almost devoid of muscovite clasts, of
the 29 dates of phengite (Cl/K $<$ 0.01), only 3 are older than
34~Ma while the remaining dates define two peaks, a major peak at 26~Ma
and a minor one at 32~Ma consistent with the peak date given by
phengite in the mylonite. These \textit{in situ} data are consistent
with those provided by a previous step-heating study focused on
ultramylonites that also highlighted some contamination by excess argon
related to the presence of Ca-rich impurities in the dated mineral
populations and limited argon loss possibly due to hydrothermal
alteration  \citep{Simon-Labricetal2009}. In our example, the
correlation of \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages
and deformation intensity within and between the three dated sections
shows that the distribution of Alpine ages is primarily controlled by
deformation textures. The data also show that the presence of Alpine
ages in the three dated rock sections is correlated with the intensity
of phengite blastesis, particularly important in high-strain zones
along which fluids circulated. Considering that temperatures reached
during the Alpine orogeny in the Oisans massif did not exceed 
300--350~\textdegree C, it is very likely that the ages retained by
newly-formed phengites represent true crystallization ages,
provided
that they evolved in a closed system, without being contaminated by
excess (extraneous or inherited) argon.  Phengite dates are
distributed on two Oligocene peak ages that are interpreted to record
the progressive localisation of deformation along the highly strained
tectonites in the Oisans massif during E--W shortening  \citep[see][for
a more complete discussion]{Bellangeretal2015}. 

In the eastern Pyrenees, the mylonitic shear zones occurring in the
Variscan basement at Cap de Creus have been the subject of detailed
structural studies 
\citep[e.g.][]{Carrerasetal1977,Carrerasetal2010,Druguetetal1997,Druguetetal2009} 
and are generally attributed to the end of the Variscan orogeny. This
assumption was recently challenged by a
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar\break \mbox{geochronological} study that argues
for a Jurassic age for these relatively low-temperature 
$({\sim}450~\text{\textdegree}\mathrm{C})$ shear zones, with possible
Tertiary reactivation \citep{Vissersetal2016,Vissersetal2020}. However,
the significance of these ages remains largely uncertain due to the
large grain size of the dated muscovite 
$(180\ndash250~\rmmu\mathrm{m})$ compared to the size of synkinematic
micas $({<}100~\rmmu\mathrm{m})$. Some new \textit{in situ} and
step-heating $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar geochronological data
are presented below that allow us to propose an Eocene age for the most
obvious mylonitic deformation in the Cap de Creus massif\break
\citep{Monieetal2018}.

The first sample is an ultramylonitic Variscan pegmatite  [299~Ma U--Pb
zircon age,  \citealp{vanLichterveldeetal2017}] that was collected in
the core of a 2~m wide NW--SE dextral shear zone in the Cap de Creus
peninsula, close to the locality sampled by  \citet{Vissersetal2016}.
The mica population contains some rare muscovite clasts but is
dominated by fine-grained synkinematic phengitic muscovite
(Si $=$ 3.1--3.2) formed at the expense of Variscan muscovite by
fluid-assisted dissolution--precipitation mechanisms 
(Figure~\ref{fig9}a). 

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}Ultramylonitic pegmatite from the Cap de Creus
peninsula;  (a)~thin section with a scarce sheared and recrystallized
clast of muscovite in a fine matrix of quartz, albite and phengitic
muscovite;  (b)~age spectra on two granulometric fractions
representative of the two mica populations;  (c)~distribution of situ
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates obtained on a thick section
\citep{Monieetal2018}.}
\vspace*{5pt}
\end{figure*}

This population was studied using stepwise and \textit{in situ} heating
experiments on two size fractions and on a thick section made
perpendicular to the foliation and parallel to the lineation,
respectively. The age spectrum of a single muscovite clast
$(300\ndash400~\rmmu\mathrm{m})$ shows a saddle-shaped pattern with
dates of 101--133~Ma while about 30 grains of synkinematic phengitic
muscovite  $(80\ndash100~\rmmu\mathrm{m})$ give a plateau-like date of
42 ${\pm}$ 1~Ma (Figure~\ref{fig9}b).  \textit{In situ} analyses of
micas along the foliation plane yield dates ranging from 43~Ma to 83~Ma
with a peak at 45~Ma in the cumulative probability diagram 
(Figure~\ref{fig9}c). This peak age validates the  ${\sim}$42~Ma
plateau-like date of the fine-grained mica fraction as no value below
42~Ma was obtained. Older \textit{in situ} dates confirm that small
relics of Variscan muscovite, still containing some inherited argon,
are present along the foliation, but are not optically distinguishable
from synkinematic micas. All these new dates are much younger than the
Jurassic ages (159--175~Ma) reported by  \citet{Vissersetal2016} and
point to an Eocene age for the main ductile activity of shear zones in
the Cap de Creus peninsula. They also show how critical the choice of
samples and minerals to date is in mylonites (strain intensity, grain
size, mineral habit\,{\ldots}) and how useful the \textit{in situ} approach is
to identify the different mica generations compared to the other
methods of argon extraction. This is particularly true for the total
melting approach which is biased by the selection of the largest mica
grains in the sheared rocks, which is not the case for the
fabric-preserving \textit{in situ} approach that allows to target micas
that differ in size, chemistry or textural habit.

This is confirmed by the study of mylonitic shear zones in the Roses
granodiorite, 15 km southwest of the previous shear zone. This Variscan
magmatic body  [291~Ma U--Pb zircon age, \citealp{Druguetetal2014}] is
affected by zones of heterogeneous and localised mylonitic deformation
resulting from the coexistence of brittle and ductile deformation
mechanisms and fluid-induced mineralogical softening reactions. In the
most deformed facies, these reactions induce a complete replacement of
plagioclase by epidote ${\pm}$ phengite, and magmatic biotite by more
magnesian metamorphic biotite and white mica  (Figure~\ref{fig10}a) at
a temperature of  450--500~\textdegree C. 

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}Sheared granodiorite from the Roses massif; 
(a)~thin section showing the coexistence of prekinematic magmatic and
synkinematic metamorphic biotite;  (b)~distribution of situ
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates across a gradient of
deformation showing some biotite relics;  (c)~distribution of situ
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates within two sections of
mylonitic granodiorite (red square) and shear band (blue square)
\citep{Monieetal2018}.}
\end{figure*}

\textit{In situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates were
obtained on two protomylonitic and mylonitic samples, both showing an
increasing gradient of deformation and metamorphic recrystallization.
In the protomylonite, \textit{in situ} biotite dates range from 69~Ma
to 47~Ma from the least to the most deformed domains, with a peak age
of about 49~Ma for the more sheared zone  (Figure~\ref{fig10}b). In the
mylonite, biotite from the least deformed portion yields a range of
values between 41.5~Ma and 47.5~Ma with a peak date of 44~Ma for newly
crystallized biotite. In the shear band with abundant synkinematic
phengite, \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates are
mainly between 39~Ma and 42~Ma with a peak at ${\sim}$40~Ma\unskip\break 
(Figure~\ref{fig10}c). These texturally and chemically controlled
\textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses indicate
that deformation and associated metamorphic recrystallizations play a
prominent role in the distribution of dates at the scale of a
centimeter section, with younger dates in the most deformed domains.
Furthermore, the preservation of these
deformation--recrystallization--chemistry--age relationships again
suggests that the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates recorded by
biotite and phengite in these mylonites should be considered as
crystallization ages rather than cooling ages. In the mylonite, the 4
Myr time lag between the crystallization of metamorphic biotite in the
host rock matrix and the crystallization of phengite in the shear band
may represent the time required to switch from a diffuse to a localized
deformation regime. These data suggest an Eocene age for the
mylonitisation of the Roses granodiorite, in agreement with the data
reported above in the Cap de Creus peninsula. On a regional scale, it
is proposed that these NW--SE striking mylonitic strike-slip structures
accommodated the end of the Pyrenean compression 
\citep[e.g.][]{Teixelletal2018,Ternoisetal2019}.

Recently, new Th--U/Pb and $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
geochronological data were obtained in the Agly massif 
\citep{Aumaretal2022}, which has long been the subject of debate as to
the influence of Cretaceous deformations on its architecture. The study
focused on the Lower Gneiss Unit that shows a highly ductile
extensional deformation with a NNE--SSW oriented stretching lineation
that is also found in the Mesozoic sedimentary series to the north. In
the basement gneisses, the original high-temperature Variscan
deformation was overprinted by this new extensional deformation that
evolved continuously from a ductile regime at relatively high
temperature  $({\geq}500~\text{\textdegree}\mathrm{C})$ to a brittle
regime at a very low temperature (150/200~\textdegree C). Synkinematic
minerals located within shear bands  provided consistent Th--U/Pb
monazite and $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar mica dates with a
94--127~Ma age  bracket interpreted as dating mylonitic deformation
during the Cretaceous rifting episode that preceded the building of the
chain. An example of stepwise heating and \textit{in situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar data is shown in
Figure~\ref{fig11}.\looseness=-1

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}(a)~Mylonitic leucogranite  [291~Ma U--Pb monazite
age,  \citealp{Vanardoisetal2022}] with a magmatic muscovite clast in a
matrix of synkinematic fine-grained biotite and muscovite;  (b)~age
spectra of muscovite and biotite clasts from a protomylonitic gneiss;
(c)~distribution of situ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates
obtained on four thick rock sections of mylonites. A Kernel density
bell-shaped curve has been drawn for each mica population 
\citep{Aumaretal2022}.}
{\vspace*{3pt}}
\end{figure*}

Several samples of proto- to ultramylonites\unskip\break
(Figure~\ref{fig11}a) were
collected throughout the massif and regardless of the degree of
mylonitisation, muscovite and biotite yield a cluster of well-defined
plateau ages of 104--105~Ma  (Figure~\ref{fig11}b). These ages were
obtained on mica clasts preserved in the\unskip\break 
mylonitic matrix and the lack
of intra- and inter-sample age gradient suggests that the dated micas
experienced a full reset of their K--Ar chronometer, which is supported
by the U--Th/Pb age of 104--127~Ma for synkinematic monazite in the same
samples \citep[see][]{Aumaretal2022}.  \textit{In situ}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses were carried on 4 thick
sections of proto- to ultra-mylonites and the distribution of ages
obtained on both large prekinematic mica clasts and tiny synkinematic
micas is shown on  Figure~\ref{fig11}c. The main observation is that
texturally earlier micas show older dates than newly crystallized micas
in the foliation, mainly between 102 and 112~Ma for the clasts $(n=12)$
and between 94 and 102~Ma $(n=17)$ for the new micas. This age range
(112--94~Ma) is interpreted to cover a large part of the ductile
deformation in the Agly massif. Lastly, it can also be noted that these
\textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses provide no
evidence for the significant presence of an inherited Variscan argon
component in the dated micas. These results from the Agly massif are
contrasting with those of the Oisans and Cap de Creus massifs for which
some micas in the high strain zones still contain inherited argon 
(Figures~\ref{fig8}b,c and~\ref{fig9}c). There are several explanations
for the results obtained in the Agly massif: a higher  temperature
$({\geq}500~\text{\textdegree}\mathrm{C})$ at the beginning of the
deformation supported by the micro-fabric of quartz, feldspar and
garnet in the mylonites, a more penetrative and long-lived mylonitic
deformation  (${\sim}$40~myrs) in an extensional tectonic context, more
extensive fluid-rock interactions highlighted by the widespread
Cretaceous hydrothermal activity in the Northern Pyrenees 
\citep[e.g.][]{Boutinetal2016} and\unskip\break 
attested by a notable contribution
of chlorine-derived  $\mbox{}^{38}$Ar in some
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar analyses.

\section{$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar geochronology of brittle
deformation}\label{sec3}

\begin{figure*}
\includegraphics{fig12}
\vspace*{5pt}
\caption{\label{fig12}Fault-rock types and evolution of related
deformation mechanisms with depth. Brittle deformation corresponds to
the temperature range of low temperature thermochronometers: Zircon
Fission Tracks (ZFT), Zircon (U--Th)/He (ZHe), Apatite Fission Tracks
(AFT) and Apatite (U--Th)/He (AHe). Closure temperature of each
thermochronometric system is reported; PAZ: partial annealing zone;
PRZ: partial retention zone  
\citep[modified after][]{MalusaFitzgerald2019}.}
\vspace*{6pt}
\end{figure*}

Classically, the dating of brittle deformation is indirectly inferred
from the dating of geological units that predate and postdate the fault
activity. More recently, with the development of low temperature
thermochronometric tools, fault motion is also inferred from
differential thermal histories of the footwall and the hangingwall
rocks \citep[][and references therein]{MalusaFitzgerald2019}.  Rapid
cooling or exhumation of wall rocks are often explained by fault motion
responsible for isotherm advection. However direct
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar (or K--Ar) dating of fault motion is
possible even in the low temperature upper crust corresponding to the
partial annealing zone of apatite fission tracks 
(Figure~\ref{fig12}). $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar (K--Ar) dating
method has been successfully applied on fault gouges using
synkinematic/authigenic clay minerals that form during the activity of
shallow crustal faults from above the ductile--brittle transition
\citep[e.g.][]{vanderPluijmetal2001}, i.e. in a range of temperature
between  ${\sim}300~\text{\textdegree}\mathrm{C}$, corresponding to the
muscovite--illite transition, to 
${\sim}140\ndash60~\text{\textdegree}\mathrm{C}$ corresponding to the
temperature range of illite-rich illite--smectite (I/S) interstratified
mineral formation \citep{HainesvanderPluijm2012}.  The
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar (K--Ar) dating method is very useful
to provide direct constraints on the timing of shallow crust faulting
but also it can yield information regarding the thermal history of 
the\unskip\break
fault-zone rocks. Note that during the last decade, the U--Pb
geochronology of calcite has been widely developed to address the
timing of brittle faulting and~fracturing  
\citep[][for a~review]{RobertsHoldsworth2022}.

\subsection{Some methodological aspects}\label{sec3.1}
Synkinematic/authigenic clay minerals are characterized by their micron
size (2 to $0.2~\rmmu\mathrm{m}$ size fractions are commonly analyzed, and
sometimes down to the 0.05 or up to  
$20~\rmmu\mathrm{m}$ size fractions) that
favors a $\mbox{}^{39}$Ar recoil effect during irradiation 
\citep{ReuterDallmeyer1987,Onstottetal1995}. The distance of
$\mbox{}^{39}$Ar recoil during irradiation was estimated to be a mean
of  $0.08~\rmmu\mathrm{m}$ \citep{Onstottetal1995}, which has
significant effects on small crystallites. One must keep in mind that
$\mbox{}^{39}$Ar recoil distance is also a function of fast neutron
energy in the irradiation facility, and therefore the amount of argon
loss is also dependent on this energy. Although the fraction of
$\mbox{}^{39}$Ar lost by recoil effect generally increases with
decreasing grain size,  \citet{Dongetal1995} also showed that the
amount of $\mbox{}^{39}$Ar recoil is rather dependent on the degree of
crystallization of clay particles. During irradiation, the amount of
$\mbox{}^{39}$Ar loss can reach 30\% of the total amount of
$\mbox{}^{39}$Ar produced which results in apparent
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages older than K--Ar ages. This
limitation favored K--Ar dating of clay minerals for a long time
\citep{Claueretal2012,Clauer2013}, despite this method presents two
main drawbacks: argon and potassium are measured on separated clay
aliquots, and it does not permit to discriminate 
illite-rich mineral
generations resulting from multistage crystallization. In the 90s, a
sample encapsulation technique was developed in order to measure
the $\mbox{}^{39}$Ar lost by clay particles during irradiation and to
correct $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages 
\citep{Folandetal1992,Dongetal1995,Onstottetal1995}. Each sample
(${\sim}$1~mg) is encapsulated and sealed under vacuum within a quartz
tube that traps the $\mbox{}^{39}$Ar released during neutron
irradiation. The $\mbox{}^{39}$Ar loss can then be measured by
breaking or laser drilling the quartz tube under vacuum in the sample
chamber to correct the bulk $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar age
obtained by classical step-heating $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
method.  \citet{Dongetal1995} defined a ``retention age'' (RA)
corresponding to the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar total age that
omits the fraction of $\mbox{}^{39}$Ar lost during irradiation and a
``total gas age'' (TGA) including $\mbox{}^{39}$Ar recoil. 
\citet{Dongetal1995} also showed that the fraction of $\mbox{}^{39}$Ar
lost by recoil is a linear function of illite crystallinity. For thick
crystallite, i.e. ${>}$50~nm, the ``retention age'' and the
``total gas age'' are close or identical. For thin crystallites, i.e.
${<}$10--15~nm, the ``total gas age'' can be considered as the
best estimate of the crystallization age. For crystallites with
intermediate thickness, crystallization ages fall between the
``retention age'' and the ``total gas age''.  \citet{Fitz-Diazetal2016}
proposed to correct $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages by taking
into account the average illite crystallite thickness
(ICT)---considered as the number of TOT layer and related to illite
crystallinity---and the corresponding recoiled $\mbox{}^{39}$Ar. This
correction is particularly useful for samples containing authigenic
illite with typical ICT between 10 and 50~nm.

At last,  \citet{Linetal2000} showed that the recoil effect is also
influenced by the incident angle of the neutron flux with the 
$c$-axis of
clay particles. A soft compaction of clay particles before \textit{in vacuo}
sealing is thus recommended to significantly reduce the recoil effect 
\citep{Dongetal1995,Claueretal2012}. During encapsulation there is a
risk to heat the samples while sealing the quartz tube. The sealing is
realized with a gas torch and in order to prevent heating the sample,
the base of the tube containing the sample is placed either in iced
water  \citep{Claueretal2012,Hall2015} or in a metallic holder cooled
with liquid nitrogen.  \citet{Claueretal2012} questioned the accuracy
of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages compared to K--Ar ages and
argued that the lack of standardization of encapsulation processes may
be a source of bias in $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of
clay minerals and this should be addressed. To the whole, the
encapsulation systems have been constantly improved and are now used
routinely in several laboratories. For example, at G\'{e}osciences
Montpellier, we developed an encapsulation system allowing
encapsulating clay samples under very low vacuum before their
irradiation, i.e. vacuum  of  $10^{-9}$ torr is currently reached
(Figure~\ref{fig13})  \citep{AbdElmolaetal2018,Munchetal2021}. 
Reproducibility tests of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages on
different clay aliquots and comparisons with K--Ar ages show the
reliability of this encapsulation technique, with smaller error margins
than for the K--Ar method.

\begin{figure}
\includegraphics{fig13}
\caption{\label{fig13}Sketch of the encapsulation device developed at
the G\'{e}osciences Montpellier noble gas laboratory. The quartz tube
containing the charcoal is immersed in liquid nitrogen to~improve the
vacuum in the encapsulation line.}
\end{figure}

A second issue with $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar (or K--Ar) dating
of clay minerals from gouges is related to tectonic processes occurring
within shallow crust
faults:\unskip\break  
fragmentation of host rocks followed by
grain-size reduction and authigenesis of clay minerals, resulting in a
mixture of inherited/detrital and authigenic/synkinematic minerals and
in the measurement of a mixed age. Depending on the lithology of the
wall rock, authigenesis will correspond either to the illitisation of
interstratified illite/smectite minerals present in sedimentary host
rocks or to the neocrystallization of discrete illite in the case of
metamorphic or magmatic host rocks, devoid of clay minerals
\citep{HainesvanderPluijm2008}  (Figure~\ref{fig14}). From the
ductile/brittle transition, i.e. at temperature around  300~\textdegree
C, newly formed illite is mainly of  $2\mathrm{M}_{1}$ polytype (polytypism is a
variety of polymorphism common in layered minerals) with minor 
$1\mathrm{M}/1\mathrm{M}_{\mathrm{d}}$ 
polytype and their relative proportion are inverted with
decreasing temperatures  (Figure~\ref{fig14}). Below 
${\sim}200~\text{\textdegree}\mathrm{C}$,  
$1\mathrm{M}/1\mathrm{M}_{\mathrm{d}}$ 
polytype is the
only newly formed polytype and with decreasing temperatures the
proportion of illite in I/S minerals decreases and ordering of illite
layers within I/S also decreases (Figure~\ref{fig14}). Thus at shallow
depth, the proportion of inherited/detrital and newly formed phases can
be resolved through quantitative X-ray diffraction analysis of
different clay size fractions. 

\begin{figure*}
\includegraphics{fig14}
\caption{\label{fig14}Mineral transformations within fault gouges to
form illite-rich clay minerals suitable for
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar (K/Ar) dating 
\citep[modified after][]{HainesvanderPluijm2008}.  (A)~Illitization of
illite--smectite (I/S) interstratified minerals present
within the wall rock; this process results in higher contents of I/S
minerals within the gouge than in the wall rock. (B)~Growth of
authigenic discrete 
$1\mathrm{M}_{\mathrm{d}}$ illite in fault gouges; this process results in
a mixture of authigenic/synkinematic $1\mathrm{M}_{\mathrm{d}}$ illite and inherited/detrital
$2\mathrm{M}_1$ illite ($=$ muscovite) from the wall rock.}
\end{figure*}

For less shallow crustal faults, but still above the brittle--ductile
transition, the problem of dating a mixture of inherited/detrital and
authigenic/synkinematic illite type minerals is much more difficult to
solve. Indeed, with X-ray diffraction techniques, muscovite forming at
temperatures above  300~\textdegree C is undistinguishable from $2\mathrm{M}_1$
illite polytype forming at temperatures down to 
${\sim}200~\text{\textdegree}\mathrm{C}$  (Figure~\ref{fig12}). As a
consequence, if muscovite is present within host rocks both
authigenic/synkinematic and inherited/detrital $2\mathrm{M}_1$ polytypes are mixed
within the gouge rocks and their relative proportion cannot be
estimated. At temperatures near  200~\textdegree C, 
$1\mathrm{M}$ polytype
authigenesis can also occur together with $2\mathrm{M}_1$ but the proportion of
detrital/authigenic can still not be estimated. 

Like for $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of ductile
deformation, a very thorough and precise mineralogical study of gouge
samples must be conducted before dating them. In the case of clay
minerals, this mineralogical study requires X-ray diffraction
techniques on both oriented and non-oriented powders in order to
determine the mineralogy and quantify the proportion of different
mineralogical phases. Scanning and transmission electron microscope
studies are also necessary to determine the mineralogy of clay minerals
and in some case their chemistry that can also be analyzed by electron
microprobe but only for well-crystallized coarse clay minerals.

\begin{figure*}
\includegraphics{fig15}
\caption{\label{fig15}Examples of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
illite step-heating age spectra for vacuum-encapsulated clay gouge size
fractions and illite age analysis plots 
\citep{Pevear1999,vanderPluijmetal2001} of
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar illite TGA's vs. \% $2\mathrm{M}_1$ (detrital
illite) of different clay-size fractions of two samples from late
shallow crustal faults occurring within metamorphic units of the Rif
chain  \citep[modified after][]{Munchetal2021}.  (A)~sample M10-29
showing classical spectra for all fractions and the corresponding
illite age analysis plot.  (B)~Sample CEU12-03 showing complex spectra
for two coarse fractions with low temperature staircase steps largely
overshooting the ``total gas'' and ``retention'' ages as well as the
age of muscovite from host rocks; the TGA's of corresponding fractions
have been removed from illite age analysis (dots between brackets). On
illite age analysis plots, the black dot~on the right axis corresponds
to the mean age of muscovite within the metamorphic host rocks.}
\end{figure*}

\subsection{Age interpretation}\label{sec3.2}
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar age spectra obtained on encapsulated
clay minerals differ from classical age spectra of magmatic or
metamorphic minerals due to $\mbox{}^{39}$Ar recoil effect during
irradiation  \citep{Dongetal1995,Halletal1997,Halletal2000} but also to
complex mixture of minerals with different age, polytype and
crystallinity, without mentioning the possibility that the dated
fractions may contain other types of potassium-rich minerals than
phyllosilicates. In classical $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
studies, samples have a size above  $50~\rmmu\mathrm{m}$ in order to
avoid recoil induced $\mbox{}^{39}$Ar loss and hopefully yield spectra
with a plateau comprising more than 50\% of total $\mbox{}^{39}$Ar
released and at least 3 steps. An illite spectrum shows different
features: (1)~a first zero age step corresponding to the recoil gas
fraction, (2)~staircase steps with increasing apparent ages from near
zero at low temperature  (3)~a more or less hump-shaped age pattern
with apparent ages that exceed the ``retention age'' and sometimes an
age drop at highest temperatures  (Figure~\ref{fig15}A). This third
part of the spectrum sometimes looks like a seemingly good plateau but
should be interpreted with great caution and as providing a meaningless
intermediate age as in the case of hump-shaped spectra obtained for
mixtures of two mica generations  \citep{WijbransMcDougall1986}. To
estimate the age of a fault gouge motion, only the TGA's, eventually
corrected for an ICT factor, of the different clay size fractions with
well-identified polytype proportions must be used. Usually, TGA's
decreases with grain size and with the relative proportion of
inherited/detrital and newly formed/synkinematic minerals of a given
polytype. By plotting TGA's against the percentage of detrital
polytype, the age of the pure authigenic phase can be determined with
the ordinate intercept (0\% of detrital component) of the best fitting
line. This is the so-called ``Illite Age Analysis'' of 
\citet{Pevear1999} and  \citet{vanderPluijmetal2001} that has been
applied on numerous shallow crust faults in various tectonic settings 
[e.g.
\citealp{Duvalletal2011,HainesvanderPluijm2008,HainesvanderPluijm2010,HainesvanderPluijm2012,Munchetal2021,Ringetal2017,Zwingmannetal2010a,Zwingmannetal2010b}; 
Figure~\ref{fig15}A]. The ages calculated reflect the most recent
period of activity of the faults along their\break exposure.

However,  \citet{Solumetal2005} demonstrated that the illite age
intercept calculated using the illite age analysis also varies from the
fault core to the damage zone of a single fault, as a function
of the intensity of the deformation and the proportion of authigenic
illite. This result highlights that a thorough structural study of the
fault zone that is to be dated is crucial and that samples selected for
illite dating must come from the most deformed part of the fault. As
authigenic illite or I/S growth in fault gouge occurs in a well
constrained temperature range, it is important to combine fault gouge
dating with thermochronological study of host rocks that constrains
their cooling history. Low-temperature thermochronometric studies often
emphasize one or more tectonic phases as the cause of accelerated
exhumation and highlight the role of major faults. The combined use of
thermochronometric tools and fault gouge dating allows the age of fault
activity to~be~better constrained
\citep{Duvalletal2011,Munchetal2021,Milesietal2022,Parryetal2001,Tagami2012}. 
\looseness=-1

In some cases, the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar age spectra of
illite are more complex and the illite age analysis is not so
straightforward. For example, some spectra from coarse fractions of
gouge samples exhibit rather a saddle shape for the high temperature
steps, i.e. after the staircase increase of apparent ages from zero,
and in some cases the ``total gas age'' of these fractions has even
been found older than the mean age of muscovite from the metamorphic
host rocks  \citep{Munchetal2021}  (Figure~\ref{fig15}B). The shape of
the spectra may be interpreted in terms of excess of radiogenic
$\mbox{}^{40}$Ar  \citep{McDougallHarrison1999} that probably
originates from inherited minerals because these spectra were only
obtained for coarser fractions in which the proportion of
inherited/detrital muscovite is higher. On the other hand,
$\mbox{}^{40}$Ar in excess could also have been incorporated during the
growth of authigenic illites from fluids circulating within the fault 
\citep[e.g.][]{Roquesetal2020}. As the shape of the spectra allows
detecting such excess of radiogenic $\mbox{}^{40}$Ar, it must be
considered before integrating the ``total gas age'' of each fraction in
an illite age analysis. In some cases, with particularly complex shape
of spectra, this can result in the rejection from the illite age
analysis of the TGA(s) of the fraction(s) exhibiting such excess
$\mbox{}^{40}$Ar  (Figure~\ref{fig15}B). 

In order to better interpret the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar age
spectrum of authigenic illites,  \citet{Claueretal2012} applied a
correction of $\mbox{}^{39}$Ar recoil on step-heating experiments by
distributing the recoiled $\mbox{}^{39}$Ar on each step in proportion
to its representative percentage on the retention age spectrum. The
goal was to reconstruct an original spectrum, not altered by the
$\mbox{}^{39}$Ar recoil loss during the sample irradiation, which can
be interpreted using the plateau age definition. The recoil-corrected
spectrum of  \citet{Claueretal2012} is still similar to the spectra
obtained from mixtures from which these authors defined one or two
``limited plateaus'', not necessarily corresponding to 3 concordant and
consecutive steps.  \citet{Claueretal2012} claim that the ``limited
plateaus'' represent different $\mbox{}^{39}$Ar reservoirs and several
illite generations for which they calculate a corrected age. Even if we
agree that the complex spectra can reflect the mixture of several
illite generations, other processes (fluid interaction, excess argon,
mixture with other clay minerals or K-bearing inherited phases) can
also lead to complex spectra. Above all, the correction applied is
rather basic and the calculation of plateau ages for the different
illite generations on a single spectrum is highly questionable.
According to  \citet{Fitz-Diazetal2016} an age interpretation based on
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar plateau ages is meaningless for most
fine-grained clays because the grain size highly influences the shape
of the spectra. However, the shape of the age spectra of different
clay-size fractions from a single sample is obviously useful to depict
the sample crystallization history in terms of mixture, excess
$\mbox{}^{40}$Ar incorporation and is a crucial point to take into
consideration before performing an illite age analysis of a fault
gouge. This is a huge advantage of the step-heating
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method \mbox{compared} to the
\mbox{complementary} K--Ar method that cannot provide such
\mbox{information.}\looseness=1

\begin{figure}
\vspace*{3pt}
\includegraphics{fig16}
\vspace*{4pt}
\caption{\label{fig16}Examples of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
illite step-heating age spectra for vacuum-encapsulated clay gouge size
fractions mainly constituted of $2\mathrm{M}_1$ polytype 
($1\mathrm{M}$ polytype is solely
present in the ${<}0.2~\rmmu\mathrm{m}$ fraction) from the T\^{e}t
fault in Eastern Pyr\'{e}n\'{e}es  \citep{Milesi2020}.}
\end{figure}

More complex cases come from the deeper brittle fault zones that were
active at temperatures comprised between  ${\sim}$200 and 
${\sim}300~\text{\textdegree}\mathrm{C}$  (Figure~\ref{fig16}). There,
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar (K--Ar) ages decrease with decreasing
clay-size fraction, illite crystallinity and increasing
$\mbox{}^{39}$Ar recoil loss  \citep{Milesi2020}. The illite age
analysis cannot be applied because inherited and neo-formed illites are
$2\mathrm{M}_1$ polytype and the age of the finest fraction can only be considered
as a maximum age as it is frequently a mixture of mainly $2\mathrm{M}_1$ with minor
$1\mathrm{M}$ polytype. The age of coarse size fraction is potentially also a
mixed age but the proportion of detrital muscovite and authigenic
illite cannot be determined. However, the shape of the spectra can help
to resolve if there is a mixture or not. The age of the medium size
fraction may either represent a mixed age with a higher proportion of
authigenic $2\mathrm{M}_1$ illite or a cooling age of the same mixture as in the
coarse fractions. This issue can be even more complicated if the host
rock is detrital and was deeply buried at equivalent temperatures,
between 200 and  300~\textdegree C, before faulting. In this case, the
gouge can contain a complex mixture of detrital muscovite, $2\mathrm{M}_1$ illite
formed in anchizonal conditions during burial and
authigenic/synkinematic $2\mathrm{M}_1$ illite. The age of fault motion is then
very difficult to resolve and in such a case the stratigraphic age of
host rocks as well as their clay mineralogy and the regional geological
constraints have to be considered in order to correctly interpret the
age of fault motion  \citep{Solumetal2005,AbdElmolaetal2022}.

\section{Pseudotachylytes}\label{sec4}
The long-term activity of faults is marked by a succession of events
that are difficult to decipher precisely in a continuous way,
particularly in the seismogenic upper crust. Among the various fault
rocks, pseudotachylytes are witnesses of frictional flash melting
associated with high-velocity co-seismic ruptures most often in the
shallow and brittle domains of the lithosphere. They occur as glassy
veins of variable thickness and are frequently associated with
cataclasites and ultra-cataclasites, resulting in a complex, frequently
asymmetric, zonation of the veins. Their melt origin is supported by
the presence of a glassy material and different microstructures such as
microlites, spherulites, vesicles, flow structures, vein injections,
and sulfide droplets that are decisive criteria to differentiate them
from crush-origin pseudotachylytes  \citep{Lietal2022}. Because of the
high cooling rates implied by their rapid solidification and their high
potassium content, pseudotachylytes represent prime targets to date
co-seismic displacement by the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
method 
\citep[e.g.][]{Kelleyetal1997,SherlockHetzel2001,Mulleretal2002,DiVincenzoetal2004,Souquiereetal2011,Menantetal2018,Mittempergheretal2022}. 
Step-heating analysis of pseudotachylyte fragments has produced
variously successful results due to the very heterogeneous character of
the veins (abundant host rock clasts, partial devitrification,
inclusions\,{\ldots}) and the often polyphased history of fault zones 
\citep[e.g.][]{Torgersenetal2022}. The use of the \textit{in situ}
laser probe $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating method has
partially overcome some of these drawbacks by focusing the analyses on
particular zones rich in molten glass and poor in clasts, such as the
vein margins or the injection veins. Below  (Figure~\ref{fig17}) are
some examples showing some characteristics of pseudotachylytes and some
data obtained in the western Alps (Ivrea zone, Dent Blanche) using both
step-heating and \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
analyses.

\begin{figure*}
\includegraphics{fig17}
\caption{\label{fig17}(a)~Pseudotachylyte in a gabbro of the Ivrea
zone. Note the injection vein perpendicular to the main vein composed
of cataclasites on the left and glassy, black pseudotachylyte on the
right;  (b)~Cathodoluminescence image of the contact between a
pseutotachylyte vein and a gneiss host-rock cut by an injection vein
(Dora Maira). The melt glass (in blue) is concentrated at the very
contact with the undeformed host-rock and within the injection vein;
(c)~BSE image of a pseudotachylyte in a paragneiss of the Ivrea zone
showing disoriented mica microlites around a corroded quartz; 
(d)~$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar age spectra of two pseudotachylyte
samples (injection veins) from the Valpelline unit  [Dent Blanche,
\citealp{Menantetal2018}] showing low and high devitrification effects
respectively. Note that the age spectrum of the pseudotachylyte (in
red) overlaps the Rb--Sr age of phengites in a nearby shear zone; 
(e)~distribution of \textit{in situ}\ $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar
dates within the injection vein shown in (a). Relatively large errors
are due to the low amount of potassium in the gabbro host-rock
\citep{Souquiereetal2011};  (f)~inverse isochron diagram of the same
sample showing an agreement between peak and intercept ages and an
initial atmospheric $\mbox{}^{40}$Ar/$\mbox{}^{36}$Ar composition (no
excess argon).}
\end{figure*}

Therefore, the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of
pseudotachylytes and associated tectonites appears essential to better
understand the mechanics of earthquakes and the links with tectonic
activity recorded by ductile and brittle faults in the mid to upper
crust. The examples of  Figure~\ref{fig17} highlight the
microstructural complexity of pseudotachylytes and the difficulty to
use them as precise $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar geochronometer of
seismic ruptures. The behavior of this chronometer remains to be better
understood and the effects of various parameters will require
consideration in future studies, including slip velocity at rupture and
amount of generated melt, mechanical and chemical zonation of the
veins, lithology and age of the host rock, nature and distribution of
clasts and fragments extracted from the host rock, nature and
distribution of neo-formed phases, presence of fluids, alteration
phases or devitrification effects.

\section{Conclusion}\label{sec5}
This overview of some recent developments in the
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dating of deformation in fault zones
shows that the interpretation of geochronological data is not so
straightforward and that it must be supported by a good knowledge of
the field, and detailed
\mbox{(micro-)} structural and petrological data. For
ductile deformations at temperature 
${<}500\ndash550~\text{\textdegree}\mathrm{C}$, the different examples
reported in Part 3 show that the $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar ages
recorded by newly formed synkinematic micas (and more rarely amphibole)
have to be considered as crystallization ages rather than cooling ages.
Frequently, these newly formed minerals are mixed with prekinematic
minerals inherited from the original host rock or from an earlier phase
of deformation along the fault zone. In this particular case, the 
\textit{in situ} laser probe $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar method
is very powerful since it allows the dating of the different K-bearing
mineral generations identified on microstructural and chemical grounds.
The behavior of the inherited minerals is strongly dependent on the
$P$--$T$-fluids conditions that prevail during deformation, on the size and
chemistry of these minerals, on the presence or not of short diffusion
pathways, on the closed/open system behavior of the studied tectonites.
In some favorable geological setting, the K--Ar chronometer of inherited
minerals can be fully reset while in other 
cases, only a limited
resetting can be observed. For deeper conditions of deformation (e.g.
high-pressure low temperature terranes), the closed-open system
behavior of the host rock can be an critical parameter to consider
since it can result in excess argon contamination of the newly
crystallized minerals due to limited fluid mobility in a closed system 
\citep{Laurentetal2017}. For all these situations, the integration of
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar dates with independent
geochronometers (Rb--Sr, U--Pb) may be particularly useful for
reconstructing $P$--$T$--$d$--$t$ paths in deformed rocks.

For brittle faults, clays from fault gouges are the first choice target
for $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar investigations. However, due to
$\mbox{}^{39}$Ar recoil effect during irradiation, to complex mixture
in the fault gouge of minerals with different age, polytype and
crystallinity, to complex fluid rock interactions, the interpretation
of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar data is not always simply a matter
of processing a binary mixture between a single generation of $2\mathrm{M}_1$
phyllosilicate inherited from the host rock and an authigenic 
$1\mathrm{M}$ clay
synkinematic fraction. By contrast with the K--Ar, the
$\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar step-heating method has the potential
to reveal the different argon reservoirs that contribute to the argon
release and to eliminate samples with a too complex isotopic signature,
such as those contaminated by excess argon. The step-heating approach
can also bring information on complex mixtures of inherited and
authigenic phyllosilicates in fault gouges with a multiphase activity.
The coupling of $\mbox{}^{40}$Ar/$\mbox{}^{39}$Ar studies in fault
gouges with multimethod thermochronological studies (fission tracks,
U--Th--Sm/He) in the host rocks may shed light on this repeated motion of
brittle faults during exhumation processes using thermal and tectonic
modeling (QTQt, Pecube). This aspect remains to be more systematically
explored in orogens.

\section*{Conflicts of interest}
Authors have no conflict of interest to declare.

\section*{Acknowledgements}
Part of this work (pseudotachylytes, T\^{e}t fault) has received
fundings from different research programs (3F, TelluS) of the Institut
National des Sciences de l'Univers, CNRS. St\'{e}phane Scaillet, Michel
Faure and an anonymous reviewer are thanked for their comments that
improved the quality of a first\unskip\break 
manuscript. 

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