Tectonic Evolution of the Nevado‐Filábride Complex (Sierra de Los Filábres, Southeastern Spain): Insights From New Structural and Geochronological Data

Abstract

The high‐pressure metamorphic Nevado‐Filábride Complex (NFC) in the Betics mountain range of southeastern Spain exhibits continental and ocean‐derived tectonic units, which are key for understanding the geodynamic evolution of the Western Mediterranean. We address the current debate in the definition of tectonic units, the emplacement of (ultra)mafic rocks, and the timing of burial metamorphism by conducting a structural study combined with single grain fusion ⁴⁰Ar/³⁹Ar dating of white micas in structurally critical outcrops of the eastern Sierra de Los Filábres. One older ⁴⁰Ar/³⁹Ar age population (38–27 Ma) is found at distance from the main shear zones in the relics of an early foliation, while a younger ⁴⁰Ar/³⁹Ar population (22–12 Ma) is dominant in the vicinity of these shear zones, where the early foliation is obliterated. Both age groups are interpreted as the record of deformation or fluid‐induced recrystallization during distinct fabric‐forming events, while alternative scenarios are discussed. A key observation is the presence of an ophiolitic mélange, which—together with new and published geochronological data—allows for a new tectonic hypothesis. This considers Paleogene subduction beneath a Jurassic oceanic lithosphere, followed by the continued subduction of NFC and overlying ophiolites below the Alpujárride Complex. Exhumation during westward slab roll‐back led to the formation of an extensional detachment system that obliquely cut nappe contacts. Although the timing constraints for high pressure‐low temperature (HP‐LT) metamorphism in the NFC remain inconclusive, the new tectonic hypothesis provides a solution that can account for both Paleogene and Miocene ages of HP‐LT metamorphism.

Full text

1. Introduction
The Betic-Rif orogen in the Western Mediterranean region (Figure1a) has been instrumental for understanding
the role of slab dynamics and continental subduction-exhumation processes (Booth-Rea etal., 2015; Jolivet
etal.,2021; Platt, Allerton, et al.,2003; Spakman et al., 2018). In a simplified form, the orogen consists of
an Eocene and younger metamorphosed nappe stack, derived mainly from subducted continental crust (the
internal Betic-Rif or the Alboran zone), which was emplaced during Miocene times over an imbricated, mostly
non-metamorphosed sedimentary cover of the Iberian and North African margins (the external Betic and external
Rif zones), as well as over the Atlantic oceanic crust in the Gulf of Cadiz (e.g., Azañón etal.,1998; Balanyá
& García-Dueñas,1987; Martínez Martínez, 1986; Platt, Allerton, etal.,2003; van Hinsbergen etal.,2020).
Tectonic reconstructions and seismic tomography images have shown that the burial and exhumation history was
associated with 400–800km of subduction, even though the Africa-Iberia absolute plate convergence was only
100–200km since Eocene times (Booth-Rea etal.,2007; Faccenna etal.,2004; Jolivet & Faccenna,2000; van
Hinsbergen etal.,2014). The excess in the amount of subduction is interpreted to result from slab roll-back that
included a significant amount of westward slab retreat, more or less orthogonal to the N-S convergence direction
between Africa and Iberia (Lonergan & White,1997; Rosenbaum etal., 2002). In this general framework, the
original geometry of the subduction zone (dipping N-NW vs. SE) and the exact amount of slab roll-back are still
debated, which has resulted in a number of different paleogeographic and geodynamic scenarios (e.g., Bessière
etal.,2021; Faccenna etal.,2004; Handy etal.,2010; Pedrera etal.,2020; Romagny etal.,2020; van Hinsbergen
etal.,2020; van Hinsbergen etal.,2014; Vergés & Fernàndez,2012). This debate is ultimately rooted in different
interpretations of the timing and direction of nappe stacking, metamorphism, and exhumation of the units in the
internal Betic-Rif orogen, and particularly in the Nevado-Filábride Complex of the Betics (NFC, Figure1b). The
Abstract The high-pressure metamorphic Nevado-Filábride Complex (NFC) in the Betics mountain range
of southeastern Spain exhibits continental and ocean-derived tectonic units, which are key for understanding the
geodynamic evolution of the Western Mediterranean. We address the current debate in the definition of tectonic
units, the emplacement of (ultra)mafic rocks, and the timing of burial metamorphism by conducting a structural
study combined with single grain fusion
40Ar/
39Ar dating of white micas in structurally critical outcrops of
the eastern Sierra de Los Filábres. One older
40Ar/
39Ar age population (38–27Ma) is found at distance from
the main shear zones in the relics of an early foliation, while a younger
40Ar/
39Ar population (22–12Ma) is
dominant in the vicinity of these shear zones, where the early foliation is obliterated. Both age groups are
interpreted as the record of deformation or fluid-induced recrystallization during distinct fabric-forming events,
while alternative scenarios are discussed. A key observation is the presence of an ophiolitic mélange, which—
together with new and published geochronological data—allows for a new tectonic hypothesis. This considers
Paleogene subduction beneath a Jurassic oceanic lithosphere, followed by the continued subduction of NFC
and overlying ophiolites below the Alpujárride Complex. Exhumation during westward slab roll-back led to
the formation of an extensional detachment system that obliquely cut nappe contacts. Although the timing
constraints for high pressure-low temperature (HP-LT) metamorphism in the NFC remain inconclusive, the
new tectonic hypothesis provides a solution that can account for both Paleogene and Miocene ages of HP-LT
metamorphism.
PORKOLÁB ETAL.
© Wiley Periodicals LLC. The Authors.
This is an open access article under
the terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.
Tectonic Evolution of the Nevado-Filábride Complex (Sierra
de Los Filábres, Southeastern Spain): Insights From New
Structural and Geochronological Data
Kristóf Porkoláb1,2 , Liviu Matenco1 , Jasper Hupkes1, Ernst Willingshofer1, Jan Wijbrans3 ,
Hugo van Schrojenstein Lantman1,4, and Douwe J. J. van Hinsbergen1
1Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands, 2Institute of Earth Physics and Space Science
(ELKH EPSS), Sopron, Hungary, 3Department of Earth Sciences, VU University Amsterdam, Amsterdam, The Netherlands,
4Njord Centre, Department of Geosciences, University of Oslo, Oslo, Norway
Key Points:
• White mica
40Ar/
39Ar dating yields
Paleogene ages in relic crenulations,
while Miocene ages in the vicinity of
shear zones
• Shear sense indicators imply
top-NNW nappe stacking and
top-W displacement along the Betic
Movement Zone
• A new hypothesis is formulated for
a gradual, Paleogene-early Miocene
burial of the Nevado-Filábride
Complex
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
K. Porkoláb,
kristof.porkolab@gmail.com
Citation:
Porkoláb, K., Matenco, L., Hupkes,
J., Willingshofer, E., Wijbrans, J., van
Schrojenstein Lantman, H., & van
Hinsbergen, D. J. J. (2022). Tectonic
evolution of the Nevado-Filábride
Complex (Sierra de Los Filábres,
southeastern Spain): Insights from new
structural and geochronological data.
Tectonics, 41, e2021TC006922. https://
doi.org/10.1029/2021TC006922
Received 31 MAY 2021
Accepted 5 JUL 2022
10.1029/2021TC006922
RESEARCH ARTICLE
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NFC is a composite unit that contains both oceanic and continental rocks, metamorphosed up to eclogite-facies
conditions. The eclogite-facies metamorphic fabric has yielded Eocene and Miocene geochronological ages, both
interpreted as prograde metamorphism (Augier, Agard, etal.,2005; Kirchner etal.,2016; Li & Massonne,2018;
Monié etal.,1991; Platt etal.,2006; Sánchez-Vizcaíno etal.,2001). The NFC contains a (ultra)mafic unit, where
Figure 1. (a) Large-scale geological map of the Gibraltar arc (modified after Comas etal.,1999). (b) Geological map of the study area (eastern Sierra de Los Filábres),
modified after Garcia Monzón etal.(1974), containing shear sense indicators, the locations of dating samples, and the location of other key outcrops. Plotted shear
sense indicators are representative average values of numerous measurements per location (for the shear sense data see TableS1). A-A’ is the location of the cross
section on Figure5a, presented together with a more detailed map of the cross section area (marked by blue box). (c) NNW-SSE cross section across the study area
highlighting the major structural features (sz=shear zone).

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the presence of often metamorphosed Jurassic gabbros, dolerites, (pillow)basalts, pelagic sediments, and vari-
ously serpentinized peridotites implies a dominantly ocean-derived character (Puga etal.,2011,1999). However,
a number of observations such as the presence of depleted and serpentinized mantle rocks, Al-rich xenoliths
in the metabasites, mafic dykes intruded into continental formations, and the analysis of detrital U-Pb zircon
populations suggest that the rock succession might have originated from a hyper-extended continental margin
setting (Gomez-Pugnaire & Munoz,1991; Jabaloy-Sánchez etal., 2021; Laborda-López etal.,2020; Morten
etal.,1987), similar to the the West Iberian Atlantic margin (e.g., Reston etal.,2007).
The NFC is truncated at the top by a major extensional detachment system (Betic Movement Zone or BMZ, Platt
& Vissers,1980; Martínez-Martínez et al., 2002) that separates it from the continental Alpujárride Complex
(AC). The continental-derived AC was buried to high pressure-low temperature (HP-LT) metamorphic conditions
in Eocene times (Azañón,1997; Bessière,2019; Bessière etal.,2022; Booth-Rea etal.,2002; Goffé etal.,1989;
Monié etal.,1994; Platt etal.,2005) below rock units of the structurally highest, Maláguide Complex (MC). The
AC and MC were interpreted to be derived from the AlKaPeCa microcontinental fragment that existed within the
Alpine Tethys Ocean, together with tectonic units exposed in the Kabylides units of Northern Africa and units
exposed on Sicily and Calabria (Bouillin etal.,1986; Handy etal.,2010; van Hinsbergen etal.,2014).
Two contrasting interpretations have been offered for the timing of burial in the NFC, based on different (or differ-
ently interpreted) geochronological data. One interpretation is based on Paleogene geochronological data derived
from the continental units of the NFC in terms of
40Ar/
39Ar ages on amphiboles (Monié etal.,1991) and white
micas (Augier, Agard, etal.,2005; de Jong etal.,1992), and U-Th-Pb ages on monazites (Li & Massonne,2018),
interpreted as HP-LT metamorphism in the continental NFC falling into the range of ∼48–30Ma, simultaneously
with the burial of the structurally higher AC (Bessière etal.,2022). This interpretation assumes Paleogene burial,
while other Miocene geochronological ages are thought to reflect a stage of slow exhumation (e.g., Augier,
Agard, etal.,2005; Bessière etal.,2021; Li & Massonne,2018), resulting in a correlation of the NFC with the
same paleogeographic continental unit as the AC and MC (AlKaPeCa unit, Bouillin etal., 1986). This inter-
pretation is difficult to reconcile with the limited amount of Eocene convergence inferred by paleogeographic
reconstructions (<100km, e.g., van Hinsbergen etal.,2020), which appears to be insufficient for the burial of the
entire oceanic to continental NFC domain.
Another interpretation is based on U-Pb zircon dating of mafic eclogites (Sánchez-Vizcaíno etal.,2001), Lu-Hf
garnet dating of oceanic and continental rocks (Platt etal.,2006), and
87Rb/
86Sr multi-mineral dating of one
mafic eclogite and two metapelite samples (Kirchner etal.,2016), which yielded Miocene ages (∼20–12Ma).
These ages are interpreted to reflect prograde burial metamorphism, which implies that the NFC belonged to
the Iberian continental margin that subducted below the Alboran domain (AC and MC) during Miocene times.
These studies however do not consider the Paleogene geochronological data reliable, and hence discard the inter-
pretation of Paleogene NFC burial (e.g., Behr & Platt,2012; Platt et al., 2006). Furthermore, a precise struc-
tural differentiation between nappe contacts and extensional detachments (BMZ) is still unclear across the NFC
(e.g., Martínez-Martínez etal.,2002). The current NFC and AC definition in the key area exposing the (ultra)
mafic unit (eastern Sierra de Los Filábres) follows the contrast in metamorphic grade, which was created by the
tectonic omission along the BMZ after nappe stacking (Martínez-Martínez & Azañón,1997; Martínez-Martínez
etal.,2002; Platt & Vissers,1980). Such a definition is difficult to be directly used for distinguishing paleogeo-
graphic domains stacked in a subduction zone.
In this study we aim to reconcile the tectonic architecture of the internal Betics in the key area of the eastern Sierra
de Los Filábres, by focusing in particular on the juxtaposition of continental and ocean-derived units, together
with the geometry, kinematics, and Ar/Ar age of shear zone folitations. We complement the extensive database
of previous studies by kinematic mapping and structural analyses, together with single grain fusion
40Ar/
39Ar
dating of white micas in structurally strategic outcrops. We re-evaluate the kinematics and timing of tectonic
burial in the NFC, allowing us to revisit the structural definition of tectonic units, constrain the paleogeographical
and kinematic reconstructions of the Alboran region, and provide new insights on the mechanics of Cenozoic
burial-exhumation processes.

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2. Geological Setting
The internal Betic nappe stack consists of three major nappe complexes, from top to bottom the low-grade to
non-metamorphic Maláguide (MC), the low-grade to high-grade Alpujárride (AC), and the medium to high-grade
Nevado-Filábride (NFC) (e.g., Azañón, 1997; Balanyá & García-Dueñas, 1987; Booth-Rea et al., 2002;
Martínez Martínez,1984; Platt, Whitehouse, etal.,2003; van Hinsbergen etal.,2014; Vissers,2012). The NFC
is located in the core of an elongated, E-W oriented metamorphic dome system, while the AC and the MC make
up the flanks of this structure (Figure1a). In our study area of the eastern Sierra de Los Filábres (Figure1b), only
the AC and NFC are exposed.
The AC in the Eastern Sierra de Los Filábres consists of a Paleozoic succession of metapelites (phyllites)
and quartzites, overlain by a Triassic succession of metamorphosed clastic sediments, evaporites (gypsum),
carbonates, and mafic intrusive bodies (Garcia Monzón et al., 1974). The AC is exposed on the northern
flank of the eastern Sierra de Los Filábres and has been separated from the NFC largely based on its (locally)
substantially lower metamorphic grade and on the presence of a regional-scale damage zone (e.g., Booth-Rea
etal.,2002; Garcia Monzón et al., 1974; Martínez-Martínez et al., 2002). The base of the AC is defined by
a zone of retrograde mylonites and cataclasites that represent a major extensional detachment known as the
BMZ, separating the AC in the hanging-wall from the NFC in the footwall across a significant tectonic omission
(Martínez-Martínez etal.,2002; Platt etal.,1984; Platt & Vissers,1980). The AC in the eastern Betics reached
low-temperature blueschist facies metamorphic conditions with a peak temperature of 350–450°C (Booth-Rea
etal.,2002) in Eocene times, interpreted as subduction-related metamorphism (Azañón,1997; Bessière,2019;
Bessière etal.,2022; Goffé etal.,1989; Monié etal.,1994; Platt etal.,2005). Subduction of the AC was followed
by its extensional exhumation reaching very low P/T gradients (within the andalusite stability field), result-
ing in a HT-LP metamorphic overprint during the early Miocene (Azañón etal.,1998; Booth-Rea etal.,2004;
García-Dueñas etal.,1992; Lonergan & Johnson,2002; Platt, Whitehouse, etal.,2003; Rossetti etal.,2005).
The NFC is defined as all rock units situated in the footwall of the Betic Movement Zone and is commonly
subdivided into three tectonic units characterized by upward increasing metamorphic grade across the sepa-
rating thrusts, while upward decreasing metamorphic grade within a single unit (Figure 2, Augier, Agard,
etal.,2005; Augier, Booth-Rea, etal.,2005; Booth-Rea etal.,2015; Martínez Martínez,1986; Martínez-Martínez
etal.,2002). In the eastern Sierra de Los Filábres (Figure1b), the Calar-Alto unit, equivalent of the Caldera unit
(Puga etal.,2002), is composed of a thick (up to 1,500m), likely Permian succession of medium to light-gray,
pelitic schists (Tahal formation), underlain by the dark schists of the Montenegro formation, and overlain by
Figure 2. Tectono-stratigraphic column of the study area with the results of this study and the tectonic subdivisions
used by previous studies, Version A (e.g., Booth-Rea etal.,2015; García-Dueñas etal.,1988; Martínez Martínez,1984;
Martínez-Martínez etal.,2002; Platt etal.,2006), and Version B (Puga etal.,2002,2011,2017).

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Permian to Triassic meta-conglomerates, quartzites, and carbonates, intercalated with small lenses of meta-mafic
rocks (meta-dolerites) (Figures1b and1c, Figure2, Garcia Monzón etal.,1974; Martínez Martínez,1984,1986).
This unit is structurally overlain by the previously described (ultra)mafic unit thought to contain the remnants
of a former oceanic lithosphere and those of a hyper-extended continental passive margin (Gomez-Pugnaire &
Munoz,1991; Laborda-López etal.,2020; Puga,2005; Puga etal.,2011). The (ultra)mafic succession contains
(meta)gabbros and dolerites, mantle peridotites of spinel-lherzolite composition and other, more or less depleted
mantle rocks, such as cumulus troctolites, and are crosscut by doleritic and plagiogranitic dykes, which are best
exposed in the area of Cobdar (Figure1b, Puga,2005). Furthermore, the same unit contains (meta)basalts show-
ing preserved pillow and flow structures (Puga,2005). The igneous age of the mafic rocks was determined by
U-Pb dating of (meta-) gabbros and dolerites yielding Early Jurassic ages (185±3Ma, Puga etal.,2011). The
(ultra)mafic succession is overlain by metasediments (quartzites, micaschists, and calcschists), interpreted to be
part of a pelagic sequence that formed on the ocean floor possibly during Cretaceous time (Puga etal.,2011;
Tendero etal.,1993).
The structural separation of the (ultra)mafic unit is debated. One group of studies include these rocks in a
Bédar-Macael unit that also contains Paleozoic and Mesozoic continental formations (version A on Figure2,
e.g., Augier, Agard, etal.,2005; Martínez Martínez,1984; Martínez-Martínez etal., 2002; Platt etal.,2006).
The observation of small mafic bodies and dykes intruding the continental formations of the Calar-Alto unit and
the overlying carbonates (Booth-Rea etal.,2009a,2009b; Morten etal.,1987), U-Pb age populations of detrital
zircons (Jabaloy-Sánchez etal.,2018,2021), and stratigraphic studies (Ortí etal.,2017; Simon,1987) support
the common origin of a mixed continental-oceanic NFC succession, interpreted to be derived from a former
hyper-extended continental margin (Booth-Rea etal.,2015; Gomez-Pugnaire & Munoz,1991; Laborda-López
etal.,2020). This interpretation does not account for the observations that the (ultra)mafic rock sequence contains
all the elements of a typical (although dismembered and metamorphosed) oceanic lithosphere sheet, favored by
the second group of studies defining a separate Ophiolite unit (version B on Figure2, Puga etal.,2002,2011).
Regardless the structural definition, the (ultra)mafic unit is interpreted by geophysical studies to have a signif-
icantly larger thickness (4–9km) at depth, beneath Sierra de Los Filábres (Pedrera etal.,2009). However, the
structural position and significance of this deep (ultra)mafic body is unclear.
An interesting structural situation is the contact between the (ultra)mafic unit and the overlying sequence, best
exposed in the area of Bédar (Figure1b). Here, the (ultra)mafic unit and associated metasediments are overlain
by a large Carboniferous orthogneiss (meta-granite) body (Martínez-Martínez etal.,2010), which is intruded into
metasediments (paragneisses and micaschists) of possibly pre-Carboniferous age, interpreted to be part of the same
Bédar-Macael unit (version A on Figure2, García-Dueñas etal.,1988; Martínez-Martínez etal.,2002,2010) or
in a different Sabinas unit (version B on Figure2, Puga etal.,2002,2011). The contact between the Carbonifer-
ous and underlying Jurassic (-Cretaceous?) rocks (Figure2) is interpreted to be either a thrust (Puga etal.,2002)
or the result of a large-scale recumbent fold with the orthogneiss in its core (García-Dueñas et al., 1988;
Martínez-Martínez etal.,2002,2010). This controversy is just apparent, since a recumbent fold must be sheared
along the overturned flank to explain the very rapid change in age, which results in a fold nappe type of thrusting
geometry.
2.1. Metamorphic Conditions and Kinematic Characteristics of the NFC
The Calar-Alto unit (Figure2) reached eclogite-grade peak burial metamorphic conditions in the order of
14 kbar/550°C (Augier, Agard, et al., 2005), 20–22 kbar/470–490°C (Santamaría-López et al., 2019), or
16.2–18.6kbar/519–543°C (Li & Massonne,2018). In the thick, dominantly metapelitic formation, the HP-LT
mineral paragenesis consists of garnet, chloritoid, phengite, chlorite, kyanite, and rutile (e.g., Augier, Agard,
etal.,2005; Booth-Rea etal.,2015). The micaschists and orthogneiss bodies of the Sabinas unit reached simi-
lar or slightly higher eclogite-grade peak burial conditions with thermodynamic estimations of 20kbar/520°C
(Santamaría-López et al., 2019), or 20–22 kbar/550–600°C (Ruiz-Cruz et al.,2015). The (ultra)mafic unit
(Figure2) reached similar eclogite-grade peak metamorphic conditions, developing a mineral paragenesis consist-
ing largely of omphacite, garnet, and phengite in the meta-mafic rocks (e.g., Puga etal.,1999). Thermodynamic
estimates of peak metamorphic conditions for the mafic eclogites are in the range of 14–22kbar/570–675°C
(Puga etal., 2000), 16–17 kbar/680–710°C (Padrón-Navarta etal.,2010), 20–22kbar/550–600°C (Ruiz-Cruz
etal.,2015), 16–22kbar/500–700°C (Augier, Agard, etal.,2005), or 17–19kbar/680°C (Menzel etal.,2019).

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Numerous studies have interpreted that both the continental and (ultra)mafic protoliths experienced an additional
episode of heating during their decompression, that is, exhumation (Booth-Rea etal.,2015; Li & Massonne,2018;
Puga etal., 2000; Santamaría-López etal.,2019), although retrograde P-T paths implying isothermal decom-
pression (Augier, Agard, etal., 2005) or cooling during decompression (e.g., Behr & Platt, 2012) have also
been reported. Retrograde metamorphism in the metapelites of the Calar-Alto and Bédar-Macael units is char-
acterized by the abundant growth or recrystallisation of chlorite, albite, white mica, and quartz, which define
the post-HP tectonic foliation(s) (Augier, Agard, etal.,2005; Booth-Rea et al., 2015). Following medium-to
low-temperature retrograde metamorphism, the NFC experienced significant further cooling and exhumation to
(or near) the surface during the late Miocene (Agard etal.,2011; Augier etal.,2013; Johnson,1997; Lonergan &
Johnson,2002; Vázquez etal.,2011).
The kinematics of deformation during HP-LT metamorphism in the NFC is largely unknown due to the signif-
icant amount of obliteration during the subsequent moderate-to low-temperature shearing (e.g., Augier, Agard,
etal.,2005). Retrograde mineral assemblages in the NFC overgrow a relic S0 sedimentary layering and the early,
HP-LT, S1 tectonic foliation, which is preserved in the deeper sections of the Calar-Alto unit, and is progres-
sively obliterated toward the Marchal shear zone (García-Dueñas etal.,1988), where the S2 or younger foliations
become more closely spaced (Augier, Agard, etal.,2005; Booth-Rea etal.,2015). The pervasive S2(-3) folia-
tion(s) carries stretching lineations that are dominantly associated with a top-W sense of shear during exhuma-
tion, especially close to the main extensional detachment system (BMZ, Augier, Agard, etal.,2005; Booth-Rea
etal.,2015; Martínez-Martínez etal.,2002; Platt & Vissers,1980). Augier, Jolivet, and Robin(2005) furthermore
reported the divergence of this generally westward shear direction on either limbs of the NFC dome. Retrograde
shearing in the formations of the NFC finally transitioned to cataclastic to brittle shearing and normal faulting
along the Betic Movement Zone (Augier, Jolivet, & Robin,2005; Martínez-Martínez etal.,2002).
3. Structural and Tectono-Stratigraphic Results
We complement the extensive available kinematic database by conducting a fieldwork with the aim of detailing
the geometry, kinematics, and metamorphic conditions of the main shear zones as well as making new obser-
vations regarding the present structural relationship between the original continental and (ultra)mafic proto-
liths. We build on the already established petrological data and use further field and microstructural criteria to
infer the kinematics of shearing during the established metamorphic stages. We used stretching lineations and
associated kinematic indicators such as shear bands (C-S or C’-S structures, Figures3c and4f), or asymmetric
porphyroclasts (Simpson & Schmid,1983) to infer tectonic transport directions under different metamorphic
conditions. The measured stretching lineations can be generally grouped in two categories based on minerals
that are characteristic for metamorphic facies: moderate-temperature (developed around 450°C), represent-
ing post-peak metamorphic conditions prevailing during deep burial and early exhumation (e.g., Booth-Rea
etal., 2015), and low-temperature, representing the retrograde metamorphic stage during significant decom-
pression (exhumation) in the NFC (Figures1b and3a, e.g., Augier, Agard, etal.,2005). Moderate-temperature
stretching lineations are usually defined by black and blue amphiboles and plagioclase in the metamafic rocks.
Low-temperature stretching lineations are defined when arguments for retrograde mineral growth with respect to
moderate-temperature conditions were found in the field or thin sections. These are situations when: (a) low-grade
minerals, most frequently chlorite and albite (Augier, Agard, etal., 2005) overgrow higher-grade fabrics with
no moderate-temperature lineation preserved (see also Augier, Agard, etal.,2005); (b) a moderate-temperature
stretching lineation is still preserved, but it is overprinted by retrograde lineations while keeping the same sense
of shear; (c) a moderate-temperature stretching lineation is still preserved in some foliation planes, but a second
set of stretching lineations overprint the first set with a different orientation, defined by different minerals, such
as chlorite, albite, green amphibole, white mica, quartz or calcite.
3.1. Kinematics of Deformation
A large part of our structural observations are similar to those of previous studies in terms of foliations, super-
position of folding and shearing characteristics (e.g., Augier, Agard, et al., 2005; Booth-Rea et al.,2015;
Martínez-Martínez etal.,2002). Therefore, we avoid an extensive repetitive description and focus on new kine-
matic observations that are herewith described in more detail.

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The NFC in the eastern Sierra de Los Filábres displays a gradual change of ductile structural elements from
the core of the dome (deeper sections of the Calar-Alto unit) toward its flanks, where the Marchal and Cuatro
Amigos shear zones crop out (Figures1b and 1c). The metapelites in the core of the dome show a distinct
succession of folds and related axial planar cleavages that are largely obliterated by the development of shear
zone foliation in the proximity of these shear zones. A generation of early isoclinal folds (F1) is only observed
in the metapelites of the Calar-Alto unit. These folds affect the S0 sedimentary layering, are generally rootless
and transposed into the tectonic foliation of the subsequent folds (S2, Figures4a and4e). F2 isoclinal folds are
associated with a pervasive S2 foliation. The S0, S1 and S2 foliations are generally (sub-)parallel in the areas close
to the shear zones (composite S0-2 foliation, Figures4b and4f), while in the deeper sections of the Calar-Alto unit
S2 foliation defines a strong crenulation cleavage in the meta-pelitic rocks (Figures4a and4e). Tight to isoclinal,
dominantly asymmetric folds (F3) refold the pervasive S2 foliation and F2 isoclinal folds, associated with an occa-
sionally strongly developed S3 tectonic foliation (Figure3d). In the shear zones, the tight F3 asymmetric folds
are flattened to isoclinal fold geometries and the S3 foliation becomes sub-parallel to the S2 foliation, making
their distinction difficult in places. These two generations of tight-isoclinal, cylindrical folds (F2-3) were meas-
ured in all exposed units of the NFC in our study area and show dominant E-W oriented fold axes (Figure3b),
and gently dipping (<45°) axial planes. These folds are dominantly asymmetric and have a ∼N-ward vergence
(e.g., Figures1c and3d). The exception is the El Pilar–El Chive area, where consistent southward vergence of
asymmetric F3 folds is observed in the meta-mafic and carbonate rocks (Figures4a and4d). F2-3 folds are the
product of ductile deformation (flow) and show oriented growth of plagioclase and black to blue amphiboles in
the meta-mafic rocks, implying that the formation of these folds was coeval with moderate-temperature shearing
that followed peak metamorphism (Figure3d). S2 and subordinately S3 foliation planes carry well-developed
stretching lineations. Based on field and micro-structural observations, two generations of stretching lineations
can be distinguished. Moderate-temperature stretching lineations and coupled kinematic indicators show an aver-
age top-NNW (top-330°) sense of shear, while low-temperature stretching lineations (including a transition from
moderate to low-temperature while keeping the same sense of shear) show an average top-W (top-271°) sense
Figure 3. (a) Stereographic projection of moderate and low-temperature stretching lineations and associated kinematic
indicators. Plotted shear sense indicators are representative average values of numerous measurements per location. (b)
Stereographic projection of cylindrical, asymmetric, tight-isoclinal fold axes (F2-3). (c) C-S shear fabric in garnet-micaschist
at the top of the Ophiolite unit (Cuatro Amigos shear zone) showing top-WNW sense of shear. (d) Tight, asymmetric,
N-verging fold (F3) in mafic amphibolite showing the crystal-plastic deformation (flow) of plagioclase.

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Figure 4.

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of shear (Figure3a). The more northerly directed tectonic transport during moderate-temperature metamorphic
conditions is in agreement with the dominantly E-W trending and N-verging, cylindrical, asymmetric F2-3 folds
(Figures3a and 3b, also described by Augier, Jolivet, & Robin,2005). Summarizing the kinematic observa-
tions, moderate-temperature shearing occurred dominantly toward the NNW, and shifted toward the W during
lower-temperature retrograde shearing.
3.2. Shear Zones and Tectonic Units in the Eastern Sierra de Los Filábres
The mapping of the main shear zones, complemented with new observations regarding the tectono-stratigraphic
superposition of the formations, has led to a refinement of the commonly used subdivision of tectonic units for
the eastern Sierra de Los Filábres (Figures1c and2). The base of the Calar-Alto unit is not exposed in our study
area. The top of this unit is defined by the Marchal shear zone (García-Dueñas etal.,1988). This shear zone is
often observed in mylonitic quartzites and marbles situated in the uppermost part of the Calar-Alto unit or in
the marbles and amphibolites in the lower part of the Ophiolite unit (Figures1c and2). The shear zone shows
NW-SE to E-W oriented stretching lineations and associated top-NW to W sense of shear (Figure4b). Along the
northern flank of the Sierra de Los Filábres, a repetition of the Ophiolite unit is observed (Figures1b and1c).
This repetition is explained by a reverse-sense shear zone (herewith named Torcal shear zone), which does not
exhibit consistent shear sense directions (both top-E and top-W kinematic indicators). The top of the Ophiolite
unit is marked by the basal shear zone of the Sabinas unit, herewith named the Cuatro Amigos shear zone, which
is observed in the amphibolites, micaschists, and calcschists of the Ophiolite unit, and in the Carboniferous
orthogneiss of the Sabinas unit. The shear zone exhibit mylonites and ultramylonites, where the observed stretch-
ing lineations are dominantly retrograde, trending WNW-ESE associated with a top-WNW sense of shear, as
observed, for instance by a well-developed C-S fabric in the micaschists of the Ophiolite unit (Figure3c). Given
that the significantly older Carboniferous orthogneiss tectonically overlies the Ophiolite unit, we describe it as
an initial thrusting contact between the Ophiolite and Sabinas units (the Cuatro Amigos shear zone, Figure2).
This is not incompatible with the fold-nappe solution described in detail by García-Dueñas etal.(1988), with
the exception that the overturned flank of the fold-nappe must have accommodated larger amounts of thrusting
(shearing) than hitherto assumed. The Sabinas unit is further dissected by multiple shear zones causing a complex
repetition of formations including a thin sliver of mafic amphibolites and serpentinites outcropping below Trias-
sic dolomite marbles (Figure1c).
3.2.1. Lithostratigraphy and Structure of the Ophiolite Unit
Starting from the lithological composition and the mafic-ultramafic petrography of the Ophiolite unit exten-
sively described by previous studies (e.g., Puga etal.,1999,2000,2011), our observations show that the base
of this unit generally displays Triassic meta-carbonates (dominantly calcite marbles and subordinately dolomite
marbles) folded together with thin layers of meta-mafic rocks (Figures5a and5c). The meta-carbonates are in
most sections overlain by the Jurassic mafic and ultramafic rock succession that can reach a thickness of 250m.
(Meta-)mafic rocks are dominantly fine to coarse grained amphibolites, often containing relics with the texture
of the basaltic, doleritic, or gabbroic protoliths. The amphibolites are characterized by a clearly developed, often
mylonitic foliation defined by stretched plagioclase and oriented amphibole crystals. The mineralogical associa-
tion of the amphibolites often includes epidote and garnet, which appear to be pre-to syn-kinematic with respect
to deformation in the main shear zones. Other rocks observed are large bodies of serpentinites, locally reaching
a thickness of 150m (Figure5h), serpentinite-schists (for details also see Menzel etal.,2019), and talc-schists.
The serpentinite schists generally show well-developed S2-3 foliations that are continuous with the ones in the
Figure 4. Interpreted thin section pictures showing the microstructure of geochronologically dated and other key samples. The sample location is plotted on Figure1b.
(a) Plain polarized thin section picture of the Tahal formation, outside of the Marchal shear zone (Key outcrop 8 on Figure1b), showing unoriented chlorite growth
during retrogression and the preservation of an early S0-1 foliation in relic crenulations. (b) Plain polarized thin section picture of the Tahal formation, inside the Marchal
shear zone (Key outcrop 9 on Figure1b), showing oriented chlorite growth. Older fabrics (S0, S1, possibly S2) are completely overprinted by the shear zone foliation.
C’-type shear bands imply top-W sense of shear. (c) Plain polarized thin section image of well-preserved garnet porphyroclasts in the Sabinas unit (Key outcrop 10 on
Figure1b) outside of the main shear zones. (d) Plain polarized thin section picture of a garnet pseudomorph within the Cuatro Amigos shear zone, largely replaced by
white mica and quartz (Sample three on Figure1b). (e) Cross-polarized picture of Sample 1 showing the preservation of the early S0-1 foliation in crenulations and the
unoriented growth of chlorite. Large mica crystals define the S0-1 foliation, while small micas define the S2 foliation. (f) Cross-polarized picture of Sample 5 showing a
mylonitic foliation and top-W C’-type shear bands. (g) Cross-polarized picture of Sample 6 showing a band of fine-grained mylonite associated with top-WNW sense
of shear in the orthogneiss of the Sabinas unit. (h) Cross-polarized picture of Sample 4 showing mylonite bands associated with top-WNW sense of shear directly below
the BMZ.

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surrounding rocks. In the Cobdar area (Figure1b), previous studies have described extensively the structure,
composition, and petrography of mantle rocks, such as peridotites or cumulus troctolites, affected by a signif-
icant degree of metasomatism and alteration from serpentinites to talc, together with gabbros, dolerites, and
pillow basalts (e.g., Puga etal.,1999,2011,2017). The crustal rocks are cut by numerous dykes of (porphyric)
basalts, dolerites, and plagiogranites (or plagio-gneisses when metamorphosed) that locally retain the typical
sheeted-dykes geometry or composition (Figure5g). The mafic and ultramafic rocks are overlain by calc-schists,
garnet-micaschists, and quartzites, interpreted to represent a syn-to post-rift deep-water successions deposited on
top of the oceanic rocks (Figures5a, 5b, 5h and5i, Puga etal.,2011; Tendero etal.,1993). These metasediments
cover both the mafic and ultramafic rocks of the Ophiolite unit (Figures5a and5b). The different lithologies
of the Ophiolite unit display various vertical superpositions of different rock types due to the effect of several
successive deformation phases and is significantly omitted in or near the vicinity with the BMZ or associated
exhumation shear zones.
One key observation not described previously is that the (meta)mafic, ultramafic, and (meta)sedimentary forma-
tions of the Ophiolite unit are often mixed in a mélange that often still preserves the olistostrome structure of
Figure 5. Structure and composition of the Ophiolite unit. (a) Cross section through the ophiolite unit at the El Pilar location
(for map-view trace of the section see subplot (b) on this figure, and Figure1b). Note the consistent southward vergence of
asymmetric folds that affect thin slivers of metamafics and the metacarbonates together. (b) Geological map detail showing
the area of the cross section. (c) SSW-verging asymmetric fold in shallow-water carbonates (limestone marble). The carbonate
layer is underlain by fine-grained mafic amphibolite displaying several mafic blocks of similar composition. (d) (Meta)
Gabbro block in pelitic matrix in the mélange formation of the Ophiolite unit. See hammer for scale. (e) Calcite marble
block in mafic-pelitic matrix in the mélange of the Ophiolite unit. (f) Interpreted thin-section picture of a chloritoid-talc-
kyanite schist sample taken from the metamorphosed ophiolitic mélange. The association between chloritoid (cld),
amphibole (amp), talc, kyanite (ky), white mica, quartz, and plagioclase indicate a mixed sedimentary-altered mafic protolith
affected by high-pressure metamorphism. (g) Plagiogranite dykes in dolerite at Cobdar. (h) Thick body of serpentinite
covered by quartzites and micaschists. (i) Calcschists and micaschists that cover the mafic and ultramafic formations of the
Ophiolite unit.

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the original protolith, despite the superimposed shearing and metamorphism (Figures5a and5b). The mélange
is made up largely by a talc, serpentinite, or meta-siltic and pelitic matrix (Figure5f), surrounding centime-
ters to tens of meters sized (meta)sedimentary and (meta)mafic blocks, such as calcite or dolomite marbles
(e.g., Figure5e), quartzites, various types of schists, metamorphosed basalt, dolerite, gabbro (e.g., Figure5d), or
meta-conglomerates. The (meta)mafic blocks often preserve the original gabbroic, doleritic, or basaltic texture
(e.g., Figure5d) and the surrounding matrix is typically rich in talc (Figure5f), serpentinite, and meta-mafic
minerals, mixed with meta-sedimentary blocks or matrix, sometimes including secondary gypsum. Thin section
analysis shows the dominance of chloritoid, talc, kyanite, and amphiboles in the fine-grained mélange matrix
(Figure5f). The proportion of mafic and siliciclastic material is highly variable within the mélange. The S2-3
foliation is independent of the contacts between different lithologies within the mélange, generally crosscut-
ting throughout the matrix and blocks of various composition, suggesting that the formation of the mélange
predate the D2 tectono-metamorphic event. The best preserved section (from El Pilar toward Lubrin along the
main road) displays a typical ophiolitic mélange facies, with mafic blocks mixed together with other blocks
of meta-sediments, enclosed in a talc-and serpentinite-rich sedimentary matrix (Figures 5a, 5b, 5d and 5e).
The mélange is cut by a series of dominantly NW-dipping, late normal faults, but still retains the original olisto-
strome structure (Figures5a, 5b, 5d and5e). The mélange formation outcrops at numerous locations in the eastern
Sierra de Los Filábres (Figure1b), and exhibit various grades of metamorphism and style of deformation. The
deformation observed in the Ophiolite unit is mostly similar with the one observed in all other units. The excep-
tion is the area of the El Pilar and El Chive (Figure1b), where the meta-mafic and ultramafic rocks and the ophi-
olitic mélange are better exposed and preserved. In this area, a generation of asymmetric folds has been observed,
which have a consistent top-SSW vergence (Figures5a and5c), in contrast to the typical N-ward vergence of F2-3
asymmetric folds observed elsewhere.
3.2.2. Additional Tectono-Stratigraphic Observations in the Low-Grade Unit
Along the northern flank of the Sierra de Los Filábres, a tectonic omission creates a significant offset from
high-grade to low-grade metamorphic conditions (the low-grade unit in this area commonly interpreted as the
AC, e.g., Augier, Agard, etal.,2005; Booth-Rea etal.,2015; Martínez-Martínez etal.,2002), separated by a zone
containing originally high-grade rocks that were retrogressed during mylonitization and cataclasis (faulting),
which mark the previously mapped BMZ of Platt and Vissers(1980). The retrogressed mylonites in the footwall
largely consist of garnet-micaschists that overly the mafic and ultramafic rocks of the Ophiolite unit (Figure6a).
These garnet-micaschists are characterized by the retrograde growth of chlorite, calcite, and quartz at the expense
of the peak metamorphic mineral assemblage (Figure6b). Stretching lineations carried by chlorite are associated
with clear top-W shear sense indicators. The retrograde mylonites are often reworked as cataclasites and cut by
numerous normal faults that post-date the cataclastic foliation (Figure6c). The transport direction of the cata-
clastic shear bands and normal faults are also top-W, in agreement with the ductile kinematic indicators in the
retrograde mylonites (Figure6c).
In the hanging-wall of the BMZ, the low-grade formations show significant lithological similarity to the underly-
ing formations of the NFC. The basal formation of the low-grade unit consists of thick successions of metapelites
and quartzites that are the low-grade equivalents of the Permian strata of the Calar-Alto unit. The metapelites only
show the grade of phyllitic cleavage in terms of fabric development, while the quartzites developed a less closely
spaced tectonic foliation compared to those of the Calar-Alto unit. In some sections, the BMZ is directly overlain
by non-metamorphosed or very low-grade dolerites and basalts in the hangingwall that preserved their igneous
textures and mineral composition (Figures6d and6e). These mafic rocks may correspond to a non-metamorphic
equivalent of the Jurassic meta-mafic rocks (fine and coarse-grained amphibolites) in the Ophiolite unit located
beneath the BMZ. These mafic rocks are also overlain by dolomites, which do not show a developed tectonic foli-
ation or signs of metamorphic recrystallization, attesting to relatively low metamorphic facies (lower greenschist
facies and below). The dolomites could be the protolith equivalents of the Triassic dolomite marbles observed
in the NFC.
3.2.3. Strain Distribution During Exhumation (Retrograde Metamorphism)
Mapping of structural and metamorphic features with special attention to the superposition of moderate and
low-temperature fabrics allowed us to delineate zones where strain localization during retrograde metamor-
phism took place in our study area. A large amount of deformation was accommodated along the BMZ exten-
sional detachment system (around 110km of extension, Martínez-Martínez et al., 2002), as evidenced by the

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earlier described tectonic omission from the higher-grade footwall to the low-grade hanging-wall (see also
Martínez-Martínez etal.,2002; Platt & Vissers,1980).
Furthermore, the NFC in the footwall of the BMZ records strain localization during retrograde metamorphism
(Agard etal.,2011), reflected in the development of retrograde shear fabrics across the dome and in the concen-
tration of strain in the top part of the NFC. The metapelites in the core of the NFC preserve an early S0-1 folia-
tion, overprinted by the dominant S2 foliation (Figures4a and4e). Both foliations are overgrown by abundant
and dominantly unoriented, retrograde chlorite (Figures4a and4e), implying that the core of the NFC was not
affected by significant deformation during the growth of chlorite (for a detailed study also see Agard etal.,2011).
In contrast, the top 100m of the same metapelite formation directly underlying the top-WNW Marchal shear
zone shows oriented chlorite (and albite) growth, defining the dominant stretching lineation and main foliation
close to and within the shear zone (Figure4b). Hence, the Marchal shear zone shows activity during retrograde
metamorphism of the NFC, but this deformation did not propagate deeper into the Calar-Alto unit, producing a
transition from oriented to unoriented chlorite growth from the flanks to the core of the dome. A similar trend is
observed in the upper two tectonic units of the NFC (Ophiolite and Sabinas units). In the proximity of the Cuatro
Amigos shear zone, the higher-temperature fabric of the garnet-micaschists is generally replaced by retrograde
white mica, chlorite, and quartz (Figure4d). Furthermore, the oriented growth of chlorite at the expense of the
higher-temperature fabric is observed in the micaschists and metamafics of the Ophiolite unit, as well as in the
orthogneiss of the Sabinas unit. The orthogneiss also exhibits bands of ultramylonites associated with top-WNW
Figure 6. (a) Interpreted satellite image showing a typical exposure of the main extensional detachment (BMZ). The
location of the exposure is given on Figure1b (Key outcrop 7). (b) Cross-polarized thin section picture of the retrogressed
garnet-micaschist directly underlying the BMZ. The asymmetry of the garnet+quartz+white mica clast suggests top-WNW
sense of shear. (c) Retrograde mylonite reworked as a cataclasite within the main damage zone of the BMZ. Cataclastic
shear bands and normal faults are dipping toward the WNW. (d) Cross-polarized thin section image of the non-metamorphic
dolerite overlying the BMZ. The dolerite preserved its original igneous texture and composition. (e) Outcrop image of the
non-metamorphic dolerite.

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sense of shear and growth of retrograde minerals such as sericite or chlorite (Figure4g). Further away from the
shear zone, garnets are typically better preserved (Figure4c), retrograde stretching lineations are less frequent,
and the dominant higher-grade sense of shear (top-NNW) is better preserved (Figure1b). Hence, the Cuatro
Amigos shear zone also shows activity during the retrograde metamorphism of the NFC, similarly to the Marchal
shear zone.
4. White Mica Chemistry
The chemistry of the white micas dated by
40Ar/
39Ar method (Section5) was constrained by microprobe meas-
urements, which allowed to characterize their Si versus Mg+Fe content (Figure7c, the entire microprobe dataset
is available in the TableS3). Variable Si content is generally interpreted as a result of the pressure-dependent
Figure 7. Relationship between metamorphic fabrics and different mica generations based on macroscopic and microscopic
(see Figure4) observations, together with the chemistry of mica populations dated by
40Ar/
39Ar. (a) Schematic 2D
representation of the metamorphic fabrics and related mica generations in the relatively deep sections of the Calar-Alto unit
(sample 1). Here the early S0-1 foliation is preserved, often defined by relatively large (>250μm) white mica grains, while
the younger tectonic foliation is constituted by substantially smaller white mica crystals. Consequently, the 250–500μm
grain-size selection for the dating favored the sampling of the S0-1 fabric. (b) Schematic 2D representation of the metamorphic
fabrics and related mica generations near or within the main shear zones of the eastern Sierra de Los Filábres (samples 2–6).
Here the early S0-1 foliation is not preserved, hence all dated mica grains belong to the younger foliation(s), which developed
during shear zone activity. (c) Si versus Fe+Mgc.p.f.u. (cations per formula unit) plot showing that all samples are
characterized by a dispersion of Si content between ∼3.2 and ∼3.35.

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Tschermak substitution reaction, which represents a solid solution exchange reaction between muscovite and
celadonite as end members, that is, KAl2[AlSi3]O10(OH)2 and K[(Mg,Fe)Al][Si4]O10(OH)2, respectively. In
common phengites, Si content is typically between 3.0 and 3.5c.p.f.u. (cations per formula unit), where low-Si
phengites are characteristic for low-pressure, while high-Si phengites are indicative of high-pressure (blueschist
and eclogite facies) conditions (Massonne etal.,1995; Massonne & Schreyer,1987). This pressure dependence
of meta-pelite phengite composition in principle can be used as a first order guide for a distinction within each
sample of different phengite generations.
We measured the composition of at least 10 mica grains in samples 1, 3, 4, 5, and 6. In sample 1, micas taken from
the S0-1 and S2 foliations (Figure7a) were measured separately, while in other samples micas were selected from
the dominant shear zone foliation (S2+). Figure7c shows that the Si content varies between 3.15 and 3.4c.p.f.u.,
with every sample reaching maximum values of 3.3 or higher (Figure7c). The dataset shows no major differences
between the mean values of each sample, but shows different degree of dispersion: for example, sample six is
much less dispersed than sample 3 (Figure7c). In case of sample 1, micas derived from the S0-1 and S2 foliations
show no difference in the mean Si content, while the S2+ micas show a significantly larger dispersion (higher
maximum and lower minimum values).
5.
40Ar/
39Ar Dating
5.1. Sampling Strategy and Methodology
40Ar/
39Ar dating of white micas was performed with the objective to constrain the timing of metamorphic fabric
formation and the activity of shear zones in our study area by selecting six samples from key areas of the east-
ern Sierra de Los Filábres (Figure1b, see Figures4d– and4h for thin section images of dated samples). We
selected one sample from the core of the NFC dome located 1–2km away (vertically) from the main shear zones
(Figures4e and7a), as well as five samples taken from different shear zones located along the flanks of the
dome (e.g.,4f–4h and7b). This strategy allowed exploring potential age differences between the S0-1 foliation,
preserved only in the sample located in the core of the dome, and subsequent (S2 and higher) shear zone foli-
ations. We applied the
40Ar/
39Ar dating of large (≥250μm) white mica crystals with the multiple single grain
fusion dating method (Uunk etal.,2018). The applicability of white mica
40Ar/
39Ar single grain fusion dating
approach for the detection of tectono-metamorphic events has been proven by numerous works (Lips etal.,1998;
Lister & Forster,2016; Porkoláb etal.,2019; Uunk etal.,2018; Wijbrans etal.,1990). The resetting of white mica
Ar-systems may be achieved by thermal diffusion (allowing to constrain the timing of cooling below the closure
temperature) or by deformation-induced or fluid-assisted (re)crystallization, which allows to constrain the timing
of metamorphic fabric formation (e.g., Villa,1998; Wijbrans & McDougall,1986). Temperature-dependent reset-
ting of white micas is a gradual process, and depends not only on the temperature itself but also on the duration
of the overprinting event (Wijbrans & McDougall,1986), as well as on the grain radius (de Jong etal.,1992).
Hence, in case the metamorphic temperature was high enough and maintained long enough to resetall (small
and large) mica crystals, homogenous age distribution of the dated single grains are expected (i.e., a single
tectono-metamorphic event will be detected by the dated grains). In case one criteria was only partially sufficient,
partial resetting may take place, producing heterogeneous age distribution, that is, detecting multiple resetting
events and mixed ages in between (Lister & Forster,2016; Uunk etal.,2018; Warren etal.,2012). Complete
resetting of the white mica Ar-system by thermal diffusion is thought to require temperatures higher than typical
values of greenschist-blueschist facies metamorphism (>500°C, Harrison et al.,2009; Warren etal.,2012). If
temperatures were not high enough (or the duration of heating not long enough) to reset the white mica Ar-system
by thermal diffusion, some (or all) of the grains can still be reset by deformation-induced or fluid-assisted crys-
tallization due to the formation of a tectonic foliation, producing similar homogenous or heterogeneous grain age
distributions (Lister & Forster,2016; Porkoláb etal.,2019; Villa,1998).
White mica crystals of the crushed samples were separated from 250 to 500μm sieve fractions using a Faul
vibrating table and heavy liquid density-based separation. Carbonate and dust contamination were removed by
HNO3 treatment of the samples. Mica separated from six samples were selected for single grain fusion dating.
The samples were packed in aluminum foil packages and stacked in an aluminum tube that was irradiated for
18hr in the CLICIT facility of the Oregon State University TRIGA Reactor. For both irradiations the neutron
flux was monitored by standard bracketing with the DRA sanidine standard with an age of 25.52±0.08Ma,
modified from Wijbrans etal.(1995) to be consistent with Kuiper etal.(2008). Single grain fusion experiments

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were carried out in the Vrije University Amsterdam argon geochronology laboratory with 25W CO2 laser heat-
ing samples loaded on Cu-trays (185 individual 2mm diameter, 3mm deep holes for single grains). The sample
holder was connected to a three-stage extraction line and a quadrupole mass spectrometer (Schneider etal.,2009).
Figure 8. Results of single grain fusion white mica
40Ar/
39Ar dating. (a–f) Plots of
40Ar/
39Ar ages, where numbers indicate sample codes (for the map location of dated
samples see Figure1b). Black circles represent individual dated grains. Black lines show the relative probability, that is, the cumulated 1-sigma normal distributions of
every grain age in the samples. Peaks in the black lines hence show most likely dates of resetting in the samples. (g) Cross-section through our study area showing the
projected locations of the dated samples.

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Data was reduced in ArArCalc 2.50 (Koppers,2002). Procedure blanks were monitored and diluted air shots were
measured in the sequence to track mass discrimination. Furthermore, we have constrained the chemistry of white
micas in the samples dated with
40Ar/
39Ar by microprobe measurements at Utrecht University, the Netherlands.
Compositional analyses were acquired on an electron microprobe (JEOL JXA-8530F) for the following elements:
Si, Al, Mg, Ca, Fe, Na, K, Mn, and Ti. For more details on the microprobe methodology we refer to the “method”
sheet of TableS3 in Supporting InformationS1.
5.2.
40Ar/
39Ar Dating Results
The dated samples show a variety in age results as well as in the distribution of the single grain ages within
individual samples. A group of three samples (samples 2, 3, and 4) display homogenous single grain age distri-
butions with a relative probability peak at ∼15Ma (Figures8b–8d). The grains that define the 15Ma probability
peak show ages between 12 and 22Ma. Some grains with significant deviation from the mean are still present
in these samples, showing older (20–25Ma) or younger (10–12Ma) ages. Sample 2 shows an outlier grain with
substantially older (57Ma) age. Sample 1 also shows a largely homogenous age distribution; however, the peak
relative probability of the ages is significantly older, ∼33Ma (Figure8a). The grains that define the probability
peak are between 27 and 38Ma, while two outlier grains are present in between 40 and 45Ma, and one grain at
70Ma. Sample 5 contains four grains close to 20Ma, and further two at 70Ma (Figure8e), while sample 6 shows
the most heterogeneous age distribution with three groups of grains at 12–14Ma, 17–24Ma, and 28–30Ma
(Figure8f).
The location and microstructure of the dated samples highlight the possible importance of the age differences
between Sample 1 and all the other samples (Figures7a, 7b and8g). Sample 1 was collected from the pelitic
schist formation of the Calar-Alto unit, relatively far from the major shear zones in the area. This sample is the
only one to preserve S0-1 foliation in crenulations (Figure7a), and thus probably preserved earlier mica genera-
tion(s) compared to the other samples (Figure7b). Furthermore, the grain size of the white micas in the relic S0-1
foliation (typically 100–300μm) is substantially larger than the grain size of white micas in the pervasive S2 foli-
ation (typically 50–150μm, Figures4e and7a). Hence, the dated micas belong to the older S0-1 foliation, as all the
selected single grains were larger than 250μm (Figure7a). The mean age of 33Ma of this sample is significantly
older than the mean age of the other samples (15–20Ma) which were collected near (sample 6) or from the major
shear zones (samples 2, 3, 4, 5, Figure8g). In these samples the oldest S0-1 foliation is completely obliterated by
the superposed foliation and shear fabric development due to shear zone activity, resulting in the formation of S2
or younger shear zone foliations (Figure7b). Samples 2, 3, and 5 were collected from micaschists of the Ophiolite
unit, incorporated into the shear zone that emplaced the Carboniferous formations of the Sabinas unit on top of
the Ophiolite unit. The shear zone was later likely reactivated (or progressively used) during the exhumation of
the NFC (see Section3.2.3). Sample 4 was collected from the retrograde garnet-micaschists incorporated in the
BMZ that separates high-grade and low-grade units, while sample 6 was collected from a mylonitic horizon of
the Carboniferous orthogneiss of the Sabinas unit.
6. Discussion
6.1. Interpretation of
40Ar/
39Ar Ages and Timing of Tectono-Metamorphic Events
Numerous studies have shown that Ar-loss in white micas is a complicated process, where thermal diffusion
might only play a limited role in the resetting of the Ar-system, particularly when significant deformation or
fluid flow is present in the given rock volume (Harrison etal.,2009; Lister & Forster,2016; Uunk etal.,2018;
Villa,1998; Warren etal., 2012). Our results are in agreement with in-situ laser
40Ar/
39Ar white mica ages
reported by Augier, Agard, etal.(2005), which shows older ∼45–25Ma ages for the preserved S0-1 foliation
when compared with the ∼22–12Ma ages in places where such foliation was overprinted by the fabric of a shear
zone. Our results and those of Augier, Agard, etal. (2005) indicate that structural criteria (preservation of a
tectonic foliation) correlate with the age results, which implies that temperature is not the dominant mechanism of
resetting. We hence argue that the dominant resetting mechanism in our samples was deformation- and possibly
fluid-induced (re)crystallization during the development of tectonic foliations. Along these lines white micas
constituting an older foliation (S0-1) in the metapelites of the Calar-Alto unit yield older age clusters compared to
samples where micas constitute younger fabrics (S2 or younger foliations developed during shear zone activity).

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Furthermore, these samples are located at a vertical distance of ca. 1–2km below the main shear zones (Marchal
shear zone, see Figures 1c and 1d in Augier, Agard, etal.(2005)) and do not exhibit evidence for significant
deformation during the activity of these overlying shear zones (22–12Ma). Hence, we assume a very limited
deformation- or fluid flow-induced Ar-loss in these samples during the activity of the main shear zones. It follows
that peak temperatures (ca. 470–550°C) in the Calar-Alto unit (Augier, Agard, etal.,2005; Li & Massonne,2018;
Santamaría-López etal.,2019) were not high enough (or not sustained long enough) to facilitate efficient thermal
diffusion in the metapelites.
Several studies have suggested that excess argon might be responsible for the presence of older than early
Miocene white mica
40Ar/
39Ar ages in the NFC (Behr & Platt,2012; de Jong,2003; De Jong etal.,2001; Kirchner
etal.,2016; Platt etal.,2006). Excess argon incorporation in white micas has been attributed to sub-microscopic
illitization during late-stage hydraulic fracturing, where the density of the fracture network controls the grade
of illitization and hence the amount of excess argon incorporation on the scale of individual grains (De Jong
etal.,2001). This process resulted in highly varying
40Ar/
39Ar ages in samples of the Carboniferous orthogneiss,
somewhat similar to sample 6 in our study selected from the same rocks (Sabinas unit, Figures2 and 7). In
this sense, significant age variations in sample 6, and possibly the significantly older, outlier grains in the
other samples could also be explained by excess argon incorporation, not only by partial resetting (inherited
argon). However, sample 1 shows a consistent, homogenous distribution of older ages between 27 and 38Ma,
which do not exhibit the typical variance caused by excess argon. In addition, the effect of excess argon (if pres-
ent) is expected to be more profound in the coarser grained rocks such as the micaschists of the Ophiolite unit
(samples 2, 3, 4, 5, yielding 12–22Ma grains) compared to the fine grained schists of the Calar-Alto unit (sample
1, yielding 27–38Ma grains), therefore predicting that sample 1 should be less affected by this process (De Jong
etal.,2001). Consequently, the above described correlation of the observed structural criteria (preservation of an
older foliation far from the shear zones) with the older age clusters demonstrated in Augier, Agard, etal.(2005)
and in this study is a more logical explanation for the variety of age data than assuming an unclear presence of
excess argon in those samples. All these arguments considered, we interpret our white mica
40Ar/
39Ar ages as a
record of two different fabric forming periods, that is, (re)crystallization of white micas as a result of deforma-
tion events: (a) a Paleogene fabric evolution recorded by Sample 1 in the Calar-Alto unit (38–27Ma), and (b)
an early-middle Miocene fabric evolution recorded by the samples within or in the proximity of the main shear
zones (22–12Ma). Outlier grains (four grains in all samples) with significantly older age results (60–70Ma) and
the significant age variation (heterogenous distribution) in sample 6 are possibly the result of inherited or excess
argon in the samples.
Accounting for the full range of existing geochronological ages (Figure9) shows that our Paleogene white
mica
40Ar/
39Ar ages and those of Augier, Agard, etal.(2005) are in agreement with amphibole
40Ar/
39Ar and
monazite U-Th-Pb data (Li & Massonne,2018; Monié et al., 1991). Miocene U-Pb zircon, Lu-Hf garnet,
and
87Rb/
86Sr multimineral ages that are supplemented by thermodynamic data constraining the P-T conditions
of the dated minerals outline a second episode of prograde, HP-LT metamorphism (Kirchner etal.,2016; Platt
etal.,2006; Sánchez-Vizcaíno etal.,2001). Accepting both sets of ages and their interpretations as burial-related
metamorphism, one could argue that the NFC might have experienced a long-lasting, multi-episode (at least two)
burial process, starting in the Paleogene and finishing during the early Miocene (see Section6.4 for discussion
on the possible geodynamic scenarios).
6.2. Superposition of Miocene Thrusting and Extension: The Mechanics of Exhumation and the
Structural Definition of the NFC and the AC
Our kinematic data suggest that the main nappe contacts of the eastern Sierra de Los Filábres formed during
moderate-temperature shearing after peak metamorphic conditions, with a kinematics of top to NW-NNW in
present-day orientation. Furthermore, these shear zones display significant evidences of activity during the retro-
gression (marked by oriented chlorite growth) and exhumation that took place with a general top to WNW-WSW
kinematics (in present-day coordinates). The transition from higher to lower metamorphic temperatures during
the activity of these shear zones appears to be accompanied also by a change in the pressure-dependent Si content
of the phengites of the shear zone foliations, ranging from 3.2 to 3.4 c.p.f.u (Samples 3, 4, 5, 6 on Figure7c).
The kinematic observations imply that the shear zones progressively rotated from top to NW-NNW shearing
possibly during burial and early exhumation to top to WNW-WSW shearing during the subsequent exhumation

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to mid-crustal levels. This significant change in the sense of shear during
exhumation implies a different strain and stress pattern for mid-crustal levels
compared to the subduction zone, most likely related to the localization of
the main, top-WNW-WSW extensional detachment system (BMZ) at the
brittle-ductile transition zone (BDTZ, Martínez-Martínez etal.,2002). Our
results hence indicate that the exhuming NFC nappes were captured by the
BMZ when reaching the BDTZ, which facilitated their further (final) exhu-
mation and resulted in oblique shearing with respect to the nappe stacking
direction (Figure10). This interpretation is consistent with existing extrusion
models of the NFC, where the formation of the BMZ at the brittle-ductile
transition zone took place coevally with the contractional exhumation (i.e.,
extrusion) of the NFC (Behr & Platt,2012; Booth-Rea etal., 2005, 2015).
These interpretations are further supported by the observation of E-W trend-
ing upright folding, related to N-S contraction, interpreted to be coeval to
and also younger than the formation of the BMZ (Augier, Agard, etal.,2005;
Martínez-Martínez etal.,2002). Therefore, in agreement with these studies,
we interpret the exhumation of the NFC to be the result of an interplay between
the extrusion of subducted crust in the deeper segments of the subduction
zone, and coeval upper crustal extension in the orogen (Figure10).
Established models of extrusion have shown that nappe formation and exhu-
mation of deeply subducted continental material creates uplift and associ-
ated extension in the orogenic upper crust (Chemenda etal., 1996,1997;
Hacker et al., 2000; Herwegh et al.,2017; Platt, 1986). Applying these
models to the Betics implies that the initial buoyancy-driven extrusion of
Figure 9. Summary of published geochronological results constraining the timing of tectono-metamorphic events in
the NFC.
Figure 10. Schematic 3D sketch of the early exhumation (extrusion) stage
of the NFC in the early-middle Miocene showing the superposition of nappe
stacking and extensional detachment (BMZ) formation. Note that the original,
∼top-NW nappe contacts were obliquely cut by the ∼top-W BMZ that
localized at the brittle-ductile transition zone. Therefore, it is possible that
low-grade and high-grade metamorphic equivalents of the same formations
from the same nappe units are juxtaposed by the BMZ.

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the subducted NFC may have triggered uplift and hence the localization of the BMZ, which in turn accom-
modated the extrusion of the subducted crust. The extrusion of the NFC and the localization of the BMZ was
facilitated by the westward migration of the slab, that created space for rock exhumation as well as for astheno-
spheric upwelling, which resulted in the observed high-T overprint (reheating) of the NFC during exhumation
(Booth-Rea etal.,2005,2015; Santamaría-López etal.,2019). The highly oblique (almost orthogonal) kinemat-
ics of the top-W or SW BMZ compared to the original, top-NW nappe stacking direction observed in Sierra de
Los Filábres (Figure10, Behr & Platt,2012; Martínez-Martínez etal.,2002; Platt, Allerton, etal.,2003) is more
compatible with an orogen-parallel (or trench parallel) extension mode that creates elongation domes associated
with orogen perpendicular shortening during orogenic building (e.g., Brun,1983; Rey etal.,2011), as previously
suggested for the NFC (e.g., Behr & Platt,2012; Jolivet etal.,2021; Martínez-Martínez etal.,2002), or observed
elsewhere, such as in case of the Tauern or Danubian windows of the Eastern Alps and South Carpathians (e.g.,
Matenco & Schmid,1999; Scharf etal.,2013).
The NFC and AC are not only defined as tectonic units, but also as paleogeographic units juxtaposed by thrusting
in a subduction zone. Their use in paleogeographic interpretations must be based on the modern nappe structure
(i.e., different units bounded by thrusts). However, the current definition of the AC and the NFC within our study
area is also based on the contrast in metamorphic grade created by the tectonic omission along the BMZ, instead
of the original thrusts (Martínez-Martínez etal.,2002). Using a detachment between the AC and NFC as a nappe
definition would be valid if the extensional detachment reactivated exactly the original nappe contacts, with an
opposite sense of shear. This way of reactivating nappe contacts has been interpreted elsewhere in the Mediterra-
nean to be an important mechanism of exhumation of metamorphic rocks (e.g., Jolivet etal.,2010). However, in
case of the Betics, the highly oblique kinematics of the superposed nappe stacking and extension implies that the
BMZ obliquely cut through the nappe-stack (Figure10), creating a tectonic omission between the NFC and the
AC, and possibly reactivating pre-existing shear zones during the late-stage formation of the extensional dome
(see also Martínez-Martínez etal.,2002).
Due to the oblique displacement along the BMZ with respect to the original nappe stacking shear zones, low-grade
metamorphic formations that originally belong to the NFC are also expected to be observed in the hangingwall
of the BMZ, overlying their high-grade stratigraphic equivalents in the footwall (Figure10). Examples for such
formations could be the low-grade or non-metamorphosed basalts, dolerites, and gabbros (Figure6), which are
the protoliths of the high-grade amphibolites or eclogites in the Ophiolite Unit across the BMZ. Similarly, the
low-grade lithologies commonly assigned to the AC along the northern flank of the eastern Sierra de Los Filá-
bres such as Paleozoic phyllites and Triassic dolomites have high-grade equivalents below the BMZ (schists and
dolomite marbles). Therefore, we suggest that due to the oblique superposition of nappe stacking and extension,
the low-grade formations in our study area are not necessarily part of the AC, but may also be part of the NFC in
terms of paleogeographic provenance, while also justifying the existence of high-grade AC metamorphic rocks
outside our study area. Ultimately, we conclude that a separation between the BMZ creating the observed tectonic
omission and the nappe stacking reflecting the paleogeographic provenance of continental units should be further
pursued in the Sierra de Los Filábres and elsewhere in the internal Betics.
6.3. Origin and Emplacement of the (Ultra)mafic Unit
The mafic-ultramafic rock succession of the NFC is most commonly interpreted in the context of a magma-poor,
hyper-extended (Iberian-derived) continental margin, which contains intruded mafic dykes as well as areas
with mantle rocks exhumed to the seafloor (e.g., Booth-Rea etal., 2015; Gomez-Pugnaire & Munoz, 1991;
Laborda-López etal.,2020). The same rock succession is also interpreted as a separate tectonic unit (Ophiolite
unit, Puga etal.,2002,2011), representing the metamorphosed remnants of an independent ophiolite sheet. Our
observations are consistent with large parts of both interpretations, which in our view are not exclusive.
A key observation is the presence of a mixture of sedimentary and (ultra)mafic rocks in an original protolith,
interpreted as an ophiolitic mélange. The observation of the S2-3 foliations in serpentinites (serpentinite schists)
that is coherent with those of the neighboring metasedimentary rocks indicates that the mélange formation
predates the D2 tectono-metamorphic event. The combination of an olistostrome and a shear zone is typical for an
ophiolitic melange, which forms by gravity emplaced (ultra)mafic blocks and rocks scraped from the underlying
continental formations. The different blocks are buried in a sedimentary matrix derived partly from alteration
reactions of (ultra)mafic minerals, and are thrusted by ophiolite sheets during their obduction over continental

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margins (Figure11c). Such ophiolitic mélanges have been described from ophiolite complexes worldwide, such
as the ones observed in the Dinarides or Hellenides (Andersen et al., 1990; Schmid et al., 2008; Yilmaz &
Maxwell,1984). The mélange of the (ultra)mafic unit cannot simply represent a brittle shear zone where the
different lithologies are mixed in the damage zone, because the sedimentary olistostrome fabric of the original
protolith is still preserved in some outcrops (e.g., Figures5d and5e), although significantly affected by the subse-
quent shearing during burial and furthermore crosscut by late extensional shear zones and normal faults during
exhumation.
The interpretation of an ophiolitic mélange associated with the obduction of ophiolites does not exclude the pres-
ence of a hyper-extended continental margin, whose continental mantle could have been connected to—a prob-
ably narrow or embryonic—oceanic lithosphere (Figure10b). Therefore, there is no conflict between ophiolite
and hyper-extended continental margin interpretations, as both can be part of the same paleogeographic domain.
6.4. Possible Geodynamic Scenarios
The geodynamic evolutionary model of the NFC critically depends on the interpretation of geochronological
data. The current scientific understanding of isotopic closure during a multiphase tectono-metamorphic evolution
is incomplete; hence, geochronological data do not offer a direct single solution yet. As a consequence of this
uncertainty, arguments have been put forward for the invalidation of either Paleogene or Miocene HP-LT fabric
age data. For example, Behr and Platt(2012) or Kirchner etal. (2016) argues that Paleogene Ar/Ar ages are
unreliable due to excess argon in white micas, while Bessière etal.(2022) argues that successful dating of burial
requires exceptionally preserved HP-LT fabrics based on samples from the AC. In our view, arguments for and
against the validity of the two age groups are not conclusive yet, which warrants the consideration of different
timing scenarios for the subduction of the NFC, depending on the interpretation of the age data. A further variable
in the geodynamic solutions is the origin of the Ophiolite unit. Our observations of the ophiolitic mélange suggest
that subduction of the NFC below the AC and MC may have been preceded by the obduction of the Ophiolite unit.
In this case, the burial evolution of the continental NFC must contain two main phases. Three possible geody-
namic scenarios in this overall context are given below.
6.4.1. Eocene Subduction-Slow Exhumation
If Paleogene geochronological ages (Figure9) are accounted for to reflect tectonic burial and metamorphism,
there are two possible solutions. The first solution considers Paleogene burial followed by slow exhumation during
the Miocene, hence relating Miocene ages to exhumation (Augier, Agard, etal.,2005; Bessière,2019; Bessière
etal.,2022; Li & Massonne,2018; Monié etal.,1991). This interpretation correlates the NFC with the same pale-
ogeographic continental unit as the AC and MC or the Upper Sebtides in the Moroccan Rif belt (AlKaPeCa unit,
Bouillin etal., 1986), which record similar Paleogene ages of metamorphism (Bessière etal.,2022; Marrone,
Monié, Rossetti, Aldega etal., 2021; Marrone, Monié, Rossetti, Lucci etal.,2021; Monié et al.,1994; Platt
etal.,2005). This solution is consistent with currently available kinematic and P-T data; however, it is diffi-
cult to reconcile with the very limited (<100km) convergence in the Eocene, considering that all three of the
AC, Ophiolite unit, and continental NFC must be stacked and subducted below the MC. This solution also
requires the assumption that Miocene ages of HP-LT fabrics in the NFC (Kirchner etal.,2016; Platt etal.,2006;
Sánchez-Vizcaíno etal.,2001) belong to insufficiently preserved HP-LT mineral assemblages.
6.4.2. Gradual Eocene-Early Miocene Burial and Fast Exhumation
The second solution is a long-lasting, gradual burial of the NFC, starting in the Paleogene and terminating
during the early Miocene, followed by fast exhumation. This solution can account for the full range of published
geochronological data of HP-LT fabrics (not discarding any published data as unreliable), which points toward
both Paleogene (Augier, Agard, etal.,2005; Li & Massonne,2018; Monié etal.,1991) and Miocene (Kirchner
etal.,2016; Platt etal.,2006; Sánchez-Vizcaíno etal.,2001) episodes of HP-LT metamorphism; an idea that has
already been proposed by Li and Massonne(2018) based on monazite dating. Multiple HP-LT episodes during
a gradual burial history is also consistent with our mica chemistry data, which shows that both the S0-1 (Pale-
ogene) and S2+ (Miocene) foliations contain high-Si phengites, generally characteristic for HP metamorphism
(Figure7c). In this scenario, the Paleogene burial can be related to the obduction of an oceanic lithosphere
connected with a hyper-extended continental margin over the continental NFC. The obduction of oceanic lith-
osphere or arc-continent collisions are commonly associated with regional burial metamorphism of continental

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Figure 11. Tectonic reconstruction of the Western Mediterranean region highlighting the relevant tectonic units of the Betics, modified after van Hinsbergen
etal.(2020) in order to visualize the hypothesis of long-lasting, gradual burial of the NFC. (a) Snapshot at 45Ma showing the Paleogene subduction of the NFC and the
AC below dominantly oceanic (Ophiolite unit) and continental (MC) upper plate segments, respectively. (b) Schematic cross-section through the 45Ma reconstruction
snapshot. (c) Schematic sketch of the proposed obduction of the hyper-extended margin connected to a piece of oceanic lithosphere, magnified from the cross-section
of (b), highlighting the possible positions of different lithologies (mafic and ultramafic rocks capped by syn-post-rift sediments, ophiolitic mélange, and the downgoing
continental formations), observed in the continental NFC and Ophiolite unit. (d) Snapshot at 20Ma showing the initial early Miocene subduction of the NFC below the
Alboran domain containing the MC and the AC. (e) Schematic cross-section through the 20Ma reconstruction snapshot.

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margins (e.g., Agard & Vitale-Brovarone,2013; Goffé etal.,1988; Porkoláb etal.,2021; Pourteau etal.,2013;
van Hinsbergen etal.,2016). In this scenario, the Paleogene white mica
40Ar/
39Ar ages of the relic S0-1 foliation
in the continental Calar-Alto unit can be explained by burial metamorphism during the obduction of the Ophiolite
unit (Figure11c), whose record is preserved only in the deeper sections of the Calar-Alto unit, at far distances
from the Miocene shear zones (Figures7a and8g). A detailed explanation and related paleogeographic recon-
struction for this scenario is presented in Section6.5. We emphasize that more petrological work is needed for
validating or invalidating this model (similarly to the other scenarios); in particular, by clearly demonstrating
long-lasting or multi-episode HP-LT fabric growth. We also note that mica chemistry (Figure7c) alone is not
sufficient to prove the existence of multiple HP-LT fabrics, as the Si-content—while being pressure-sensitive—
also depends on the chemical composition and the temperature (Massonne & Schreyer,1987).
6.4.3. Early Miocene Subduction and Fast Exhumation
In the third scenario, where Paleogene geochronological ages are not interpreted as burial metamorphism, the
Miocene subduction of the entire (continental and oceanic) NFC remains the simplest solution (Booth-Rea
etal.,2015; Kirchner etal.,2016; Platt etal.,2006; Sánchez-Vizcaíno etal.,2001). Given the currently incom-
plete understanding of isotopic closure and the related overall debate on the reliability of different geochronolog-
ical data, we do not exclude this interpretation. However, the presence of a metamorphosed ophiolitic mélange
stacked in the NFC units and the structural dependence of Ar/Ar ages are not straightforward to reconcile with
this interpretation.
6.5. Tectonic Reconstruction for the Gradual Paleogene—Early Miocene Burial Hypothesis
Reconstructions of the Betics orogen have placed the continental NFC either in the AlKaPeCa terrane (i.e.,
a pre-subduction microcontinent or extensional allochthon), to explain Paleogene burial ages (solution of
Section6.4.1, e.g., Bessière etal.,2021; Vissers etal.,1995), or on the SE-Iberian margin, to explain Miocene
burial ages (solution of Section6.4.3, e.g., Moragues etal.,2021; Platt etal.,2006). Based on the combination
of our results and literature data (e.g., Figure9), we suggest that these interpretations are not mutually exclusive,
and we envisage a model of long-lasting burial for the NFC that explains both Paleogene and Miocene ages of
prograde metamorphism.
The Paleogene burial ages of the continental NFC place these units within the AlKaPeCa terrane, which differs
from recent paleogeographic reconstructions (e.g., van Hinsbergen etal., 2020; van Hinsbergen etal.,2014).
The burial of the NFC and AC are simultaneous, and given the very low convergence rates of only mm/year
in the Eocene, this requires that the NFC and AC were lateral paleogeographic units that were both part of the
AlKaPeCa terrane (part of the same continental block or laterally separated blocks, Figure11a). The AC was
buried below the MC that was presumably either the most distal forearc of Iberia, or a continental nappe system
that was also derived from AlKaPeCa. There is no structurally higher unit above the MC to give a conclusive
answer. The NFC was buried below oceanic crust (the Ophiolite unit), and we therefore envisage that the subduc-
tion zone transitioned from along the Iberian continental margin into the oceanic crust of the Alpine Tethys,
such that a narrow strip of oceanic crust was located in the upper plate, below which the NFC subducted in
Eocene times (Figures11a–11c).
The Miocene burial of the NFC plus the overlying ophiolites below the AC requires that the pivot point around
which the trench rotated was located between the two units (Figure11d). As a result, the NFC and the overlying
Ophiolite unit were part of the Iberian margin (lower plate), whilst the AC was part of the upper plate, that is, the
Alboran domain (Figures11d and11e). The Alboran domain started to thrust over the composite NFC–Ophiolite
unit in the Early Miocene, with an almost opposite vergence compared to Paleogene thrusting (Figures 11a
and11d). Peak metamorphic conditions in the (ultra)mafic rocks were achieved during this Miocene stage. This
Miocene history is almost the same as in van Hinsbergen etal.(2020). The difference is that instead of assuming
an undeformed Iberian margin, our reconstruction implies that the Iberian margin already hosted an Eocene
fold-thrust belt related to ophiolite obduction, prior to the Early Miocene subduction below the Alboran domain.
This model accounts for all previously interpreted Paleogene and Miocene episodes of prograde metamorphism
(subduction) in the continental formations of the NFC, corresponding to Paleogene and Miocene geochronolog-
ical data, and only one, Paleogene phase of subduction in the AC, while keeping the consistency with all other
geological and geophysical data described in van Hinsbergen etal. (2020). It also explains the presence of an

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ophiolitic mélange associated with the (meta-)mafics and ultramafics of the Ophiolite Unit by incorporating a
Paleogene obduction (Figure11c). The Paleogene obduction might also play an important role in the rheological
weakening of the Iberian margin, which might be important for facilitating the westward rollback of the slab
along the margin (Chertova etal., 2014). In our reconstruction, the NFC undergoes ∼25° clockwise rotation
following the early Miocene subduction. As the initial Miocene burial direction was top-W (Figure 11d), the
presently observed NW-SE orientation of the moderate-temperature stretching lineations are explained by the
clockwise rotation (the originally E-W trending lineations are rotated to NW-SE). Furthermore, this rotation
might also explain the observed top-SSW vergence in and above the ophiolitic mélange formation (Figures5a
and5c): the Paleogene vergence of obduction was top-SE (Figure11a), which rotates to a more southerly (∼top
S) direction after the early Miocene.
7. Conclusions
We presented the results of a structural analysis and single grain fusion
40Ar/
39Ar dating of white micas to address
the tectonic and paleogeographic evolution of the Betics mountain range in SE Spain. We refined the tectonic
subdivision of the eastern Sierra de Los Filábres to document strain localization during the burial and exhumation
of the NFC. Our results show that the main nappe contacts in the NFC were active during the Miocene continen-
tal subduction-exhumation cycle both at moderate-temperature metamorphic conditions following burial and at
retrograde conditions during the late exhumation stage of the NFC. The kinematics of shearing along the nappe
contacts gradually changed from top-NNW to top-W as the rocks were being exhumed and captured by the top-W-
WSW extensional detachment (BMZ) localized at the brittle-ductile transition zone. The observed change in the
sense of shear during exhumation suggest that the BMZ obliquely cut the inherited nappe contacts, hence the
low-grade formations exposed along the northern flank of the Sierra de Los Filábres do not necessarily belong to
the AC, but may also belong to the NFC in terms of paleogeographic provenance. We documented the existence
of an ophiolitic mélange formation, which suggests that the (ultra)mafic succession of the NFC (Ophiolite unit) is
a dismembered ophiolite sheet originally emplaced by an obduction process. White mica
40Ar/
39Ar ages correlate
with the preservation or obliteration of an early, relic, metamorphic fabric in the rocks of the NFC. Our white
mica
40Ar/
39Ar ages hence largely record Ar-loss due to deformation/fluid-induced recrystallization during two
fabric-forming periods at 38–27Ma and 22–12Ma. The combination of our results with previously published
geochronological and petrological data in the NFC allows for three different geodynamic solutions due to the
currently limited understanding of isotopic closure during a multiphase tectono-metamorphic evolution. These
are (a) Eocene burial and slow exhumation; (b) our newly proposed hypothesis, Eocene-Early Miocene gradual
burial and fast exhumation; and (c) Early Miocene burial and fast exhumation. The newly proposed hypothesis of
Eocene-Early Miocene gradual burial (subduction) accounts for Paleogene obduction of the Ophiolite unit over
the continental formations of the NFC, which explains Paleogene geochronological data in the continental NFC
as well as the presence of an obduction mélange in the Ophiolite unit.
Data Availability Statement
Structural data for this research (data of stretching lineations with associated shear sense directions and fold axes)
are available in TableS1 in Supporting InformationS1. The description and coordinates of the dated samples
are available from TableS2 in Supporting InformationS1. The microprobe data of dated white mica popula-
tions are available from TableS3 in Supporting InformationS1. The supporting tables are available from the
Zenodo data repository platform (https://doi.org/10.5281/zenodo.5707184).
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Acknowledgments
The research presented in this paper
was funded by the European Union's
MSCA-ITN-ETN Project SUBITOP
674899. Hans de Bresser and Frits Hilgen
are gratefully acknowledged for their field
introduction to the geological world of
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