# Interactions of plutons and detachments, comparison of Aegean and Tyrrhenian granitoids

Laurent Jolivet, Laurent Arbaret, Laetitia Le Pourhiet, Florent

Cheval-Garabedian, Vincent Roche, Aurélien Rabillard, Loïc Labrousse

► To cite this version:

Laurent Jolivet, Laurent Arbaret, Laetitia Le Pourhiet, Florent Cheval-Garabedian, Vincent Roche, et al.. *Interactions of plutons and detachments, comparison of Aegean and Tyrrhenian granitoids*. 2021. insu-03240172

HAL Id: insu-03240172

https://insu.hal.science/insu-03240172

Preprint submitted on 28 May 2021

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1 Interactions of plutons and detachments,

2 comparison of Aegean and Tyrrhenian granitoids

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4

5 Laurent Jolivet¹, Laurent Arbaret²,³,⁴, Laetitia Le Pourhiet¹, Florent Cheval-

6 Garabedian²,³,⁴, Vincent Roche¹, Aurélien Rabillard²,³,⁴, Loïc Labrousse¹

7

8 ¹Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre Paris, ISTeP UMR 7193, F-75005

9 Paris, France

10 ²Université d'Orléans, ISTO, UMR 7327, 45071, Orléans, France

11 ³CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France

12 ⁴BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France

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14 **Abstract:** Back-arc extension superimposed on mountain belts leads to distributed normal 15 faults and shear zones, interacting with magma emplacement in the crust. The composition of 16 granitic magmas emplaced at this stage often involves a component of crustal melting. The 17 Miocene Aegean granitoids were emplaced in metamorphic core complexes (MCC) below 18 crustal-scale low-angle extensional shear zones and normal faults. Intrusion in such contexts 19 interacts with extension and shear along detachments, from the hot magmatic flow within the 20 pluton root zone to the colder ductile and brittle deformation along the detachment. A 21 comparison of the Aegean plutons with the Elba Island MCC in the back-arc region of the 22 Apennines subduction shows that these processes are characteristic of pluton-detachment 23 interactions in general and we discuss a conceptual emplacement scenario, tested by numerical 24 models. Mafic injections within the partially molten lower crust above the hot asthenosphere 25 trigger the ascent within the core of the MCC of felsic magmas, controlled by the strain 26 localization on persistent crustal scale shear zones at the top that guide the ascent until the brittle 27 ductile transition is reached during exhumation. Once the system definitely enters the brittle 28 regime, the detachment and the upper crust are intruded while new detachments migrate upward 29 and in the direction of shearing. Numerical models reproduce the geometry and the kinematic 30 evolution deduced from field observations.

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# 1. Introduction

In the deep parts of orogens, the flow of melts is coupled with ductile deformation and controlled by buoyancy and tectonic forces (Brown, 1994; Brown and Solar, 1998; Brown, 2007). Migmatites, which are weak crustal material as long as they are kept at high temperature, are the source of magmas that concentrate within plutons of various sizes. On the other hand, interactions between magmatism and lithospheric deformation, and more specifically interactions of plutons with crustal-scale tectonics, depend first of all upon the rate of magma production and, to a second order, strain rates. The rate of magma transfer to the crust is indeed so large compared to tectonic strain rates that the construction of plutons is thought in a first approach to be little influenced by the tectonic setting, especially when small plutons are concerned (de Saint Blanquat et al., 2011).

<figure><img src="image_4.png" /><figcaption>Figure 1: Tectonic map of the Aegean region showing the successive generations of granitoids since the late Cretaceous. Modified from Jolivet et al. (2015).</figcaption></figure>

The Miocene Aegean plutons (figure 1), emplaced in an extensional context within metamorphic core complexes (MCCs), may however depart from this behaviour. Despite a moderate volume, they have indeed recorded the complete evolution from syn-tectonic magmatic flow to localized mylonitic deformation along the main detachment (Faure and Bonneau, 1988; Urai et al., 1990; Faure et al., 1991; Lee and Lister, 1992; Gautier et al., 1993;
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Laurent et al., 2015; Rabillard et al., 2015; Bessière et al., 2017; Rabillard et al., 2018). All of them moreover show a systematic magmatic and tectonic evolution of the host MCCs with several magmatic pulses and a series of detachments forming sequentially during exhumation (Rabillard et al., 2018). Several of them also show an association of mixed or mingled felsic and mafic magmas, with an evolution from a significant component of crustal melting toward more mafic composition, a trend that is common in post-orogenic magmas (Bonin, 2004).

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Figure 2: Tectonic map of the Central and Western Mediterranean with the distribution of recent magmatism (Savelli, 1988; Serri et al., 1993; Savelli, 2002b, a; Duggen et al., 2005; Avanzinelli et al., 2009; Savelli, 2015). Tectonic map (B) of the northern Tyrrhenian region and Northern Apennines and (C) a diagram showing the evolution of the ages of syn-rift basins, metamorphic events and magmatism along a cross-section from Corsica to the Apennines, modified from Jolivet et al. (1998).
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Whether these features are characteristic of syn-extension plutons in post-orogenic back-arc environments is the question we address in this paper, through a comparison of the Aegean plutons with those of the northern Tyrrhenian Sea and Tuscany, with a focus on Elba Island in the Tuscan archipelago (figure 2). Striking similarities can indeed be observed between the two contexts in terms of tectonic and magmatic evolution. We propose and test a scenario of formation and emplacement of plutons in a back-arc post-orogenic context below crustal-scale detachments.

## 2. Geodynamic context

The Aegean and North Tyrrhenian granitoids were emplaced during the Miocene and part of the early Pliocene in the back-arc regions of the Hellenic and Apennines subduction, respectively (Serri et al., 1993; Jolivet et al., 1998; Pe-Piper and Piper, 2002, 2007; Avanzinelli et al., 2009; Jolivet et al., 2015; Rabillard et al., 2018) (figure 3).

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<caption>Figure 3: Two lithospheric-scale cross-sections of the Aegean domain (Jolivet and Brun, 2010) and the Northern Tyrrhenian Sea and the Apennines (Jolivet et al., 1998).</caption>

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These two subduction zones started to retreat approximately at the same time, 30-35 Ma ago (Jolivet and Faccenna, 2000). A first-order change in the geodynamics of this region indeed occurred at this period, also coeval with the hard collision between Africa and Eurasia in the eastern and westernmost Mediterranean. The subducting African lithosphere, locked between two collision zones, continued to subduct northward but with a significant component of retreat.
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Since that time subduction has been continuous, with however several episodes of slab
detachment and tearing (Wortel and Spakman, 2000; Spakman and Wortel, 2004; Faccenna and
Becker, 2010; Faccenna et al., 2014). The Aegean plutons studied in this paper were emplaced
during the formation of a large tear in the subducting lithosphere between 16 and 8 Ma (Jolivet
et al., 2015). The oldest North Tyrrhenian pluton is dated around 7 Ma in Elba (Westerman et
al., 2004) and the youngest ones, Pliocene in age (Serri et al., 1993), are currently exploited for
geothermal energy in Tuscany (Rossetti et al., 2008; Rochira et al., 2018). All these plutons
contain a significant component of crustal melts and some of them are linked with migmatite
domes such as on Naxos, Mykonos and Ikaria (Jansen, 1977; Urai et al., 1990; Denèle et al.,
2011; Beaudoin et al., 2015; Vanderhaeghe et al., 2018). Mixing and mingling with mafic
magmas are also observed in some of these plutons and the general evolution shows an increase
of the mantle component with time.

Most of these plutons are associated with low-angle normal faults (LANF) and shear
zones and they were emplaced in the core of MCCs (Faure et al., 1991; Lee and Lister, 1992;
Lister and Baldwin, 1993; Daniel and Jolivet, 1995; Jolivet et al., 1998; Berger et al., 2013;
Bessière et al., 2017; Rabillard et al., 2018). These LANF and associated ductile shear zones
(we use the term "detachment" for the whole structure, brittle and ductile) started to form before
the emplacement of the plutons, in both regions. The main difference between the two regions
are the kinematics of these detachments (figure 3) (Jolivet et al., 2008) and the role of tectonic
inheritance. In the Aegean, most of the MCCs are capped by north-dipping detachments except
in the southwest where south-dipping detachments are observed. The north-dipping
detachments probably partly reactivate former thrusts related to the building of the Hellenides
orogenic wedge. In the Northern Tyrrhenian Sea and in Tuscany, all detachments dip eastward,
i.e. toward the subduction zone. In that case, the detachments cannot reactivate the former
thrusts of the internal Apennines that dip westward. Only in the case of the oldest detachments,
found in Alpine Corsica, can they correspond to reactivated thrusts. The case of Elba Island
shows the detachments cutting down-section eastward within the stack of former nappes (Keller
and Pialli, 1990; Collettini and Holdsworth, 2004). Whatever the nature (i.e. reactivated
structure or not) and the sense of shear of these detachments, the interaction with the plutons
follows a similar pattern that we recall below, first briefly for the Aegean region and then for
Elba Island (see Rabillard et al., 2018, for details on the Aegean).
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## 3. Aegean plutons

The Miocene Aegean and Menderes plutons were emplaced during a short time period between ~20 Ma and 8 Ma, the oldest cropping out in the Menderes massif and the youngest in the western part of the Aegean region (figures 1, 3) (Jolivet et al., 2015). Those occupying the Cycladic domain are all associated with detachments, either north or south-dipping (figure 4) (Grasemann and Petrakakis, 2007; Rabillard et al., 2018).

<figure><img src="image_11.png" /><figcaption>Figure 4: Five examples of the Aegean granitoids showing the interactions between deformation and intrusion, after Rabillard et al. (2018) and references therein. The left column show maps of the entire islands and the right column shows the internal fabrics of the plutons. These maps were obtained based on deformation grades observed in the field, a scale of grades was designed for each pluton to describe the gradients. Arrows show stretching lineations and sense of shear and black bars on the Tinos and Serifos plutons shows the direction of the magnetic lineation.</figcaption></figure>

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Except for Serifos and Lavrion plutons, associated with the West Cycladic Detachment System (WCDS) (Grasemann and Petrakakis, 2007; Berger et al., 2013; Scheffer et al., 2016), the plutons crop out in the core of MCCs exhumed by north-dipping detachments, such as the North Cycladic Detachment System (NCDS) (Gautier and Brun, 1994b, a; Jolivet et al., 2010) or the Naxos-Paros Fault System (NPFS) (Urai et al., 1990; Gautier et al., 1993; Vanderhaeghe, 2004; Bargnesi et al., 2013; Cao et al., 2017). The detachments upper plate is made of the Upper Cycladic Nappe, a remnant of the Pelagonian domain, made of greenschists-facies metabasites or serpentinite with, in a few cases, early to late Miocene sediments deposited during extension (Angelier et al., 1978; Sanchez-Gomez et al., 2002; Kuhlemann et al., 2004; Menant et al., 2013). The MCCs are made of various units of the Cycladic Blueschists, more or less retrograded in the greenschist-facies, or the Cycladic basement, showing HT-LP metamorphic facies and even anatectic conditions on several islands, such as Naxos, Paros, Mykonos or Ikaria (Buick and Holland, 1989; Urai et al., 1990; Buick, 1991; Keay et al., 2001; Duchêne et al., 2006; Seward et al., 2009; Kruckenberg et al., 2011; Beaudoin et al., 2015; Laurent et al., 2015; Rabillard et al., 2015; 2018). The plutons intruded these MCCs and were sheared at the top by the detachments during their emplacement (Rabillard et al., 2018).

The granitoids show a variety of facies and composition, but most of them have a crustal melting component and some are closely associated with migmatites, as on Ikaria or Mykonos (Vanderhaeghe, 2009; Kruckenberg et al., 2011; Denèle et al., 2011; Beaudoin et al., 2015; Jolivet et al., 2021). Compositions show a common trend for these plutons indicating that they crystallized primarily from I-type magmas with some contamination by the continental crust and little fractionation (figure A1, Appendix A). Field evidence show a close association of these I-type intrusions with two-micas granites (in Ikaria for instance), migmatites, or both (Ikaria, Naxos, Paros, Rheneia-Delos) (Pe-Piper et al., 1997; Pe-Piper, 2000; Pe-Piper et al., 2002; Vanderhaeghe, 2004; Bolhar et al., 2010; Bolhar et al., 2012; Bargnesi et al., 2013; Beaudoin et al., 2015; Laurent et al., 2015; Jolivet et al., 2021). Tinos, Ikaria and Serifos granitoids were emplaced in several magma batches with an evolution through time, characterized by more and more mafic compositions and a decrease of the grain size (Grasemann and Petrakakis, 2007; Ring, 2007; Bolhar et al., 2010; Petrakakis et al., 2010; de Saint Blanquat et al., 2011; Bolhar et al., 2012; Beaudoin et al., 2015; Laurent et al., 2015; Rabillard et al., 2015; Ducoux et al., 2016). On Serifos and Naxos, the farthest parts of the pluton from the detachment show an enrichment in mafic enclaves and evidence for magma
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169 mixing and mingling in the roots of the rising plutons (Rabillard et al., 2015; Bessière et al., 2017; Rabillard et al., 2018).

171 A common evolution is observed in several of these plutons during their interaction with the system of detachments exhuming their host MCC (Rabillard et al., 2018). A series of two or three detachments is observed (figure 4, figure 5).

<figure><img src="image_14.png" /><figcaption>Figure 5: Details of the two detachments on Mykonos and Serifos. A: northeastern Mykonos (see location on figure 5), B: Southwest Serifos. The Mykonos (MD) and Livada (LD) detachments on Mykonos and the mineralized veins and normal faults (baryte and iron-hydroxides) - grey- are after Menant et al. (2013). The Kàvos Kiklopas (KKD) and Meghàlo Livadi (MLD) detachments on Serifos are after Grasemann and Petrakakis (2007) and Ducoux et al. (2016).</figcaption></figure>

182 The deepest one is mostly ductile and has started to act long before the granitic intrusion that ultimately intrudes it. The upper detachments are mostly brittle and are locally intruded by dykes and sills emanating from the main pluton. When a sedimentary basin is present, it is deposited on top of the uppermost detachment during extension and can be partly affected by mineralized veins (Menant et al., 2013). All plutons show a gradient of shearing deformation toward the detachment with an evolution from ductile to brittle (Figure 4). The maps shown in figure 4 were drawn after detailed field observations and the construction of a scale of up to 7 grades of progressive deformation, from non-deformed granitic texture to ultra-mylonites, with the progressive appearance of foliation, stretching lineation, localization of C and C' shear
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bands (Berthé et al., 1979; Lister and Snoke, 1984), for details see Rabillard et al. (2018). The inner parts of the plutons show mixing of acidic and mafic magmas and a co-magmatic deformation, co-axial with the post-solidus deformation along the detachment (Rabillard et al., 2015; 2018). The flow of magma is thus oriented by the regional strain field. Serifos shows (i) a decrease of grain size through time with an inner facies with smaller grain size and finally fine-grained dykes and (ii) evidence for hydrothermalism in the root zone of the pluton, suggesting that the magmatic system was open upward with a possible volcano-plutonic system (Rabillard et al., 2015; 2018).

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## 4. North Tyrrhenian plutons

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The Monte Capanne pluton on Elba island (figure 6) is the oldest of a series of plutons cropping out in the Tuscan archipelago and onshore Tuscany (Serri et al., 1993; Westerman et al., 2004; Avanzinelli et al., 2009).

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<caption>Figure 6: Tectonic map and cross-section of Elba Island showing the main tectonic units and the main extensional shear zones and detachments, modified after Bianco et al. (2015).</caption>

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Details of the internal structure of the Monte Capanne intrusions are reported based on Farina et al. (2010).

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These plutons belong to magmas migrating from west to east between the end of the Oligocene and the Quaternary, mimicking the migration of the Apennines thrust system and the HP-LT metamorphism of the internal Apennines and Tuscan Archipelago (Serri et al., 1993; Jolivet et al., 1998) (figure 2). This situation is thus very similar to the Aegean Sea. The decrease of the time lag between the recording of HP-LT metamorphism or the activation age of the thrust front and the magmatism has been interpreted as a consequence of slab steepening during retreat (Jolivet et al., 1998; Brunet et al., 2000). Pluton ages decrease eastward from ~8 Ma to 2-3 Ma (figure 2). Among the youngest plutons are those powering the active geothermal fields of Larderello and Monte Amiata (Camelli et al., 1993; Brogi et al., 2003; Rossetti et al., 2008). The oldest plutons are observed offshore on Elba (Monte Capanne and Porto Azzuro plutons), Monte Cristo and Giglio islands (Westerman et al., 1993) (figure 2). These four plutons are granodiorites/monzogranites and they all display a contamination with crustal magmas with a main source thought to be lower crustal anatexis (Serri et al., 1993; Innocenti et al., 1997). They were emplaced within an overall extensional context during the rifting of the Northern Tyrrhenian Sea in the back-arc region of the Apennines (Jolivet et al., 1998). First evidenced in Alpine Corsica and on Elba island, a series of east-dipping low-angle detachments controlled the kinematics of extension along the Corsica-Apennines transect from the Oligocene onward (Jolivet et al., 1998). Extension is shown to migrate from west to east with time and it is active at present in the highest altitude regions of the Apennines just west of Corno Grande peak with however west-dipping normal faults (D'Agostino et al., 1998). The youngest east-dipping low-angle normal faults are seismically active in the Alto Tiberina region (Collettini and Barchi, 2002, 2004; Pauselli and Ranalli, 2017). Evidence for top-to-the east shearing deformation is found within the plutons of the Tuscan archipelago, but the detachments crop out nicely mostly on Elba island (Keller and Pialli, 1990; Daniel and Jolivet, 1995; Collettini and Holdsworth, 2004; Liotta et al., 2015).

## 4.1. Monte Capanne pluton, Elba Island

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Elba, the largest island of the Tuscan archipelago, shows the relations between peraluminous magmatic bodies and two east-dipping low-angle shear zones cutting down-section within the Tuscan nappe stack emplaced before extension started (figure 6) (Keller and Pialli, 1990; Bouillin et al., 1993; Pertusati et al., 1993; Daniel and Jolivet, 1995; Westerman et al., 2004; Bianco et al., 2015). Five thrust packages (complexes I to V) are separated by west-dipping low-angle reverse faults (Trevisan, 1950; Barberi et al., 1967; Perrin, 1975; Pertusati
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et al., 1993; Bianco et al., 2015; Bianco et al., 2019). Long thought free of any HP-LT imprint, at variance with the nearby Gorgona and Giglio islands where the presence of Fe-Mg-carpholite attest for blueschist-facies metamorphism (Rossetti et al., 1999a; 1999b), the nappe stack has only recently revealed HP-LT parageneses along the east coast of the island (Bianco et al., 2015). Through a correlation with the HP-LT units of Gorgona (Rossetti et al., 1999), where $^{40}$Ar/$^{39}$Ar dating on micas yielded ages around 25 Ma (Brunet et al., 2000), the Elba blueschists are attributed to the Oligocene-Early Miocene.

The Nappe stack is intruded by the shallow-level San Martino and Portoferraio porphyries coeval with the Monte Capanne pluton (figure 6), spanning a short period between 8 and 6.8 Ma, showing that the magma has intruded the detachment in a late stage (Saupé et al., 1982; Juteau et al., 1984; Ferrara and Tonarini, 1985; Bouillin et al., 1994; Westerman et al., 2004). The Monte Capanne intrusion makes the major part of the western half of the island and the highest peak. It is surrounded by a contact metamorphic aureole developed at the expense of the nappe stack (Duranti et al., 1992; Dini et al., 2002; Rossetti et al., 2007; Rossetti and Tecce, 2008). The metamorphic parageneses within the aureole suggest an emplacement at a depth of 4-5 km (Dini et al., 2002; Rocchi et al., 2002; Farina et al., 2010; Pandeli et al., 2018). The pluton shows an internal deformation with a gradient of shearing toward the east attested by the magnetic fabric, stretching lineation and sense of shear (Bouillin et al., 1993; Daniel and Jolivet, 1995). The pluton and the metamorphic aureole are separated from the nappe stack by an east-dipping low-angle shear zone (Capanne shear zone) evolving into a brittle east-dipping fault (eastern border fault) (Daniel and Jolivet, 1995). Syn-kinematic contact metamorphism minerals coeval with top-to-the east kinematic indicators attest for the syn-kinematic nature of the intrusion (Daniel and Jolivet, 1995; Pandeli et al., 2018).

The eastern part of the island shows granitic dykes emanating from the buried younger Porto Azzuro pluton intruding the Calamiti schists complex (Complex I, figure 6) (Daniel and Jolivet, 1995; Maineri et al., 2003; Musumeci and Vaselli, 2012). Here too, evidence for top-to-the east shearing at the time of intrusion have been described (Daniel and Jolivet, 1995) (figure 7). The pluton and the Calamiti schists are topped by the Zuccale low-angle normal fault that cuts down-section across the entire nappe stack with clear evidence of top-to-the east shearing (figure 8) (Keller and Pialli, 1990; Keller et al., 1994; Collettini and Holdsworth, 2004).

The main facies of the Monte Capanne pluton exhibits a constant, peraluminous, monzogranitic composition (Poli et al., 1989; Dini et al., 2002; Gagnevin et al., 2004) while the mafic microgranular enclaves (MME) varies from tonalitic-granodioritic to monzogranitic. The
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278 leucogranitic dykes are syenogranitic in composition (Gagnevin et al., 2004). Gagnevin et al. 279 (2004) proposed a multiphase magmatic emplacement from peraluminous magmas issued from 280 melting of a metasedimentary basement and hybridized with mantle-derived mafic magmas 281 whose heat supply possibly enhanced wall-rock assimilation. In addition, injection of mantle- 282 derived magma in the Sant' Andreas facies would have triggered extensive fractionation and 283 mixing of the basic magma with the resident monzogranitic mush (Poli and Tommasini, 1991).

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Figure 7: Photographs of the Sant' Andrea facies in the Monte Capanne pluton and of the
deformation along the eastern margin of the pluton, in the pluton itself and in the contact
metamorphic aureole. A: general view of the orientation of K-feldspar megacrysts and some
mafic enclaves. B: Detailed view of the oriented K-feldspar megacrysts (horizontal plane). C:
zoom on the orientation of K-feldspar megacrysts (vertical plane). D: Cotoncello dyke. E:
cluster of K-feldspar megacrysts in the vicinity of the Cotoncello dyke. F: Mylonitic foliation
within the Monte Capanne shear zone. G: Sigmoidal foliation and top-to-the-east sense of
shear within the metamorphic aureole of the Monte Capanne pluton. H: detailed view of syn-
kinematic contact metamorphism garnets in veins perpendicular to the regional stretching
direction.
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<figure><img src="image_22.png" /><figcaption>Figure 8: Photographs of the Zuccale detachment and its internal structure (see Collettini and Holdsworth, 2004). Upper: overview of the detachment fault. Lower left: Detail of the contact zone and the truncated foliation in the hanging wall. Lower right: detail of the shear bands indicating top-to-the east kinematics.</figcaption></figure>

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The internal magmatic structure of Monte Capanne pluton has been described based on the abundance of large alkali-feldspar phenocrysts (Farina et al., 2010). Three main facies corresponding to different magma batches emplaced within a too short period to be discriminated based on geochronology are reported with downward fining of grain size (figure 6). The largest grain size characterizes the upper Sant' Andrea facies that mainly crops out in the northwest of the pluton, while the finest grain size is observed in the lower San Piero facies cropping out mainly in its eastern part, within the zone affected by the most intense shearing. These three facies delineate an asymmetric dome-shaped bulk structure compatible with the general top-to-the east sense of shear. In the westernmost part of the Monte Capanne pluton near Sant' Andrea, mafic products are observed as large enclaves, with evidence of magma mixing and mingling. These mafic enclaves are mostly found in the Sant' Andrea facies that was emplaced first. Their occurrence in the westernmost part of the plutonic body, the farthest from the detachment, with a geometry similar to that observed on Serifos island in the Cyclades (Rabillard et al., 2015), suggests that they are associated with the root of the pluton.

Assuming that the three main felsic facies correspond to three successive intrusion batches, one observes an evolution toward finer grain size through time, an evolution that is compatible with progressive exhumation and also with a shorter residence time in the magma chamber, suggesting opening of the magmatic plumbing toward the surface leading to volcanic activity, as recorded above the detachment. The last episodes of intrusive activity are seen as a series of felsic dykes striking N-S or NE-SW, due to eastward extensional brittle deformation while the pluton was at near solidus conditions.

## 4.2. Orientation of K-feldspar megacrysts
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A detailed study of the orientation of feldspar megacrysts was conducted along the shore near Sant'Andrea (figures 7, 9, 10, 11). The pluton is there characterized by a high concentration of megacrysts and of mafic enclaves reaching several meters in size. Megacrysts show euhedral shapes in general.

<figure><img src="image_24.png" /><figcaption>Figure 9: Photographs of the root zone of the Sant'Andrea facies in the melange zone in and around the Cotoncello dyke. A: Schlieren with cross-bedding. B: Mush zone with cluster of K-feldspar megacrysts and mafic enclaves. C: Isolated blob of mush zone with large K-feldspar megacrysts. D: Folded alternation of leucocratic and melanocratic layers with schlierens. E: Detail of D, schlieren. F: detail of D: schlieren tube.</figcaption></figure>

Although the orientation of megacrysts is quite stable at the scale of a few hundred meters, in the westernmost region the presence of the large enclaves is associated with a disorientation of the megacrysts (figure 10A), indicating increasing tortuosity of the flow wrapping around them associated with local turbulence in pressure shadows. Smaller enclaves are in general aligned with the megacrysts. Some of the mafic enclaves with lobate shapes show a sharp boundary with the felsic matrix, suggesting quenching of a hot mafic magma within the cooler felsic magma (Fernandez and Barbarin, 1991; van der Laan and Wyllie, 1993; Fernandez and Gasquet, 1994).
<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>Figure 10: Photographs of large mafic enclaves and melange zones in the westernmost part of the Sant'Andrea facies showing the disorientation of K-feldspar megacrysts and melange facies (mingling).</figcaption></figure>

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Other enclaves are less mafic and show evidence of magma mingling-mixing. The disaggregation process of the mafic magma responsible of theses enclaves could happened either in a deep-seated magmatic chamber (Christofides et al., 2007), or more likely in the ascent conduit as a result of remelting of chilled mafic margins (Fernández and Castro, 2018) and subsequent viscous fingering dynamics (Perugini et al., 2005). Megacrysts contain inclusions of biotite, plagioclase and quartz and show euhedral shapes in general, although resorption surface has been noticed (Gagnevin et al., 2008). Other enclaves are less mafic and show evidence of magma mingling-mixing. These enclaves are associated with an aureole where feldspar crystals are concentrated, showing that the assimilation of the enclave occurred at the magmatic stage. Megacrysts are sometimes included within the mafic enclaves, showing that they were already present before the solidification of enclaves and thus providing evidence
<figure><img src="image_27.png" /></figure>

of low viscosity contrast between the enclaves and the host magma at the magmatic stage. All these observations suggest that this western zone is a mixing between a mafic magma of mantle origin and a felsic magma partly issued from crustal anatexy and that this part of the pluton is close to the main feeder. This conclusion is confirmed by AMS (anisotropy of magnetic susceptibility) showing that the magnetic foliation and lineation are steeper there than anywhere else in the pluton (Bouillin et al., 1993).

<figure><img src="image_28.png" /><figcaption>Figure 11: Detailed study of the orientation of K-feldspar megacrysts in the Sant-Andrea facies with foliation trajectories.</figcaption></figure>

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Further to the east, still within the Sant' Andrea facies, the main granite is intruded by a N-S syeno-granitic dyke-like structure near Cotoncello headland, made of a finer-grained facies and a lower concentration of megacrysts and enclaves (figure 7D, figure 9). In its vicinity, the host granite contains folded schlierens with cross-bedding (Figure 9A). Within these large schlierens, the megacrysts are aligned parallel with the folded foliation of biotite-rich layers. From place to place, decametric megacryst-rich mush zones enriched in decametric and rounded mafic enclaves occur in this host facies (Figure 9B). In addition, isolated blobs of mush, characterized by an irregular shape, are observed in the coarse-grained, megacryst-poor domains (Figure 9C). These blobs originate from the disruption of preexisting mush zones within the root zone by subsequent magma injection as illustrated by the dyke-like structure (Rodríguez and Castro, 2019). This structure is composed of three successive injections characterized by undulating and fuzzy boundaries (Figure 9D). The westernmost injection (injection Ia, figure 9D) shows folded alternating leucocratic flow-sorted layers made of quartz, K-feldspar and plagioclase with more melanocratic layers rich in biotite that can be described as schlierens (Figure 9E). These schlierens are folded and cross-cut by a subsequent and final
<figure><img src="image_29.png" /></figure>

injection (Figure 9D, injection II). In the easternmost injection (Figure 9D, injection Ib), K-feldspar megacrysts are accumulated and their orientation defines a concave upwards foliation. Such mineral fabric is similar to those described by Rocher et al. (2018) in finger and drip structures developed at the margins of the Asha pluton (NW Argentina) and interpreted as mechanical accumulation in a downward localized multiphase magmatic flow. In addition, this megacrysts accumulation is associated at its top with ring schlieren that could represent a cross-section of a schlieren tube (e.g. Žák and Klomínský, 2007) (figure 9F). Ring schlierens are also associated with drip structures in the Asha pluton among others (Paterson, 2009; Rocher et al., 2018). The most external rim between the host body H and injection Ia (Figure 9D) is associated with a reaction zone with recrystallization of quartz and K-felspar (Figure 9E, white arrow). Outside the injections, the mineral fabric shown by the K-feldspar megacrysts tends to reorientate parallel to the rims.

All these observations point out to an injection of a low viscosity, crystal poor, magma with a viscosity contrast of about one order of magnitude lower with respect to its host magma (Wiebe et al., 2017). Mineral fabrics and accumulation, folded and ring schlieren indicate that the structures were formed by localized multiphase magmatic flow when the crystallizing host magma remained partially molten, probably containing around 50% of crystals (Weinberg et al., 2001). The Cotoncello dyke-like structure is thus co-magmatic with the Sant'Andrea facies, but the pluton was already enough crystallized to allow the formation of N-S cracks in the crystal mush capable of transmitting tectonic stress where the magma was injected.

Between the Cotoncello dyke and the root zone to the west, the proportion of enclaves and megacrysts is everywhere high. Systematic measurements of felspar megacrysts were made (figure 11). Mineral foliation and lineation represents the main orientation distributions of the orientation of (010) faces and [001] major axis of the measured crystals, respectively. At the scale of a few hundred meters the fabric shows a consistent pattern with a low-angle north-dipping foliation more prominent in regions poorer in mafic enclaves. The lineation is in average E-W trending. Late mafic and acidic dykes strike perpendicular to the lineation. Within the mélange zone, the mineral fabric is often perturbated approaching enclave swarms. Then, the fabric becomes more uniform with variations around an average ENE-WSW trend from N30 to N100°E for the long axes of megacrysts.

As the megacrysts were formed in early magmatic conditions (Vernon, 1986; Vernon and Paterson, 2008), they were in suspension within the melt. Such a preferential orientation is due to a rigid rotation of isolated crystals within a viscous matrix submitted to magmatic flow (Fernandez and Laporte, 1984). In the present case, the various observations attesting for a co-
<figure><img src="image_30.png" /></figure>

420 magmatic fabric show that the preferential orientation of the megacrysts foliation results from
421 a fossilization of the magmatic flow. The large-scale variations of the foliation attitude suggest
422 in addition that the E-W to ENE-WSW flow was laminar in general, except in the immediate
423 vicinity of the large enclaves where the flow wrapping around these stronger bodies was more
424 turbulent.

425 These detailed observations show that the internal magmatic fabric of the pluton is similar
426 in orientation with its overall tectonic fabric, including the sub-solidus deformation along the
427 eastern margin due to the detachment with a main stretching direction oriented WNW-ESE, as
428 shown by magnetic susceptibility studies (Bouillin et al., 1993) and deformation features near
429 the main eastern contact within the eastern extensional shear zone (Daniel and Jolivet, 1995).
430 This focussing of the pluton fabric, from the magmatic stage to the brittle stage around an E-W
431 stretching direction compatible with the extensional shear along the main detachment, suggests
432 that the magmatic flow was oriented parallel to the main direction of extension active at crustal
433 scale since the magmatic stage. A continuum is thus observed from the magmatic stage to the
434 sub-solidus deformation and the localization of the detachment, and this continues during the
435 emplacement of the younger Porto-Azzuro pluton and the formation of the Zuccale low-angle
436 normal fault. This evolution recalls that of the Aegean plutons summarized above.
437

# 5. Discussion and modelling

## 5.1. *Synthesis of observations*

The coaxiality of the structures measured in the Monte Capanne pluton from its magmatic
stage to the tectonic overprint is similar to observations made on the Cycladic plutons,
especially Ikaria and Serifos where a similar gradual transition is observed from the magmatic
stage to the localisation of strain along the main detachment. The similarity goes further as the
root of the pluton shows a mixture of mafic and felsic facies. On Serifos (figure 4), field
observations show that the root of the pluton is characterized by vertical or steep dykes and
some of them are dilacerated by the top-south flow while the magma is still viscous (Rabillard
et al., 2015). Moving toward the detachment, the sub-solidus deformation takes over with a N-
S trending stretching lineation and top-south kinematic indicators. A similar evolution can be
observed in the Raches pluton of Ikaria island in the Cyclades (Laurent et al., 2015). Emplaced
below to top-to-the north detachment, the magma shows a steep foliation in the south far from

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<figure><img src="image_31.png" /></figure>

the detachment and it flattens toward the north to become parallel to the detachment plane. Evidence of co-magmatic stretching and shearing parallel to the regional stretching direction is observed in the southern side of the pluton and sub-solidus mylonitization and ultra-mylonites on the northern side. A similar situation can be described in the case of the Naxos granodiorite (Bessière et al., 2017). All cases show the syn-kinematic character of the pluton, the best evidence being the syn-kinematic contact metamorphism.

<figure><img src="image_32.png" /><figcaption>Figure 12: Schematic section showing a conceptual model of the relations between syn-kinematic plutons and the detachments, based on the examples of the Aegean and North Tyrrhenian plutons. Modified from Rabillard et al. (2018).</figcaption></figure>

The Monte Capanne pluton thus shows clear similarities with the Aegean plutons. Figure 12 shows a simplified scheme of the geometrical and kinematic relations between detachments and plutons based on the examples of the Aegean and the Northern Tyrrhenian, modified from Rabillard et al. (2018). The root zone of the pluton, characterized with an association of mafic and acidic magmas, shows a steeper upward magmatic flow and evidence of co-magmatic stretching and shearing parallel to the regional direction of extension with a kinematics similar to that of the main detachments. During the emplacement of the pluton, the magma chamber progressively opens toward the surface and the granitoids evolve toward finer-grained facies. Progressive extension and exhumation is accompanied by the inflation of the pluton and injection of dykes across the ductile detachment. New detachments are formed above sequentially.

## 5.2. A conceptual model based on published numerical experiments

This evolution can be compared with numerical models. Thompson and Connolly (1995) summarize the three ways of melting lower continental crust as (1) supplying water to the crust to lower the solidus, (2) decreasing pressure and (3) providing additional heat to the lower crust. They also state that extension alone of a thickened crust is unlikely to reach the conditions of
<figure><img src="image_33.png" /></figure>

482 lower crustal melting unless some additional heat is given by the mantle. Back-arc regions
483 above retreating slabs, where the lithosphere is thinned and the asthenosphere advected upward
484 underneath the crust seem to the first order to fit these conditions (Roche et al., 2018).

485 Schubert et al. (2013) have explored numerically the effect of the injection of molten
486 mafic material in an extending crust. They show that the injection of this hot material in the
487 lower crust will induce melting and trigger the formation of felsic magmas that will then ascent
488 along steep normal faults all the way to the upper crust, forming the observed plutons. This is
489 a situation that can easily be compared with the Aegean or the Tuscan archipelago where the
490 granitoids are associated in their root zones with coeval mafic magmas and the felsic plutons
491 ascend along low-angle detachments. In figure 13, we propose a further conceptual model based
492 on numerical experiments of post-orogenic extensional deformation with low-angle shear zones
493 (Huet et al., 2011).

<figure><img src="image_34.png" /><figcaption>Figure 13: Conceptual model of the succession of events leading to the emplacement of a plutonic system below an active series of detachments, based on Huet et al. (2011) and Schubert et al. (2013).</figcaption></figure>

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<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

In this series of numerical experiments, the thermal gradient and Moho temperature were varied as well the rheological stratification with either a classical rheological stratification or an inverted crustal structure resulting from the formation of the pre-extension nappe stack, the latter setup being used in figure 13, see also Labrousse et al. (2016) for more details on the dynamics of this system with inverted rheological profiles. This latter choice is designed to mimic the Aegean orogenic wedge where the Cycladic Blueschists Unit is sandwiched between the Cycladic Basement and the Upper Cycladic Unit (UCU) (Huet et al., 2009; Jolivet and Brun, 2010; Ring et al., 2010). The UCU belongs to the Pelagonian paleogeographic domain and is largely composed of an ophiolite, denser and stronger than the CBU (Labrousse et al., 2016) as well as other basement lithologies (Reinecke et al., 1982; Katzir et al., 1996; Soukis and Papanikolaou, 2004; Martha et al., 2016; Lamont et al., 2020). Asymmetric lateral boundary conditions are applied with 1 cm/yr on the left side and no displacement of the right side as in Tirel et al. (2004). The upper surface is free and the base is driven by hydrostatic forces. No prescribed discontinuity is introduced in the model, strain localization is only due to the use of random noise in the cohesion value of the upper crust (for more details, see Huet et al., 2011). The results shown here represent a case where the rheological stratification is inverted and the thermal gradient is high, a likely situation in the Aegean or Tyrrhenian post-orogenic and back-arc contexts.

The conceptual model of the interactions between the numerical model dynamics and the intrusions is that we assume that a batch of mafic magmas, issued from partial melting of the mantle, is injected at the base of the lower crust where it triggers the melting of felsic materials. This leads to the formation of migmatites and collection of the felsic melts in a rising pluton progressively caught in the detachment dynamics as it reaches the upper parts of the crust. The felsic magma is thus deformed while it is still partly liquid and then mylonitized once it has cooled down below the solidus. While extension proceeds, the overburden is removed by the activity of the detachment and the molten material that comes next is injected in lower pressure conditions and finds a faster access to the surface because of extension, thus leading to smaller grainsize plutonic facies and probable volcanism at the surface. While the system of detachments migrates toward the right and a new dome forms, the same situation can be reproduced and a new pluton is emplaced below a detachment further to the right, closer to the active detachment. This evolution is reminiscent of the evolution of Elba Island with the formation of the Monte Capanne pluton in a first stage and the Porto Azzuro pluton in a later stage.
<figure><img src="image_37.png" /></figure>

## 5.3. Testing the concept with numerical experiment

This conceptual model is now tested with new numerical experiments involving the emplacement of magmas like in Schubert et al. (2013), but in a different situation where low-angle detachments form, to see whether the introduction of a low-viscosity material in the model developed by Huet et al. (2011) would drastically change the system dynamics or not. This has been done for figures 14, 15 and 16.

<figure><img src="image_38.png" /><figcaption>Figure 14: Snapshots of the numerical experiments with two different liquidus temperatures (950°C and 1000°C) from 2 to 12.5 Myr. White line limits the molten lower crust with melt ration above 40% (pluton).</figcaption></figure>

The kinematics of exhumation produced by the nappe stacking experiments of Huet et al. (2011) produces extension along long-lived detachment better resembling Mediterranean example than diapiric spreading models that are produced by models with no intermediate weak layers as in Tirel et al. (2004, 2008) or Rey et al. (2009). Hence, in order to test how molten rocks interacts with detachments, we decided to build on our experience and start from this set up which is a 210 km wide model domain submitted to 1cm/yr of extension on its left side for 10 Myr or more with an initial lithospheric column constituted from 25 km upper crust, 10 km weak middle crust, 15 km thick lower crust overlying 40 km of lithospheric mantle. The Moho located at 50 km depth is initially at a temperature of 830°C. We have taken the same

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555 rheological parameters which are reported in Table A1 (Appendix A). The four major 556 differences with Huet et al. (2011) are:

557

558 i) Erosion and sedimentation applied on the top boundary,

559 ii) the deforming Wrinkler foundation at the LAB has been replaced by inflow of 560 asthenospheric material with higher thermal diffusivity to simulate small scale 561 convection and keep the base of the lithosphere at 1300°C during the experiments 562 as it was the case in Huet et al. (2011) study,

563 iii) the numerical code used in this study is pTatin2d (May et al., 2014, 2015) that solves 564 the same momentum equation

565 $\nabla \cdot \sigma = \rho g$

566 For velocity $v$, as well as heat conservation

567

568 $-\nabla \cdot (-\kappa \nabla T + vT) + H = \frac{\partial T}{\partial t}$

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570 for Temperature T as Huet et al. (2011). However, it uses an incompressible visco-plastic 571 rheology minimizing the stress between a dislocation creep regime and Drucker Prager failure

572

573 $\nabla \cdot v = 0$

574 $\sigma = \min \left( \sin \phi + 2 C \cos \phi, \frac{1}{\epsilon n} A \frac{-1}{n} e \frac{Q+VP}{nRT} \right)$

575 to evaluate an effective viscosity:

576 $\eta_r = \frac{\sigma}{2\epsilon}$

577 instead of visco-elasto-plastic rheology based on dislocation creep and Mohr Coulomb failure

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579 criteria.

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581 iv) We added a simplified parametrization in order to account for the mechanical effect 582 of a melt in the simulations. The melt fraction $M_f$ has been introduced as a linear 583 function of solidus ($T_s$) and liquidus ($T_l$) temperature

584

585 $M_f = \min \left( \max \left( \frac{T - T_s}{T_l - T_s}, 0 \right), 1 \right)$
<figure><img src="image_40.png" /></figure>

586
587 following Gerya and Yuen (2003). Based on melt fraction, the density and viscosity of passive
588 markers are modified following algebraic averaging for density

589
590 $\rho = M_f \rho_m + (1 - M_f) \rho_r$,
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592 and harmonic averaging for viscosity

593
594 $\eta = \left( \frac{M_f}{\eta_m} + \frac{1-M_f}{\eta_r} \right)^{-1}$.
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596 The solidus dependence on pressure P (in GPa) is implemented following wet granite
597 solidus of Miller et al. (2003), but we also added a variable temperature offset ΔT to account
598 for more mafic granitic composition as follows:

599
600 $T_s^c = 590 + \frac{250}{10(P + 0.1)} + \Delta T$

601
602 and the dependence of liquidus to pressure is modeled following:

603
604 $T_l^c = T_s^c|_{P=0} + 10 + 200P$.

605
606 The mantle is also allowed to melt following Hirshmann et al. (2000) solidus law

607
608 $T_s^m = -5.904P^2 + 139.44P + 1108.08$

609
610 $T_l^m = T_s^m + 600$

611
612 All solidus and liquidus are represented on figure A2 (Appendix A) as a function of
613 pressure and temperature. For low melting temperature of the lower crust (wet granite solidus
614 with ΔT up to 100°C), the crust is largely molten in the initial conditions and buoyancy effects
615 dominate forming “spreading domes” in the classification of Huet et al. (2011), and this despite
616 the presence of a weak middle crustal layer. For higher melting temperature (ΔT from 150°C)
617 melt proportion remains below the 8% melt connectivity threshold described by Rosenberg
618 and Handy (2005) during most of the simulation. We interpret elements with markers that never
<figure><img src="image_41.png" /></figure>

618 crossed that critical threshold as migmatites. In that case, melting does not disrupt the typical
619 asymmetric detachment kinematics observed in Elba and in the Cyclades that was well
620 reproduced by Huet et al. (2011) study. In the late stage of deformation, when the lithospheric
621 mantle is sufficiently attenuated by boudinage, the temperature of the lower crust reaches a
622 sufficient temperature to melt more generously, generating plutons which we define as markers
623 with a melt proportion greater than 40%.

<figure><img src="image_42.png" /><figcaption>Figure 15: Zoom of the last 5 Ma of evolution of the model of figure 15 with a liquidus temperature of 1000°C from 15Myr to 19.8 Myr (black dotted rectangle on figure 15 for location).</figcaption></figure>

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With a temperature of 950°C at the surface for the liquidus, the molten layer is initially thicker (figure 14) and strain develops into a spreading dome geometry like in the models of Tirel et al. (2004, 2008) or Rey et al. (2009), with symmetric strain pattern and limited strain localization. Shear is indeed progressively relocalized on newly formed shear zones at the top of newly exhumed hot material at the structure axis. When the temperature of the liquidus is higher, reaching 1000°C in surface conditions (figure 14), the molten layer is initially thinner, the deformation is persistently more localized on a detachment on one edge of the dome structure and the model evolves with a detachment on the side of a dome with a limited rate of partial melting in the lower crust (<8%). The deformation ultimately migrates to form a second dome where a syn-kinematic low viscosity body develops with a melt ratio >40%, which we interpret as analogous to a granite intrusion. The late evolution of this second dome shows a strongly asymmetric geometry with the shape of the syn-kinematic intrusion controlled by the asymmetry in strain pattern and a low-angle shear zone (figure 15). Figure 16 shows a structural interpretation of the final step of the model for the second dome highlighting this asymmetry: the dome is bounded by two antithetic crustal scale persistent shear zones, with steep and shallow dip, and an eccentricity of the intrusion feeding pipe within the dome. This overall asymmetrical strain and intrusion localization in the case of limited partial melting rate hence reproduces the main features of the strain and intrusion pattern described in the field in the Cyclades and Elba.

<figure><img src="image_44.png" /><figcaption>Figure 16: Structural cross-section based on the most evolved stage of the numerical model at 19.8 Myr.</figcaption></figure>

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Although the model does not show the details of the interactions between the dome and the pluton, which involve percolation of melts, drainage through migmatites and dyke-swarms, and progressive intrusion of the detachment by the rising granite, the overall geometry and kinematics is similar with the natural case. The observed geometry is better reproduced in runs with higher crustal melting temperatures and limited melt production. Low temperature melting reactions for the continental crust are the wet solidus and the muscovite dehydration solidus, while biotite and hornblende dehydration melting reactions could represent higher temperature melting reactions (Weinberg and Hasalová, 2015). The model does not show either the role of mafic injections at the base of the model. In the case of the Aegean and Monte Capanne, we have postulated that mafic melts, generated by partial melting of the mantle in the arc and back-arc region, intrude the lower crust and trigger the generation of felsic melts that then rise within the dome. This is an additional input of heat in the model that would also localize the weakest layers and thus the deformation and likely favor the evolution of the model in the same direction as in the model presented here with a lower melting temperature. Similar evolution can arise with an additional input of water in the lower crust. This water may originate from amphibole-rich gabbros (sanukitoides) that could act as water donors enhancing lower crustal partial melting to further produce secondary I-type granites (Castro, 2020). In the Aegean arc, whereas amphibole bearing, I-type granites (Naxos and Serifos granodiorites among others) likely reassemble secondary I-type granites as described by Castro et al. (2020 and references therein), this water origin remains elusive as amphibole-bearing mantle magmas are not yet evidenced interacting with the migmatites, except in the Mykonos-Delos-Rheneia MCC (Jolivet et al., 2021).

The metamorphic parageneses associated with contact metamorphism in the case of Elba suggests a depth of emplacement of the pluton of 4-5 km. In the numerical model, the genesis of the pluton starts in the lower crust at a larger depth. The contact metamorphism observed in the field characterizes the upper part of the pluton after it had risen within the crust, which may explain this apparent contradiction.

One additional factor has not been considered in this study, the heterogeneity of the crust inherited from earlier tectonic events. It has been shown by numerical experiments that dipping heterogeneities in the crust mimicking structures inherited from nappe stacking help localizing deformation, favoring the development of asymmetrical extensional structures and the development of MCCs (Le Pourhiet et al., 2004; Huet et al., 2011; Lecomte et al., 2011; 2012). Using this sort of initial conditions with melting would also favor the localization of
<figure><img src="image_46.png" /></figure>

687 deformation on a single detachment. It may alternatively favor the development of several 688 asymmetric domes with low-angle detachments. Future studies should focus on testing such 689 initial conditions and also test these processes in 3-D.

690 The comparison of the Aegean and North Tyrrhenian plutons shows that the model of 691 interactions between plutons and detachments proposed by Rabillard et al. (2018) is 692 reproducible in similar contexts in different regions and can thus be probably generalized. The 693 comparison of this field-based conceptual model with numerical models moreover suggests that 694 this conceptual model is physically feasible. It requires the concomitance of post-orogenic 695 extension, thus extension set on an orogenic wedge, and a back-arc context to provide the 696 necessary heat and water for the generation of magmas in the mantle and the crust. The 697 possibility of a slab tear would be even more favorable as it increases the possibilities of 698 advecting hot asthenosphere directly below the extending crust (Roche et al., 2018).

699 As shown in the 3-D numerical experiments of Roche et al. (2018), slab retreat and back-arc extension lead to the boudinage of the lithosphere with a spacing of ~100 km which will 700 then localize the formation of crustal domes and detachments. This type of evolution may 701 explain the formation of the lines of domes observed in the Aegean and the Menderes Massif. 702 Whether the boudinage also focuses the collection of mantle-derived magmas below the domes 703 is a question that should be addressed by further modelling. This question is important also 704 because the interactions between plutons and detachments described here and in Rabillard et 705 al. (2018) may provide guides for geothermal exploration. The case of the Menderes Massif is 706 exemplary of the intimate relations between active geothermal fields and crustal-scale 707 detachments (Roche et al., 2018). The case of the Tuscan Archipelago is partly similar with the 708 geothermal fields of Larderello and Monte Amiata developed above recent shallow plutons in 709 a context of asymmetric extension with top-to-the east low-angle shear zones (Jolivet et al., 710 1998; Brogi et al., 2003; Rochira et al., 2018). The Monte Capanne and Porto Azzuro plutons 711 on Elba are associated with hydrothermal activities and mineralization (Maineri et al., 2003; 712 Rossetti and Tecce, 2008; Liotta et al., 2015) that make them good exhumed analogues of active 713 geothermal fields, a situation that is found also in the Cyclades with the mineralizations 714 observed on Mykonos or Serifos in the Cyclades (Salemink, 1985; St. Seymour et al., 2009; 715 Menant et al., 2013; Tombros et al., 2015; Ducoux et al., 2016). 716 717

718 6. Conclusions

719
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The comparison between the Aegean and Tyrrhenian Miocene plutons shows striking similarities in their interactions with coeval detachments. These plutons were all emplaced underneath low-angle ductile shear zones and brittle detachments in a post-orogenic back-arc environment where extra heat is provided by advected asthenospheric mantle. The roots of those felsic plutons show a mixing with mafic magmas. The magmatic fabric is steep in the vicinity of the roots and shallows toward the detachments. The plutons record substantial stretching and shearing coaxial with the regional deformation while they still contain a significant amount of melt. In sub-solidus conditions, the granitoids are then mylonitized when approaching the detachment, until the formation of ultra-mylonites and pseudotachylytes. In both cases too, the felsic magma intrudes the detachment and invades the upper plate. A migration of detachments is then observed from the deep and ductile detachments to more brittle and surficial ones. Late magmatic batches show a smaller grain size compatible with an opening of the magma chamber toward the surface suggestive of the volcano-plutonic context of these plutons. To account for these similarities suggesting that this model can be generalized we proposed a conceptual model where mafic magmas batches are injected in the lower crust of an extending orogenic wedge in a back-arc region with low-angle detachments. These mafic injections trigger the melting of the lower crustal felsic material and the ascent of felsic plutons in the crust, controlled by the low-angle detachments. The migration of the detachments through time explains the migration of plutons and detachments observed in the Tuscan Archipelago and in Tuscany from the Late Miocene to the Late Pliocene, as well as in the Aegean. This conceptual model is tested with a numerical approach showing the impact of melt supply in the development of the dome strain pattern. The observed asymmetry of strain localization and intrusion are reproduced for a limited melting rate, while a higher melting rate would lead to the development of a completely different dome structure. The geometry and kinematics observed in the field are well reproduced by the model. These intimate interactions between plutons and detachments can be foreseen as useful guides for the prospection and understanding of geothermal and associated mineralization.

**Author contribution:** Laurent Jolivet, Laurent Arbaret, Florent Cheval-Garabedian, Vincent Roche and Aurélien Rabillard did the field work in the Aegean, Loïc Labrousse and Laetitia Le Pourhiet designed the modeling procedure and ran the numerical experiments. All authors contributed to the writing of the manuscript.

**Competing interests:** The authors declare that they have no conflict of interest.
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752 **Acknowledgments:** This paper is a contribution of the ERC Advanced Research Grant 753 RHEOLITH (grant agreement No.290864), of Institut Universitaire de France and Labex 754 VOLTAIRE.

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1225

1226
<figure><img src="image_64.png" /></figure>

1227 Appendix A Table A1

1228

<table><thead><tr><th></th><th></th><th>Sediments<br>(2)</th><th>Upper<br>crust (1)</th><th>Middle<br>crust(2)</th><th>Lower<br>crust(1)</th><th>Lithospheric<br>mantle(3)</th><th>Asthenospheric<br>mantle(3)</th></tr></thead><tbody><tr><td>Density(kg.m<sup>-3</sup>)</td><td>ρ<sub>r</sub></td><td>2400</td><td>2700</td><td>2700</td><td>2700</td><td>3300</td><td>3300</td></tr><tr><td>Pre exp. Factor<br>(MPa<sup>-1</sup>s<sup>-1</sup>)</td><td>A</td><td>2.4</td><td>3.3</td><td>2.4</td><td>3.3</td><td>3.5</td><td>3.5</td></tr><tr><td>Activation<br>energy(kJ)</td><td>Q</td><td>156</td><td>186</td><td>156</td><td>219</td><td>532</td><td>532</td></tr><tr><td>Stress exponent</td><td>n</td><td>6.7 10<sup>-6</sup></td><td>2 10<sup>-6</sup></td><td>6.7 10<sup>-6</sup></td><td>1.3 10<sup>-3</sup></td><td>2.5 10<sup>4</sup></td><td>2.5 10<sup>4</sup></td></tr><tr><td>Melt<br>density(kg.m<sup>-3</sup>)</td><td>ρ<sub>m</sub></td><td>-</td><td>2400</td><td>2400</td><td>2400</td><td>2800</td><td>2800</td></tr><tr><td>Melt viscosity</td><td>η<sub>m</sub></td><td>-</td><td>10<sup>17</sup></td><td>10<sup>17</sup></td><td>10<sup>17</sup></td><td>10<sup>12</sup></td><td>10<sup>12</sup></td></tr><tr><td>Heat production<br>(Wm<sup>-3</sup>)</td><td>H</td><td>1.67 10<sup>-10</sup></td><td>1.67 10<sup>-10</sup></td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Thermal<br>diffusivity</td><td>κ</td><td>10<sup>-6</sup></td><td>10<sup>-6</sup></td><td>10<sup>-6</sup></td><td>10<sup>-6</sup></td><td>10<sup>-6</sup></td><td>5 10<sup>-6</sup></td></tr></tbody></table>

1229

Table A1: Values and notation for variable physical parameters. Parameters for dislocation creep come from 1 (Ranalli and Murphy, 1987), 2 (Hansen and Carter, 1982) and 3 (Chopra and Paterson, 1984). Other constant parameters include: friction φ and cohesion C which varies linearly with plastic strain in range [0,1] respectively from 30° to 20° and from 20 MPa to 2 MPa; Coefficient of thermal expansion and compressibility that are set to 3.10<sup>-5</sup>K<sup>-1</sup> and 10<sup>-11</sup> Pa<sup>-1</sup>.

1236
<figure><img src="image_65.png" /></figure>

1237

Appendix A, figure A1 and A2:

1238

<figure><img src="image_66.png" /></figure>

1239

1240

Figure A1: MgO v. SiO2 Harker diagram showing the negative correlation between whole rock MgO and SiO2 content for three, I-type hornblende-biotite bearing, representative Aegean granites. Data from Delos intrusion (Pe-Piper et al., 2002), Серифос (Salemink, 1985) and Naxos (Pe-Piper et al., 1997). Mixing/mingling processes between mafic mantle-derived melts and acid magmas produce composite batholiths (Poli and Tommasini, 1990) as illustrated by the case of the Elba Island magmatic complex showed for comparison (See Dini et al. 2002 for explanation). MME = Mafic Microgranular Enclaves.

1249
<figure><img src="image_67.png" /></figure>

<figure><img src="image_68.png" /></figure>

<figure><img src="image_69.png" /><figcaption>Figure A2: Solidus and liquidus as a function of pressure and temperature for mantle and crust (950 and 1000°C).</figcaption></figure>

1250
1251
1252
1253