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The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

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Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The T RSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
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Michard etal. Swiss Journal of Geosciences (2022) 115:19
https://doi.org/10.1186/s00015-022-00419-8
ORIGINAL PAPER
The Maira-Sampeyre andVal Grana
Allochthons (south Western Alps): review
andnew data onthetectonometamorphic
evolution oftheBriançonnais distal margin
André Michard1* , Stefan M. Schmid2, Abdeltif Lahfid3, Michel Ballèvre4, Paola Manzotti5, Christian Chopin6,
Salvatore Iaccarino7 and Davide Dana7
Abstract
Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolu-
tion of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the
Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map
accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the
distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent
deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and
was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation
related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their
former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-
Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations,
seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-
Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and
correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones
involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dron-
ero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present
a new metamorphic map based on published and new petrological data, including new thermometric data obtained
by Raman spectroscopy of carbonaceous material (RSCM). The TRSCM values range from ~ 400 °C to > 500 °C, going
from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the
Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after
the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-
Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
Keywords: Briançonnais distal margin, Jurassic rifting, Cretaceous-Paleocene extension, Metamorphic wedge, Raman
spectroscopy of carbonaceous material (RSCM), Exhumation, High-pressure metamorphism
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«Je regarde […] les observations de détails comme
l’unique base d’une connoissance solide mais je voudrois
qu’en observant ces détails, on ne perdit jamais de vue les
grandes masseset les ensembles et les ensembles et que
la connoissance des grands objets et de leurs rapports
Open Access
Swiss Journal of Geosciences
Editorial handling: Vladica Cvetković.
*Correspondence: andremichard@orange.fr
1 Université Paris-Sud (Orsay), 10 rue des Jeûneurs, 75002 Paris, France
Full list of author information is available at the end of the article
19 Page 2 of 43
A.Michard et al.
fût toujours le but que l’on se proposât en étudiant leurs
petites parties.»—H.B. de Saussure, 1779, p. 3
1 Introduction
Being in the center of Europe and remarkable for their
wonderful summits (Mont Blanc, 4807 m a.s.l.), the
Western Alps captivated European geologists and min-
eralogists from the late eighteenth century (De Saussure,
1779) onward. At the advent of plate tectonics in the lat-
est 1960’s, the geological study of the Western Alps took
a new start. e close association of ophiolites and con-
tinent-derived crustal units and the processes of thrust-
ing and metamorphism in the internal zones of the belt
became more intelligible than in the time of Argand’s
pioneering work (Argand, 1911, 1924). e descrip-
tion of the Western Alps belt (Fig.1) in terms of rifting,
drifting, subduction and collision began in the 70’s (see
Frisch, 1979 and references therein), then benefited from
deep seismic profiles, followed by tomographic studies
in the following decades (e.g., Roure et al., 1990, 1996;
Schmid and Kissling, 2000; Lippitsch etal., 2003; Handy
etal., 2021). Knowledge about present-day oceanic ridges
and passive margins was applied to understand Alpine
ophiolites and inverted Mesozoic margins (Lemoine
etal., 1985, 1986; Lemoine and Trümpy, 1987; Lagabri-
elle and Lemoine, 1997). e general occurrence of high-
pressure (HP), low-temperature (LT) metamorphism
was recognized in the Internal (Penninic) zones (Bearth,
1959, 1966). is was followed by the discovery of ultra-
high-pressure (UHP) metamorphism in a continental
crustal slice of the southern Dora-Maira massif (Chopin
1984, 1987; Goffé and Chopin, 1986), and in the Zermatt
metaophiolites further to the north (Reinecke, 1991).
Geochronological studies established that UHP-HP
metamorphism developed progressively from the Late
Cretaceous to the Eocene (Duchêne et al., 1997; Hun-
ziker etal., 1992; Monié, 1990; Rubatto etal., 1999; Tilton
etal., 1991), coeval with the subduction of Alpine Tethys
and European margin lithosphere beneath the Africa-
related Adria microplate and followed by rapid cooling
and decompression (Rubatto and Hermann, 2001; Avigad
etal., 2003).
During the last two decades, several papers deciphered
the most critical areas of the Western Alps (e.g., Agard
etal., 2001; Dal Piaz etal., 2001; Le Bayon and Ballèvre,
2006; Gasco etal., 2011; Compagnoni etal., 2012; Angi-
boust et al., 2012; Manzotti et al., 2014, 2018; Weber
et al., 2015; Ballèvre et al., 2018, 2020; Groppo et al.,
2019). Other papers proposed structural, tectonic and/
or metamorphic syntheses of Western Alps in the frame
of other parts of the Tethyan belt (e.g., Michard et al.,
1996; Oberhänsli etal., 2004; Schmid etal., 2004, 2017;
Rosenbaum and Lister, 2005; Diehl etal., 2009; Handy
etal., 2010; Bousquet etal., 2008, 2012; Dumont etal.,
2011; Kissling and Schlunegger, 2018; Le Breton etal.,
2020; Agard, 2021; Gnos etal., 2021). e structure of
the belt is roughly deciphered down to 100 km depth
(Fig. 1C) and its paleogeography restored in its major
aspects (Fig. 1B). Indeed, the Western Alps have been
and still are the “mother” of subduction-collision oro-
gens, and its HP-LT Internal Zones are characteristic
of “Ampferer-type”, amagmatic subduction of a narrow
oceanic domain and adjacent thinned continental crust,
opposite to the “Benioff-type”, arc-related subduction of a
large ocean (Agard, 2021; McCarthy etal., 2020). Regard-
ing the dynamics of plate convergence, the Alps fall into
the category of slab-pull orogens (Faccenna etal., 2013;
Jolivet etal., 2021), consistent with the narrowness of the
subducted oceanic lithosphere.
Despite the permanent research activity in the belt, a
small but critical area (framed in Fig.1A) of the Internal
Zones remained unexplored since the late 1960’s. is
area of the Cottian Alps (the southern part of the French-
Italian Alps between the Torino and Cuneo transects) is
crossed by the valleys of the Maira and Grana rivers, and
mainly occupied by Triassic-Jurassic series that tectoni-
cally overlie the southern Dora-Maira units. e area has
been made famous by the work of Franchi (1898) describ-
ing the occurrence of determinable Liassic ammonites
in the lowest “calcescisti lucenti” (in French “Schistes
lustrés”, which are glittery calcareous-clastic metasedi-
ments) above Upper Triassic dolostones (similar to the
“Hauptdolomit facies” of Eastern Alps) in Val Grana. One
of us (A.M.) mapped the area at scale 1:50,000 in the lat-
est 50’s and published a geological monograph (Michard,
Fig. 1 a Sketch tectonic map of the Western Alps with location of the studied Maira-Sampeyre and Grana Allochthons (MGA), after Schmid
et al. (2017), modified, with traces of the seismic profiles ECORS-CROP, NFP20-W and CIFALPS. Outline of the geophysical Ivrea body after Ceriani
et al. (2001). Eclogite domain boundary after Malusà et al. (2011). Ac: Acceglio village; Alp Mar: Alpes maritimes; Arg: Argentera-Mercantour;
A-C: Arnasco-Castelbianco; Amb: Ambin; Ar: Arvieux; Bel: Belledonne; Can: Canavese Line Ch: Chaberton-Grande Hoche; DB: Dent Blanche; DM:
Dora-Maira; Em-Ub: Embrunais-Ubaye; G: Gondran; GM: Grande Motte; GP: Gran Paradiso; IL: Insubric (Peri-Adriatic) Line; MH: Monferrato Hills;
MR: Monte Rosa; Mrg: Marguareis; Mt Bl: Mont Blanc; NB: Nappe de la Brèche; Pel: Pelvoux; PF: Penninic Front; R: Rochebrune; RB: Roc del Bouchet;
RC: Roche des Clots and Péouvou units; SVL: Sestri-Voltaggio Line; TH: Torino Hills; Vi, Viso; Z: Zermatt. b Restoration of the Alpine domain in
cross-section before plate convergence, after Manzotti et al. (2014) and Ballèvre et al. (2020). c Sketch lithospheric-scale cross-section along the
CIFALPS transect, after Malusà et al. (2021), modified after Michard et al. (2004), Schmid et al. (2017), Ballèvre et al. (2020). Acronyms as above with Br:
Briançonnais; SL: Schistes lustrés
(See figure on next page.)
Page 3 of 43 19
The Maira-Grana Allochthons
Fig. 1 (See legend on previous page.)
19 Page 4 of 43
A.Michard et al.
1967) immediately before seminal publications founded
the paradigm of plate tectonics (Isacks et al., 1968; Le
Pichon, 1968). Michard (1967) proposed a stratigraphic
continuity between Liassic “Schistes lustrés” (linked to
the shallow-water, epicontinental Triassic deposits) and
overlying “Schistes lustrés”, which host lenses of serpen-
tinites and metabasites and are rich in metashales and
metacherts. Soon afterwards this stratigraphic inter-
pretation was questioned (Lemoine, 1971; Schumacher,
1972; Michard and Schumacher, 1973) as it could not be
reconciled with the growing wealth of new observations
in support of an oceanic origin for the ophiolite-bearing
“Schistes lustrés” in terms of the sedimentary cover of an
oceanic basement (Piedmont-Liguria, or Piemonte-Ligu-
ria Ocean), formed during the Late Jurassic-Cretaceous
(Lemoine et al., 1970; Elter, 1971; Decandia and Elter,
1972; Lemoine et al., 1978; Lagabrielle, 1981; Lemoine
et al., 1986). Unfortunately, the Grana and Maira val-
leys have not been re-studied by modern geologists after
the 1970’s, except for a survey of the metamorphism by
Bousquet etal. (2008) and a map of the northern border
of Val Maira by Mondino (2005).
e present paper first of all presents a critical review
of the geology of this key region in the light of modern
geological knowledge. Additionally, we also present new
field and laboratory results, particularly concerning met-
amorphism, including a set of Raman Spectroscopy of
Carbonaceous Material (RSCM) data for the first time.
We pay special attention to the dominantly Triassic and
Liassic units cropping out along the Maira and Grana val-
leys, hereafter labeled as the Maira-Sampeyre and Val
Grana (in short, Maira-Grana) Allochthons (MGA) and
to their relationships with the surrounding tectonic units.
ese are the southern Dora-Maira continental units to
the northeast (interpreted as the most distal parts of the
Briançonnais terrane; Ballèvre et al., 2020); the Mon-
viso ophiolites and Queyras ophiolite-bearing Schistes
lustrés (Piemonte-Liguria Ocean) to the northwest and
west, and the Briançonnais units further to the west and
southwest. In the studied area the latter are mainly rep-
resented by units derived from the “Acceglio Zone”, here
labeled Acceglio sensu lato (s.l.) paleogeographic domain,
deprived of Triassic carbonates (Debelmas and Lemoine,
1957; Lemoine, 1960; Lefèvre and Michard, 1976; Mich-
ard etal., 2004).
e combination of published and new structural and
petrological-thermometric data presented below allow
us to discuss the relationships of the Maira and Grana
allochthonous units (MGA) with other similar, so-called
“Prépiémontais” (Prepiedmont, Prepiemonte), Triassic-
Liassic units (Deville, 1986; Ellenberger, 1958; Lemoine,
1967; Pantet etal., 2020) as well as with the Schistes lus-
trés and various units derived from the Briançonnais
sensu lato domain (Acceglio s.l. and Dora-Maira units).
In the discussion, we will pay special attention to the
extensional events that took place from the Triassic up
to Late Cretaceous-Eocene in the Briançonnais domain
s.l.. ese events that framed the Briançonnais passive
margin remain poorly studied so far with respect to those
that framed the conjugated Adria margin (e.g., Bertotti
etal., 1999; Manatschal and Bernoulli, 1999; Mohn etal.,
2010, 2012; Sutra and Manatschal, 2012; Beltrando etal.,
2015; Chenin etal., 2017).
2 Geological setting
It is recognized that the complex structure of the West-
ern Alps (Fig.1A) formed at the expense of the follow-
ing three major domains of the Mesozoic paleogeography
(Fig. 1B) inherited from the Pangea breakup (Handy
etal., 2010; Schmid etal., 2017; Kissling and Schlunegger,
2018; Le Breton etal., 2020).
e External Zones (Helvetic-Dauphinois-Provençal
zones) are derived from the proximal part of the Euro-
pean passive margin, which comprises a crustal base-
ment exposed in the uplifted External Crystalline
Massifs, and a detached, folded Triassic-Neogene cover
(Fig. 2A; Bellahsen et al., 2014; Nibourel et al., 2021).
e External Zones are bounded to the east by the major
out-of-sequence thrust of the Internal or Penninic Zones
(“Front Pennique”, FP; Ceriani etal., 2001) of Oligocene
age (equivalent of the “Briançonnais Frontal rust’’ first
described by Tricart (1986), which connects to the NE
with the Rhône-Simplon detachment (Campani et al.,
2014) bounding the Central Alps culmination or Lepon-
tine Dome, and to the SE with the sinistral Argentera-
Cuneo Line (Schmid et al., 2017). At an earlier stage
(late Eocene) some internal Penninic units were already
thrust over the External Zones west and in front of the
FP, namely the Helminthoid Flysch nappes of the Embru-
nais-Ubaye and Alpes Maritimes, and the Prealps nappes
north of Mont Blanc.
e Briançonnais paleogeographic domain (sensu lato)
represents the distal part of the thinned European margin
that was separated from the proximal part by the partly
oceanic, SW-ward narrowing Valais-Subbriançonnais
rift during the Cretaceous (Beltrando et al., 2012; De
Broucker etal., 2021; Loprieno etal., 2011). On the scale
of lithospheric plates, the Briançonnais s.l. domain is
regarded as a terrane rifted from Europe during the Early
Cretaceous, loosely connected to Corsica, and displaced
NNW-ward by ~ 200 km along the proximal European
margin during the Eocene (Frisch, 1979; Stampfli etal.,
1998; Schmid and Kissling, 2000; Handy et al., 2010).
Units derived from the Briançonnais s.l. domain form the
classical Briançonnais nappes of the Briançon area (Bri-
ançonnais sensu stricto) and its northern equivalents,
Page 5 of 43 19
The Maira-Grana Allochthons
the Siviez-Mischabel Nappe (Genier etal., 2008; Pantet
etal., 2020; Scheiber etal., 2013). e Mesozoic-Eocene
series of the Briançonnais s.str. are characterized by thick
Triassic carbonates (Fig. 2B; Debelmas and Lemoine,
1957; Lemoine, 1960, 1961; Deville, 1986), which have
been partly eroded during the late Early-Middle Jurassic
emersion that affected most of the Briançonnais domain
(Debelmas, 1987; Lemoine et al., 1986). e Acceglio
s.l. tectonic units (also labeled in the literature “Ecailles
intermédiaires” after Lemoine (1961), and “Ultrabrian-
çonnais” after Lefèvre and Michard, 1976) whose Trias-
sic carbonates have been totally or almost totally eroded
(Fig.2C) are generally located in a more internal position
with respect to the units derived from the Briançonnais
s. str. ey are in close neighborhood with other units
whose facies differs from that of the Briançonnais s.str.
in being characterized by thick Upper Triassic dolos-
tones followed by Liassic and Dogger (?) syn-rift series
rich in bedded carbonate breccias then sands (Fig. 2D).
Since the latter series have been regarded as transitional
to the “Piémontais” ophiolitic domain they were labeled
“Prépiémontais” units (Ellenberger, 1958; Lemoine,
1967)—a classical terminology with varied subsequent
adaptations (see for details Additional file1). Hereafter
we use the term “Prepiemonte” for these units, which
might include in some cases Upper Jurassic to Upper
Cretaceous deep-sea sedimentary facies (Dumont etal.,
1984) but clearly belong to the Briançonnais s.l. domain.
Although separated from the Classical Briançonnais units
(Briançonnais s.str.) by the ophiolitic Schistes lustrés
nappes in map view (Fig.1), the Internal Crystalline Mas-
sifs are widely regarded as derived from the pre-Triassic
basement of the Briançonnais domain s.l. (e.g., Ballèvre
etal., 2018). e lowermost, monometamorphic units of
the Gran Paradiso and Dora-Maira massifs (Money and
Pinerolo units, respectively) compare with the “Zone
houillère” of the classical Briançonnais s.str. (e.g., Man-
zotti etal., 2015, 2016; Ballèvre etal., 2018, 2020). ey
are overlain by units involving both monometamor-
phic orthogneisses and polymetamorphic schists, which
derive from a Variscan basement. e Dora-Maira poly-
metamorphic rocks record a pre-Alpine amphibolite-
facies Barrovian metamorphism (Vialon, 1966; Chopin
etal., 1991; Sandrone etal., 1993). In the eclogitic unit of
northern Dora-Maira (Muret unit), Nosenzo etal. (2021)
constrain the pre-Alpine P–T evolution from about 4–5
kbar, 500°C to 6–7 kbar, 650°C, consistent with Bar-
rovian metamorphism, which they date ~ 325 Ma. e
Acceglio s.l. derived units, most Prepiemonte type units
and the Internal Crystalline Massifs were subducted
and affected by HP to UHP metamorphism. In terms of
Fig. 2 Stratigraphic columns of the main paleogeographic domains exposed in the southern Western Alps. a European proximal margin
(Dauphinois) after Lemoine et al. (1986). b Briançonnais s.str. (Classical Briançonnais of Briançon area). c Acceglio s.l. units after Michard et al. (2004).
d Prepiemonte units after Dumont et al. (1984). e Piemonte-Liguria oceanic domain after Lemoine et al. (1986) and Deville et al. (1992)
19 Page 6 of 43
A.Michard et al.
their paleogeographic setting, they are grouped hereafter
under the name “Briançonnais distal margin”.
e Briançonnais s.str., Acceglio s.l. and Prepiemonte
units continue southeast of the Argentera-Cuneo Line in
the Ligurian Alps, although in a different structural set-
ting (Seno etal., 2005). Units derived from the Briançon-
nais and Prepiemonte domains are also recognized in
the French-Swiss Prealps nappes, where they correspond
to the “Médianes Rigides” and “Nappe de la Brèche”,
respectively (e.g., Lemoine, 1967; Mosar, 1989). Notice
that these rootless, non or hardly metamorphic Prealps
nappes also include Subbriançonnais-Valaisan and Pie-
monte-Liguria elements (“Médianes plastiques”, ophi-
olites and flyschs, respectively).
e Piemonte-Liguria domain whose stratigraphy is
summarized in Fig.2E includes three complexes of units
that all derive from the Piemonte-Liguria Ocean (north-
eastern Alpine Tethys; Stampfli etal., 1998). ese com-
plexes of units differ by their metamorphic grade (Agard,
2021). e most internal, i.e., the Monviso and Zermatt
complexes, are eclogitic, the latter being locally affected
by UHP metamorphism (Reinecke, 1991). e Queyras
and Tsaté complexes are more external and their meta-
morphism ranges within the blueschist-facies (Dev-
ille etal., 1992; Agard etal., 2001; Bousquet etal., 2008,
2012; Manzotti etal., 2021). Lastly, the Gets nappe of the
French-Swiss Prealps and Chenaillet ophiolitic klippe
east of Briançon are hardly affected by greenschist-facies
metamorphism. According to Manatschal et al. (2011)
and Li et al. (2013), the Chenaillet ophiolite is derived
from the western part of the Piemonte-Liguria Ocean
margin. However, since it lacks significant Alpine defor-
mation and because it tectonically overlies the Queyras
complex, Schmid et al. (2017) inferred an upper plate
position of this klippe in an intra-oceanic subduction
scenario, and hence placed the Chenaillet ophiolites at
the eastern margin of the Piemonte-Liguria Ocean. e
ophiolites of the Gets nappe, only metamorphosed under
sub-greenschist facies conditions (Bill etal., 2001a) origi-
nate from an oceanic branch of Alpine Tethys (Elter etal.,
1966) that formerly extended between the Sesia conti-
nental extensional allochthon and the distalmost Adria
margin (Canavese; e.g., Beltrando etal., 2015; Festa etal.,
2020).
e Piemonte-Liguria Ocean opened slowly during the
late Bathonian to late Kimmeridgian at least (De Wever
and Caby, 1981; Bill etal., 2001b; O’Dogherty etal., 2006;
Cordey and Bailly, 2007) and reached a width of ~ 400km
during the Early Cretaceous (Stampfli etal., 1998; Handy
etal., 2010, their Fig.8b; Li etal., 2013; Le Breton etal.,
2020), although higher estimates have also been pro-
posed (Agard, 2021; Vissers etal., 2013). e protoliths of
the metasedimentary series associated with the ophiolite
bodies typically derive, from bottom to top, from ophi-
olitic breccias, radiolarites, micritic limestones, black
shales, and calcareous-pelitic turbidites (flysch), ranging
in age from Bathonian to Late Cretaceous, respectively
(Fig.2E; Lemoine and Tricart, 1986; Deville etal., 1992;
Lemoine, 2003; Li etal., 2013). ese metasediments are
the genuine, most extended “Schistes Lustrés”, contrast-
ing with the restricted Schistes lustrés-like Prepiemonte
sequences, as quoted above (Sect. 1). e highest unit
in the Vanoise area, namely the Pointe du Grand Val-
lon unit, is a blueschist-facies meta-flysch devoid of any
metabasite, which yielded planktonic foraminifera of
upper Maastrichtian age; it could be an equivalent of the
Helminthoid Flyschs of Piemonte-Liguria origin (Deville
etal., 1992).
West of the southern Cottian Alps, the Upper Creta-
ceous Helminthoid Flysch constitutes the main part of the
Embrunais (-Ubaye) nappes (Fig.2). Subbriançonnais and
Briançonnais slivers form the lowest units of these nap-
pes that emplaced upon the External Zones by the end
of the sedimentation of the Oligocene foredeep deposits
(Kerkhove etal., 1984; Dumont etal., 2011). e Embru-
nais Helminthoid Flyschs would originate from the inner-
most Piemonte-Liguria domain and belong to the upper
plate in the subduction setting like the Chenaillet klippe;
their thick, turbiditic sedimentation is thought to record
the subduction onset at ~ 90Ma (Stampfli etal., 1998).
e Africa-derived Adria microplate (Argand’s “Afri-
can promontory”) occupies an upper-plate position in
the Alpine orogen east and north of our area of inter-
est. Along the Central and Eastern Alps transect, upper
crust and Mesozoic cover of the proximal margin of
Adria are exposed in the Southern Alps, and the Ivrea
Zone exposes its verticalized lower crustal and upper-
most mantle underpinnings that originate from the Adria
distalmost margin (Handy etal., 1999). e occurrence
of a thick slice of mantle, i.e., the Ivrea mantle wedge
(or “geophysical body”) belonging to the thinned Adria
margin beneath the outcropping parts of the Ivrea Zone,
and further to the south in the subsurface underneath
the Dora-Maira units, is well constrained by geophysi-
cal data (Fig.1A; Berckhemer etal., 1968; Masson etal.,
1999; Schmid and Kissling, 2000; Zhao etal., 2015, 2016;
Schmid etal., 2017). e distal part of the Adria margin
is also exposed in the Lower and Middle Austroalpine
nappes of the Eastern Alps close to the Central Alps cul-
mination (Manatschal and Bernoulli, 1999; Mohn etal.,
2010), and in the strongly dismembered sediments of
Canavese Zone belonging to the Southern Alps (Belt-
rando etal., 2015; Elter etal., 1966; Ferrando etal., 2004;
Festa etal., 2020). e Sesia-Dent Blanche nappes, which
constitute the highest tectonic elements of the Western
Alps northwest of the Canavese Zone (or Line), probably
Page 7 of 43 19
The Maira-Grana Allochthons
derive from an extensional allochthon detached from the
Adria distal margin since they underwent high-pressure
metamorphism (Manzotti etal., 2014).
e most internal parts of the Western Alps arcuate
belt do not crop out south of the area of the Sesia Zone,
being covered by the deposits of the Tertiary Piemonte
basin (Carrapa and Garcia-Castellanos, 2005). e Early
Oligocene, lowermost beds of this basin unconformably
overlie Briançonnais-derived crystalline basement and
the roots of the ophiolites of the Western Alps, including
the ophiolitic units of the Ligurian Alps, as well as much
of the NE-verging Apenninic units (Fig. 1A; Lorenz,
1986; Mosca etal., 2010; Molli etal., 2010; Maino etal.,
2013; Marroni etal., 2017; Piana etal., 2021). Ligurian
flysch units of the frontal Apennines (Elter etal., 1966)
crop out in the Monferrato-Torino Hills as a result of
north-directed Mio-Pliocene thrusting and folding that
also affected the Ligurian Alps (Schmid etal., 2017).
3 Structural data
3.1 Methodology fordrawing anew structural map
e outlines of the major lithological units presented
in a new structural map of the southern Cottian Alps
(Fig. 3) were taken from the Geological/Environmen-
tal map of Piemonte, scale 1:250,000, compiled by Piana
etal. (2017). Our input consisted in delineating the vari-
ous tectonic units based on their distinct structural and
petrological characters (see Sect.3.2). For this purpose,
we used the following maps at the scale 1:50,000: (i) the
map of the Maira and Grana valleys published by Mich-
ard (1967) and complemented by the unpublished thesis
of Schumacher (1972) in the upper Val Grana, and (ii)
the Larche and Aiguille de Chambeyron sheets of the
Geological map of France (Gidon, etal., 1977, 1994) for
covering the western fringes of the studied area, which
include the Acceglio-Longet antiformal band and the
overlying Schistes lustrés (also mapped by Lefèvre and
Michard, 1976, and Lefèvre, 1982). However, in contrast
to the map of Michard (1967), we defined tectonic con-
tacts by separating continental sedimentary sequences
that include Triassic carbonates at their base from jux-
taposed oceanic sequences that include serpentinites,
despite of the apparent structural and metamorphic con-
tinuity of the units. Such tectonic contacts are frequently
well-marked by lenses of serpentinites or cargneules (also
spelled cornieules = evaporitic tectonic breccias; Masson,
1972; Fudral etal., 2010). Admittedly, where such mark-
ers are lacking, tracking a tectonic limit between ocean-
derived Schistes lustrés (Cretaceous) and Schistes lustrés
(Jurassic) that are part of the Prepiemonte sedimentary
sequence remains uncertain in places.
e mapped area comprising the MGA was extended
to the southern Dora-Maira units (Val Varaita and Val
Po) in the NE, to the southern Monviso complex in the
NW and to the Acceglio s.l. units in the W and SW. e
tectonic contacts within the Dora-Maira units and west-
erly adjacent Monviso ophiolite and Queyras Schistes
lustrés were taken from the literature (see below). e
Geological map of Italia, scale 1:100,000, sheet Dronero-
Argentera, 2d edition (Malaroda et al., 1971) was also
consulted; it does not differ substantially from Michard
(1967) in the MGA area, and its background is consist-
ent with that of the 1:250,000 Piemonte map (Piana etal.,
2017).
3.2 Tectonic units andtheir mutual relationships
Figure3 illustrates the major tectonic units in map view
and cross-sections are shown in Figs.5 and 6. Metamor-
phic data are presented in map view and cross-section in
Figs.10 and 13, respectively. Table1 will help the reader
with a summary of the nomenclature used hereafter (for
more details, please see Additional file1).
e Sanfront-Pinerolo (SP) is the lowermost of the
Dora-Maira units (Argand, 1911; Vialon, 1966), and then
of the whole studied area. e SP unit includes graph-
ite-rich conglomeratic schists considered for long as
Carboniferous, which indeed yielded a most recent zir-
con population with an age of ~ 330Ma (Manzotti etal.,
2016). ese metasediments are intruded by fine-grained
orthogneiss derived from Permian granodiorite, locally
dated at about 290Ma (Bussy and Cadoppi, 1996). Addi-
tionally, conglomeratic quartzite and meta-dolostones
and marbles occur in the Sanfront-Monte Bracco area,
referred to Permo-Triassic clastics and Triassic carbon-
ates, respectively; these are considered as relics of the
Mesozoic cover of the SP unit. e Alpine metamorphic
grade reached garnet-blueschist to eclogite facies condi-
tions (Avigad etal., 2003; Groppo etal., 2019).
e Brossasco-Isasca (BI) unit is made up of a poly-
metamorphic complex of mica-schists, marbles and
metabasites that are associated with mono-metamorphic
orthogneisses derived from Permian granites, whose
metasomatized parts yielded the famous coesite-bear-
ing lenses of pyrope whiteschists (Chopin, 1984; Cho-
pin etal., 1991; Kienast etal., 1991; Schertl etal., 1991;
Henry, 1990; Biino and Compagnoni, 1992; Henry etal.,
1993; Chopin and Schertl, 1999; Compagnoni & Rolfo,
2003; Castelli et al., 2007, 2014; Campomenosi et al.,
2021). is UHP unit overlies the HP Sanfront-Pinerolo
unit along a former major thrust contact that was over-
printed by ductile late extensional structures indicating
top-SW normal faulting (Fig.6) under more moderate
P–T conditions (Avigad etal., 2003). e small “San Chi-
afreddo unit”, defined by Compagnoni et al. (2012), is
mapped here as part of the BI unit.
19 Page 8 of 43
A.Michard et al.
Fig. 3 Structural map of the southern Cottian Alps with traces of cross-sections Figs. 5, 6, 12 and location of map Fig. 10. Background map:
Piemonte geological map: 250,000 (Geoportale Arpa; Piana et al., 2017), modified. Cargneule (cg) is only distinguished in the Pradleves fault
zone. See Table 1 for details on tectonic units and shear zones. Acronyms: AL, Acceglio s.l. units (mainly Acceglio-Longet band); BF: Bersezio fault;
BI, Brossasco-Isasca UHP unit; CBR: Classical Briançonnais units (Briançonnais s. str. domain); CF: Colletto fault; CLSZ, Cima Lubin shear zone; GS,
Giulian-Sea Bianca unit; Drn, Dronero unit (1, lower; 2, upper); LB, Longet-type breccias; MV, Monviso complex (eclogite-facies metaophiolites and
metasediments); QN, Queyras nappe (blueschist-facies ophiolitic Schistes lustrés); RS, Rocca Solei unit; R, Ricordone unit; SBR, Subbriançonnais units;
SDSZ, San Damiano shear zone; SP, Sanfront-Pinerolo unit (Carboniferous); VG, Val Grana unit (Middle Triassic-Jurassic); VM-S, Val Maira-Sampeyre unit
(Permo-Triassic to Middle Triassic); VPSZ, Valmala-Piasco shear zone
Page 9 of 43 19
The Maira-Grana Allochthons
Table 1 Structural units of the South Cottian Alps (see map Fig. 3)
Acro-nyms Name of tectonic units Paleogeographic domain Main lithologies Metamorphic facies
QN Queyras nappe Piemonte-Liguria
Ocean Radiolarites, marbles and calcschists with ophi-
olite bodies Blueschist
MV Monviso ophiolite complex Mainly ophiolites with minor
metasediments Eclogite
VG Val Grana unit “Prepiemonte” – type facies of the Briançonnais
s.l. distal margin Mesozoic metasediments
(Ladinian-Pliensbachian) Blueschist
VM-S Val Maira – Sampeyre unit Permian-Mesozoic metasediments
(Upper Permian-Ladinian) Blueschist
(cf. GS further N) (cf. Giulian-Sea Bianca unit)
CLSZ Cima Lubin shear zone Shear zones representing tectonic mélanges of
oceanic and margin units Mesozoic metasediments (dolostones, calcs-
chists) and minor ophiolite bodies Blueschist
SDSZ San Damiano shear zone Blueschist
VPSZ Valmala-Piasco shear zone HT- blueschist
Drn2 Upper Dronero unit Dora-Maira crystalline Massif representing part
of the former basement of the Briançonnais s.l.
distal margin
Polycyclic schists and monocyclic metased. and
granites (Permian) Quartz eclogite
Drn1 Lower Dronero unit Polycyclic basement Blueschist
R Ricordone unit Polycyclic basement Quartz eclogite
RS Rocca Solei unit Polycyclic basement Quartz eclogite
BI Brossasco-Isasca unit Polycyclic basement Coesite eclogite
SP Sanfront-Pinerolo unit Monocyclic metasediments (Carboniferous- Tri-
assic) + Permian
felsic intrusives
Garnet blueschist to
eclogite
AL Units of the Acceglio- Longet band (Acceglio
anticline, Rocca Corna and Pelvo d’Elva units) and
their southern equivalents
Acceglio-type facies domains of the Briançon-
nais s.l. distal margin (Acceglio, "Ecailles intermé-
diaires" or Ultrabriançonnais
auct.)
Monocyclic metasediments (Up. Carbon.-Up.
Cretac.-Paleoc., with Middle Triassic—Liassic
gap) + Permian felsites
Polycyclic schists (scarce)
HT- blueschist
CBR Classical Briançonnais nappes or Briançonnais s.
str. (various nappes, see Fig. 6)Briançonnais proximal margin (= Briançonnais
s.str. domain) Cf. Acceglio units but with thick Middle ± Upper
Triassic dolostones
± Lower Liassic (rare) + Eocene
“Flysch noir”
Greenschist to LT- blueschist
SBR Subbriançonnais units Valaisan and Subbriançonnais rift
domains Up. Triassic (evaporites) – Eocene sediments Non- metamorphic
19 Page 10 of 43
A.Michard et al.
e following two units overlie the coesite-eclogite
facies Brossasco-Isasca unit and reached quartz-eclog-
ite facies conditions: the lower one is labeled Rocca
Solei (RS) unit and the next higher Ricordone (R) unit
(equivalent to “Units II and III, respectively, in Henry
etal., 1993). Both display lithological associations simi-
lar to those of the BI unit (Compagnoni & Rolfo, 2003).
in slivers of quartzites, dolostones and calcschists are
pinched along the tectonic contact between Rocca Solei
and Ricordone units (Henry etal., 1993). ese Mesozoic
slivers connect with similar Mesozoic slivers that are part
of the Valmala-Piasco shear zone.
e Valmala-Piasco shear zone (VPSZ) is a major
mélange-type shear zone that truncates the top parts of
both Ricordone and Rocca Solei units, and locally even
the Brossasco-Isasca unit (Figs.3, 6). In addition to lenses
of Triassic carbonates (Piasco, Rossana) the VPSZ also
displays serpentinites (Venasca-Serravalle quarry), meta-
basites and calcschists (Balestro et al., 2020; Michard,
1967). During the latest stages of its tectonic evolution
this shear zone represented an extensional detachment
that exhumed the eclogitic units in the footwall with
respect to the overlying, blueschist-facies Lower Dronero
unit (Drn1). e overall trace of the VPSZ in map view
underlines the dome-shaped structure of the eclogitic
units of the southern Dora-Maira units.
e Dronero unit, originally defined by Michard etal.
(1993) and later tentatively divided into lower and upper
Dronero units (Drn1 and Drn2, Fig.3) by Balestro etal.
(1995), includes polymetamorphic garnet-chloritoid
schists that originate from pre-Permian basement, meta-
granitoids (orthogneiss, sometimes with whiteschists)
and metabasites similar to those observed in the Ricor-
done and Rocca Solei units (Balestro etal., 1995; Henry
etal., 1993), as well as ankerite-chloritoid silvery schists
of presumably Permo-Carboniferous age or Lower Per-
mian age (Balestro et al., 2020; Henry, 1990; Michard,
1967). According to Balestro etal. (1995), eclogite assem-
blages are only preserved in the metabasites of the upper
Dronero unit whereas the lower unit only shows garnet-
blueschist-facies assemblages.
A second major mélange-type shear zone, referred to
as San Damiano shear zone (SDSZ), also involves tec-
tonic lenses of calcschists, metabasites and continent-
derived rocks (gneiss, Triassic carbonates). It is found
in the hangingwall of the southwestern margin of the
lower Dronero unit. is shear zone omitted the eclog-
itic upper Dronero unit in its footwall, suggesting that it
also accommodated late-stage normal faulting. It closely
resembles the Valmala-Piasco shear zone in terms of its
mixed lithological content.
e Val Maira-Sampeyre (VM-S) unit is the lower tec-
tonic unit amongst the Maira-Sampeyre and Val Grana
Allochthons. It is found in the hangingwall of the San
Damiano shear zone. e base of the VM-S unit, namely
the “Sampeyre unit” as defined by Michard (1967) can
be followed all the way from Val Maira to Val Varaita.
Note that we interpret the Sampeyre unit sensu Mich-
ard (1967) no longer as a part of the Dora-Maira units.
Rather it represents a lower subunit (Sampeyre subunit)
of the larger Val Maira-Sampeyre unit. is lower subunit
is made up of conglomeratic quartzites (Permo-Triassic)
and fine-grained quartzites (Lower Triassic). It is sepa-
rated from the overlying Anisian-Ladinian carbonates
of the Val Maira subunit (corresponding to “Unité I” in
Michard, 1967) by a folded fault along which the litholo-
gies of the upper Val Maira subunit are wedged out in
map and profile view (see Figs.3, 6). In Fig.4 we propose
that the folded fault between the two subunits possibly
represents a reactivated paleo-fault. e stratigraphic
column of the Val Maira-Sampeyre unit is presented for
convenience as Additional file1: Fig. S1A.
e Cima Lubin shear zone (CLSZ), originally labeled
“Unité II” in Michard (1967), tectonically separates the
Val Maira-Sampeyre unit from the overlying Val Grana
unit described below. It represents a third mélange-type
ophiolite-bearing shear zone. is lithologically hetero-
geneous shear zone involves tectonic lenses of silvery
mica-schists (Permian?), quartzites (Lower Triassic),
cargneules and dolostones (Middle-Upper Triassic), as
well as calcschists and metabasites. To the north, the
CLSZ continues into the so-called “Roccenie formation”
(Mondino, 2005), a metasedimentary sequence that over-
lies the Giulian-Sea Bianca Unit (GS, Fig.3) and directly
underlies the basal serpentinites of the Monviso ophi-
olitic unit (Agard etal., 2001; Angiboust etal., 2012; Cas-
telli etal., 2014; Lombardo et al., 1978; Schwartz etal.,
2000b). e overall steep westward dip of the CLSZ in
the north, from the Po-Varaita crest to the Varaita-Maira
crest, gives way further to the south and across the Maira
valley to a shallow, southwestward to southward dip
locally deformed by kilometer-size late-stage folds. ese
are: (i) an antiform north of the Stroppo village in Val
Maira (Fig.3); (ii) the Pradleves antiform in Val Grana
(Fig.5), and (iii) the Valgrana antiform (next to the local-
ity Valgrana in lower Val Grana) exposing a tectonic win-
dow of (Val Maira) dolostones below the CLSZ mélange
and Val Grana dolostones (Fig.3). ese late-stage folds
formed during the back-folding and—thrusting that took
place during phase D3 discussed later.
e Val Grana (VG) unit exposes Triassic carbonates
that predominate in the east, i.e., in the lower Grana val-
ley, whereas the Jurassic strata dominate northwest of
the Colletto transverse fault, which operated as a nor-
mal wrench (tear) fault during late-stage folding of the
unit (Fig.3). e carbonate sequence of the Val Grana
Page 11 of 43 19
The Maira-Grana Allochthons
unit includes Ladinian dolostones and subordinate sandy
limestones, black cherts and meta-cinerites, followed
upward by Carnian-Norian dolostones. e transition
to the Jurassic beds is marked by Rhaetian-Hettangian
coral and cherty limestones. ese are overlain by the
meta-calcarenites with bedded to chaotic breccias whose
Fig. 4 Panorama (a) and block-diagram (b) showing the relationships between the meta-carbonates of the Val Maira subunit and the underlying
meta-clastic, Permo-Triassic and Lower Triassic Sampeyre subunit. The folded contact between the two subunits may be interpreted as an inverted
former normal fault within the Briançonnais margin. Note the occurrence of dolomitization fronts in the Ladinian formations and the abundance of
cargneules in the Anisian-Ladinian
19 Page 12 of 43
A.Michard et al.
base is Sinemurian-Lower Pliensbachian (Narbona val-
ley, Fig.8c; Sturani, 1961; Ellenberger etal., 1964; Mich-
ard, 1967; see also Additional file1: Fig. S1B). e Norian
dolostones reappear in Val Maira where they form the
core of the kilometer-scale, east-verging Monte Bettone
anticline (Figs.3, 8e, f) formed during D3 back-folding
and -thrusting as well. ere, an angular unconform-
ity occurs between the tilted Norian beds and overlying
Rhaetian-Hettangian formation (Additional file 1: Fig.
S3).
e Maira-Sampeyre and Val Grana Allochthons are
obliquely overlain by two lithologically different tectonic
units derived from the Piemonte-Liguria Ocean: (i) the
Monviso (MV) ophiolitic complex, consisting of eclogitic
units that dominantly expose metagabbros and metaba-
sites with subordinate oceanic metasediments (breccias,
marbles, calcschists), underlain by a thick basal sole of
serpentinites (Lombardo etal., 1978; Deville etal., 1992;
Schwartz etal., 2000b, 2013; Agard etal., 2001; Angiboust
etal., 2012; Castelli etal., 2014; Angiboust and Glodny,
2020), and, (ii) the ophiolite-bearing Queyras nappe
(QN), consisting of three units (not separated in Fig.3) of
blueschist-facies oceanic metasediments hosting boudi-
naged kilometric meta-ophiolite bodies (Lagabrielle and
Fig. 5 Semi-schematic geological cross-section of the Maira-Sampeyre and Grana Allochthons and adjoining units. See Fig. 3 for location of
the topographic line; the deepest parts of the section are projected from the SE over 1–5 km, complying with the thicknesses of the geological
formations. The entire section has been affected by blueschist-facies metamorphism before its exhumation under greenschist-facies conditions.
Stratigraphic abbreviations: L-MJ: Lower-Middle Jurassic coarse/chaotic carbonate breccias; LP: Lower Pliensbachian cherty marbles; LSi: Lower
Sinemurian siliceous marbles; MT: Middle Triassic, with 1: Anisian-Ladinian evaporitic carbonates, and 2: Ladinian dolostones; Pmv: Permian
meta-volcanics; PT: Permian–Triassic conglomerates (Verrucano Piemontese, “anagenites”); R-H: Rhaetian-Hettangian coral marbles; Tq: Lower
Triassic (Werfenian) quartzites; USi: Upper Sinemurian turbiditic carbonate breccias and marbles; UT: Upper Triassic dolostones;. Lithostratigraphic
abbreviations: cg: cargneules (with dolostone/marbles blocks, particularly in the Pradleves cargneule zone); cs: calcschists; dbr: dolostone breccias;
mb: metabasite; ms: mica-schist; mtbs: meta turbiditic black shales; og: orthogneiss; qdbr: quartzite and dolostone breccias (Longet breccias, LB in
Fig. 3); sr: serpentinite; wcm: whitish cherty marbles
Fig. 6 Orogen-scale profile across the Cottian Alps. See Fig. 3 for the trace of the profile, and text in this section and in the Discussion (Sect. 5). AA:
Acceglio antiform; AC: Aiguille de Chambeyron unit; AM: Aiguilles de Mary unit; CC: Ceillac-Chiappera unit; CH: Châtelet unit; MA: Marinet unit; MGA:
Maira-Sampeyre and Grana Allochthons; PE: Pelvo d’Elva antiform; PH: Peyre Haute unit; Pv: Péouvou « Prepiemonte» unit; RC: Roure-Combrémond
units; RO: Rouchouze unit; RP: Rocca Peroni unit; ST: Sautron unit; SZ: shear zone with tectonic mélange
Page 13 of 43 19
The Maira-Grana Allochthons
Polino, 1988; Deville etal., 1992; Schwartz etal., 2000a,
b; Tricart and Schwartz, 2006; Herviou et al., 2021).
Between the Upper Po and Varaita valleys (Figs.3, 6), the
Queyras nappe tectonically overlies the Monviso com-
plex along a west-dipping, late-stage extensional fault
(Ballèvre etal., 1990; Schwartz etal., 2007).
In turn, the Monviso complex overlies the Cima
Lubin shear zone and the Val Maira-Sampeyre unit or
its northernmost equivalent, the Giulian-Sea Bianca
(GS) unit (Balestro et al., 2011; Michard, 1967) along
another west-dipping late extensional fault that can be
traced southward into the hangingwall of the Cima Lubin
shear zone (Fig.3). However, immediately south of the
Varaita-Maira watershed (Sampeyre Pass), the Mon-
viso serpentinites disappear in map view, mainly due
to the mega-lens geometry of the whole Monviso com-
plex, and additionally to a minor north-verging normal
fault. Only rare potential remnants of the thick Monviso
complex are found further south as lenses in the Cima
Lubin shear zone. us, south of the Sampeyre Pass
and down to the alluvial plain of Cuneo, it is the Quey-
ras nappe that directly overlies the Val Grana unit (see
profile of Fig.6 around and west of Monte Bettone, and
discussion, Sect. 5). e metasedimentary series of the
Queyras nappe include Upper Jurassic meta-radiolarites
and Tithonian-Berriasian marbles, Lower Cretaceous
metamorphosed turbiditic black shales and monoto-
nous calcschists dated as Upper Cretaceous (Tricart
and Schwartz, 2006; Herviou etal., 2021 and references
therein).
e Val Grana unit and overlying Piemonte-Liguria
units are bounded to the west and SW by the back-
thrusted Acceglio-Longet (AL) antiformal band derived
from the Acceglio s.l. paleogeographic domain (Lefèvre
and Michard, 1976; Schwartz et al., 2000a). is band
of dominantly siliceous rocks forms the hangingwall of
the major D3 synform depicted in Fig.6. Another major
backthrust located at the margin of the studied area, west
of Acceglio locality, brings the Roure and Combrémond
units, with their sedimentary cover, similar to the Acceg-
lio-Longet units (Le Guernic, 1967; Michard etal., 2004),
over a second synform occupied by the Queyras rocks
(Fig.6). e tight anticlines (Acceglio and Pelvo d’Elva)
and associated slivers that form the Acceglio-Longet
band, and the two Roure and Combrémond units repre-
sent previously stacked slivers derived from one and the
same Acceglio s.l. paleogeographic domain (see Discus-
sion, Sect.5.2.4).
Upper Cretaceous-Paleocene polygenic chaotic brec-
cias (Longet-type breccias; LB in Fig.3) associated with
light gray and occasionally siliceous marbles appear in
the tectonic contact with the Acceglio s.l. units (Pelvo
d’Elva inverted limb and Longet area). ese puzzling,
chaotic breccias occur as discontinuous tectonic lenses
at the very bottom of the Queyras Schistes Lustrés, in
terms of the original nappe stack that formed during D2,
i.e., before D3 (Leblanc, 1962; Lemoine, 1967; Lefèvre
and Michard, 1976; Gout, 1987). Some of these breccia
lenses could have been detached from their Acceglio s.l.
substrate during thrusting of the Queyras nappe (see Dis-
cussion, Sect.5.2.4).
3.3 Setting oftheMaira-Grana Allochthons atthescale
oftheorogen
In order to put the tectonic units described above into
an orogen-scale context we constructed the profile pre-
sented in Fig.6. Its trace indicated in Fig.3 extends fur-
ther to the SW in direction of azimuth 210° towards Col
de Larche and ends at Col de la Bonette.
e westernmost part of the profile shows the Pen-
ninic Front, i.e. a major fore-thrust formed during D3
that also thrusts the previously emplaced Embrunais-
Ubayae nappe stack. is part of the profile is a slightly
modified version of the profile published on the 1:50,000
map, sheet Larche (Gidon et al., 1977), complemented
with along-strike projections of the Subbriançonnais
units exposed in Valle Stura di Demonte, based on the
1:100,000 geological map sheet Argentera (Malaroda
et al., 1971). Lateral projections are based on the pro-
nounced NW oriented axial plunge of the edge of the
Argentera external massif that also affects the neigh-
boring Briançonnais units. e slope of the Penninic
Front follows the constraints provided by crustal P-wave
tomography provided by Diehl etal. (2009) and Schmid
etal., (2017, their Fig.2a).
e part of the section between Col de Larche and
Acceglio was constructed by using the French 1:50,000
map, sheets Larche and Aiguille de Chambeyron (Gidon
etal., 1977 and 1994, respectively). e along-strike SE-
ward projection of a section exposed along the Upper
Ubaye valley into the section of Fig. 6 is based on an
axial plunge of 11° towards azimuth 313° across the Bri-
ançonnais s.str. units, the Roure-Combrémond Acceglio
s.l. unit and the Péouvou Prepiemonte unit. e projec-
tion of the Acceglio-Longet anticlines, however, used an
axial plunge of 9° towards azimuth 340°. Both these val-
ues correspond to the average plunge of the D3 retro-
folds extracted from pole figure data provided by Caron
etal. (1973), Lefèvre and Michard (1976) and Michard
etal. (2004) that apply to these two sectors of the pro-
file. In this part of the profile the Briançonnais nappe pile
became tilted into a steeply foreland-dipping orientation
due to D3 backfolding.
e Roure-Combrémond and Acceglio-Longet units
are interpreted as being connected at depth around a
19 Page 14 of 43
A.Michard et al.
F3 synform with the Queyras Piemonte-Liguria units
in the core, following Michard etal., (2004; their Fig.4).
We refer to this synform as the external Queyras syn-
form. However, the exact geometry of the structures
around this synform at depth are unknown and largely
conceptual.
e link of the Acceglio s.l. units with the area exposing
the MGA units buried at depth is provided by the inter-
nal Queyras synform. According to our interpretation
this synform separates the steeply SW-dipping units of
the various Briançonnais s.l. units described above from
the more flat-lying part of the profile located NE of the
synform, which exposes a normal way up nappe stack.
is interpretation is firmly supported by the vergence
of large-scale F3 folds affecting the MGA units discussed
earlier (see detailed profile of Fig.5) and the vergence of
the spectacular Mte Bettone backfold crossed by the pro-
file of Fig.6 (see also Fig.8e) and located NE of the axial
plane of this mega-fold.
For constructing the part of the section NE of the inter-
nal Queyras synform, located outside map sheets Larche
and Aiguille de Chambeyron, we used the detailed map
of Michard (1967) covering the Maira-Sampeyre and
Val Grana Allochthons and adjacent areas, and the map
compilation presented in Fig. 3 modified after Piana
etal. (2017) that also covers the Dora-Maira units. e
westernmost part of the section shows that the Maira-
Sampeyre and Val Grana Allochthons, together with
the underlying units constituting the Dora-Maira units,
form a normal way-up original nappe stack located in the
lower limb of the internal Queyras synform. e units of
the Dora-Maira massif define an asymmetric antiform
related to a hypothetical backfold, partly concealed by
the Plio-Pleistocene deposits of the Tertiary Piemonte
basin (Cassano et al., 1986; Pieri & Groppi, 1981). e
existence of such a backfold is a corollary of the neces-
sity to root the Piemonte-Liguria oceanic units along the
W-dipping interface between the top of Ivrea Zone of the
Adria plate and the base of the Dora-Maira units (Debel-
mas etal., 1983; Schmid etal., 2017).
3.4 Meso- andmicrostructures
e MGA and neighboring units have all been affected
by (U)HP-LT metamorphism, ranging from the coesite-
eclogite-facies in the Brossasco-Isasca unit of the south-
ern Dora-Maira “massif” to blueschist-facies in the
Queyras and Acceglio s.l. units (Goffé etal., 2004; Bous-
quet etal., 2008; see Sect.4 for details). erefore, their
lithologies exhibit the usual, polyphase structure of rocks
that were subducted, then exhumed, and eventually
affected by the latest collisional events. Figure7 provides
a brief overview of the orientation of mesostructures that
have been analyzed in terms of superimposed phases
of deformation by previous workers in the areas of the
southern Dora-Maira units (Henry et al., 1993), the Val
Grana and Val Maira transects (Horrenberger & Mich-
ard, 1978; Schumacher, 1972), and in the Acceglio-Longet
antiformal band (Lefèvre and Michard, 1976; Michard
etal., 2004).
e stereoplots from the Dora-Maira basement rocks
(Fig.7a, b) show a weakly dispersed flat-lying foliation
that overprints earlier high-P parageneses and formed
during top-to-SW shearing occurring under retrograde
greenschist-facies conditions, associated with the exhu-
mation of the Dora-Maira units along low-angle exten-
sional faults (Avigad et al., 2003; Henry et al., 1993).
Cataclasites and pseudotachylites are locally hosted in
the top-to-the SW/W mylonitic foliation (Dana, 2020;
Ferré etal., 2015; Henry etal., 1993; Zechmeister etal.,
2007). According to Rubatto and Hermann (2001), green-
schist-facies conditions in the Dora-Maira units prevailed
during the 33–30 Ma time interval. is time interval
approximately coincides with the timing of D3 back-fold-
ing and—thrusting in the Val d’Aosta sections(35–31Ma
according to Bucher etal., 2004) as well as with the age
of fore-thrusting at the Penninic Front behind the Pel-
voux Massif (34–31Ma according to radiometric dating
by Simon-Labric et al., 2009). Although the structural
relation of such normal faulting within the Dora-Maira
nappe stack with D3 backthrusting affecting the adja-
cent Val Grana and Val Maira units remains unclear we
assume this extensional exhumation to be part of the D3
deformation phase.
e authors of Fig.7c, d presented stereoplots cover-
ing Mesozoic units. ey all described a first schistos-
ity mostly parallel to bedding (S0-1), overprinted by an
S2 foliation that is axial-planar to isoclinal D2 folds. In
general, S1 and S2 are hard to be distinguished in the
absence of F2 folds. Hence, F1 and F2 can be confused in
many cases. Likewise, fold axes related to D2 and D3 are
often hard to separate. However, S3 foliations related to
backfolding can often easily be discriminated from ear-
lier structures. A penetrative S3 foliation is observed in
incompetent lithologies that underwent intense shear-
ing during D3 leading to the progressive rotation of fold
axes into parallelism with the direction of shearing within
S3. is is manifested by the distribution of F3 fold axes
within a great circle (Fig.7e, f, h). e directions of trans-
port were derived according to a method described by
Caron etal.(1973) that utilizes the great circle distribu-
tion of pre-D3 lineations (not shown in Fig.7e, f, h). is
clearly documents the intensity of shearing in incompe-
tent lithologies, i.e., within the micaceous Permian–Tri-
assic beds of the Acceglio-Longet Band (Fig.7h) and in
the underlying Schistes lustrés of the Queyras nappe
(Fig. 7e, f). e large-wavelength, NW-trending D3
Page 15 of 43 19
The Maira-Grana Allochthons
anticlines formed by stiff slabs of quartzite or dolostone
are illustrated in the Acceglio (Fig. 7g) and Narbona-
Campomolino (Fig.7d) diagrams, respectively. e effect
of this D3 folding is less visible in the diagram of Fig.7c
from the Val Maira Anisian-Ladinian weak formations
derived from an evaporitic sequence.
Figure8 illustrates typical structures observed in the
Maira-Sampeyre and Val Grana Allochthons at the out-
crop and landscape scale. e occurrence of superim-
posed structures is particularly clear in the case of the
Lower-Middle Jurassic beds near Colletto, 1km NW of
Campolino (Fig.8a, c, d). At the larger scale, competent
horizons appear to be only affected by late stage D3 open
Fig. 7 Orientation of the structural elements in the studied area. Stereoplots (Wulff net, lower hemisphere) simplified after Henry et al., 1993 (a,
b), Horrenberger & Michard, 1978 (c, f), Schumacher, 1972 (d, e) and Lefèvre and Michard, 1976 (g, h). In g, the black dots mainly correspond to S3
fanned cleavage, but possibly also to the folded S2 foliation
19 Page 16 of 43
A.Michard et al.
folds such as in case of the slab of Triassic dolostones
(Rocca Caire and Campomolino anticlines; Fig.8c).
However, at the meter scale (Fig.8a, d) as well as under
the microscope (Fig.9a, b) the structures exhibit super-
imposed foliation and crenulation planes that reveal
multi-stage deformation. e dominant (regional) folia-
tion overprints a previous foliation, which is sheared
obliquely or microfolded within microlithons bounded
by the younger (S3) foliation (Fig. 8a, b). In line with
Schumacher (1972) and Caron et al. (1973), we label
these foliations S2 and S3, respectively, assuming that a
previous S1 foliation formed sub-parallel or parallel to
bedding S0. In contrast, the dolostone slabs of the same
unit generally show scattered, strait phengite lamellae
devoid of preferred orientation at the microscopic scale.
e development of a tectonic cleavage in the dolos-
tone beds has been only observed in the Mte Bettone
tight anticline (Fig.8e, f). Detachment of the incompe-
tent Liassic formations above their competent Triassic
base during the deformation process is documented by
decametric recumbent folds located in the Rhaetian beds
downstream Campomolino (Schumacher, 1972).
In the Anisian-Ladinian formations of the Val Maira
sub-unit, the early metamorphic foliation S1-2, which
nearly parallels bedding, is severely deformed by recum-
bent folds and minor thrust faults (Fig.8h). e protoliths
of this formation are dolomitic limestones interbedded
with evaporitic clays (Michard, 1967), which acted as a
detachment horizon between the Val Maira Middle Tri-
assic formations and their Lower Triassic base (Sampe-
yre subunit) notwithstanding the role of an early normal
fault (Fig.4). ese lithologies are likely at the origin of
the 400m-thick cargneule zone of Pradleves (Figs.3, 5).
In the Queyras Nappe, whose metamorphic grade
compares with that of the underlying Val Grana unit (see
chapter4), the meso- and microstructures observed in
the Upper Jurassic-Cretaceous Schistes Lustrés also com-
pare with those in the underlying Early-Middle Jurassic
sequences. Tight F2 folds folding S0/S1 are well illus-
trated in the metacherts of the Mte Plum klippe (Fig.8g);
their axial plane is deformed by open F3 folds at a large
scale (see cross-section, Fig.5). e planar structure of
banded competent glaucophanite (see microscopic fea-
tures in Fig.9g) is probably defining a less deformed S2
foliation. In contrast, the S3 foliation is particularly pen-
etrative in the Ussolo calcschists sampled at short dis-
tance of the Acceglio-Longet backthrust (Figs.8b, 9b). In
the Ussolo outcrops, ankeritic calcschists display pseu-
domorphs of lawsonite prisms (“lawsonite A”-type of
Lefeuvre etal., 2020), which contain relics of the pre-S3
foliation (Fig.9e).
On the western and southwestern border of the
Maira-Sampeyre and Val Grana Allochthons and west
of the main (internal) D3 synform, the Acceglio-Longet
antiformal band (Fig. 3) also offers superimposed
microstructures involving a poorly preserved S1 folia-
tion, a well-marked S2 foliation locally associated with
isoclinal folds, and a dominant S3 crenulation-type
foliation (Schumacher, 1972; Lefèvre and Michard,
1976; Michard et al., 2004). e jadeite orthogneiss
at the bottom of the overturned Pelvo d’Elva unit (see
Fig.10 for location) formed at the expense of a Permian
(?) alkaline granite (Lefèvre and Michard, 1965). It now
displays a typical mylonitic S/C’ structure (Fig. 9h).
is D3 microstructure developed during the retro-
grade alteration of jadeite into muscovite and albite and
the concomitant quartz recrystallization in the pressure
shadows (see also the macroscopic view of the same
sample in Additional file2: Fig. S1).
e microstructures in the southern Dora-Maira
eclogite- and blueschist-facies schists are compre-
hensively described in Henry etal. (1993) and Avigad
etal. (2003). ere, the Alpine nappe contacts are over-
printed by mylonitic microstructures, which devel-
oped under retrograde greenschist-facies conditions.
Foliation and C’ shear bands all indicate ductile top-SW
extensional overprint (Fig. 7). As mentioned earlier,
the main greenschist facies foliation “Sm” described by
Henry etal., (1993; their Fig.7) indicating extensional
unroofing developed roughly at the same time as the
(See figure on next page.)
Fig. 8 Outcrop and landscape scale structures. a Penetrative S2 foliation with flattened quartz lenses crenulated by late foliation S3; locally, minor
shear planes C oblique on S3 indicates top-NE shear sense; Battuira (44.4274 × 7.2128), Val Grana Upper Liassic-Dogger(?) turbiditic limestones.
b Tight crenulation schistosity (S3) entirely transposing bedding and earlier foliation (S2) in the Queyras calcschists beneath the Pelvo d’Elva
backthrust; Ussolo (44.4910 × 7.0260). c Campomolino open anticline (F3) seen from the crest north of Punta Castellar (44.4067 × 7.1837); Val Grana
unit, Sinemurian bedded limestones and breccias topped by the Pliensbachian Bercia limestones. d Close-up of carbonate breccias to the SW of
(c); F3 folds (deforming a thin breccia bed and minor calcite veins) in foliated marbles showing lenticular texture and tilted, flattened dolostone
pebbles; Val Grana Middle-Upper Liassic member west of Punta Castellar above San Magno (44.4041 × 7.1771). e Western (normal) limb of the
Monte Bettone F3 anticline from the Vallone d’Elva (hazardous!) road; a minor parasitic anticlinal fold is seen below the major hinge; Norian bedded
dolostones of Val Grana unit. f Detail of an oblique cleavage (S3) that exceptionally affects the dolostone beds, slightly oblique (N130E) with respect
to the fold axis in map view (N160E); lowest Vallone d’Elva (44.4980 × 7.1010). g Tight similar folds (F2) deforming S0-1 with tilted axial plane due to
later major folding (F3, not shown) in Upper Jurassic meta-cherts of the Queyras klippe, eastern cliffs of Monte Plum (44.4297 × 7.2303). h Folding
of an early metamorphic foliation by superimposed recumbent F3 (?) folds; Anisian-Ladinian dolomitic limestones and argillites associated with
cargneules west of Lottulo, Val Maira subunit (44.4940 × 7.2113)
Page 17 of 43 19
The Maira-Grana Allochthons
Fig. 8 (See legend on previous page.)
19 Page 18 of 43
A.Michard et al.
S3 foliation related to the D3 back-folding event in the
overlying Maira-Sampeyre and Val Grana Allochthons,
although the kinematics are different (top-SW normal
faulting in the Dora-Maira units vs. top-NE thrusting in
the other units).
4 Metamorphic mineralogy andRaman
geothermometry
4.1 Mineral composition ofmetamorphic rocks
4.1.1 Summary ofpublished results
Blue amphibole (“gastaldite”), lawsonite and jadeite have
been described by Franch (1898, 1900) in the Acceglio-
Longet antiformal band and the bordering ophiolitic
Schistes lustrés in Val Grana (Rio Grande and Mte
Ruera; Fig.10), as well as chloritoid (“sismondine”) in the
Schistes lustrés between Elva and Stroppo (Val Maira).
Lefèvre and Michard (1965) described the partly retro-
gressed minerals jadeite, glaucophane and lawsonite in
the Acceglio-Longet band (see Additional file2: Figs. S1,
S2). Michard (1967) found the mineral assemblages (“par-
ageneses”) quartz-phengite-lawsonite ± chlorite ± chlori-
toid in the MGA, and glaucophane-lawsonite-chlorite in
the juxtaposed Schistes lustrés metabasites. He pointed
out that these HP-LT minerals become progressively
altered to higher-T, lower-P minerals when approach-
ing the Dora-Maira units. For example, lawsonite is well
preserved in the metabasites of the Acceglio-Longet
band, next to jadeite-bearing orthogneiss (Lefèvre and
Michard, 1976). However, it is altered into epidote, albite,
white mica ± chlorite in the metatuffites interlayered
in the Norian dolostones of the Pradleves anticline (Val
Grana; see Additional file2: Fig. S3) and in the Ladinian
schists of the Val Maira sub-unit (Fig.9c).
In the recent literature, the MGA and overlying
Schistes Lustrés of the Queyras Nappe were altogether
considered to belong to the blueschist-facies zone of
the Western Alps, with peak metamorphic conditions
of ~ 300–400 °C, 8–15 kbar (Goffé et al., 2004; Ober-
hänsli etal., 2004; Bousquet etal., 2008, 2012). e meta-
morphic facies of the Acceglio-Longet antiformal band
was classified as upper blueschist-facies (transitional
to eclogite-facies), with peak metamorphism at about
450 °C, 12–14 kbar (Goffé et al., 2004; Michard et al.,
2004; Schwartz et al., 2000a). Goffé (2002) and Goffé
etal. (2004) gave a precise description of the retrograde
evolution towards the greenschist-facies based on Fe–
Mg carpholite (Fe, Mg)2+Al2Si2O6(OH)4 in the metape-
litic rocks. Fresh carpholite fibers occur in the innermost
units of the classical Briançonnais (Ceillac-Chiappera
unit; see also Michard et al., 2004), relic fibers in most
of the Queyras Schistes lustrés, and entirely transformed
fibers in the easternmost Queyras units, next to the Mon-
viso units (Fig.10).
In the Colle di Sampeyre area, Mondino (2005)
described scattered occurrences of omphacite-garnet and
glaucophane-lawsonite associations in metabasites found
within both the lowest Queyras Schistes lustrés and the
Cima Lubin shear zone. In other words, metamorphic
grade reaches the blueschist-eclogite transitional facies
around the southern tip of the Monviso serpentinites.
Further to the north, and outside of our study area, the
P–T conditions in the Queyras Schistes lustrés have
been shown to evolve from ca. 12 kbar, 330°C (low-T
blueschist) in the western (uppermost) unit towards
ca. 18–20 kbar, 450–470 °C (blueschist-eclogite facies
transition) found in the most internal (lowest) unit,
which overlies the Monviso eclogitic complex (Agard
et al., 2001; Herviou et al., 2021; Lefeuvre et al., 2020;
Schwartz etal., 2013). Quartz-eclogite to coesite-eclog-
ite facies conditions of metamorphism are well-known
in the Dora-Maira units beneath the MGA, as reported
above (chapter3.2). Greenschist-facies retrogression also
affected these HP(UHP) eclogitic units (e.g.,Avigad etal.,
2003; Castelli etal., 2014; Henry etal., 1993).
4.1.2 New results
We collected 49 samples in the Varaita, Grana and Maira
valleys, among which 42 have been studied (Table 2)
from the point of view of their mineralogy/petrology,
and 29 for Raman geothermometry. Petrological studies
were carried out in two laboratories: (i) Ecole Normale
Supérieure, Paris, for samples AC1-40 (optical micros-
copy) and 41 (electron microscopy); and (ii) Geosciences
Rennes for samples GR20 85-91 collected in the Mte
Plum klippe. RSCM measurements were performed at
the French Geological Survey, Orléans.
Fig. 9 Microphotographs of key structures. Abbreviations for mineral according to Whitney and Evans (2010). a Superimposed foliations
S2 and S3 in the calcschists/black schists of Battuira, Val Grana unit (sample AC01; same outcrop as Fig. 8a). b Pressure solution foliation (S3)
bounding microlithons with crenulated previous S2 foliation in Liassic calcschists at Ponte Marmora, Val Grana unit (sample 38-M2111). c White
mica (wm)-epidote-albite pseudomorph after lawsonite (?) in Middle Triassic carbonaceous schists (sample 35-M2108, Val Maira subunit). d
Chloritoid-phengite association in quartz-mica-schist from the Cima Lubin tectonic mélange in the core of Pradleves anticline (sample AC09M). e
Pseudomorph after lawsonite (?) including traces of a previous foliation (S2 ?), now transposed into S3 (sample 40-M2113, Queyras Schistes lustrés
above Ussolo; cf. Fig. 8b). f Metachert with blue amphibole (glaucophane) overgrown by green amphibole (actinolite), Mte Plum klippe (sample
GR20-89; cf. Fig. 8g). g Metabasite from the same klippe as (f), sample GR20-87. h: Jadeite-bearing mylonitic orthogneiss from the Acceglio-Longet
band west of Pelvo d’Elva (sample 41 = 73Bel13; location Fig. 10); see Additional file 2, Figs. S1, S2 for a macroscopic view of the S/C’ mylonitic
structure and more petrological details
(See figure on next page.)
Page 19 of 43 19
The Maira-Grana Allochthons
Fig. 9 (See legend on previous page.)
19 Page 20 of 43
A.Michard et al.
While the main mineralogical/textural characters of
typical samples from the MGA were reported in chap-
ter3.4 (Fig.9a–d, h), an additional study of the jadeite
orthogneiss from the basement of the Pelvo d’Elva unit
is presented in Additional file2. is rock was com-
pared (Lefèvre and Michard, 1976) to the Sapey Gneiss
of Vanoise whose age was first regarded as possibly Per-
mian but subsequently shown to be Upper Ordovician
(452 ± 5Ma; Bertrand etal., 2000).
Here we report details regarding the mineralogy and
texture of the metamorphic rocks that form the Mte
Plum klippe of the Queyras Nappe. We paid particular
attention to this tectonic element as its significance has
been debated in the early 70’s (Lemoine, 1971; Michard
and Schumacher, 1972). All samples described below,
except serpentinite and marble, were taken in the Mte
Plum scree, which offers a vast array of material, but
can be correlated to the outcrops.
Serpentinites (sample GR 20 85) are massive rocks
displaying a brecciated appearance (Fig. 11). No dis-
tinct layering, nor alternating layers with different sizes,
can be seen. A mineralogical distinction between clasts
and matrix is not apparent. We conclude that the brec-
ciation is of tectonic rather than sedimentary origin as
proposed by Michard (1967). e thin section displays
relics of either pyroxene or olivine, in a matrix of fine-
grained serpentine, chlorite and/or talc.
Metabasites (samples GR 20 86-88) are finely lami-
nated-foliated, displaying varied combinations of sodic
amphibole (“glaucophane”; Additional file 3: Fig. S1),
epidote, Fe-rich chlorite, titanite and sulfides. e
main foliation is defined by the shape fabric of blue
Fig. 10 Metamorphic mineral distribution from the literature and RSCM geothermometry of the samples studied in this work. See Fig. 3 for
location, and Table 2 for details. Samples AC01 to AC27 are shown as 1 to 27 for clarity. The TRSCM value plotted at Ussolo corresponds to the mean
results obtained from samples 39, 40a and 40b.- 1: Goffé (2002); 2: Lefèvre and Michard (1976), Schwartz (2000); 3: Henry et al. (1993), Castelli et al.
(2014); 4: Balestro et al. (1995); 5: Angiboust et al. (2012), Schwartz et al. (2013)
Page 21 of 43 19
The Maira-Grana Allochthons
Table 2 Sample locations and RSCM results (TRSCM)
Sample Locality Coordinates (degrees) Nature Formation, unit, Fig. # R2 T_RSCM(°C)
Mean SD Mean SD SE N
AC01 L Battuira 44.4274/7.2128 Calcschists (cs) Valliera meta-lmsts,
(VGU). Figures 6a, 7a0.5 0.02 416 11 3 10
AC02 L Valliera soprana 44.4255/7.2130 Quartz-schist Valliera beds (VG unit) 0.4 0.03 461 16 5 10
AC03 L Valliera-Colletto 44.4153/7.2173 Liassic lmst-cs Narbona Member (VG u.) 0.46 0.04 428 6 1 11
AC04 L P Colletto 44.4127/7.2175 Meta-siliceous lmsts Hettangian/ Lower Sinemurian (VG
unit) 0.45 0.06 441 29 8 11
AC06 M ~ 300 m NW Pta Castellar 44.4069/7.1848 Metabasite Contact QN/VGU
AC07 L ~ 700 m NNE Pta Castellar 44.4100/7.1903 Meta-siliceous lmsts with dolostone
grains Top of Mid-Upper Liassic Lmbr (VG unit) 0.56 0.6 390 °C 28 9 10
AC08 L Above Chiappi 44.4041/7.1771 Meta-calcaro-dolomitic banded, coarse
breccia Main part of “Lmb” (VG u.) Fig. 6d 0.63 0.05 358 25 6 13
AC09 M 1st bridge upstream Pradleves 44.4128/7.2652 Silvery quartz-schist Permian (?), part of CLSZ. Figure 7d 0.54 0.07 400 35 2
AC11 L
AC11 M Lottulo 44.4910/7.2301 Yellowish calcschist Anisian-Lower Ladinian (VM-S unit).
Figure 6h0.28 0.07 507
512 29
34 9
10
10
AC12 L Lower Vallone Elva 44.4980/7.1010 Dolostone with oblique cleavage Norian M. Bettone (VG u. in Val Maira).
Figure 6f0.30 0.03 506 14 4 11
AC13 M Upper Vallone Elva 44.5400/7.0801 Schistes lustrés QN above M. Bettone 0.29 509 26 5
AC14 LM 100 m W Sampeyre Pass Ctd-bearing black schists Queyras nappe 0.41 0.03 455 16 5 11
AC15 LM 300 m W Sampeyre Pass 44.5521/7.1173 Idem Queyras nappe 0.44 0.03 442 17 5 11
AC16 L Costa Cavallina 44.5464/7.1234 Lmst in calcschists (chocolate table) CLSZ 0.35 0.05 482 25 6 15
AC17 LM Costa Cavallina 44.5439/7.1294 Serpentinite/meta-rodingite?
AC18 LM East of C. Cavallina 44.5378/7.1453 Bedded metabasite
AC19 L Below S Martino 44,5160/7.1140 Schistes lustrés CLSZ 0.36 0.10 478 48 10
AC20 L Below Plum klippe 44.4260/7.2220 Schistes lustrés with lmst beds QN ? Valliera cs (?) 0.33 0.06 502 25 13
AC21 L M. Plum south slope 44.3313/7.2129 M. Plum lmst stuck over serpentinite QN klippe (?) 0.41 0.04 458 18 5 5
AC24 L Acceglio anticline
core 44.4878/7.1775 Metasedimentary intercalation; Permian
(?) Acceglio-Longet band
(AL unit) 0.36 0.05 480 25 7 11
AC26 L Valmala 44.4912/7.2304 Calcschists VPSZ 0.23 0.12 534 56 15 13
AC27 L Above Valmala Mica-schist Lower (?) Dronero unit 0.52 0.03 407 15 5 10
28 (M21-01) Battuira 44.4274/7.2128 Triassic carbonate VPSZ 0.5 0.04 414 21 5 14
30 (M21-03) Valliera soprana 44.4255/7.2130 Triassic dolostone SDSZ
31 (M21-04) San Damiano Podio 44.4938/7.2644 Quartz-mica-schist SDSZ 593 24 7 10
32 (M21-05) San Damiano Podio 44.5112/7.1513 Quartz-mica-schist SDSZ 0.23 0.1 564 39 10
33 (M21-06) Caudano-Centenero road 44.5108/7.1503 Middle Triassic dolostone Val Maira subunit
34 (M21-07) Caudano-Centenero road 44,5108/7,1503 Middle Triassic dolostone Val Maira subunit
35 (M21-08) Centenero village (from a trench) 44.5095/7.1546 Middle Triassic black shales Val Maira subunit. Figure 7c 0.43 0.03 455 9 2 13
19 Page 22 of 43
A.Michard et al.
Table 2 (continued)
Sample Locality Coordinates (degrees) Nature Formation, unit, Fig. # R2 T_RSCM(°C)
Mean SD Mean SD SE N
36 (M21-09) Centenero village (from a trench) 44.5095/7.1546 Middle Triassic black shales Val Maira subunit 0.41 0.04 455 19 6 10
37 (M21-10) Ponte Marmora 44.4906/7.0924 Liassic calcschist VG unit in Val Maira 0.39 0.05 463 24 7 10
38 (M21-11) Ponte Marmora
power station 44.4906/7.0924 Liassic calcschists VG unit in Val Maira 0.39 0.06 465 29 8 10
39 (M21-12) Above Ussolo 44.4910/7.0260 Cretaceous (?) Schistes lustrés Queyras nappe 0.58 0.01 381 8 2 13
40 (M21-13)
(a &b sam-
ples)
Above Ussolo 44.4910/7.0260 Cretaceous (?) Schistes lustrés Queyras nappe Figs. 6b, 7e 0.54 0.01 387
399 9
24 3
75
10
GR20-85 Below Mte Plum 44.4316/7.2139 Serpentinite Sole of M. Plum klippe
GR20-87 & 88 Monte Plum-Bars la Chiau 44.4320/7.2197 Metabasite Klippe of the Queyras nappe
GR20-89 Monte Plum-Bars la Chiau 44.4300/7.2246 Banded metachert Klippe of the Queyras nappe Figs. 6g, 7f
GR20-90 Monte Plum-Bars la Chiau 44.4300/7.2246 Marble Klippe of the Queyras nappe
GR20-91 Monte Plum-Bars la Chiau Banded metabasite Klippe of the Queyras nappe. Figure 7g
41 (Bel73-13) Grangie Sagneres, summit 2985
west of Pelvo d’Elva Jd-orthogneiss
(Permian?) Acceglio-Longet band (AL). Figure 7h
Unit acronyms (AL, etc.) as in Fig.3 and Table1. Other abbreviations: cs, calcschists; Lmbr: Middle Liassic breccias; lmst: limestone (protolith); u.: unit. Figures showing outcrop or thin section relative to any sample are
indicated for convenience. R2: Raman parameter proposed by Beyssac etal. (2002) to quantify graphitization processes; N: number of Raman spectra; SD: standard deviation; SE: standard error (SE = SD/N)
Page 23 of 43 19
The Maira-Grana Allochthons
amphiboles, and the alignment of tiny crystals of titan-
ite. Epidote seems to partly overgrow this foliation. Tight
folds defined by the shape fabric of amphiboles show that
the main foliation is a composite one. Another sample
of metabasite (sample GR 20 91) displays a quartz vein,
parallel to the foliation, and shows compositional layers a
few mm thick.
Metacherts (sample GR 20 89) are finely foliated and
layered rocks, the main constituents being quartz, blue
amphibole (elongated, defining the foliation, strongly
pleochroic, displaying occasionally boudinaged crys-
tals), chlorite (pale green, with low interference color,
so a magnesian variety), epidote and opaques (mostly
sulfides). e glaucophane (Additional file3: Fig. S1) is
commonly overgrown by actinolite (Fig.9f). Some lay-
ers are richer in quartz, some almost exclusively made of
quartz. White mica is occasional, and present in larger
quantities in a few quartz-rich layers, defining very thin
plates parallel to the main foliation. Microprobe analyses
show a typical phengitic composition (Si = 3.19 pfu).
e marble (sample GR 20 90) displays a foliation with
mm-sized darker spots in relief on the weathered sur-
face (fossils? albite? lawsonite?). In thin section, the spots
are recognized as calcite aggregates that do not present
diagnostic features allowing the identification of their
precursor.
To conclude, the Mte Plum outcrops offer the same
mineral assemblages as the neighboring Schistes lustrés,
consistent with their interpretation as a klippe of the
Queyras nappe.
4.2 Raman spectroscopy ofcarbonaceous material (RSCM)
e grade of graphitization of carbonaceous material
(CM) is used to evaluate maximum temperatures reached
in the sampled material using the empirically calibrated
RSCM method. is method, based on the characteri-
zation of the CM structures by Raman spectroscopy,
allows one to calculate the peak temperature in the range
200–650°C with a precision generally better than 50°C
(Beyssac etal., 2002; Lahfid etal., 2010). e peak tem-
perature (TRSCM) recorded by CM during the thermal
transformation process is insensitive to the retrograde
path of rocks or the overprint events. During the last
decades, RSCM geothermometry has been frequently
used to decipher the thermal evolution of the internal
Western Alps, particularly that of the Piemonte-Liguria
metasedimentary “Schistes Lustrés” and the Briançon-
nais s.l. (Beyssac etal., 2002; Gerber, 2008; Gabalda etal.,
2009; Plunder etal., 2012; Negro etal., 2013; Angiboust
etal., 2012, 2014; Schwartz etal., 2013; Lanari etal., 2012;
Lefeuvre etal., 2020; Manzotti etal., 2021). We applied
this empirical geothermometer to 29 samples selected
among the metasedimentary rocks (according to their
apparent richness in carbonaceous material) of the MGA
and juxtaposed units (Fig. 10). Details on the analytical
method can be found in Delchini etal. (2016).
e results are first presented in Fig.10 and Table2.
In the Val Grana unit, 9 samples yielded results between
358 ± 25 and 506 ± 13 °C, with the lowest TRSCM val-
ues observed in the southernmost part of the unit. Four
samples from Middle Triassic beds of the Val Maira-
Sampeyre unit yielded higher results, ranging between
455 ± 19 °C and 512 ± 33 °C. Significantly, the highest
TRSCM values correspond to the deepest, Anisian-Ladin-
ian part of the Val Maira subunit whereas the lowest val-
ues correspond to the uppermost, Ladinian part.
In the Queyras “Schistes lustrés, the two samples from
the most external part of the nappe yielded low TRSCM
values (381 ± 8 and 387 ± 6 °C) with respect to those
from the inner part of the nappe and the Mte Plum klippe
(five results from 442 ± 16°C to 509 ± 25°C ).
Six samples were collected in the mélange-bearing
shear zones. In the Cima Lubin shear zone, TRSCM values
of 478 ± 48°C and 400 ± 35°C were obtained in Val Maira
and Val Grana, respectively. Higher values at 593 ± 24
and 564 ± 39 characterize the quartz-schists samples
from the San Damiano shear zone. e Valmala-Piasco
shear zone only yielded discordant values at 534 ± 56 °C
and 414 ± 22°C.
Fig. 11 Serpentinite from the sole thrust of the Mte Plum
klippe: lens-shaped, unfoliated serpentinite bodies of various size
surrounded by a fine-grained serpentinite matrix showing a mylonitic
foliation (Sm), likely polyphase (S2–S3). Location: ~ 50 m to the NE of
sample AC21 in map Fig. 10 and point “sr” in cross-section Fig. 5
19 Page 24 of 43
A.Michard et al.
Fig. 12 Peak P–T metamorphic conditions from the literature (1–14) and TRSCM values (this work) along a cross-section of the southern Cottian Alps. See trace of cross-section in Figs. 3 and 10.
Caution: the two scales (P on the left, T on the right) are independent. Western part of cross-section (Briançonnais nappes west of Acceglio) after Gidon et al. (1977); central part after Michard
(1967); eastern part (Dora-Maira units) after Avigad et al. (2003). Acronyms as Fig. 3, with CC: Ceillac-Chiappera unit; C-E: Cretaceous-Eocene; MA: Marinet-Aiguilles de Mary; MB: Mte Bettone; Ol:
Oligocene (“Grès d’Annot”); QN: Queyras nappe divided into LPU1/LPU2/LPM = Liguria-Piemonte Upper1 & 2/Medium units in Herviou et al. (2021); Ro: Rouchouze; RP: Rocca Peroni; sr,: serpentinite;
St: Sautron; T-J: Triassic-Jurassic. 1: Michard et al. (2004); 2: Houfflain and Caby, 1987; Schwartz et al. (2000a); 3:Agard et al. (2001), Goffé et al. (2004); 4: Schwartz et al. (2013), Lefeuvre et al. (2020),
Herviou et al. (2021); 5: Angiboust et al. (2012); 6: Mondino (2005); 7: Henry et al. (1993); 8: Castelli et al. (2014); 9: Chopin and Schertl (1999); 10: Castelli et al. (2007); 11: Ferrando et al. (2009); 12:
Campomenosi et al. (2021); 13: Avigad et al. (2003); 14: Groppo et al. (2019)
Page 25 of 43 19
The Maira-Grana Allochthons
Additionally, a Permian schist sample from the Acceglio
anticline yielded a TRSCM value at 480 ± 25°C.
Leaving aside an isolated result from the lower Dronero
unit, the TRSCM values obtained in the present work are
all consistent with the blueschist-facies mineral assem-
blages reported in the study area (Sect.4.1).
5 Discussion
5.1 The South Cottian metamorphic wedge
e MGA and neighboring units are part of the tectonic-
metamorphic wedge of the Western Alps. In the deep
regional cross-section (Fig.6), they appear as two distinct
units separated by a major shear zone (SZ) made up of a
tectonic mélange of continental and oceanic rocks (Cima
Lubin Shear Zone). In the following, our discussion will
concern the entire wedge except for its most external
part, i.e., the Briançonnais s.str. nappes described else-
where (Gidon, etal., 1977, 1994; Lefèvre, 1982; Michard
etal., 2004). Hence, the discussion will also concern the
deepest units of the wedge, which belong to the southern
Dora-Maira massif units and include the coesite-bearing
Brossasco-Isasca unit.
5.1.1 Metamorphism
Figure12 shows the peak pressure and temperature val-
ues that characterize the tectonic units exposed in the
study area. Part of these values refer to the peak pres-
sure conditions of metamorphism and are compiled from
the literature, while the peak temperature values result
from the RSCM analyses carried out in this work (chap-
ter4.2). e latter values are shown with more detail in
a complementary diagram TRSCM/distance along the
same transect (Fig.13). Figure12 illustrates the increase
of metamorphic grade in the Permian-Mesozoic conti-
nental units derived from the Briançonnais s.l. paleogeo-
graphic domain, when going from the external classical
Briançonnais units (Sautron and Marinet-Aiguilles de
Mary: ~ 6–7 kbar, 300–340 °C) to the more internal
Ceillac-Chiappera unit (~ 10 kbar, 330°C), to the Acceg-
lio-Longet band (~ 12–14 kbar, 430–480 °C). Within
this antiformal band, we note a reasonable fit between
our TRSCM result (480 ± 25°C) and those inferred from
the mineral assemblages (430 ± 20 °C, Michard et al.,
2004; 450 ± 25 °C, Schwartz et al., 2000a). Petrological
estimates of peak P–T conditions are not available for
the Triassic-Liassic series of the Maira-Grana Alloch-
thons. However, TRSCM values from the Val Grana
unit are in the same range as those from the Acceglio-
Longet band, mainly grouped between 420 and 470°C,
with slightly higher T in the northernmost and possibly
deeper rocks (samples #38 and 12, Fig. 13). Likewise,
the Val Maira-Sampeyre unit shows TRSCM values in the
range ~ 460–510°C.
e Dora-Maira units underlying the MGA across the
San Damiano Shear Zone (Fig. 6) exhibit contrasting
P–T values that do not correlate with tectonic position
(Fig. 12). e lower Dronero unit records blueschist-
facies P–T conditions comparable to those of the Val
Maira-Sampeyre unit (Henry et al., 1993), while the
overlying upper Dronero unit exhibits eclogite-facies
assemblages (Balestro etal., 1995). Beneath the Valmala-
Piasco Shear Zone, the Rocca Solei unit and underlying
Brossasco-Isasca unit exhibit Alpine quartz-eclogite and
coesite-eclogite-facies conditions, respectively (Chopin
and Schertl, 1999; Castelli et al., 2007; Ferrando et al.,
2009; Groppo et al., 2019; Campomenosi et al., 2021).
e lowermost Sanfront-Pinerolo unit shows upper-blue-
schist to eclogite-facies conditions of equilibration (Avi-
gad etal., 2003; Groppo etal., 2019). Our TRSCM value
for sample #26 from the Valmala-Piasco Shear Zone
(534 ± 56 °C) compares favorably with the peak tem-
perature at 500–520°C inferred by Groppo etal. (2019)
for the underlying Rocca Solei eclogites. In contrast, the
available TRSCM values for the San Damiano Shear Zone
(564 ± 39°C and 593 ± 24°C) appear significantly higher
than the temperature close to 450–470°C in the lower
Dronero unit during blueschist-facies metamorphism
(Groppo etal., 2019). is apparent discrepancy cannot
be explained yet and requires complementary analyses.
e P–T conditions of metamorphism of the Pie-
monte-Liguria nappes have been thoroughly ana-
lyzed north of our study area (Agard etal., 2001, 2009;
Schwartz et al., 2013; Lefeuvre et al., 2020; Herviou
etal., 2021). Our TRSCM results for the Queyras Schistes
lustrés samples #39–40 and #13–16 (Fig. 13) compare
with those obtained further to the north by Schwartz
et al. (2013) in their Medium Temperature Blueschist
(MT-BS) unit that overlies the Acceglio-Longet Band,
and in their High Temperature Blueschist (HT-BS) unit
Fig. 13 Diagram TRSCM /distance from Chiappera (west of Acceglio
village) along the SW–NE transect of Fig. 12 (see trace in Fig. 10).
Localities from the Grana valley in italics. Small numbers in the graph
indicate the analyzed samples (see Table 2)
19 Page 26 of 43
A.Michard et al.
overlying the Monviso Complex, respectively. ese tem-
perature values are equivalent within uncertainty to the
peak metamorphic temperatures inferred by Lefeuvre
etal. (2020) and Herviou etal. (2021) for their LPU2 and
LPM units, which correspond to the MT-BS and HT-BS
units of Schwartz etal. (2013), respectively. e Monviso
Complex contrasts with the Queyras units by its higher,
eclogite-facies grade of metamorphism, bordering the
coesite stability field (550 °C, 26–27 kbar) in the Lago
Superiore subunit but reaching lower peak P–T condi-
tions in the Monviso subunit (500°C, 22–24 kbar; Angi-
boust etal., 2012; Schwartz etal., 2013). e TRSCM value
of 478 ± 48°C we obtained in the calcschists of the Cima
Lubin Shear Zone in the southern prolongation of the
Monviso Complex is consistent within uncertainty with
the values obtained by Schwartz etal. (2013) in the Mon-
viso eclogites.
5.1.2 Deformation phases: fromearly thrusting
tobackfolding
At the outcrop and thin section scales (see chapter3.4),
and in line with Schumacher (1972) and Caron et al.
(1973), we recognized three major “phases” of deforma-
tion D1 to D3 in the metamorphic units that constitute
the South Cottian wedge.
D1 produced S1 that is subparallel or parallel to litho-
logical contrasts (S0). F1 folds are rare and mostly miss-
ing. is primary foliation S0-1 is folded by F2 folds. e
D1 phase occurred under peak-pressure conditions in the
blueschist-facies MGA and overlying Queyras Schistes
lustrés (Caron et al., 1973; Schumacher, 1972). In the
underlying eclogite-facies units of southern Dora-Maira,
Henry etal. (1993) recognized an early (U)HP foliation
deformed by syn-greenschist facies folds, whereby the
correlation of these folds with the structures in our area
of investigation remains unclear.
D2 is occasionally well defined by isoclinal to tight folds
(Fig.8g), which transpose the earlier foliation S1 into the
main foliation S2. e early HP minerals are deformed
in the hinges of such F2 folds and partly transformed
into greenschist-facies minerals (Fig.9f). erefore, the
new foliation S2 developed during exhumation of the
blueschist-facies units. On a larger scale, exhumation
of high-pressure rocks is commonly seen to go hand in
hand with nappe stacking associated with buoyant uplift
and/or extrusion of nappe bodies within the subduction
channel together with erosion and collapse of the upper-
most parts of the wedge (e.g., Chemenda et al., 1996;
Malavieille etal., 1998; Bucher etal., 2004; Brun and Fac-
cenna, 2008). Notice that distinguishing F2 from subse-
quent F3 folds is often not easy (Fig.9a). In the case of
the Dora-Maira continental units, the equivalent defor-
mation phase related to exhumation is represented by
syn-greenschist facies mylonitic structures (Henry etal.,
1993; Avigad etal., 2003).
D3 in the units SW of and overlying the Dora-Maira
massif corresponds to the back-folds and -thrusts clearly
visible at the large scale (Fig. 6) and generally associ-
ated with an S3 foliation. F3 back-folds are observed at
all scales, up to kilometric (Figs.4, 8e, 8h). S3 commonly
appears as a crenulation and/or pressure solution cleav-
age associated with scarce greenschist-facies recrystal-
lizations (Fig. 9a, b, e); in incompetent lithologies it is
pervasive and associated with very large strains related
to top-ENE to –NE shearing. e S3 schistosity swings
around from a NNW-SSE strike to a WNW-ESE strike
in the northwest and southeast parts of the study area
(Figs. 3, 7), respectively. is is the result of late-stage
oroclinal bending of the southernmost Western Alps in
connection with the rotation of Corsica-Sardinia after
about 20Ma ago (e.g., Schmid etal., 2017). e D3 back-
thrusting structures are absent within the Dora-Maira
massif that underwent extensional unroofing during the
greenschist-facies D3 event. e Pradleves cargneule
zone (Fig. 5) could result from the hydraulic extrusion
(Fudral etal., 2010) of the Triassic evaporitic rocks (brec-
ciated in the sole of the Val Grana unit) between the Pra-
dleves-Rocca Caire and Valgrana D3 anticlines.
5.1.3 Timing ofthetectono‑metamorphic evolution
e available data from the literature point to a Mid-
dle-Upper Eocene age for the peak metamorphic con-
ditions in both the continental and oceanic units of the
working area. e eclogites from the Monviso complex
equilibrated at ~45 Ma (Rubatto and Hermann, 2003;
Rubatto and Angiboust, 2015) or ~51 Ma (Garber etal.,
2020), while the overlying metasedimentary unit yielded
40Ar/39Ar ages from ~60 to ~50–45 Ma (Agard et al.,
2002). e age of UHP metamorphism in the Brossasco-
Isasca unit is now established at ~35 Ma (Gebauer etal.,
1997; Rubatto and Hermann, 2001; Gauthier-Putallaz
etal., 2016; Xiong etal., 2021). e other, surrounding
Dora-Maira units, for which geochronological data have
long been lacking, show a trend of peak metamorphic
ages younging downwards in the nappe stack, from ~40
Ma to ~33 Ma, according to rutile U-Pb dating by Bonnet
etal. (2022). All the Dora-Maira units were therefore part
of the subducting Briançonnais margin beneath the Pie-
monte-Liguria oceanic material and the overlying leading
edge of the Adria plate at about 40–33 Ma. Hence, it is
clear that the Briançonnais s.l. distal margin was deeply
subducted in the Late Eocene. e oldest structural
imprints attributed to a poorly defined D1 phase could
have occurred under peak burial conditions affecting the
Briançonnais margin. In the southern Dora-Maira units,
Page 27 of 43 19
The Maira-Grana Allochthons
UHP and HP planar-linear and shear fold structures have
been preserved in eclogite boudins and escaped the later
lower-grade, greenschist-facies overprint (Henry, 1990;
Henry et al., 1993). Unfortunately this strongly parti-
tioned deformation did not yield any kinematic informa-
tion about the earliest movements leading to exhumation.
Nappe emplacement together with folding structures
attributed to the D2 phase at a smaller scale, are already
associated with retrogression of the HP-LT mineral
assemblages to greenschist-facies minerals, retrogression
that will continue during the subsequent D3 phase asso-
ciated with backfolding and extensional unroofing. In
other words, building of the tectono-metamorphic wedge
mostly occurred at crustal depths although it initiated
within the subduction channel (see next subsection) dur-
ing the D1 phase. Radiometric dating of the D2 and D3
phases are not available for our working area. However,
radiometric data for D2 (43–39 Ma) are available in an
area further to the north along the ECORS-CROP tran-
sect (Villa etal., 2014) suggesting that D2 followed soon
after peak metamorphism, which is also likely to be the
case for our study area.
In the Gran Paradiso massif, the northern equivalent
of the Dora-Maira units (Fig.1A), 42–41 Ma may be so
far the best age bracket available regarding peak meta-
morphism (Manzotti etal., 2018). In the corresponding
transect, Bucher et al. (2004) estimated D3 backfolding
to have occurred between 35 and 31 Ma, an estimate
that most likely also applies to the Dora-Maira transect,
since an along-strike change in the age of this event is
hard to imagine. Exhumation and cooling below ~250°C
took place at around 30 Ma based on few on-site zircon
fission-track ages available in the Dora-Maira UHP unit
(Gebauer etal., 1997) and those from the Gran Paradiso
massif (Hurford and Hunziker, 1989; Malusà etal., 2005;
Rosenberg et al., 2021). Hence, 30 Ma would approxi-
mately date the crossing of the ductile-brittle transition,
which would mark the end of the D3 phase. On-site apa-
tite fission-track ages along a N-S transect across the
Dora-Maira units yielded ages between 27 and 13 Ma
(Beucher etal., 2012). An additional constraint regarding
the timing of exhumation around 30 Ma is that (i) during
the Early Oligocene, the lowermost beds of the Piemonte
Basin started to unconformably overlie the blueschist-
facies Briançonnais units and the oceanic units of the
Ligurian Alps and Northern Apennine junction (Lorenz,
1986; Mosca etal., 2010; Molli etal., 2010; Maino etal.,
2013; Marroni etal., 2017; Piana etal., 2021); (ii) Jourdan
etal. (2013) report that short-lived fast erosional exhu-
mation occurred at about 30–28 Ma ago according to an
analysis of sedimentary deposits in the fore and retro-
deeps of the Alps. In summary, exhumation of the wedge
units close to the surface was completed at around 30 Ma
ago.
5.1.4 Exhumation scenario
e mechanisms that permit (U)HP-LT continental and/
or oceanic units previously subducted down to man-
tle depth to be subsequently exhumed up to the surface
without major T increase has been repeatedly explored
since the late 70’s (see Guillot etal., 2009 for review). By
the earliest 90’s, the discussion around the exhumation of
the Dora-Maira UHP and HP eclogitic units launched by
Chopin (1987) concentrated on two end-member mecha-
nisms, (i) extrusion in the “subduction zone” followed by
corner flow” in the wedge (Henry, 1990; Michard etal.,
1993), and (ii) “metamorphic core complex” mechanism
of extensional thinning of a previously thickened crust
(Avigad, 1992). According to the latter author, the contact
of the Brossasco-Isasca UHP unit onto the Sanfront-Pin-
erolo blueschist-facies unit represents a (former) thrust,
unlike the various contacts of the quartz-eclogite unit
(Rocca Solei or Dronero; see Fig.12) over the Brossasco-
Isasca unit, considered as low-angle normal faults since
they omit parts of the intermediate metamorphic facies.
Fig. 14 Interpretation of the origin of the southern Cottian Alps
metamorphic units at the expense of the subducted Briançonnais
distal margin. The half-arrows feature the locus of detachment where
each unit was sampled. Acronyms as follows: A: Acceglio-Longet; BI:
Brossasco-Isasca; CC: Ceillac-Chiappera; MA: Marinet; MV: Monviso
complex.RS: Rocca Solei; SP: Sanfront-Pinerolo; SL: Schistes lustrés
(Queyras nappe); VG: Val Grana; VM-S: Val Maira-Sampeyre; The
subduction channel is left “open” (white) for clarity; it is supposed
to contain a mélange of Schistes lustrés-type metasediments and
meta-ophiolites (mostly serpentinites) from the Piemonte-Liguria
Ocean. The ophiolitic Schistes lustrés are divided into a lower-plate
accretionary prism and an upper-plate system (e.g., Chenaillet;
Schmid et al., 2017; Ballèvre et al., 2020; Agard, 2021). The occurrence
of a southern prolongation of the Sesia-Dent Blanche unit in the
Cottian transect is dubious but shown here to better suggest the
correlation with this northern transect. Late Eocene–Oligocene
emplacement of Helminthoid flysch nappes over External
Briançonnais after Kerckhove (1963) and Kerckhove et al. (1984).
Length and thickness of the Briançonnais crust inspired by the
natural examples (e.g., Chenin et al., 2017). Approximate position of
isotherms 400 °C and 600 °C after Zhao et al. (2020)
19 Page 28 of 43
A.Michard et al.
Unfortunately, no kinematic marker can be directly
linked to thrusting under UHP conditions. All the above-
mentioned contacts have been similarly overprinted by
more steeply inclined top-SW normal fault zones exhib-
iting greenschist-facies shear bands (Henry, 1990; Henry
etal., 1993).
In this discussion, we take advantage of the more
recent papers dedicated to exhumation processes in
the Alpine belt (e.g., Agard etal., 2002; Levi etal., 2007;
Guillot etal., 2009; de Sigoyer etal., 2004; Huet et al.,
2009; Burov etal., 2014; Gross etal., 2020; Agard, 2021;
Candioti etal., 2021). ereby we adopt the concept of
exhumation in three steps, largely in line with Michard
etal. (2004). A first step takes place within the “subduc-
tion channel” (well-described in Guillot etal., 2009; this
name replaces that of “subduction zone” used in Michard
etal., 1993). is is followed by a second step involving
nappe stacking and nappe refolding, which forms the col-
lisional wedge. e third step is the extensional collapse
of the wedge, which starts while the wedge is still grow-
ing and is accompanied by erosion. Figure14 illustrates
a potential scenario for the Maira-Grana Allochthons
and adjoining units of the Briançonnais s.l. distal margin.
ey were deeply encroached in the subduction beneath
Adria during the first step, and this was followed by the
transition towards a second step leading to nappe stack-
ing. In this qualitative model we assume that the Maira-
Sampeyre and Val Grana Allochthons (VM-S + VG
units) detached from their Variscan basement and were
temporarily accreted to the hangingwall of the subduc-
tion channel while the Briançonnais crust continued sub-
ducting down to greater depth, giving birth to the high-P
metamorphism of the Dora-Maira basement units. is
hypothesis offers an explanation for the widespread lack
of Triassic metasediments above the Dora-Maira units,
except for scarce, minor quartzite and dolomite slivers
along contacts between basement units (e.g., Valmala-
Piasco shear zone) or linked to the Sanfront-Pinerolo
unit (Sanfront area, Fig.3).
e tectonic sketch featured in Fig.14 represents but
one possible interpretation of the structure of the Brian-
çonnais s.l. distal margin during the transient stage of an
early step in the exhumation of its detached units. Admit-
tedly it is not the only possible interpretation. For exam-
ple, Butler (2013) and Ballèvre etal. (2020) proposed that
the distal part of the Briançonnais margin involved crus-
tal boudinage resulting in at least one extensional alloch-
thon (future Dora-Maira units) separated from the main
part of the marginal crust by exhumed serpentinized
mantle rocks. A similar pre-orogenic setting was also
suggested by Méresse etal. (2012) for the Corsica tran-
sect, based on the occurrence of “mixed” metasedimen-
tary breccias displaying both oceanic and continental
clasts. In the Western Alps, “mixed” breccias have been
described by Dumont et al. (1984) in the Prafauchier
sequence (Upper Jurassic?) above the “Prepiemonte”
Rochebrune unit or next to it (Barféty etal., 1995). Simi-
lar breccias could occur in the uppermost beds of the Val
Grana unit (Valliera beds) west of Mte Plum (Michard,
1967, p. 241). However, we consider the extensional dis-
ruption of continental allochthons like those observed at
the toe of the Iberian margin (e.g., Sutra and Manatschal,
2012, their Fig.3B) as plausible, but unproven in the case
of the south Cottian transect.
5.2 Restoring theBriançonnais s.l. distal margin
In the previous section, we presented a schematic sce-
nario of the main units that have been sampled and
exhumed on top of the still subducting Briançonnais
distal margin (Fig.14). Now we try to restore the origi-
nal layout of the basement and cover continental units
from the Triassic to the Late Cretaceous, i.e., during the
extensional evolution of the Briançonnais passive margin.
However, it is first necessary to precisely define the vari-
ous tectonic units representative of this margin.
5.2.1 Relationships betweentheMaira‑Sampeyre andVal
Grana units
We defined in the MGA the Val Grana unit on top and
the Val Maira-Sampeyre unit below (Fig.5). ey are
separated from each other by the thick and continu-
ous Cima Lubin Shear Zone (CLSZ), which includes
a mélange of ophiolite lenses and continental schists
analogous to those of the Dronero unit. Moreover, the
TRSCM cluster around 420-470°C in the Val Grana unit,
and around 460-510 °C in the Val Maira unit (Fig.13).
is difference of ~50°C is likely beyond error and sug-
gests a difference of burial ranging between 4 and 6
km (assuming a geotherm between 10°C and 8°C/km,
currently accepted in the area; see, e.g., Michard etal.,
2004; Groppo etal., 2019; Angiboust and Glodny, 2020;
Agard, 2021) between these units during their blues-
chist-facies metamorphism. e Val Maira-Sampeyre
unit itself overlies the Dronero unit across the interven-
ing San Damiano Shear Zone (SDSZ) that also includes
ophiolites similar to the CLZS. We may infer that the
Val Grana and Val Maira-Sampeyre units have been
located a few kilometers away from each other along
the subducting Briançonnais distal margin before being
detached and thrusted with ophiolites and basement
slivers of the mélanges above and below (Fig.14). e
sedimentary sequence of the Val Maira-Sampeyre unit
encompasses the Lower and Middle Triassic whereas
that of the Val Grana unit includes Middle and Upper
Triassic to Lower and possibly Middle-Upper Jurassic
Page 29 of 43 19
The Maira-Grana Allochthons
sequences (Michard, 1967; Additional file 1: Fig. S1).
erefore, it is tempting to propose that the Val Grana
sequence could correspond to the upper part of the
incomplete Val Maira-Sampeyre sedimentary sequence.
In this perspective, we may propose that the Val Grana
unit detached from the Val Maira-Sampeyre unit and
ceased subducting by underplating at a depth ~50 km
while the truncated lower unit kept subducting for
some kilometers more. Detachment of the Val Grana
unit would have occurred on some Ladinian tuffite
beds.
Alternatively, we could hypothesize that the Val
Grana unit detached from the Val Maira-Sampeyre unit
across a low-angle normal fault during the late exten-
sional thinning of the Briançonnais distal margin (Late
Cretaceous-Paleocene; chapter 5.2.3). Such a normal
fault could be compared with the normal fault (sub-
sequently inverted) that separated the Val Maira sub-
unit from its former Sampeyre base (Fig.4). However,
significant Upper Cretaceous-Paleocene breccias that
could have characterized such faults have not been
observed yet, neither in the Val Maira sub-unit nor in
the Giulian-Sea Bianca unit, its northern equivalent
(Fig.3; Balestro etal., 2011).
5.2.2 Early Jurassic rifting: theVal Grana record
e Val Grana sequence is well known since many years
for its chaotic to bedded breccias partly dated by Sine-
murian and Pliensbachian ammonites (Franchi, 1898;
Sturani, 1961; Michard and Sturani, 1963; Ellenberger
et al., 1964; Michard, 1967). erefore, this sequence
clearly compares with the “Prepiemonte” units repeat-
edly described since the seminal works of Ellenberger
and Lem