Exhumation controlled by transcurrent tectonics: the Argentera–Mercantour massif (SW Alps)

Guillaume Sanchez,1 Yann Rolland,1 Marc Jolivet,2 Stephanie Brichau,3 Michel Corsini1 andAndrew Carter4

1GEOAZUR, UMR 6526, Universite de Nice Sophia-Antipolis, 28 Av de Valrose, BP 2135, 06108 Nice, France; 2GEOSCIENCES Rennes,

UMR CNRS 6118, Universite de Rennes 1, Batiment 15, Campus de Beaulieu, CS 74205, F-35042 Rennes Cedex, France; 3LMTG, UMR

5563, UR 154 CNRS, Universite Paul-Sabatier IRD Observatoire Midi-Pyrenees, 14, av. Edouard Belin, 31400 Toulouse, France; 4School of

Earth Sciences, Birkbeck College, Malet Street, London WC1E 7HX, UK

Introduction

The combination of low-temperaturethermochronometers provides con-straints on the thermal and exhuma-tion histories in mountain ranges.Over the last decade, development ofthe (U-Th) ⁄He isotopic technique onapatite (AHe) has allowed the latestages of exhumation through shallowcrustal levels to be constrained(T < 70 �C; Farley, 2000). This meth-od is particularly sensitive to processessuch as brittle tectonics and erosion(Ring et al., 1999; Willett, 1999;Molnar, 2004).In the Alpine orogen, large com-

bined apatite fission track (AFT) and(U-Th) ⁄He databases are now mostlyavailable in central, north-westerndomains and in the Ligurian Alps(Bistacchi et al., 2000; Foeken et al.,2003; Fugenschuh and Schmid, 2003;Bertotti et al., 2006; Tricart et al.,2007; Glotzbach et al., 2008; Reineck-er et al., 2008; Vernon et al., 2009).However, only very scarce (U-Th) ⁄Heages have been obtained from thesouthern branch of the externalAlpine arc, restricted to the junction

between the Western Alps and theLigurian basin. Consequently, thisarea is less well documented in termsof final cooling and exhumation his-tories than the northern part of thebelt. Here, we present new AFT andAHe data for the SW Alps, specificallythe Argentera–Mercantour ExternalCrystalline Massif (ECM). These newdata combined with structural con-straints show Neogene ongoing exhu-mation related to transcurrenttectonics in the SW Alps.

Tectonic setting

The Alpine belt results from Europe–Adria convergence since the Creta-ceous (Coward and Dietrich, 1989;Vialon et al., 1989). After a phase ofoceanic subduction from the Late Cre-taceous (c. 96 Ma) to the Eocene(45 Ma) (Dal Piaz et al., 1972, 2003;Agard et al., 2002), continental sub-duction and underthrusting of theEuropean passive continental marginbelow the Adriatic plate induced theformation of an orogenic wedgebounded by the Penninic Front, duringthe Lower Oligocene (Tricart, 1984;Ceriani et al., 2001; Simon-Labricet al., 2009). Following this stage ofrapid nappe stacking, continuousdeformation of the European marginwas driven by large dextral strike-slipfault systems. The Insubric Linemainly accommodated the oblique

indentation of the Adria microplate(Fig. 1; Vialon et al., 1989; Ciancale-oni andMarquer, 2008) and drasticallymodified the geometry of the internaldomain, which was subjected to exten-sional and transtensional deformation(e.g. Tricart et al., 2006). The recent tocurrent tectonic evolution of the Alpsis debated. Three main models areproposed: (i) westward indentation ofAdria (e.g. Tapponnier, 1977), (ii)anticlockwise rotation of Adria (e.g.Vialon et al., 1989), or (iii) gravita-tional collapse of previously thickenedcrust (e.g. Sue et al., 2007).Specifically, in the area studied

(Figs 1 and 2), the Argentera–Mer-cantour ECM (Malaroda et al., 1970)has been cross-cut by N140�E dextralshear zones since the early Miocene(c. 22 Ma; Corsini et al., 2004). Thesemylonitic shear zones were reactivatedin a brittle regime (Baietto et al.,2009) and exhumed from 10 to15 km deep during the last 6 Ma(Bigot-Cormier et al., 2006). Recentand ongoing right-lateral faultingalong the southern branch of thisfault system, the Jausiers–Tinee fault(JTF on Fig. 2), is evidenced by (i)metre-scale offsets observed alongriver drainage distributaries andHolocene glacial polished surfaces(Sanchez et al., 2010a,b) and (ii)current seismicity, which is predomi-nantly of strike-slip character, asshown by focal mechanisms aligned

ABSTRACT

New apatite fission track (AFT) and (U-Th) ⁄ He ages from theArgentera–Mercantour External Crystalline Massif of the SW Alpsdocument an exhumation history along transcurrent dextralfaults. Significant AFT age differences across the strikes of themain faults have been obtained from the NE (12.9 Ma) to the SW(5.2 Ma) and are linked to vertical uplifts along these right-lateral transpressional structures. Such segmentation is notobserved in AHe ages. AHe ages have preserved younger ages(4–5 Ma) on the eastern hangingwall side of the High Durance

extensional system. Such an exhumation history is interpreted toresult from a transition of transpressional to transtensionalregimes at 8–5 Ma during continuing Adria–Europe convergence.These data show that recent extensional deformation in the SWAlps can be related to the development of a transtensionaldomain in the Ubaye–Embrunais depression linked to ongoingstrike-slip deformation along the Argentera–Mercantour.

Terra Nova, 23, 116–126, 2011

Correspondence: Guillaume Sanchez,

GEOAZUR, UMR 6526, Universite de

Nice Sophia-Antipolis, 28 Av de Valrose,

BP 2135, 06108 Nice, France. Tel.:

+33 492 07 68 05; fax: +33 492 07 68

16; e-mail: guillaume.sanchez@unice.fr

116 � 2011 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2011.00991.x

on the JTF fault (Jenatton et al.,2007). This dextral active JTF faultand the potentially active Serenne–Bersezio fault merge into the activeHigh Durance extensional system inthe NW part of the Argentera–Mer-cantour ECM (Fig. 2; Sue and Tri-cart, 2003; Champagnac et al., 2006;Sanchez et al., 2010b). At the scale ofthe SW Alps, this active fault networkdefines a zone of transtensional basindepression (Ubaye–Embrunais zone),on the sides of which the high-eleva-tion Pelvoux and Argentera–Mercan-tour ECM are exhumed (Sanchezet al., 2010b). Further to the SE, theseN140�E dextral faults link to E–Wthrusts in a transpressional domain

along the SE boundary of the Argen-tera–Mercantour ECM and connectto the Saorge–Taggia fault (Baiettoet al., 2009; Sanchez et al., 2010b).

Exhumation history

Recent AFT and AHe ages obtainedin the other Western and Central AlpsECM range between 2.2 and 10.5 Maand 1.7 and 6.4 Ma, respectively(Fugenschuh and Schmid, 2003; Tri-cart et al., 2007; Glotzbach et al.,2008; Reinecker et al., 2008; Vernonet al., 2009). These young ages ofdenudation, as well as the high relativepresent-day elevation of the ECM,indicate recent mountain-building

processes along the Alpine Externalarc. This has been mostly attributed totectonic processes such as thrustingbelow the ECM (Leloup et al., 2005)and extensional or dextral transpres-sive reactivation of the Penninic FrontalThrust (Fugenschuh and Schmid,2003; Rolland et al., 2007; Tricartet al., 2007; Glotzbach et al., 2008).Recently, climatic processes inducingvariations in denudation rate in thelast 5 Ma (Cederbom et al., 2004;Champagnac et al., 2007, 2008; Ver-non et al., 2008) have also beeninvoked, ignoring the fact that someparts of the orogen underwent slowand steady exhumation during the last15 Ma (Bertotti et al., 2006).

Fig. 1 Geological map of theWestern Alps, modified after Bigi et al. (1990) and Polino et al. (1990). The External zones comprise: 1,TheDauphinois zone, which is limited by the Penninic Frontal Thrust (PFT), the Subalpine Frontal Thrust (Digne–Castellane, Nice)and the Jura Frontal Thrust. It comprises (1a) external crystalline massifs formed during the Variscan period and (1b) theirMesozoic(Trias–Cretaceous) sedimentary cover; and 2, the Upper Cretaceous Helminthoid flysch. The internal zones comprise: 3, theBrianconnais and Piemontais zones, which are made of metamorphic rocks from (3a) the European passive continental margin and(3b) theTethyan oceanic domain; 4, theAustro-Alpine units, includingmainly theDentBlanche nappe (DB) and the Sesia Lanzo zone(SL), which represent the Adriatic continental margin; and 5, Oligocene to Quaternary molasse basins in the Alps periphery. Blackandwhite dashed lines represent themain active fault system; in the SWpart of thewesternAlpine arc they represent principal faults inthe Argentera–Mercantour ECM. External crystalline massifs: Mt Bl., Mont Blanc; Arg., Argentera–Mercantour. InternalCrystallineMassifs: DM,Dora-Maira; GP,Grand Paradis;MR:MonteRosa; PFT: Penninic Frontal Thrust; Ca: Canavese line. Theblack rectangle is the study area located in Fig. 2. Projection latitude ⁄ longitude coordinates according to the Lambert II extendedsystem.

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� 2011 Blackwell Publishing Ltd 117

Fig.3

Pliocene

MioceneMesozoic and Paleogene Permo-TriasExternal Cristallin massifInternal Cristallin massif

Geological Formations

Structural Features

Strike slip faultNormal faultThrust faultPenninic Frontal Thrust

HDF

SFVTF

STF

PFT

BF

JTF

MDF

PFEF-GVF

866 800 906 200 945 600 985 000 1 024 400

2 009 400

1 970 000

1 930 600

1 891 200 F

FF

10.3 – 7.714.3 – 5.7

AFT (Ma)

Literature data

AHe(i) 14.7 – 3.6

(i) 13.1 – 27.3

Dora-Maira

Viso

(iii) 8.6

Ophiolites

(ii)

RF

VF

PLF

Fig. 2 Structural map of the recent fault network in the SW Alps (Sanchez et al., 2010a,b). The black dashed rectangle is the studyarea located in Fig. 3. Black dashed ellipses and related dates refer to AFT (bold) and AHe (italic) data quoted from (i) Tricart et al.(2007), (ii) Foeken et al. (2003) and (iii) Schwartz et al. (2007). BF: Bersezio fault; PFEF-GVF: Pont du Fosse-Eychauda–GrandVallon fault; HDF: High Durance fault; JTF: Jausiers–Tinee fault; MDF: Middle Durance fault; MSBF: Monaco–Sospel–Breilfault; PFT: Penninic Frontal Thrust; PLF: Peille–Laghet fault; RF: Rouaine fault; SF: Serenne fault; STF: Saorge–Taggia fault;VF: Vesubie fault; VTF: Valletta fault. Projection latitude ⁄ longitude coordinates according to the Lambert II extended system.

Table 1 AFT data.

Sample Altitude (m) Lithology N qd · 105 cm)2 qs · 105 cm)2 qi · 105 cm)2 U (ppm) P (v2) % Central age (±1r) (Ma)

Northern transect

CF.02 2690 Sandstone 18 12.20 (12073) 1.89 (104) 60.56 (3337) 55.23 97.8 6.1 ± 0.6

CF.04 2435 Sandstone 16 12.17 (12073) 1.35 (63) 45.69 (2129) 41.92 99.9 5.8 ± 0.7

CF.05 2755 Gneiss 15 12.14 (12073) 1.32 (84) 29.87 (1903) 28.32 98.5 8.6 ± 1.0

CF.07 2568 Gneiss 24 12.11 (12073) 0.49 (41) 18.11 (1525) 17.05 100 5.2 ± 0.8

CF.09 2301 Gneiss 19 12.09 (12073) 1.10 (86) 33.61 (2625) 31.52 90.6 6.4 ± 0.7

CF.11 2042 Gneiss 20 12.06 (12073) 0.90 (117) 32.51 (4213) 31.02 84.6 5.4 ± 0.5

CF.13 2281 Gneiss 18 12.03 (12073) 1.60 (98) 39.71 (2430) 41.34 96.4 7.8 ± 0.8

CF.15 2053 Gneiss 23 12.00 (12073) 0.88 (64) 26.23 (1902) 26.88 100 6.5 ± 0.8

CF.17 1724 Gneiss 17 27.80 (27798) 0.65 (41) 30.14 (1905) 12.92 100 9.6 ± 1.5

CF.19 1494 Gneiss 13 27.78 (27798) 0.86 (61) 36.85 (2620) 15.71 91.1 10.4 ± 1.4

Southern transect

CF.22 1364 Gneiss 19 27.42 (27798) 1.21 (87) 41.46 (2977) 17.25 99.9 12.9 ± 1.4

CF.28 2030 Micaschiste 18 27.80 (27798) 1.37 (58) 69.30 (2931) 29.06 100 8.9 ± 1.2

CF.35 951 Gneiss 35 27.80 (27798) 0.78 (49) 39.47 (2467) 16.05 99.7 8.9 ± 1.3

N is the number of grains analysed. qd is the CN5 dosimeter track density with the number of tracks counted in brackets. qs and qi are sample spontaneous and

induced track densities, respectively, with the number of tracks counted in brackets. U is the calculated mean 238U content of the sample. P(v2) is the probability of

chi-squared for v degrees of freedom. Apatite grains were separated using conventional density and magnetic techniques. The apatite samples for FT analysis were

irradiated in the FRM 11 thermal neutron facility at the University of Munich, Germany. FT were counted on an optical Zeiss microscope, using a magnification of

1250 under dry objectives at the University of Montpellier and Birkbeck College, London. Apatite fission track (AFT) ages were obtained using the standard external

detector method and the zeta calibration approach (f = 322 ± 5), obtained on Durango, Fish Canyon and Mont Dromedary apatite standards (Hurford, 1990; Hurford

and Green, 1983). AFT ages given in this table and the text are central ages; errors are quoted at ±1r. Significantly older AFT ages are obtained in this article than by

Bogdanoff et al. (2000). Those authors used the pioneer �method of populations�, which may explain this discrepancy (see Gallagher et al., 1998 for further details

concerning the limitations of this method). Their data are not considered in our interpretation because of this disagreement. Furthermore, confined track lengths

cannot be measured because of the lack of fission tracks due to young FT ages and on occasion low U concentration.

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118 � 2011 Blackwell Publishing Ltd

Table 2 AHe data.

Sample

no.

Alt.

(m) Lithology

He

(nmol g)1)

U

(p.p.m.)

Th

(p.p.m.) FT

He raw

age (Ma)

He corr.

age (Ma)

Error

Abs. SD Comments

Northern transect

CF-05 2755 Gneiss 0.419 22.84 2.05 0.80 3.3 4.1 0.3

0.412 22.38 2.19 0.78 3.3 4.2 0.3

0.405 25.87 1.91 0.79 2.8 3.6 0.3

0.160 8.58 3.11 0.76 3.2 4.2 0.3

Mean 0.349 19.92 2.32 0.78 3.2 4.0 0.1 0.3

CF-07 2568 Gneiss 0.396 16.25 3.19 0.72 4.3 5.9 0.4

0.101 4.18 1.34 0.73 4.2 5.7 0.4

0.118 5.79 1.10 0.77 3.6 4.6 0.3

Mean 0.205 8.74 1.87 0.74 4.0 5.4 0.2 0.7

CF-09 2301 Gneiss 0.373 21.05 1.26 0.76 3.2 4.2 0.3

2.087 20.82 2.22 0.76 18.0 23.6 1.7 i

0.544 11.27 1.03 0.82 8.7 10.6 0.7

Mean 0.459 16.16 1.15 0.79 6.0 7.4 0.4 4.5

CF-11 2042 Gneiss 0.137 11.82 2.84 0.75 2.0 2.7 0.2

0.111 9.29 1.56 0.79 2.1 2.7 0.2

0.201 14.47 2.17 0.77 2.5 3.2 0.2

Mean 0.149 11.86 2.19 0.77 2.2 2.8 0.1 0.3

CF-13 2281 Gneiss 0.228 14.02 0.88 0.75 2.9 3.9 0.3

0.121 7.34 1.14 0.73 2.9 4.0 0.3

0.174 8.84 1.65 0.70 3.5 4.9 0.3

Mean 0.174 10.07 1.22 0.73 3.1 4.3 0.2 0.6

CF-15 2053 Gneiss 0.186 13.03 2.25 0.69 2.5 3.3 0.2

0.067 6.27 0.44 0.71 1.9 2.4 0.2

0.113 8.23 0.56 0.78 2.5 3.1 0.2

Mean 0.122 9.17 1.08 0.72 2.3 2.9 0.1 0.5

CF-17 1724 Gneiss 0.251 14.56 2.06 0.75 3.1 4.1 0.3

0.360 18.66 2.79 0.72 3.4 4.8 0.3

0.393 18.61 2.70 0.77 3.8 4.9 0.3

Mean 0.335 17.28 2.52 0.75 3.4 4.6 0.2 0.4

CF-19 1494 Gneiss 0.450 19.96 4.62 0.78 3.9 5.0 0.4

0.506 25.27 6.53 0.77 3.5 4.5 0.3

0.797 17.91 6.42 0.72 7.5 10.4 0.7 i

Mean 0.478 22.62 5.58 0.78 3.7 4.8 0.2 0.4

Southern transect

CF-22 1364 Gneiss 0.670 20.90 2.35 0.79 5.7 7.3 0.5

1.244 41.59 3.03 0.76 5.4 7.1 0.5

0.702 19.25 2.53 0.75 6.5 8.7 0.6

Mean 0.872 27.25 2.64 0.77 5.9 7.7 0.3 0.9

CF-30 1500 Micaschiste 0.648 14.09 2.65 0.76 8.1 10.7 0.7

0.843 25.62 4.72 0.73 5.8 8.0 0.6

0.822 18.69 2.77 0.74 7.8 10.5 0.7

Mean 0.771 19.46 3.38 0.74 7.2 9.7 0.4 1.5

CF-32 1205 Micaschiste 0.587 8.63 1.62 0.81 12.0 14.7 1.0 i

0.410 10.00 2.00 0.77 7.2 9.3 0.7

0.189 4.86 1.49 0.79 6.7 8.4 0.6

Mean 0.299 7.43 1.75 0.78 6.9 8.9 0.4 0.6

CF-38 2127 Gneiss 0.87 21.54 3.75 0.77 7.1 9.3 0.7

0.56 13.60 2.08 0.80 7.3 9.2 0.6

0.49 13.08 2.79 0.79 6.5 8.3 0.6

Mean 0.639 16.07 2.87 0.78 7.0 8.9 0.4 0.6

CF-40 1927 Gneiss 1.214 31.73 3.56 0.76 6.9 9.0 0.6

1.518 46.61 3.04 0.72 5.9 8.2 0.6

1.366 34.86 1.52 0.77 7.1 9.2 0.6

Mean 1.366 37.73 2.71 0.75 6.6 8.8 0.4 0.6

Measurements were made by degassing apatite multigrain (3 or 4) aliquots through laser heating and evaluating 4He on a quadrupole mass spectrometer at the

University of Kansas, IGL. Grains were retrieved, dissolved in HNO3, spiked with 230Th and 235U and analysed for U and Th by ICP-MS at the University and Birkbeck

College, London, Thermochronometry Research Group. For the raw AHe ages, an alpha-ejection correction was applied based on measured grain dimensions (Farley

et al., 1996) using the procedure of Gautheron et al. (2006). The estimated analytical uncertainty for AHe ages based on age standards is about 7% for Durango

apatite (2r). These are the default uncertainty values used for a sample unless the standard deviation from the sample replicate ages is higher in which case the latter

is used. Ages in italics are anomalous owing to some inclusions (see column ‘Comments’) and are not taken into account for the AHe ages.

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At the scale of the SW Alps, thePelvoux and Argentera–MercantourECM underwent similar exhumationpaths on both sides of the Ubaye–Embrunais transtensional domain. InthePelvouxECM,AFTagesof5–6 Maare interpreted as hangingwall denuda-tion during extensional reactivation ofthe Penninic Frontal Thrust (Tricartet al., 2007). In theArgentera–Mercan-tour ECM, preliminary AFT dating attheNWboundaryof themassif suggestsuplift initiation at c. 6 Ma (Bogdanoff

et al., 2000; Bigot-Cormier et al.,2006). In the ECM cover close tothe Argentera–Mercantour, Labaumeet al. (2008) documented a full reset ofAFT ages at c. 7–8 Ma.

AFT and AHe thermochronologydating

Sampling and methods

In order to document the late exhu-mation history of the central and NW

edge of the Argentera–MercantourECM and the influence of the develop-ment of the transtensional system, weundertook low-temperature thermo-chronology using AFT and AHe cou-ples on the same samples (Gallagheret al., 1998; Ehlers and Farley, 2003).These techniques are well suited toconstraining exhumation paths in theuppermost crust because of their lowclosing temperatures (AFT: �110–60 �C, Green et al., 1986; AHe:�80–40 �C, Wolf et al., 1998).

Undifferentiated crystalline basement of the Argentera–Mercantour massif

Geological formation

Argentera–Mercantour sedimentary cover

Internal zone

Structural features

Major strike slip fault

Minor strike slip andnormal fault

Normal faultPenninic Frontal Thrust

River

954 700

969 900

98 500

977 500

1 923 800

947 100

1 954 300

1 931 400

962 300

1 939 000

1 916 400

2759

3031

2832

2712

3032

2860

2772

2274

850

1130

1700

1213

1600

CIME DE LA BONETTE

MONT AIGA

MONT TENIBRE

CIME DE COLLE LONGUE

LE GERBIER

TETE DE SIGURET

L'ENCLAUSE

Berzézio

Jausiers

St Etienne de Tinée

Le Pra

Isola

Samples (this study)AFT ages (Ma)AHe ages (Ma)

CF.096.4 ± 0.77.4 ± 0.4

Samples (Labaume et al., 2008)AFT ages (Ma)

MOb8.2 ± 0.9

AFT and AHe ages

BFVTF

JTF

1 946 600

PFT

CF.045.8 ± 0.7

CF.026.1 ± 0.6

CF.288.9 ± 1.2

CF.358.9 ± 1.3

CF.2212.9 ± 1.47.7± 0.3

CF.388.9 ± 0.4

CF.309.7 ± 0.4CF.32

8.9 ± 0.4

CF.408.8 ± 0.4

CF.1910.4 ± 1.44.8 ± 0.2

CF.179.6 ± 1.54.6 ± 0.2 CF.15

6.5 ± 0.82.9 ± 0.1

CF.137.8 ± 0.84.3 ± 0.2

CF.058.6 ± 1.04.0 ± 0.1

CF.075.2 ± 0.85.4 ± 0.2

CF.096.4 ± 0.77.4 ± 0.4

CF.115.4 ± 0.52.8 ± 0.1

BEt10.0 ± 1.2

MOb8.2 ± 0.9

RTF7.4 ± 0.9

Fig. 3 Fault network of the NW Argentera, with AFT and AHe results obtained in this study and from Labaume et al. (2008)(grey boxes). BF: Bersezio fault; JTF: Jausiers–Tinee fault; VTF: Valletta fault. Projection latitude ⁄ longitude coordinatesaccording to the Lambert II extended system.

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A total of 17 samples were col-lected along two transects cross-cutting the main NW–SE faults(Fig. 3): (i) the northern (Pra-Ferri-ere) transect was sampled in thehangingwall of the west-dippingCamp des Fourches normal fault atthe rim of the Argentera–MercantourECM; and (ii) the southern (Isola-Vinadio) transect was sampled in thecore of the massif to compare theexhumation history far from thetranstensional system. Both transectsare perpendicular to the mainNW–SE dextral faults located in thewestern part of the massif. Sampledlithologies include metamorphic andmagmatic rocks such as micaschists,gneisses, granites from the VariscanArgentera–Mercantour ECM base-ment and sandstones from the sedi-mentary cover.Technical details are described in

Tables 1 and 2. In each sample, AFTages were calculated using the Track-key software (Dunkl, 2002). EachAHe age typically comprises three orfour replicates, the mean of which isreported in Table 2.

Results

Apatite fission track ages from base-ment rocks range from 5.2 ± 0.8 to12.9 ± 1.4 Ma over the whole massif(Fig. 3). For sedimentary samples,AFT ages are in the same range(6.1 ± 0.6 and 5.8 ± 0.7) andclearly younger than their deposi-tional age. For each of the twotransects, AFT data show a generalgradient across the dextral Vallettaand Bersezio fault systems with agesglobally younging towards the SW(Fig. 4). In the northern transect, thepart west of the Valletta fault pre-serves ages that are well correlatedwith topography, ranging between5.2 ± 0.8 and 8.6 ± 1.0 Ma overan altitude difference of �1 km,while to the east the ages are olderand at a lower elevation (9.6 ± 1.5and 10.4 ± 1.4 Ma). The age differ-ence between the two sides of thefault is therefore around 4–5 ± 1 Ma.In the southern transect, similar ages of8.9 ± 1.2 Ma are obtained on bothsides of the Valletta fault, while anolder age of 12.9 ± 1.4 Ma is obtained

east of the Bersezio fault at a lowerelevation. This distribution of AFTages in groups separated by majorfaults is likely due to faulting.AHe ages are, as expected, usually

younger than AFT ages and rangefrom 2.8 ± 0.1 to 9.7 ± 0.4 Maacross the entire studied area with nocorrelation between age and elevation.Most AHe ages in the northern tran-sect are younger than 6 Ma except forone of 7.4 ± 0.4 Ma (CF.09). Sys-tematic differences of 3–6 Ma areobtained between the AFT and AHeages (Fig. 4), indicating a relativelyslow cooling rate (11 �C Ma)1). Theanomalously old AHe age (CF.09)might be explained by undetectableU-Th rich micro-inclusions (Beltonet al., 2004), as suggested by the strongAHe age variability of replicate dat-ing. AHe ages in the southern transectare older. They range from 7.7 ± 0.3to 9.7 ± 0.4 Ma and are similar withinerror to the AFT ages. This could beexplained by several processes such as(i) underestimating of fission trackannealing (FT) annealing (Hendriksand Redfield, 2005), (ii) enhanced He

0

500

1000

1500

2000

2500

30000

500

1000

1500

2000

2500

3000

0 2 4 6 8 10 12 14 16

NESW

A FT-NorthNE blockCentral blockSW block

A He-NorthA FT-S outhA He-S outh

Age (Ma)

Elev

atio

n (m

)

AFT and AHe ages vs. elevation

Fig. 4 Age vs. elevation plot constructed from AFT and AHe cooling ages obtained along the two profiles (northern and southerntransects). There is a global younging of AFT ages towards the SW across the strike of the main N140�E strike-slip faults, whichsuggests that the Argentera–Mercantour was exhumed along different blocks bounded by N140�E brittle faults. The AHe agesobtained from the northern transect, close to the domain of N–S transtensive tectonics, are younger than those obtained from thesouthern transect. This clear AHe age difference between the two transects, with no correlation with elevation, suggests a laterexhumation in the NW part of the Argentera–Mercantour ECM, related to the N–S transtensive brittle faults.

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retention (Green et al., 2006; Shusteret al., 2006), or (iii) overestimation ofHe ages owing to U-Th zonation(Hourigan et al., 2005). However, thegood reproducibility of replicate anal-yses indicates that such processes areunlikely. Thus, the overlapping ofAFT and AHe ages within error sup-ports the hypothesis of a rapid coolingevent in this transect.

Discussion: tectonic implications

Exhumation along transpressivedextral faults in the Argentera–Mercantour ECM during the Miocene(22–8 Ma)

Evidence for the onset of right-lateraltranspressive displacements is docu-mented under ductile conditions at

�15 km depth and at c. 22 Ma byphengite 40Ar ⁄ 39Ar dating and barom-etry (Fig. 5; Corsini et al., 2004). Acontinuous tectonic evolution is ob-served from ductile to brittle condi-tions along the strikes of the maindextral N140�E faults, with some slipor shear (Baietto et al., 2009). Thus,the significant AFT age differencesobtained across the Argentera–Mer-cantour from NE (12.9 Ma) to SW(5.2 Ma) may be interpreted as theresult of the relative uplift of blocksalong mainly transpressive N140�Efaults, as previously suggested byBigot-Cormier et al. (2006). This rel-ative uplift of NE blocks is alsoevidenced by the full reset of ZFTages in these domains, while SWblocks have retained their pre-AlpineZFT ages (Bigot-Cormier et al.,

2006). Combining 40Ar ⁄ 39Ar andAFT ages yields an average coolingrate of �21 �C Ma)1 from 22 to10 Ma. Further cooling below 70 �C(AHe) in the southern transect oc-curred at c. 8 Ma (Fig. 5). This exhu-mation likely occurred through pulsesof tectonic activity, as suggested bythe acceleration in cooling rates at8–9 Ma. In any case, it is likely thatthe exhumation of the Argentera–Mercantour massif occurred in anexclusively transpressive contextbetween 22 and 8 Ma (Fig. 6a). Upliftof the Argentera–Mercantour ECMwas accommodated by a combinationof vertical uplift along right-lateralstrike-slip faults and S-vergent thrustsin front of the Ivrea body (Paul et al.,2001; Schreiber et al., 2010). Infront of the ECM, the southward

0

50

100

150

200

250

300

350

400

450

05101520

Transpressional regime

Transtensionalregime

Age (Ma)

Ar-Ar phengite(Corsini et al., 2004)

Apatite Fission Tracks

Northern transectSouthern transect

Cooling rates (°C Ma–1)

Apatite (U-Th)/He

Tem

pera

ture

(°C

)

Pelvoux (ECM)Mt Blanc (ECM)

Ligurian coastLigurian Alps

Tectonicevents

Exhumationhistory Aar (ECM)

Mon Viso (Piemontais domain)Dora Maira (ICM)

Extension faulting (HDF)

Digne–Castelane thrustingT-t cooling path

AHeAFT

NE SW

15.58.0

20.0

21.0

20.0

11.0

Fig. 5 Temperature–time paths for the Argentera–Mercantour ECM, and exhumation history vs. tectonic events in the WesternAlps. The AFT and AHe ages reported in the T–t path are mean weighted ages for each compartment described in the text. Meancooling rates are also indicated for each transect. The vertical grey rectangle represents the transition from transpressional totranstensional regimes at ca. 8–5 Ma. Literature data are quoted for the tectonic and exhumation histories of the ECM and relatedsedimentary cover (Gidon and Pairis, 1992; Fugenschuh and Schmid, 2003; Tricart et al., 2007; Glotzbach et al., 2008; Reineckeret al., 2008; Vernon et al., 2009) and the evolution of the Internal zones (Fugenschuh and Schmid, 2003; Schwartz et al., 2007; Sueet al., 2007; Tricart et al., 2007), the Ligurian Alps (Bertotti et al., 2006) and the Ligurian margin (Foeken et al., 2003).

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propagation of a thrust-and-fold beltdown to the Ligurian margin isalso documented during this period(22–8 Ma, e.g. Gidon and Pairis,

1992; Figs 5 and 6a). This, along withthe relative uplift of the ECM, sug-gests cover–basement decoupling untilc. 8 Ma.

Onset of the Ubaye–Embrunaistranstensional system at 8–5 Ma

The Ubaye–Embrunais domain is atectonic depression bounded by SE-

- 4000 m

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- 4000 m

0 m

1 956 000

1 942 960

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938 880951 920

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Barcelonnette

St Etienne de Tinée

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BF

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JTF

- 4000 m

0 m

- 4000 m

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1 956 000

1 942 960

1 929 920

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991 040

(b)

(a)

22-8 Ma

8-4 Ma

External zone

Internal zoneBarcelonnette

St Etienne de Tinée

BFHDF

VTF

FF

JTF

Fig. 6 Block diagrams of the NW part of the Argentera–Mercantour ECM showing the geometry and main tectonic displacementduring (a) the Miocene (22–8 Ma) and (b) the late Miocene–Pliocene (8–4 Ma). Transpressive motions in response to thecompression of the Ivrea body occurred during the Miocene and were responsible for the important part of the Argentera–Mercantour exhumation. Transtensive motions developed in the NW part of the Argentera–Mercantour massif at around 4–5 Ma.This transcurrent motion is still active as shown by Sanchez et al. (2010a,b). Uplift and subsidence blocks are marked by thefeatures + and ) respectively. BF: Bersezio fault; FF: Fremamorte fault; HDF: High Durance fault; JTF: Jausiers–Tinee fault;PFT: Penninic Frontal Thrust; VTF: Valletta fault. Projection latitude ⁄ longitude coordinates according to the Lambert IIextended system.

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dipping normal faults along thePelvoux ECM and NW-dippingnormal faults along the Argentera–Mercantour ECM (see Tectonicsetting and Fig. 2). To the NW,E-dipping normal faults reactivatedthe Penninic Frontal Thrust, and AFTages of 5–6 Ma are obtained from thehangingwall part (Sue and Tricart,1999; Tricart et al., 2007).In the NW Argentera–Mercantour

ECM, the AFT ages obtained in thisstudy and those from Labaume et al.(2008) show similar within-error valuesin the crystalline basement and itsoverlying sedimentary cover, whichclearly shows that there was no cover–basement decoupling after c. 8 Ma.Furthermore, no systematic differencesin AHe ages were obtained across thestrike of the main strike-slip faults,indicating that there was no differentialexhumation along these after 8 Ma. Inthe northern transect, close to theUbaye–Embrunais transtensionaldomain, we detected younger AHeages (4–5 Ma) than in the southerntransect (8–9 Ma; Fig. 4). This agedifference suggests a later exhumationin the NW part of the Argentera–Mercantour ECM, which remained atT > 70 �C until c. 5 Ma (Fig. 5).Therefore, based on the symmetri-

cal structure of the SE Pelvoux andNW Argentera–Mercantour massifson either side of the Ubaye–Embru-nais depression and on their compa-rable exhumation histories, weinterpret the 4–5 Ma age clusteringat the NW rim of the Argentera–Mercantour ECM as resulting fromthe onset of a transtensional regime inthe Ubaye–Embrunais area (Figs 5and 6b). This reconciles the late denu-dation histories of both ECM with thecoincidence of extensional and right-lateral tectonics, as previously sug-gested by Tricart (2004).

Implications for the tectonic evolutionof the SW Alps

This study confirms the importance ofstrike-slip tectonics in the late(<20 Ma) exhumation of the ECM.Indeed, right-lateral tectonics havebeen ongoing in the SW Alps sinceat least 22 Ma in the Argentera–Mercantour massif and are stillreflected in the seismological data(Jenatton et al., 2007). The perma-nence of dextral tectonics invalidates

(i) the model of westward indentation,which would imply left-lateral dis-placements along the SW margin ofAdria and (ii) the gravitational col-lapse model, which would lead toextensional tectonics in the internaland external Alpine arcs. Thus, wepropose that the exhumation of ECMis controlled by dextral transcurrenttectonics, which mainly accommodatethe anticlockwise rotation of Adria.Such a model of rotation is alsosupported by palaeomagnetic datashowing an average 30� anticlockwiserotation in the internal domain (Col-lombet et al., 2002).

Acknowledgements

We wish to thank J. Schneider, D. Schre-iber, G. Giannerini and J.M. Lardeaux fortheir constructive discussions. We aregrateful to M. Manetti for his technicalhelp. Further reviews by M. Zattin, P.Tricart, C. Sue and one anonymous refereeand editorial work by G.V. Dal Piaz and C.Doglioni significantly improved the previ-ous version of this paper.

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Received 11 December 2009; revised versionaccepted 20 January 2011

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