Subduction and Obduction Processes in the Swiss Alps

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Published in Tectonophysics 296: 159-204, 1998

Subduction and obduction processes in the Swiss Alps

G.M. Stampfli a,Ł,J.Mosara,D.Marquerb, R. Marchant a, T. Baudin c,G.Borela a Institut de Geólogie et Paleóntologie, Universite´ de Lausanne, CH-1015 Lausanne, Switzerland b Institut de Geólogie, Rue Emile Argand 11, CH-2007 Neuchatel, Switzerland c BRGM, 3 Avenue Claude Guillemin, B.P. 6009, 45060 Orleans cedex 2, France

Abstract

The significance of the Briançonnais domain in the Alpine orogen is reviewed in the light of data concerning its collision with the active Adriatic margin and the passive Helvetic margin. The Briançonnais which formerly belonged to the Iberian plate, was located on the northern margin of the Alpine Tethys (Liguro-Pie«mont ocean) since its opening in the early-Middle Jurassic. Together with the Iberian plate the Briançonnais terrane was separated from the European plate in the Late JurassicÐEarly Cretaceous, following the northern Atlantic, Bay of Biscay, Valais ocean opening. This was accompanied by the onset of subduction along the northern margin of Adria and the closure of the Alpine Tethys. Stratigraphic and metamorphic data regarding this subduction and the geohistory of the Briançonnais allows the scenario of subductionÐobduction processes during the Late CretaceousÐearly Tertiary in the eastern and western Alps to be specified. HPÐLT metamorphism record a long-lasting history of oceanic subduction-accretion, followed in the Middle Eocene by the incorporation of the Briançonnais as an exotic terrane into the accretionary prism. Middle to Late Eocene cooling ages of the Briançonnais basement and the presence of pelagic, anorogenic sedimentation lasting until the Middle Eocene on the Briançonnais preclude any sort of collision before that time between this domain and the active Adria margin or the Helvetic margin. This is confirmed by plate reconstructions constrained by magnetic anomalies in the Atlantic domain. Only a small percentage of the former Briançonnais domain was obducted, most of the crust and lithospheric roots were subducted. This applies also to domains formerly belonging to the southern Alpine Tethys margin (AustroalpineÐinner Carpathian domain). It is proposed that there was a single Palaeogene subduction zone responsible for the Alpine orogen formation (from northern Spain to the East Carpathians), with the exception of a short-lived Late Cretaceous partial closure of the Valais ocean. Subduction in the western Tethyan domain originated during the closure of the Meliata ocean during the Jurassic incorporating the AustroalpineÐCarpathian domain as terranes during the Cretaceous. The subduction zone propagated into the northern margin of Adria and then to the northern margin of the Iberian plate, where it gave birth to the PyreneanÐProvençal orogenic belt. This implies the absence of a separated Cretaceous subduction zone within the Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced the subduction to jump to the SE margin of Iberia in the Eocene, creating the Apenninic orogenic wedge and inverting the vergence of subduction from south- to north-directed.

Keywords: subduction; obduction; Alps; plate tectonics; Brianc¸onnais; Tethys

Ł Corresponding author. E-mail: gerard.stampﬂ[email protected]

0040-1951 PII: S0040-1951(98)00142-5 2

1. Introduction taceous context. Then we review first the eastern Swiss Briançonnais (mainly the Tambo and Suretta Subduction and obduction processes in the west- nappes), then the Swiss and French Subbriançon- ern Alps can be approached through different meth- nais Pre«alpes Me«dianes in order to compare them ods (i.e. stratigraphic and sedimentologic analyses, and show their concomitant incorporation into the structural analyses and dating of metamorphism, Tertiary accretionary prism as exotic terranes. We etc. :::) but only a multidisciplinary effort can build discuss the collision processes and try to develop enough constraints around a still debated subject. In more irrefutable arguments concerning subduction our approach we used data from our own field work and obduction processes in the western Alps. Exist- and also a vast amount of published data. These data ing absolute ages are reviewed and placed in this sets have been analysed from a structural, petrolog- context of ongoing deformation. In the discussion ical and sedimentological point of view. Structural the HPÐLT debated ages are reassessed together with models were constrained at depth by deep seismic the location and geometry of the subduction zone data and lithospheric cross-sections (Stampfli, 1993; affecting the Alpine region. Marchant, 1993; Marchant and Stampfli, 1997b). Together with palaeontological and absolute age determinations, these data allowed the construction 2. The geodynamic framework of the western of subsidence curves and the elaboration of flex- Alps ural models. These models were incorporated and constrained by palaeo-reconstructions made in real 2.1. The plate tectonics of the western Tethyan projection using the GMAP software (Torsvik and regions Smethurst, 1994). Although there has been significant progress in The reconstructions shown in Figs. 1 and 2 are the dating of metamorphism events in the last based on a pre-break-up fit using palaeomagnetic pole decade, it is in this field that the major controversies data, Euler rotation poles, as well as magnetic anoma- are found, mainly regarding the timing of high-pres- lies from the Central Atlantic which we have com- sure metamorphism affecting both the Pie«mont and bined to reconstruct a consistent model for the Alpine Valais sutures. A review of the solid data regard- domain s.l. (Klitgord and Schouten, 1986; Srivastava ing mainly the collision between the Briançonnais and Tapscott, 1986; Rowley and Lottes, 1988; Srivas- domain and the surrounding areas is therefore nec- tava et al., 1990; Srivastava and Verhoef, 1992). essary to constrain controversial absolute ages. The This plate model takes into account the likelihood exotic position of the Briançonnais terrane between of a late Palaeozoic rifting and seafloor spread- the European Helvetic margin and the AustroalpineÐ ing of the eastern Mediterranean basin (Stampfli et Adria margin is fundamental to determine the timing al., 1991, 1998; Stampfli and Pillevuit, 1993). This of closure of the oceanic Pie«mont area and the tim- opening was concomitant with the opening of the ing of the collision between the accretionary prism Neotethys and the drifting of the Cimmerian con- and the Helvetic margin. To do so we analysed evi- tinents since the late-Early Permian. This implies dence pertaining to the Briançonnais crustal nappes also a late closure of the palaeo-Tethys (Late Per- and covers from eastern Switzerland and the Bri- mian to Triassic) on a Greek and Turkish transect ançonnais cover nappes found as exotic klippen in of the Tethyan realm, accompanied by the opening the Swiss and French Pre«alpes and compared them. of back-arc basins along the Eurasian margin (e.g. The geodynamic evolution of these domains and sur- MeliataÐHallstatt and Vardar oceans; Stampfli et al., rounding areas involved in Tertiary collision allows 1991; Stampfli, 1996; Stampfli and Marchant, 1997). us to determine what part of these terranes were The opening and closing of the Meliata ocean subducted or obducted and at what time. affected the whole Alpine history. First it created In a first section we present the plate tectonic a thermal subsidence starting in the Late Permian framework of the AlpineÐMediterranean domain to responsible for the general Triassic transgression place the Briançonnais in a global Jurassic and Cre- in the Alpine domain (e.g. Fig. 3). Then its final 3

closure in the Early Cretaceous affected the whole of older oceanic domains such as the Meliata and Austroalpine domain and can be seen as a major Vardar oceans (Channell and Kozur, 1997). element in the onset of the closure of the Alpine Tethys itself (see Section 5). 2.1.2. The Alpine Tethys rifting Late Triassic ages obtained from shear zones in 2.1.1. The Apulia–Adria problem the southern Alps (Schmid et al., 1987; Zingg et al., It is possible to show that the Apulian plate s.l. 1990; Hunziker et al., 1992) and field evidence (e.g. (Italy) suffered relatively little rotation in regard Froitzheim and Manatschal, 1996), indicate impor- to Africa since the Triassic (Channell, 1992, 1996; tant transtensive events which eventually led to the Channell et al., 1992; Channell and Doglioni, 1994). opening of the central Atlantic=Alpine Tethys ocean However, when rotating the Italian Peninsula back- and the break-up of Pangaea. Areas affected by ward using data from magnetic lineation in the At- PermoÐTriassic rifting became subsiding rim basins lantic, its pre-break-up position is clearly superposed of the Jurassic Alpine Tethys (Fig. 1). The Jurassic on the Iberian plate. This leaves open the question rifting cut in between these zones of thinned and of an Apulian plate being an African promontory already cold lithosphere. responsible for the Alpine collision. Also, the con- Subsidence patterns of the marginal areas of tinuity between the active subduction zone under the Alpine Tethys (Loup, 1992; Borel, 1995, 1998; Greece and the outer Dinarides (Wortel and Spak- Mosar et al., 1996), together with stratigraphic and man, 1992; de Jonge et al., 1994) shows that there sedimentological data allow us to place the onset of is a possible plate limit between Apulia and the the final rifting in the Sinemurian (Figs. 3 and 4), autochthonous units of Greece (Fig. 6). followed by a widespread thermal uplift phase in the Together with major problems concerning the re- Toarcian (Favre and Stampfli, 1992; Mosar et al., assembling of microplates in a pre-break-up position 1996) and followed by seafloor spreading starting in (Fig. 1), this led us to consider that the Apulian plate the early-Middle Jurassic (e.g. Bill et al., 1997). In s.l. is most likely cut into two pieces, an Apulian the North Atlantic ocean, spreading can be placed in plate s.str. to the south and an Adriatic plate s.str. to the early to middle Toarcian (Steiner et al., 1998). the north. Both pieces were joined together during A diachronous opening of the North AtlanticÐAlpine the Late Permian or Early Triassic, but further lateral Tethys system with a rotation point near the Black displacement between both microplates is likely. The Sea can be defined. Apulian plate was certainly attached to the African Thermal subsidence of the Alpine Tethys started plate until the Early Cretaceous, then it started a in the Bajocian. The progradation of carbonate plat- lateral displacement to reach its final position in the forms toward the rift is hampered by the presence of Miocene as a separate entity (Fig. 2). This implies rim basins on both sides of the Alpine Tethys: (1) to the opening of a Late Cretaceous ocean just north of the north, the HelveticÐDauphinois basin and its SW the former east Mediterranean ocean, an ocean rep- extension toward the Subbriançonnais rim basin, the resented by the Antalya and Lycian ophiolitic nappes Sardinia and the Subbetic rim basin; (2) to the south, (Stampfli et al., 1998). the Lombardian rim basin representing an aborted The Apulian plate can therefore be considered arm of the Alpine Tethys active from the Late Tri- as an exotic or rather a displaced terrane like most assic to Early Jurassic (e.g. Winterer and Bosellini, large tectonic units from the former southern margin 1981; Bertotti et al., 1993). of the Alpine Tethys: the Austroalpine, Carpathians Progradation did not succeed in filling these rim and Tisza composite terranes (Figs. 1 and 2). Unlike basins and the former rift shoulders of the Alpine Apulia, still rooted in the lithosphere, these compos- Tethys (the Briançonnais and south Helvetic domains ite terranes were decoupled at upper crustal levels in the north and the Canavese and lower Austroalpine and incorporated into the accretionary prism, their domains to the south) became drowned submarine lithospheric roots were subducted. Their composite ridges developing condensed sequences. nature comes from the fact that they record the clo- In the Subbriançonnais rim basin, a carbonate sure not only of the Alpine Tethys ocean but also platform developed in Middle and Late Jurassic time. 4 5

This platform, centred around the rift shoulder, pro- on the order of 10 to 15 Ma; Favre and Stampfli, graded mainly towards the rim basin (i.e. towards 1992; Steiner et al., 1998). the northwest, away from the oceanic realm) and is a The Bay of Biscay rifting took place during the characteristic feature of the Subbriançonnais domain rotation of Iberia (after the Valanginian from palaeo- (e.g. Septfontaine, 1983; Mosar et al., 1996). magnetic data, Moreau et al., 1992; see also Olivet, 1996). Seafloor spreading there is younger than in 2.1.3. The Valais ocean the Atlantic between Iberia and America (M0, 112 In a model where the Briançonnais domain is at- Ma) and stopped in the Campanian (A33, 83Ð71 tached to the Iberian plate (Frisch, 1979; Stampfli, Ma). In the Pyrenean region, the thermal event linked 1993; Stampfli and Marchant, 1997), it can be deter- to the emplacement of basic material at depth started mined that the opening of the Valais rift between the around 115 Ma and lasted until 80 Ma (Montigny et Briançonnais and southern France started in the Late al., 1986). Jurassic when Iberia separated from Newfoundland. Different subsidence behaviour (see Borel, 1998 In that area, the minimal Late Jurassic (160 Ma) for a review) can be noticed between the basins to M0 (112 Ma; Srivastava et al., 1990; Sibuet and related to the opening of the Atlantic between Colette, 1991) syn-rift opening would be 200 km ac- Newfoundland and Iberia (Jeanne d’Arc, Lusita- cording to the reconstruction of Malod and Mauffret nian, Provence, Valais, S Helvetic) and the basins (1990) or 350 km using that of Sibuet and Colette related to the rotation of Iberia (Biscay, Pyrenees, (1991). Discrepancies stem from the former pre-rift Aquitaine, Cantabric, western Paris basin). For the situation of Iberia with regard to Newfoundland. The latter group, thermal subsidence is active in the Late data of Srivastava et al. (1990) and of Srivastava Cretaceous where the Cenomanian is largely trans- and Verhoef (1992) allow these differences to be gressive on the former rift shoulders (e.g. in the narrowed down and their proposal was applied to Pyrenees, Peyberne`s, 1976; Peyberne`s and Souquet, our model. Similarly, a continental separation on the 1984; Simo, 1986; in Normandy, Me«gnien, 1980), order of 200 to 250 km can be calculated for the Red which implies a seafloor spreading older than the Sea before the onset of seafloor spreading. Cenomanian. For the first group, as shown by the The Valais rift was roughly parallel to the Atlantic subsidence curve established for the Pre«alpes Me«di- rift between Iberia and Newfoundland (Fig. 1) and anes (Fig. 3), the onset of thermal subsidence can be opened at the same time and by the same amount. placed in the Valanginian (132Ð137 Ma, Gradstein The oldest magnetic anomaly in this part of the At- et al., 1995; 122Ð130 Ma, Odin and Odin, 1990). lantic is M0 (Aptian). However seafloor spreading is Actually a major change of sedimentation in the inevitably slightly older than the first clear magnetic Subbriançonnais domain is found at the top of the lineation (for the central Atlantic the discrepancy is Calcaires Plaquete«s (Python-Dupasquier, 1990) and

Fig. 1. (A) EarlyÐLate Jurassic reconstruction (Oxfordian, 156 Ma, magnetic anomaly M25) of the MediterraneanÐwestern Tethys domain. Europe fixed in present-day position, orthogonal projection centred at 30¼N, 0¼E. (B) Early Cretaceous reconstruction (middle Aptian, 112 Ma; magnetic anomaly M0). AA D Austroalpine east; AF D Afyon; AP D S Apuseni (arc, back-arc); BA D Balkhanides; BT D Bator (back-arc); BD D Beydaghlari; BE D Betic; BY D Beysehir; BR D Briançonnais; BU D Bucovinian; BU¨ D Bu¬kk, Fatric; BV D Budva; CB D central Bosnia; CIM D Cimmerian blocks; CN D Carnic; CP D Calabria, Peloritani; CV D Canavese; DA D Dacides; DO D Dobrogea; DR D Drina-Ivanjica; DU D Durmitor; EP D east Pontides; FA D Fatric; GE D Gemeric; GT D Gavrovo-Tripolitza; GT D Getic; HA D Hadim, East Taurus; HE D Helvetic rim basin; HK D high karst; IO D Ionian; IS D Istanbul; JA D Jadar; JU D Julian Alps; KA D Kalnic; KB D Kabylies; KC D Kura south Caspian; KF D Kotel flysch; KO D Korab; LA D Lagonegro; LI D Ligurian; LJ D lower Juvavic; LO D Lombardian rim basin; MA D Magura margin; MA D Mani; ME D Mesek west Tizia; MI D Mirdita; ML D Maliak; MN D Menderes; MO D Moesia; NA D north Apenninic; NCA D North Calcareous Alps; OT D Othrys-Evvia ophiolites; PA D Panormides; PE D Penninic; PI D Piemontese; PK D Paikon (intra oceanic arc); PL D Pelagonian; PN D Pienniny; RH D proto-Rhodope (prism); RI D Rif; Sa D Sakarya; SA D south Apennines; SB D Subbetic rim basin; SE D Sesia, Austroalpine west; SI D Sicanian; SK D south Karawanken; SL D Slavonia; SR D Srednogorie; ST D Strandja; SZ D Szarvasko (arc, back-arc); TA D west Taurus; TDR D Trans-Danubian range; TI D Tirolic, Bavaric; TT D Tatric; Tu D Tuscan nappes; TZ D Apuseni east Tizia; UJ D upper Juvavic; VE D Veporic; WCA D western Carpathian. 6

Fig. 2. Late Cretaceous reconstructions. (A) Santonian=Campanian boundary (84 Ma; magnetic anomaly 33). (B) Maastrichtian (69 Ma; magnetic anomaly 30). See Fig. 1 for legend. 7

Fig. 3. Synthetic subsidence curves for the Pre«alpes Me«dianes, modified from Mosar et al. (1996) and Borel (1998). The subsidence program used to derived the curves is from R. Schegg. Timescale from Gradstein et al. (1995). 1 D thermal subsidence of the HallstattÐ Meliata rift; 2 D Alpine Tethys rift thermal expansion; 3 D Alpine Tethys thermal subsidence; 4 D Valais rift thermal expansion; 5 D Valais thermal subsidence; 5a D onset of closure of the Valais ocean; 6 D flexural bulge and Cretaceous Valais subduction uplift; 7 D flexural trough; 8 D obduction.

can be dated as Barremian (121Ð127 Ma, timescale deep marine re-sedimented material is found in the of Gradstein et al., 1995; 114Ð116 Ma, timescale of Albian and early Cenomanian deposits (Kerckhove, Odin and Odin, 1990). Therefore, spreading in the 1969; Kerckhove and Lereus, 1987). A total opening Valais ocean could have lasted from the BarremoÐ of 200 km during the rifting phase and more or less Aptian to the AlboÐCenomanian, as the rotation of the same amount during spreading can be envisaged Iberia and seafloor spreading in the Bay of Biscay for the Valais ocean. since the Albian implies a concomitant closure of Both Valais margins disappeared into the lower the Valais ocean. This closure generated a hiatus in Penninic suture zone and only the surrounding areas the Subbriançonnais basin (event 5a in Fig. 3) and (rim basins, rift shoulders) can give clues about their westward, in the French Subbriançonnais, important evolution. The Pre«alpes Me«dianes Lower Cretaceous 8

Fig. 4. Stratigraphy and origin of the oceanic series found in the accretionary sequences of the western Swiss Alps. Timescale from Gradstein et al. (1995). It can be seen that the Cretaceous accretionary units found nowadays in the Pre«alpes (Nappes Supe«rieures) are mainly derived from the northern half of the Liguro-Pie«mont ocean and the Tsate« accretionary units from the southern half. The star symbol represents recently dated gabbros from the Gets nappe (Bill et al., 1997). 9

sequence, representing the southern shoulder area, (Ackermann, 1986), Paleocene sediments seem to be is condensed (Python-Dupasquier, 1990) but stayed absent, but Paleocene fauna is reworked in the flysch. in relatively deep water conditions throughout the Large conglomeratic influxes started again in the ex- Cretaceous period. This region was formerly part ternal domain during the Lutetian with the deposition of the Liguro-Pie«mont ocean Jurassic passive margin of the Meilleret flysch (Homewood, 1974). The con- and was brought down to depth, close to the CCD, by tinuity of inversion-related deposits from the Campa- thermal subsidence during the late-Middle Jurassic nian to Lutetian in the external Valais=Ultrahelvetic (Borel, 1995, 1998). The Early Cretaceous Valais domain precludes any final closure of this domain tectonic=thermal uplift was therefore insufficient to before that time (40 Ma). cause it to emerge. Subduction of the buoyant young Valais ocean An oceanic connection with the Atlantic existed crust certainly increased the intra-plate stress within through the Valais=Pyrenean system. The similitude the European margin, and could be responsible for of facies between the Rheno-Danubian flysch and the widespread inversion phase affecting large ar- the Valais sequence from the Albian to Late Creta- eas of the European plate already during the Late ceous (Stampfli, 1993, and references therein), and Cretaceous (Ziegler, 1990; Ziegler et al., 1995). the continuity of deep water clastic facies in these two domains allows them to be assigned to the same 2.2. Exotic terranes in the Alps position with regard to the European margin. The presence of contourites and strong and changing cur- 2.2.1. The Briançonnais terrane rent directions along the basin (Hesse, 1974) suggest The separation of the Iberian plate from North a connection with the major oceanic domains. In the America in the Late Jurassic implies a separation of Ligurian domain such turbiditic deposits are absent, the Iberian plate from Europe too. This separation, the AlboÐCenomanian formation being dominated as we have seen, is brought about by the Pyrenean by anoxic black-shale deposits (Fig. 4). pull-apart rift system extending eastward into the Valais or North Penninic suture zone (Frisch, 1979; 2.1.4. Valais subduction Stampfli, 1993). The rotation of Iberia since the TuronoÐSenonian The Briançonnais domain (Figs. 1 and 2) was implies the closure of the Valais ocean and Provence attached to Corsica=Sardinia and therefore to the basin since that time too (Fig. 2). Subduction-related Iberian plate (Stampfli, 1993). Its former position sediments from the Valais ocean have not yet been was thus more to the (present) southeast than usu- dated (see below), but in Provence, the Ciotat con- ally supposed (e.g. Dercourt et al., 1993) and the glomerate records major clastic input coming from Valais trough was never connected to the Vocontian the southeast at that time (Philip et al., 1987). First trough (as proposed by Dercourt et al., 1985 and stages of inversion in the French external Alps (De- by Yilmaz et al., 1996). The most internal south voluy region) are also dated as TuronoÐSenonian Helvetic domains (Ultrahelvetic), the lower Penninic (Huyghes and Mugnier, 1995). A flexural bulge de- Lepontine nappes and external Valais zone (i.e. zone veloped in Provence during the Late Cretaceous, the Subme«diane and the Bu¬ndnerschiefer) are therefore flexural basin became lacustrine during the Maas- considered as former elements of the northern Alpine trichtian (Debrand-Passard and Courbouleix, 1984). Tethys oceanic margin trapped by the eastward dis- In the external Swiss Alps the Late Cretaceous placement of the Briançonnais terrane together with Niesen conglomerates (CampanianÐMaastrichtian; the Iberian plate. This displacement induced a du- Ackermann, 1986) could be related to the flexure plication of this northern margin in the present-day of the Helvetic margin (Fig. 14), although an ori- Alpine orogen. The Valais ocean (sometimes re- gin from a major inversion of the northern Valais ferred to as the North Penninic ocean) is therefore a margin is more likely. The Niesen conglomeratic composite oceanic strip comprising a small segment input seems to stop in the Maastrichtian (migration of the Early Jurassic margin to the north (Fig. 1), of the flexural bulge), it is followed by the Ches- represented by the Bu¬ndnerschiefer and elements of selbach flysch whose age reaches the late Lutetian the Zone Subme«diane (see below). 10 11

2.2.2. The Austroalpine terranes onwards (Puga et al., 1995). Resedimentation of the The drifting of Iberia in the Early Cretaceous as Dorsale Calcaire (former rift shoulder of the south- well as the final closing of the remnant Meliata do- ern margin of the Iberian plate) in the Rif flysch main induced the subduction of the Liguro-Pie«mont basin starts in Cenomanian time, grading into major part of the Alpine Tethys ocean in the Late Cre- olistostrome deposits in the Maastrichtian (Gu¬beli, taceous. This is marked by HPÐLT metamorphism 1982; Thurow, 1987) which we relate to the major (Hunziker et al., 1992) of elements pertaining to the transform linking the Atlantic and the Vardar re- toe of the Austroalpine margin (Fig. 5) and of the ac- gions, cutting in between Adria and the Austroalpine cretionary prism s.str. (Tsate« nappe, Fig. 4; Marthaler domain (Fig. 2). In EoceneÐOligocene times the and Stampfli, 1989; Stampfli and Marthaler, 1990; Alboran (Iberian) plate locally collided with North Deville et al., 1992; Fudral, 1996). This means that Africa, which was accompanied by major terrane most of the southern margin of the Alpine Tethys displacements and the late Tertiary opening of the is made of exotic terranes, some of them (most of Algero-Provençal and Tyrrhenian oceans, liberating the Austroalpine?) possibly derived from the Meli- the Alboran blocks from their Iberian motherland ata prism. In the western Alps, some of these were (Fig. 6). Here too, lateral displacements generated obducted (Dent-Blanche, Arolla-Valpelline) and dis- duplication of palaeogeographic elements, creating placed westward along the margin, others were only pseudo-oceanic sutures. We propose a displacement partially subducted (Sesia) and formed the back stop toward the southwest of the internal Betic domains of the Alpine Tethys accretionary prism. Other ele- (together with the Rif) subsequently incorporated ments were subducted and underplated at different into the Betic orogen as a terrane. depths and suffered important eclogitic metamor- The Iberian margin is considered an active margin phism (internal massifs: Monte Rosa, Grand Paradis, developing back-arc spreading at least since Eocene Dora Maira, see discussion below). The westward time, whilst the African margin is considered a pas- displacement of the Austroalpine elements (Frank, sive margin. Terrane displacement was related to the 1987; Ratschbacher and Neubauer, 1989; Tru¬mpy, slab roll-back of the Alpine Tethys and the local 1992; Froitzheim et al., 1994) is in part related to detachment of this slab. tectonic escape movements within the Meliata accretionary prism (Froitzheim et al., 1997), although it is mainly due to an eastward-directed relative move- 3. The Briançonnais terrane in eastern ment of Apulia, as seen in Fig. 2, following the Switzerland onset of subduction of the Vardar ocean under the Rhodope. 3.1. Geological setting

2.2.3. The Alboran terranes 3.1.1. Introduction The Alboran plate (Rif, internal Betic, Kabylies, We shall now review in detail the geodynamic Peloritan, Calabria, SardiniaÐCorsica microplates; evolution of the Brianc¸onnais terrane. Its exotic po- Wildi, 1983) formed the southern margin of the sition is fundamental for determining the timing of Iberian plate (Fig. 1). This margin was affected by closure of the oceanic Pie«mont area and the timing deformation processes from the Early Cretaceous of the collisional events. We start ﬁrst with the Bri-

Fig. 5. Evolution of the structural bulge during the subduction of the EuropeanÐIberianÐBriançonnais plates. As the slab subducts the lithosphere becomes thicker, the dark grey part of the lithosphere corresponds to the thickness at the time of the reconstruction for the Alpine Tethys segment, the light grey to final thickness before the slab detachment around 35 Ma. It can be seen that the Briançonnais domain entered the flexural bulge around 90 Ma and was in the subduction trough around 50 to 45 Ma. Ar D Arosa (Dent-Blanche); Br D Briançonnais; He D Helvetic; I-M D internal massifs (Monte Rosa; Grand Paradis); Pi D Pie«mont ocean; SA-Iv D southern Alps-Ivrea; SH D South Helvetic; Va D Valais ocean; Vl-Se D Valpelline (Dent-Blanche)-Sesia; Z-S D Zermatt-Saas ophiolite. Flexural model from M. Burkhard (Neuchaˆtel): dc D density of crust; et D elastic thickness; wo D maximum depression; xo D zero elevation position of bulge; xb D maximum elevation position of bulge; wb D height of bulge. 12 13

ançonnais basement from eastern Switzerland where et al., 1994, 1996). The second main result is the recent studies allow a precise determination of the description of a new unit, the Starlera nappe, lying structural and metamorphic history. on the reduced autochthonous cover of Tambo and The central Swiss Alps can be divided into the Suretta (Baudin et al., 1995) (Fig. 7). three following structural domains: (1) an external The Tambo and Suretta nappes form a thin crys- part, the Helvetic; (2) an internal part, the Penninic talline sliver, each about 3.5 km thick and covered zone; and (3) the South Alpine units (e.g. Coward by a reduced autochthonous sedimentary cover. They and Dietrich, 1989). The Penninic zone corresponds are mainly composed of old crystalline basement and to the internal parts of the Alpine mountain belt. small occurrences of Early Permian granites (Roffna This area is constituted of imbricate stacks of sed- and Truzzo granites). These nappes are bounded by iment covers and basement slices. Deformation of different PermoÐMesozoic covers (Gansser, 1937). these domains is related first to oceanic subduction (1) The Misox zone, between Tambo and Adula, processes starting in the Early Cretaceous, followed which is the southern extremity of the North Pen- by incorporation of exotic terranes and finally col- ninic Bu¬ndnerschiefer and flysch domain (Stein- lision of the two continents in the Tertiary. This mann, 1994a) and does not exceed 800 m in thick- implies several phases of deformation and system- ness. This zone consists mainly of calcareous schists atic refolding of former structures. with some extremely stretched lenses of gneisses Early cartographic and petrological work (Staub, (Gadriol gneiss) and basic rocks (MORB of Valais 1916, 1924; Wilhelm, 1933; Gru¬nenfelder, 1956; and=or Pie«mont origin; Du¬rr et al., 1993; Steinmann, Gansser, 1937; Zurflu¬h, 1961; Blanc, 1965; Stroh- 1994a,b). (2) The Splu¬gen zone, between Tambo and bach, 1965; Weber, 1966; Streiff et al., 1976), and Suretta and the Avers and Schams nappes at the recent detailed mapping and stratigraphic observa- top of Suretta. The Splu¬gen zone shows important tions (Suretta nappe, Milnes and Schmutz, 1978; thickness variations. Schams nappes, Schmid et al., 1990; Schreurs, 1993; Tambo nappe, Baudin et al., 1993; Mayerat, 1994; 3.1.2. Polycyclic basements Suretta cover, Baudin et al., 1995) allow the struc- The polycyclic basements are essentially com- tural units in this part of the Alps (Fig. 7) to be posed of metapelites and metagreywackes (parag- clearly defined. neiss and micaschists) including some lenses of The Suretta and Tambo nappes belong to the east- mafic rocks (amphibolites) with local migmatites ern part of the Briançonnais terranes, together with (Wilhelm, 1921; Staub, 1924; Gansser, 1937; Gru¬- their cover, the Schams and Starlera nappes in the nenfelder, 1956; Schaerer, 1974). north (Fig. 7). A new detailed tectonic framework The basement rocks of Suretta are intruded by for the frontal part of the Suretta nappe and of the pre-Alpine magmatic bodies (Fig. 7), and an Early Tambo nappe is described in Marquer et al. (1996) Permian subvolcanic intrusion, the Roffna granite, in and in Baudin et al. (1993), respectively. One of the the frontal part (Marquer et al., 1996). The Roffna main results of these studies is the strong heteroge- porphyry has a high-level intrusive and volcanic neous deformation undergone by the basement rocks origin (Gru¬nenfelder, 1956; Milnes and Schmutz, during the Tertiary tectonics, leading to lenses of pre- 1978) and forms the frontal part of the Suretta nappe. served material surrounded by mylonite zones at all Until now, its age has been considered to be 350 scales. This peculiar geometry makes a precise de- Ma (207Pb=206Pb; Hanson et al., 1969) but new age formation-metamorphism analysis along zones with determination (in progress) and the similarity of a strain gradient (such as mylonite zones) and in most of these volcaniclastic facies with those of the areas of weak Alpine deformation possible (Marquer PermoÐTriassic cover, directly overlain by Triassic

Fig. 6. Present-day plate tectonic setting and distribution of major tectonic units in the Alpine and western Mediterranean areas. Be D Bern; Bo D Bologna; Ge D Geneva; Gn D Genova; Gr D Grenoble; In D Innsbruck; La D Lausanne; Ma D Marseilles; Mi D Milano; Ni D Nice; To D Torino; Zu¨ D Zu¬rich. (Modiﬁed from Stampﬂi and Marchant, 1997.) 14

Fig. 7. (A) Cross-section of the internal eastern Swiss Alps showing the structure of the Suretta and Tambo Briançonnais nappes. (B) Simplified structural map of the internal eastern Swiss Alps. 1 D European units of the lower Penninic structural domain (Adula); 2 D Bu¬ndnerschiefer of the Valais domain; 3 D Briançonnais basement nappes: Tambo and Suretta north of Engadina line, Chiavenna, Gruf, Belinzona-Dascio south of the line; 4 D Briançonnais covers, allochthon: Starlera (white); autochthon (black); 5 D Schams nappe; 6 D Avers Schistes lustre«s (Pie«mont oceanic accretionary units); 7 D Austroalpine domain; 8 D Bergell granite; 9 D south Alpine domain. (C) Synthetic tectonic scheme of cross-section (A). 15

carbonate, indicate a Permian age. The intrusive relationships between the foliated basement and the Roffna granite show that the basement rocks present a strong deformation, trending NNEÐSSW, and a high-grade metamorphism inprint older than Early Permian. Lenses of mafic rocks are found in the Suretta basement (Timun complex; Zurflu¬h, 1961; Blanc, 1965) and new metamorphism-deformation studies allow two distinct subduction events for this basement to be defined: one pre-Alpine HPÐHT and the second Alpine HPÐLT (Biino et al., 1997). The Tambo basement consists mainly of polycyclic rocks which are intruded in the south by the Truzzo granite. Much of this basement is characterised by strong pre-Alpine deformation. In the northern part of the nappe, alignment of lenses of Fig. 8. Synthetic lithological section of the Suretta nappe. 1 mafic and ultramafic rocks are especially well de- D micaschist and paragneiss; 2 D amphibolite, prasinite; 3 D orthogneiss; 4 D Roffna gneiss; 5 D albitic micaschists; 6 D veloped. These amphibolites with a few preserved conglomeratic micaschists; 7 D quartzites; 8 D lower polygenic pyroxeneÐgarnet assemblages (pyroxenites or eclog- breccias with calcschist matrix; 9 D Cornieules; 10 D dolomites ites) testify to a pre-Alpine HPÐHT metamorphism and limestones; 11 D dark fetid limestones; 12 D white marbles; (Biino et al., 1997). In other parts of the nappe, 13 D upper polygenic breccias with white marble matrix; 14 D these mafic rocks have suffered a complete retro- upper polygenic breccias with white calcschist matrix. Modified after Baudin et al. (1995). grade metamorphism which produced prasinites or ovardites. The monocyclic basement in the Tambo nappe is represented by the Truzzo granite, well- that monocyclic PermoÐTriassic rocks are more abun- exposed in the southern part of the nappe (Blanc, dant in the Tambo and Suretta nappes than previously 1965; Weber, 1966; Gulson, 1973; Marquer, 1991). recognised (Baudin et al., 1991). They generally over- This granitic complex is 293 š 14 Ma old (Rb=Sr lie the basement, but can also be found as thin wedges whole rock dating; Gulson, 1973) and has suffered pinched between slabs of crystalline rocks at the front Alpine deformation only. It appears as an origi- or in the roof of the nappe, and also at the base of the nally isotropic and homogeneous body of porphyritic overlying Suretta unit (Fig. 8). The age attribution of granite (with centimetric K-feldspar porphyroclasts), the PermoÐTriassic cover is based on its stratigraphic crossed by many Alpine shear zones (Marquer, position between old basement and Mesozoic carbon- 1991). The top of the granite is generally over- ate series, and its facies, which is similar to that of the lain by polycyclic rocks except in the southeastern Briançonnais zone (Staub, 1958a). Conglomerates at part of the area, where it is directly covered by the the bottom of the stratigraphic pile show lithologi- PermoÐTriassic unit (see below). cal similarities with the classic Verrucano facies and pass progressively into more quartzitic formations. In 3.1.3. Cover units places, the series seems to begin with an augengneiss Autochthonous cover. The chemical composition and a few metres thick and generally shows strong vol- textures of the Roffna granite are close to those caniclastic tendencies with gneiss-like rocks rich in of rhyoliticÐgranitic rocks and they are associated K- and Na-feldspars. One can observe a clear facies with volcanic effusives (metatuffs) unconformably convergence between the latter rocks and the Roffna overlying the basement of both the Tambo and the porphyry. Suretta nappes. This monocyclic Permian volcani- This PermoÐTriassic sequence is followed up- clastic cover progressively grades into conglom- wards by pure quartzite present only on the Suretta erates, subsequently metamorphosed into chlorito- nappe, probably Scythian in age. On Tambo and albitic gneiss. Our recent field work has revealed Suretta, a strongly reduced carbonate series, with 16

polygenic breccia, lying unconformably on the older No age determinations were possible and the in- sediment or directly on the basement exhibits a Bri- terpretation of the given ages has to be taken with ançonnais-type stratigraphy (Baudin et al., 1995). caution. They are based on a facies comparison with For a long time, the carbonate Mesozoic cover of the the typical French Briançonnais series (Barfe«ty et Tambo nappe (Splu¬gen zone) and the Suretta nappe al., 1992). Alternatively to the hypothesis of an early were regarded by most previous authors (with the ex- syn-orogenic history for the 4th e«caille described by ception of Staub, 1958b) as only Triassic. Our recent Barfe«ty et al. (1992), this series is interpreted here as field work has shown the presence of a complete a syn-rift sequence (Fig. 9). cover in a normal position, systematically overlying the PermoÐTriassic cover. It compares well with 3.2. Tectonic evolution of the eastern Penninic Alps other areas of the Briançonnais (Barrhorn series, Sar- tori, 1990; and Vanoise cover, Jaillard, 1988) char- 3.2.1. Introduction acterised roughly by the same stratigraphic sequence The Alpine nappe pile was created in a subduction starting with Middle Triassic dolomite, followed by zone environment during the closure of the Pie«mont the Dogger or Malm breccias, Malm layered marbles and Valais oceans. The Austroalpine nappes were and Cretaceous (up to Palaeogene?) phengite-rich thrust towards the west during the Late Cretaceous marbles or calcareous schists and breccia. These fa- oceanic subduction phase (see review in Froitzheim cies are also found in the Schams nappe, located et al., 1994), whereas the Penninic units were em- in front of Tambo=Suretta basement units (Schmid placed by thrusting towards the northwest in the et al., 1990; Mayerat, 1994). The thickness of the early Tertiary (e.g. Schmid et al., 1996). The upper Mesozoic Splu¬gen zone varies from a few metres up Penninic units represent an orogenic wedge, consist- to several hundred metres. ing of underplated basement and sedimentary slices detached during the oceanic closure (e.g. Marquer et Allochthonous cover. The sedimentary rocks above al., 1994). During the early Tertiary the Misox zone the Suretta and Tambo nappes are regarded as a was subducted below the Tambo and Suretta north- series which can be divided into a reduced au- ern Briançonnais border. The overall structure of this tochthonous cover overlain by a more complete al- area was already recognised by Milnes (1974), based lochthonous cover, the Starlera nappe (Baudin et al., on structural observations in the Suretta nappe. On 1995). It must be pointed out that the PermoÐTriassic a larger scale, the basement slices were thought to and the carbonate series of the autochthonous cover be a combination of thrusts and huge recumbent are frequently separated by a cornieule layer which folds (Milnes, 1974; Milnes and Schmutz, 1978). developed from Middle Triassic dolomites (and Recent structural and seismic investigations in the evaporites?) belonging to the Starlera nappe. The eastern Swiss Alps showed that the nappe geometry compilation of several sections, already described by appears to be related to thrust tectonics and post- Baudin et al. (1995), allows the stratigraphy of this nappe refolding (Schmid et al., 1990; Pfiffner, 1990; nappe to be summarised from the bottom to the top Pfiffner et al., 1990b; Schreurs, 1993; Marquer et al., as follows (Fig. 8). 1996). Recent investigations (Suretta nappe, Milnes Banded marbles and dolomites (Middle Triassic), and Schmutz, 1978; Marquer et al., 1996; Schams, dark stinky marbles sometimes with monogenic mi- Schmid et al., 1990; Schreurs, 1993; Tambo nappe, crobreccias (Dogger), massive ivory-white marbles Baudin et al., 1993; Mayerat, 1994; Suretta cover, (Malm) and a thick member of calcschists and brec- Baudin et al., 1995; Adula nappe, Heinrich, 1982; cias. These breccias are polygenic and contain clasts Lo¬w, 1987; Meyre and Puschnig, 1993; NFP-20- of basement, Triassic quartzites and dolomites as East, seismic lines, Pfiffner et al., 1988, 1990a; Frei well as Jurassic carbonates. At the top of the Suretta et al., 1989; Schmid et al., 1990) and tectonic models nappe, the breccia grades upwards into a thick calc- (Merle et al., 1989; Schmid et al., 1990; Marquer schist series, including large basement boulders and et al., 1994; Merle, 1994) gave different scenarios is therefore hardly distinguishable from the Avers of Tertiary tectonic evolution for this area. For the Schistes Lustre«s. Suretta and Tambo nappes, the sequence of Alpine 17

Fig. 9. Reconstruction of the eastern Swiss Briançonnais domain in the early-Middle Eocene. See text for discussion and references about the timing of events. 95 ate,18;Sher,19)adinterpreted and 1993) Schreurs, Suretta 1989; the Baltzer, Ja in 1985; and recorded are (Purdy Ma, nappe 30 ranging and ages, 45 phengite between RbÐSr Gebauer, and zircons; KÐAr (shrimp while Ma 1996) garnet; 35 (SmÐNd and Ma 1993) 37Ð44 Becker, around Cima-Lunga given and are Adula for nappes the nappes ages in metamorphism Suretta absolute HP Eocene The the and progressive north-northwest. Tambo the the Adula, to towards the linked of is stacking deforma- 10) ductile (Fig. D1 The tion 1990). 46Ð37 Odin, 1995; and al., Odin et Mid- Ma, Gradstein in Ma, likely (49Ð37 most Eocene and dle Eocene Early the below before not underplated as was Brianc zone northern dated the this Eiermann, was Therefore 1956; 1988). Arblatsch sediments, (Ziegler, the Eocene onset uppermost as PaleoceneÐEarly the closure the for ocean Valais bracket flysch, final time the minimum of a gives 1994a,b) (Steinmann, and zone oceanic Valais the to long Bu ex- the southern of the tremity constitutes zone Misox Brianc (Adula The nappes. margin the Misox European from the south nappe) suture, the narrow separates A stacking zone, 1994). crustal al., Tertiary et early (Marquer from results nappes Brianc the of Subduction 3.2.3. geometry. sequence of the following out constrains an the develop will it as thrusts because reconstructions importance palaeogeographic of tec- thin-skin is early directly tonics This nappe nappe. Suretta Starlera Suretta the the and overlies units, Tambo Suretta Splu and the the Tambo of In basement units. the autochthonous reduced or the cover either overlies tonically de cover early tectonics Thin-skin 3.2.2. 1996). al., et Nievergelt discussion event in (see in kinematics deformation overall disagreement associated D2 the some the and Marquer is concerning There 1993; models 1996). these al., 1994, Starlera et al., the Baudin et of to emplacement (according the nappe (D1 following stages D4) four in to divided be can events deformation h tcigo h dl,TmoadSuretta and Tambo Adula, the of stacking The an as defined recently 8), (Fig. nappe Starlera The cleet(adne l,19) tec- 1995), al., et (Baudin «collement nnrcifradflsh hc be- which flyschs and ¬ndnerschiefer oni el TmoadSuretta) and (Tambo realm ¸onnais gnzn oae ewe the between located zone ¬gen gr 96 tiizadJa and Steinitz 1976; ¬ger, oni ab n Suretta and Tambo ¸onnais oni terrane ¸onnais ¬ger, 18

to the closure of the Valais trough. As said before (rotation of Iberia) part of the structural fabric of the Valais accretionary prism could be Late Cretaceous, unless the Cretaceous prism was totally subducted.

3.2.4. Subduction of the European margin and syn-collision extension (related to slab-break-off?) The subsequent deformations D2, D3 and D4 did not modify the overall framework of the nappe pile. After the onset of continental collision, eastÐ west extension took place along major ductile displacement zones (e.g. Turba Mylonite Zone, Liniger, 1992; Nievergelt et al., 1996). Event D2 is a ductile and heterogeneous deformation associated with a gently E-dipping schistosity and an EÐW stretching lineation. Most of the D2 mylonitic zones cross-cut previous contacts and indicate top to the east shear- ing. As a consequence of the subvertical shortening between the subhorizontal D2 shear planes (Baudin et al., 1993), the gently SE dipping D1 foliation and the pre-Alpine foliations (previously steeply dipping Fig. 10. PÐTÐt paths for the eastern Switzerland Briançonnais towards the southeast) underwent strong SE-vergent basement nappes (see review of ages and references in Marquer folding. D2 is responsible for the large structure on et al., 1994). the top of the Suretta nappe developing both recumbent SE-vergent folds F2 with very low angles as crystallisation ages (as the Suretta nappe never between fold axes directed mainly N70 and EÐW reaches high temperatures; Ja¬ger et al., 1967). The stretching lineations. This D2 deformation is corre- white mica ages in the Suretta nappe were not se- lated with the NiemetÐBeverin phase described in lected with respect to the microstructural sites and the Suretta and the Schams nappes by previous au- could therefore not be linked precisely to the D1 thors (Milnes and Schmutz, 1978; Schmid et al., or D2 ductile deformations. Burial of the thinned 1990; Schreurs, 1993). continental crust of the eastern Briançonnais margin The Mesozoic sediments of the Schams nappes are induced in the basement nappes a ductile thrust- wrapped around the front of the crystalline basement ing D1 that propagated as isoclinal folds F1 into nappes (equivalent of the NiemetÐBeverin post-nappe the cover. D1 is associated with a strong SSEÐ fold in many works; Milnes and Schmutz, 1978; NNW stretching lineation and a top to the NNW Schmid et al., 1990; Schreurs, 1993). The rheological shear sense. Estimates of the PÐT conditions based contrast between basement and cover became very on phengitic substitution (Massonne and Schreyer, low during D2 deformation. This led to a very simi- 1987) in D1 mylonitic foliations systematically show lar style of heterogeneous deformation affecting both HPÐLT metamorphic conditions. For example, meta- basement and cover. The phengitic substitution values morphic conditions of about 12 kbar and 500¼C are measured in the D2 mylonites or in the D2 shear bands reached in the Tambo nappe (Baudin et al., 1993). indicate pressures decreasing with time and associ- In front of the Tambo and Suretta basement, the ated with a slight decrease of temperature (Fig. 10). pile of crystalline and sedimentary slabs (Areua, For example, a progressive decrease of pressure and Schams, Vignone) represents an accretionary wedge temperature from 1.1 GPa to 0.5 GPa and 550¼C to particularly well-developed in the northern Penninic 500¼C is recorded at the bottom of the Tambo nappe Bu¬ndnerschiefer and flysch (Steinmann, 1994a,b). (Baudin and Marquer, 1993) and from 1.0 GPa to 0.5 The overall geometry of the frontal slices is related GPa at 400Ð450¼C in the Roffna granite (Nussbaum et 19

al., 1998). The ductile D2 mylonites and shear zones viously described D3 phase. D3 deformation during were first created at a deep crustal level. The progres- cooling is supported by the south-dipping (north-ver- sively shallower tectonic setting of the Tambo and gent) thrusts which cross-cut the intrusion when so- Suretta nappes during D2 deformation, correspond- lidified (Rosenberg et al., 1994, 1995). Rapid cooling to isothermal decompression of about 0.5 GPa, is ing corresponding to the D3 uplift is also reported considered a syn-collision extension process caused from the Suretta and Tambo nappes from 30 Ma until by the D2 ductile vertical shortening, low-angle de- about 20 Ma (Hurford et al., 1989; Marquer et al., tachments toward the east (e.g. Turba Mylonite Zone; 1994). This ductileÐbrittle D3 deformation was gen- Nievergelt et al., 1996) and associated erosion. This erated by differential uplift of the southern part of progressive extension is defined by eastward-escap- the nappe and could be associated with the OligoÐ ing extensional structures produced by the relaxation Miocene vertical movements along the Insubric line of a buoyancy disequilibrium in an abnormally thick- which started shortly after the Bergell intrusion (Hur- ened accretionary prism (see review of exhumation ford, 1986; Heitzmann, 1987; Schmid et al., 1989). processes in Platt, 1993). The Turba mylonite, an EÐ The last extensional D4 deformation consists W extensional structure (Liniger, 1992; Nievergelt et of several NNWÐSSE brittleÐductile normal faults, al., 1996), related to the D2 event, is cross-cut by the steeply dipping towards the east-northeast and low- Bergell granodiorite (at Lavinair Crusc, Swiss geogr. ering the eastern sides with pluri-hectometric fault coordinates: 772=138), which intruded around 30 Ma throws (e.g. the Forcola fault; Marquer, 1991). This (von Blanckenburg, 1990). Therefore D2 must have late D4 extensional deformation seems to be a sym- occurred before 30 Ma. The Bergell intrusion is di- metrical structure, coeval with the last Simplon nor- rectly linked to the slab break-off (Davies and von mal faulting in the western part of the Penninic zone Blanckenburg, 1995). (Mancktelow, 1985; Steck, 1984, 1990). Although It is important to note that D2 WÐE stretching perhaps not contemporaneous with it, because of the was still active in the upper Penninic nappes while younging of the cooling ages towards the Simplon the lower Penninic nappes were progressively trans- area (Hurford et al., 1989; Hunziker et al., 1992). ported towards the north-northwest. Moreover, D2 D4 could also be linked to late transpression of the thinning could also explain part of the exhumation Apulian plate along the Alpine arc (Schmid and of the eastern part of the high-grade Lepontine ther- Froitzheim, 1993). The timing of the D4 phase is not mal dome (Bradbury and Nolen-Hoeksema, 1985; well constrained. We assume that the D4 phase may Hurford, 1986; Hurford et al., 1989; Merle, 1994). be around or younger than 20 Ma and is coeval with the dextral strike slip along the Insubric line, which 3.2.5. Subduction of the European margin and post-dates the uplift of the whole region. collision (back-thrusts and late extension) In this part of the Alps, the ongoing syn-colli- D3 and D4 deformation events occurred under sion extension, leading to D2 and D4 structures, is lower greenschist facies conditions and are much interrupted by a double event corresponding to the more localised. Event D3 consists of local staircase- Bergell intrusion and the D3 pop-up structures. The shaped folding with steeply S-dipping axial surfaces progressive deformation during D3 led to a succes- and EÐW fold axes. They are preferentially devel- sion from ductile to brittleÐductile structures. oped in the southern parts of the nappes (Baudin et al., 1993). In the root zone, a conjugate set of EÐW-directed thrusts is responsible for the pop-up 4. Western Briançonnais cover nappes structure formed by the whole Bergell area (Huber and Marquer, 1996). The D3 event is probably syn- 4.1. Geological setting of the Preálpes to post-Bergell granodiorite intrusion. Submagmatic fold-and-thrust belt deformation in the Tertiary intrusion, and the folding of the western intrusive contact (Davidson and 4.1.1. Introduction Rosenberg, 1996), indicate, in deeper levels of the We shall now move to the review of the Bri- continental crust, kinematics compatible with the pre- ançonnais sedimentary cover as exposed in western 20

Switzerland and in the French Savoie region. This Middle and Late Jurassic (Fig. 12). With the Early cover was obducted with the frontal part of the Cretaceous opening of the Valais rift to the north, the accretionary prism in Eocene times and therefore Briançonnais portion of the sedimentary basin and escaped metamorphism. All major geological events its basement evolved into a microcontinent until the are well dated by fossils and offer a perfect comple- Eocene (Frisch, 1979; Stampfli and Marthaler, 1990; mentary data set in regard to what can be defined Stampfli et al., 1991; Stampfli, 1993; Mosar et al., in the Briançonnais basement nappes in eastern and 1996). A variety of palaeotectonic features (normal western Switzerland. The reader can find more de- faults, syn-sedimentary growth structures, inversion tailed descriptions and discussions on the Pre«alpes structures; Septfontaine, 1995) developed during this Me«dianes, as well as references, in Masson (1976), palaeotectonic history. Isolated from the Iberian con- Plancherel (1979), Baud and Septfontaine (1980), tinent during the Palaeogene, the Briançonnais exotic Tru¬mpy (1980), Mosar et al. (1996), and a quite terrane was incorporated into the accretionary prism complete bibliography (more than 950 references) of the closing Alpine Tethys and the incipient Alpine on the World Wide Web site: www-sst.unil.ch=marge orogen during the LutetianÐBartonian. and co=prealps=REFEREN.htm. During its ‘Alpine’ deformation the Pre«alpes The Pre«alpes consist of several klippen along Me«dianes nappe was detached from its basement the northern front of the Swiss and French Alps and transported onto the foreland. Equivalent strati- (Fig. 11) from east of the Mythen near Luzern graphic units have been found in the Siviez-Mischa- (Switzerland) to the Klippe des Annes near An- bel and Pontis nappes of the Pennine Alps, south of necy (France), of which the Chablais Pre«alpes south the Rhone valley (Ellenberger, 1950, 1952; Sartori, of Lake Geneva and the Pre«alpes Romandes between 1987, 1990; Sartori and Marthaler, 1994; Fig. 11). Lake Geneva and Lake Thun are the most important. There the corresponding stratigraphic units (basal It was Schardt (1884, 1893, 1898) who first demon- series and lateral equivalents) remained attached to strated that the Pre«alpes were allochthonous. The their pre-Triassic basement and were intensely de- Pre«alpes Me«dianes (Fig. 11) are the largest of sev- formed. The very low-grade metamorphic conditions eral allochthonous structural and palaeogeographic (recently dated at 27Ð28 Ma, M. Jaboyedoff, Lau- units, among which one can differentiate from top sanne, oral commun., 1998) have their origin in the to bottom: (1) the Nappe Supe«rieure, which itself heat flux induced by tectonic burial beneath overrid- can be subdivided into four different units: the Gets ing nappes (Nappes Supe«rieures) in the accretionary nappe, the Simme nappe, the Dranses nappe and the prism. After having been transported on top of the Gurnigel nappe (Fig. 4); (2) the Bre`che nappe rest- developing Helvetic nappes, the Pre«alpes were em- ing on the trailing part of the Pre«alpes Me«dianes only placed in their present-day position in front of the (Figs. 11 and 12); (3) the Pre«alpes Me«dianes nappe; Alpine mountain belt during Oligocene times. Post- and (4) the Niesen nappe which exists in the merid- emplacement and out of sequence thrusting, possibly ional part of the Pre«alpes only and which today forms younger than Oligocene, is observed and can be re- the southernmost structural unit (Caron, 1972, 1973; lated to thrusting in the sedimentary substratum and Ackermann, 1979; Bernoulli et al., 1979; Matter et the basement (Stampfli, 1993, 1994; Mosar et al., al., 1980; Tru¬mpy, 1980; Caron et al., 1989). The 1996). Pre«alpes Me«dianes are separated from the Niesen nappe by the ‘Zone Sub-me«diane’ (Weidmann et al., 4.1.2. Sedimentology and palaeotectonics 1976). The Pre«alpes Me«dianes are formed by limestones, Between their original position and their present- dolomites, marls and shales ranging from Triassic day location as klippen, the Pre«alpes underwent a to Tertiary in age (Tru¬mpy, 1960, 1980; Badoux complex history of palaeotectonics and Alpine tec- and Mercanton, 1962; Plancherel, 1979, 1990; Baud tonics. Due to the opening of the Alpine Tethys and Septfontaine, 1980; Baud et al., 1989; Borel, ocean to the south, the Briançonnais sedimentation 1995; Mosar et al., 1996). The reader can find more realm of the Pre«alpes Me«dianes evolved as a rim detailed descriptions and discussions of the various basin on the northern passive margin during the formations encountered in the Pre«alpes Me«dianes in: 21

Fig. 11. Detailed map and cross-section of the Pre«alpes east of lake Geneva. UMM D Lower Marine Molasse; OMM D Upper Marine Molasse; USM D Lower Freshwater Molasse. 22

Fig. 12. Reconstruction of the Brianc¸onnais domain in Early Cretaceous and early-Middle Eocene. See text for discussion and references about the timing of events. Stretching factor from Marchant and Stampﬂi (1997b), assuming original crustal thickness of 30 km. 23

Baud (1972, 1987) for the Triassic; Thury (1973), interbedded black marls and black argillaceous lime- Mettraux (1989), Mettraux and Mosar (1989), and stone were deposited in an anoxic environment. An Borel (1998) for the Lower Jurassic (Lias); Furrer accelerated subsidence during the late Liassic re- and Septfontaine (1977), Furrer (1979), and Sept- sulted in the appearance of the deep argillaceous fontaine (1983) for the Middle Jurassic (Dogger); Cancellophycus limestone facies in the Me«dianes Weiss (1949), Isenschmid (1983), Heinz (1985), and Plastiques. This deposition continued into the Mid- Heinz and Isenschmid (1988) for the Upper Juras- dle Jurassic (Dogger) with sporadic appearances of sic (‘Malm’); Boller (1963) for the Lower Creta- oolitic or spatic and sandy limestone. Slumps and ceous (Neocomian); Caron and Dupasquier (1989), turbidites are frequent in a ramp setting dipping and Python-Dupasquier (1990) for the middle Creta- towards the north. To the south, in the Me«dianes ceous; Guillaume (1986) for the upper CretaceousÐ Rigides, a lagoonal facies developed starting in the Tertiary. Bajocian with its marly and argillaceous limestone The Pre«alpes Me«dianes are separated into two ma- containing Mytilus and coal layers. This facies is jor sedimentation realms clearly differentiated since transgressive on Triassic limestone in this merid- the Liassic and separated by two minor domains ional area. From the Middle Jurassic onward the rim (Fig. 13): to the north-northwest, in the Me«dianes basin geometry is well established, characterised by Plastiques, a large basin is marked by a thick Juras- a north-facing progradation, away from the opening sic sequence. To the south this subsiding domain Alpine Tethys in the south. turns into a ramp — associated with a continuously During the Late Jurassic 200Ð300 m of mas- active structural high — that gives way to a platform sive, thick-bedded limestone were deposited, with and lagoonal environment in the Me«dianes Rigides facies ranging from hemipelagic limestones with in- (Baud and Septfontaine, 1980). terbedded calcareous turbidites to shallow platform The sedimentary record in the two major domains carbonates in the Me«dianes Rigides (Heinz and Isen- is quite different (Fig. 13). It reflects the important schmid, 1988). These sediments rest in stratigraphic role played by the rift shoulder of the Alpine Tethys unconformity on Triassic limestones in the south- ocean since late Liassic, separating the rim basin ern part of the Me«dianes Rigides and indicate the to the north-northwest from the margin s.str. to the general drowning of the former rift shoulder at that south-southeast (Fig. 12). Sedimentary sequences of time (Borel, 1998). This ubiquitous, competent mas- Middle Triassic age in the Me«dianes Rigides are sive limestone controls the structural development of formed by massive, sometimes dolomitic limestones folds and thrusts. developed in a lagoonal and inter- to supratidal envi- Cherts are more frequent towards the top, with a ronment. In the Me«dianes Plastiques the stratigraphic progressive transition to the Lower Cretaceous (Neo- sequence starts with interbedded formations of Late comian) pelagic limestones interbedded with cherts Triassic shales and dolomites. The depositional envi- and marls. The sediments are strongly folded, mak- ronment is very shallow to lagoonal. ing it difficult to determine the original thickness Rhetian limestones including tempestites witness (estimated at 100Ð150 m). The succession thins to- the onset of crustal extension and may be as thick wards the centre of the basin and pinches out to the as 200 m (Borel, 1998). Lumachellic limestones, southeast, suggesting deposition along a slope. oolites, sandy, as well as entrochal calcarenites and An important change occurred during the late- spongolites indicate a general deepening of the de- Early Cretaceous as shown by the appearance of positional environment during early to middle Lias. pelagic and hemipelagic argillaceous, anoxic sedi- Locally, polyphase megabreccias, karsts and fissure ments and a large decrease in sedimentation rate. fillings developed (Baud and Masson, 1975; Baud The interbedded limestones and shales (grey, red et al., 1979; Mettraux and Mosar, 1989; Hu¬rlimann and green) of the Late Cretaceous=early Tertiary et al., 1996; Poinssot et al., 1997). The active de- Couches Rouges Group indicate yet another ma- velopment of syn-sedimentary normal faults created jor change in environment. These sediments are small depositional domains, with series up to sev- deposited in a drowned and starved realm, possi- eral hundred metres thick. During Toarcian times, bly reaching the CCD. Important stratigraphic gaps 24 25

(Fig. 13) during this Late Cretaceous to Eocene pe- their ramp portion. The imbricates dip gently to the riod are recorded by hardgrounds and condensed north in the Simmental area of the eastern Pre«alpes beds. The Couches Rouges Group rapidly changes Me«dianes and are steeply dipping in the western re- upward to schistose sand-rich flysch deposits of gion near Chaˆteaux-d’Oex (Fig. 11). Different types Lutetian age, thus ending the sedimentation history of backthrusts have developed: large backthrusts re- in the Pre«alpes Me«dianes. sulting from the inversion of former normal listric, syn-sedimentary faults; backthrusts associated with 4.1.3. Regional structural geology of the Preálpes pop-up structures developed in large-scale fold cores; Me´dianes and backthrusts developed at the transition between The Pre«alpes Me«dianes underwent thin-skinned ramps and flats in thrust hangingwall blocks. tectonics and show the typical characteristics of a foreland fold-and-thrust belt (Fig. 11; see Mosar 4.1.4. Transported metamorphism and internal and Borel, 1992; Mosar, 1994, 1997; Mosar et al., deformation in the Preálpes Me´dianes 1996, and references therein). The foreland prop- Numerous studies in recent years (see Mosar, agating thrust sequence developed above a basal 1988; Zahner and Mosar, 1993; Jaboyedoff and de«collement in Triassic evaporites, mainly during its The«lin, 1995) have shown that the Pre«alpes Me«- incorporation in the accretionary prism of the clos- dianes underwent a very low-grade metamorphism. ing Alpine Tethys. It is along this de«collement that This metamorphism varies from diagenesis in the the Pre«alpes have been detached from their basement north (i.e. in the Me«dianes Plastiques) to epizone in and partly carried over the Helvetic units onto the the south (i.e. in the Me«dianes Rigides, 300Ð400¼C). NNW Alpine foreland. Two major transport direc- This zonation also exists from top to bottom in tions, top-to-the-N=NW, towards the Alpine foreland the southern part of the Pre«alpes nappe stack. The have been determined. At the western termination epizonal conditions contrast with the diagenetic con- late movements to the west in the complex frontal ditions preserved in the top part of the underlying imbricate of the Me«dianes Plastiques can be demon- Niesen nappe. Metamorphism in the Pre«alpes Me«di- strated (Mosar, 1994; Mosar et al., 1996). anes is thus a transported feature! Similar disconti- The Pre«alpes Me«dianes nappe is subdivided into nuities in metamorphism occur between the Niesen the Me«dianes Plastiques, forming the frontal (NW) nappe and the underlying Ultrahelvetic units, and part of the nappe and Me«dianes Rigides, forming between the Ultrahelvetic units and the underlying the trailing (SE) part of the nappe (Fig. 11). A do- Helvetic nappes (Burkhard, 1988). main with intermediate characteristics exists between Metamorphism of the Pre«alpes Me«dianes occurred the Me«dianes Rigides and Plastiques: the Gastlosen during thrusting of the overlying nappes and subse- range. The Me«dianes Plastiques are composed of a quent burial. In order to explain the epizonal tem- succession of large-scale fault-related folds, whose peratures above 300¼C with a geothermal gradient of trends vary from EÐW in the eastern part of the ¾30¼C=km, the Pre«alpes Me«dianes would thus have nappe to NNEÐSSW, and even NÐS, in the west- to have been buried to a depth of 10 km in their trail- ern part of the nappe. Folds and associated thrust ing portion, whereas the frontal portion was probably planes die out along-strike, resulting in ‘en e«chelon’ only buried to a depth of ¾4to5km.Thesecould fold-thrust structures. The trailing part of the nappe, be minimum estimates of burial depth, since in the the Me«dianes Rigides, is formed by one major, in context of subduction in an accretionary prism the some places one or two minor, imbricated thrust geothermal gradient would have been significantly slices that dip to the N=NW. These slices form fault- lower than normal, although slab detachment in the bend-like folds that are cut by a large backthrust in Early Oligocene would have increased this gradient.

Fig. 13. Stratigraphic scheme for the Pre«alpes Me«dianes (modiﬁed from Mosar et al., 1996 and Borel, 1998). Formation names are shown; hatch pattern indicates erosion and=or non-deposition; timescale after Gradstein et al. (1995). Palinspastic reconstruction (a) modiﬁed from Baud and Septfontaine (1980). 26

Rb=Sr, K=Ar and 40Ar=39Ar dating of white micas Nappe Supe«rieure and the infra-Pre«alpes Me«dianes in the Me«dianes Rigides from Triassic and Middle me«langes (they are mostly supra-Helvetic) that are Jurassic limestone yielded ages of ¾60 to 80 Ma (the linked to the Pre«alpes Me«dianes emplacement. Gummfluh imbricate, Masson et al., 1980; Huon et al., 1988; Cosca et al., 1992; the Amselgrat imbricate, The me´lange zones. The Pre«alpes Me«dianes rest on De Coulon, 1990). Results from a recent investigation top of the Helvetic nappes in the south and the Sub- by M. Jaboyedoff and M. Cosca (oral commun., 1998, alpine Molasse and flysch in the north. Along this Lausanne) show that these ages are mixed ages and contact we observe the Ultrahelvetic and the ‘Zone that the age of the metamorphism in the Me«dianes Subme«diane’ units which have been interpreted as Rigides is early-Late Oligocene (27Ð28 Ma). me«langes similar to those formed in accretionary The internal deformation of the Pre«alpes Me«di- prisms (Jeanbourquin, 1992, 1994; Jeanbourquin et anes limestone has been quantified using the dis- al., 1992). They were subsequently strongly over- tortion of pellets and ooids as well as twinning printed by Alpine tectonics, especially during nappe of sparitic calcite (Mosar, 1989). The strain inten- emplacement. We shall review these me«langes start- sity in the frontal Me«dianes Plastiques is very low ing from the external to the internal. and dominated by transgranular deformation mecha- (1) The Ultrahelvetics can be subdivided into up- nisms, mainly pressure solution, with weak develop- per and lower Ultrahelvetic me«langes. The lower ment of calcite twins. This deformation corresponds ones are always associated with a specific Helvetic to an ‘early’ shortening parallel to bedding, em- nappe, whereas the upper ones do not show this phasised by the development of tectonic stylolites. systematic correspondence and their kinematic de- The strain increases towards the Me«dianes Rigides, velopment appears to be closely related to the evo- where intragranular deformation mechanisms, such lution of the Pre«alpes nappe stack (Jeanbourquin, as twinning (numerous large, curved and twinned 1991a,b, 1992, 1994; Jeanbourquin et al., 1992). twins) and dynamic recrystallisation, prevail. The These Ultrahelvetics are formed by a succession of most intense strain is associated with thrusting along units and slivers (or nappes) in each of which a com- the basal de«collements of the Pre«alpes Me«dianes plete (though sometimes dismembered) stratigraphic nappe and of the overlying Bre`che nappe. The max- sequence is defined (Badoux, 1963, 1965; Lempicka- imum finite extension direction is sub-parallel to the Mu¬nch and Masson, 1993). Their palaeogeographic thrust planes in their vicinity. A sub-horizontal cleav- origin is located to the south of the Helvetic domain age, which makes only a small angle to the bedding and in the external Valaisan realm (Jeanbourquin and planes, developed in the same context. Elsewhere in Burri, 1991). In the meridional Pre«alpes they are lo- the Me«dianes Rigides, the extension direction dips cated between the Pre«alpes nappes and the Helvetic at a high angle to the thrust planes, reflecting early nappes. The Ultrahelvetic me«lange is also found be- layer-parallel shortening similar to that recognised in neath (Badoux and Norbert, 1952) and in front of the the Me«dianes Plastiques. Pre«alpes Me«dianes nappe (Weidmann, 1985, 1992). (2) The Zone Subme«diane (Weidmann et al., 4.1.5. Transitions from the Preálpes Me´dianes to 1976) is found below the basal de«collement of the neighbouring structural units meridional part of the Pre«alpes Me«dianes. It forms The transition from the Pre«alpes Me«dianes nappe the contact with the underlying Niesen flysch nappe to the underlying and overlying structural units is in the central and eastern Pre«alpes Romandes and the marked by me«lange units which are often termed Ultrahelvetic units in the western part of the Pre«alpes wildflysch in the Alpine literature (Homewood and Me«dianes along the Rhone valley. This zone is a Caron, 1982). The contacts are characterised by spe- me«lange of blocks of various lithologies recording cial types of units and rocks such as me«langes and a continuous sedimentation from Triassic to Eocene. rauhwackes (cornieules) associated with anhydrites. Its origin has tentatively been located in the Valais For the Pre«alpes Me«dianes two groups of me«langes oceanic domain of which it represents the suture, can be distinguished: the supra-Pre«alpes Me«dianes therefore, north of the Pre«alpes Me«dianes deposi- me«langes mostly related to the emplacement of the tional realm (Weidmann et al., 1976; Stampfli, 1993; 27

Jeanbourquin, 1994). The presence of Liassic brec- from the Atlantic magnetic anomalies and reported cias, pelagic Dogger and Malm give a slope-to-basin along a transect perpendicular to the western Alps. character to this sequence, the presence of Urgonian- The time sequence of incorporation of the dif- type limestone allows this sequence to be positioned ferent terranes on a western Alps transect is well south of the Helvetic domain. This is one of the argu- established based on the age of associated flysch ments used by Stampfli (1993) to propose a duplica- deposits (Caron et al., 1989). tion of the north Alpine Tethys margin in the western In the Pie´mont domain on a western Alps tran- Alps transect. The Valais ocean rifting is documented sect, the Gurnigel flysch was deposited from the by the Late Jurassic=Early Cretaceous spilite and the Maastrichtian until the Middle Eocene (Caron et Albian sandstones and breccias. Very large outcrops al., 1980a,b) and followed by the chaotic complex of of anhydrites and gypsum deposits are present along likely Middle Eocene age (Steffen et al., 1993) which the Rhone valley between Ollon and Bex. These an- includes elements from the Briançonnais southern hydrites, of which some were formerly attributed to margin (Bre`che nappe). the Ultrahelvetic units, can now be considered part In the Briançonnais domain the deposition of the of the Zone Subme«diane. Superposed folding of the Me«dianes flysch (Caron et al., 1980a; Guillaume, anhydrite bands is evidence of multiphase deforma- 1986; Hable, 1997) lasted at least until Lutetian time tion (Zahner and Mosar, 1993; Mosar et al., 1996). (NP 15, 47 to 43 Ma or CP 14, 43Ð37 Ma, accord- A wildflysch forms the matrix of the zone together ing to Wei and Peleo-Alampay, 1993). It is a rather with the gypsum and cornieules. Some cannibalism distal flysch deposit, precluding any deposition on of me«langes can be demonstrated, such as the Pale- an already detached substratum. Thus the subduction ocene Trom Breccia (with basement and Tithonian of the Briançonnais domain did not take place be- limestone clasts) reworked in an Eocene wildflysch. fore the late-Middle Eocene, as is confirmed by the A Tertiary flysch with Nummulites, Discocyclines fact that metamorphic ages from the Briançonnais and Globorotalia is also present. The deformation, basement in Valais start at around 38 Ma (Fig. 10) as well as the mixing of the me«langes, has largely (Markley et al., 1995, 1998). Slivers of basement been explained by thrusting along the Pre«alpes Me«di- with part of their cover were detached from the sub- anes basal de«collement. Thus, the Zone Subme«diane ducting slab and underplated; they form presently me«lange is primarily tectonic in origin (Jeanbourquin the bulk of the middle Penninic domain (Escher et op. cit.). al., 1997). Part of the cover was detached from the (3) The ‘Flysch with Couches Rouges lenses’ basement and incorporated into the still-active accre- (Chaotic complex in Fig. 12) is found on top tionary prism to form the exotic Pre«alpes Me«diannes of the Pre«alpes Me«dianes as well as associated domain (Mosar et al., 1996). with the overlying Bre`che nappe and=or the Nappe Elements from the Valais ocean were then accreted Supe«rieure. These complexes can form rather con- and are represented by the ‘Valaisan trilogy’ (Aro- tinuous horizons and primarily contain characteristic ley, Marmontains, Saint Christophe; Burri, 1958) slivers of the hemipelagic Couches Rouges Group. made of poorly dated clastic deep-sea sediments out- cropping at present in the Sion-Courmayeur zone. 4.2. Geodynamic evolution of the western Penninic MORB from the Valais ocean are found in a few Alps places in the Valais and in eastern Switzerland in the Bu¬ndnerschiefer (Du¬rr et al., 1993). The obduction 4.2.1. Subduction–obduction processes of oceanic material could witness the Cretaceous Subduction and obduction processes affecting the very oblique subduction phase of the Valais ocean, oceanic sequences of the Pie«mont ocean are shown but no metamorphic Cretaceous ages or Cretaceous in Fig. 4. This figure together with Fig. 5 is based me«langes have been found so far. However the St. on spreading rates obtained from the central Atlantic Christophe flysch or Flysch de Tarentaise is recog- magnetic anomalies which allow the opening of the nised as sedimented on a structured substratum (e.g. Alpine Tethys to be defined. The rates of conver- Antoine, 1971; Ge«ly, 1989; Fudral, 1996) and its age gence between Africa and Europe are also derived is Campanian or younger, which fits the closure of 28 29

the Valais domain at that time due to the opening 1988; Charollais et al., 1993), but the Priabonian of the Biscay ocean (Fig. 2). HP metamorphism is fauna is described as reworked in the Nie«lard flysch recorded in these sequences but not dated yet. HP (Ge«ly, 1989). Most of the attenuated crust from the ages in eastern Switzerland (Bu¬ndnerschiefer; Goffe« Helvetic margin was subducted and little is known and Oberha¬nsli, 1992) are between 44 and 35 Ma about its syn-rift sequences. The highly metamor- (see above). Cooling ages in the Valais sequences of phic Lebendun conglomerates of the Simplon region the French Alps are centred around 33 Ma (Cannic, have been interpreted as syn-rift deposits (Spring et 1996). If a Late Cretaceous accretionary me«lange ex- al., 1992). Their obducted equivalent is represented isted it was certainly reworked and incorporated into by the sub-Niesen Dogger sequence from le Sepey younger me«lange possibly affected by HP metamor- (Homewood, 1974; Badoux and Homewood, 1978), phism. Recent microfossil findings in the internal made of pebbly mudstone, conglomerates (with Tri- Valais me«langes (the external Zone Houille`re) allow assic and basement clasts) and thick turbiditic sand- the closure of the remnant Valais ocean to be de- stones. The Monte Leone crustal nappe is considered termined. Late Eocene to possibly Early Oligocene a former tilted block from that margin. It is presently pelagic microfauna has been found in these me«langes in tectonic contact with mantle rocks from the Geis- (M. Sartori, oral commun., 1998, Geneva; Bagnoud spfad massif (Keusen, 1972; Pastorelle et al., 1995), et al., 1998) which reworked mainly the Briançon- representing denuded mantle from the toe of the nais basement (Zone Houille`re) and parts of its Juras- margin. The Zone Subme«diane elements (see above) sic cover (e.g. Pierre Avoi sequence). Palaeogene and are to be placed on this distal part of the Helvetic Cretaceous (not older than Aptian) microfauna are margin, the younger flysch deposits there are at least also reworked, but as no Cretaceous element from of Middle Eocene age. General metamorphism of the the Subbriançonnais basin are reworked this implies south Helvetic domain ranges in age from 30 to 20 (a) the presence of pelagic deposits of this age in the Ma (cooling ages; Steck and Hunziker, 1994) and Valais domain (the Valais trilogy), and (b) that the mylonites from the Helvetic nappes have been dated Subbriançonnais cover nappes (Pre«alpes Me«diannes) between 32 and 13 Ma (Kirshner et al., 1996). were still attached on their substratum in the Early Further subduction of the Helvetic rim basin led to Oligocene. In this context it is possible that part the decoupling of major parts of its crystalline base- of the ‘ophiolitic’ Versoyen sequence is effectively ment (Mont Blanc and Aiguilles Rouges massifs; of Carboniferous age (Cannic, 1996; Cannic et al., Fig. 14). This decoupling was made possible by the 1996) and reworked as major olistolites into this Late increased heat flux following slab detachment around Eocene=Oligocene me«lange. 35Ð33 Ma (Wortel and Spakman, 1992; Stampfli Incorporation of the Helvetic margin into the and Marchant, 1995, 1997). This detachment cor- prism (Fig. 14) is well constrained by the ages of responds to the emplacement of the peri-Adriatic the south Helvetic flysch (e.g. Jeanbourquin and intrusives (e.g. Bergell intrusion) and the rapid Early Burri, 1991; Jeanbourquin, 1994) and their possible Oligocene transgression on the Helvetic domain fol- equivalent in France (e.g. Nie«lard flysch; Ge«ly, 1989) lowing a temporary de-flexuring of the lower plate and the Sardona flysch in east Switzerland (Lihou, and which created an underfilling of the foreland 1996). It can be placed after the Priabonian since the basin at that time (Sinclair, 1997). This phenomenon Valais me«lange is dated as Early Oligocene and the allowed further subduction of the European plate Ultrahelvetic flysch has been dated in the western (Marchant and Stampfli, 1997a). Younger phases of Alps as Late Eocene (37 to 34 Ma; e.g. Kindler, advance of the prism into the European margin are

Fig. 14. Reconstruction of the Helvetic margin in Maastrichtian and Early Oligocene. See text for discussion and references about the timing of events. Stretching factor from Loup (1992). AR D Aiguilles rouges massif; Di D Diablerets nappe domain; DB D Dent Blanche nappe; Le D Lebendun sequence; MB D Mont Blanc massif; MF D Mont Fort nappe; Mo D Morcles nappes domain; Me´-Br D Me«dianes and Bre`ches nappes; ML D Monte Leone nappe; Ni D Niesen nappe; N-S D Nappes Supe«rieures; S-C D zone Sion-Courmayeur; S-M D Siviez-Mischabel nappe; SuM D zone Subme«dianes; V-A D VerampioÐAntigorio nappes; Wi D Wildhorn nappe domain; ZH-Po D zone Houille`re, Pontis nappes. 30

well recorded by the Molasse basin deposits (e.g. A general emersion of the HelveticÐDauphinois Schlunegger et al., 1997) and the obduction of the and Subalpine domains took place at least as early as Jura (e.g. Burkhard and Sommaruga, 1998). Paleocene time (Allen et al., 1991; Lihou and Allen, 1996; Burkhard and Sommaruga, 1998) and can be 4.2.2. Subduction of the Pie´mont ocean and related to the only flexural bulge effect. In view of evolution of the flexural bulge different transgressive patterns of the Nummulitic We reported the evolution of the flexural bulge platform onto the retreating forebulge (Herb, 1988; in time along a convergent Alpine transect as shown Menkweld-Gfeller, 1997), it could be suggested that in Fig. 5. In the Briançonnais and Helvetic domains the collision was slightly oblique and diachronous, the flexural bulge can be detected through a good being older eastward than westward. preservation of the sedimentary sequences. The flexural bulge affected the Briançonnais domain in the Late Cretaceous and could have been 5. Discussion responsible both for the major hiatuses in the sedimentary record at that time (Figs. 3 and 13) and 5.1. Incorporation of the Briançonnais terrane in for the very condensed nature of the section as the accretionary prism, implications a whole (Couches Rouges, Guillaume, 1986). The Valais ocean oblique subduction at that time would In view of the data presented above, this incorpo- have affected this area in a similar fashion, due ration can now be quite precisely placed in Middle to the high buoyancy of the young Valais oceanic Eocene time, and certainly not before 45 Ma. This crust. Therefore it is difficult to relate this uplift date comes from the age of the Me«dianes flysch and associated breccia deposits to one event or the (43Ð47 Ma) and chaotic complex (younger than 46 other. Late Cretaceous to early Tertiary breccias are Ma) as well as from the metamorphism of the Bri- rather limited in the Subbriançonnais (Hu¬rlimann ançonnais basement between 32 and 38 Ma for the et al., 1996) but quite widespread in the French Penninic nappes of the Valais and between 35 and Briançonnais (Chaulieu, 1992). The flexuring might 40 Ma for the east Switzerland nappes. Westward, in also be responsible for the emplacement of within- the French Briançonnais, Priabonian limestones fol- plate basalts in MaastrichtianÐPaleocene times in the lowed by a chaotic complex are known (e.g. Deville, Briançonnais domain (E. Deville, pers. commun., 1987), giving an even younger age of incorporation 1992; Deville, 1993), although they could also be there between 37 and 34 Ma. The most eastern Bri- related to the onset of separation of the Briançonnais ançonnais elements (i.e. Sulzfluh and Falknis cover from the Iberian plate. nappes, northeast of Tambo and Suretta) have a The distal Helvetic margin was affected by verti- lithological sequence very similar to the Pre«alpes cal movements (mainly inversion of former normal Me«dianes (Tru¬mpy, 1970, 1980). These sequences faults) already in the Late Cretaceous (Figs. 5 and comprise the Couches Rouges formations terminated 14) characterised by the Niesen flysch deposits (Ack- by a flysch sequence dated as Late PaleoceneÐEarly ermann, 1986) that we link to the Valais ocean sub- Eocene in the Falknis nappe. However, the Couches duction. The general onset of widespread inversion Rouges of the Sulzfluh nappe are also dated as Pale- on the European margin and further north starting ocene. If we extrapolate the formation ages from the in the Late Cretaceous (Ziegler et al., 1995) implies Pre«alpes Me«dianes, where the Paleocene Chenaux intra-plate stress generation due to recoupling of the Rouges formation reaches the Middle Eocene, the subducting plate with the upper plate. This is cer- Briançonnais flysch of eastern Switzerland could be tainly true for the Valais subduction where the sub- younger than Early Eocene as the fossils in such ducting oceanic material is young and buoyant and deposits are usually reworked. Tru¬mpy (1980) sug- also in the Meliata subduction zone where the Aus- gested the incorporation of these eastern Briançon- troalpine (e.g. Schweigl and Neubauer, 1997) and nais elements during his Meso-Alpine phase which inner Carpathian microcontinents (e.g. Plasienka, he places in Late Eocene. It could be suggested, 1996) started subducting in the Early Cretaceous. however, that this incorporation into the accretionary 31

prism was diachronous, starting in the Lutetian (49 ages). A younger LuÐHf age (32.8 Ma, Ducheˆne to 41 Ma, Gradstein et al., 1995; 46 to 40 Ma, et al., 1997a) obtained for the UHP rocks of Dora Odin and Odin, 1990) in the east and finishing in Maira is a cooling age (Lardeaux, oral comm. Biella, Priabonian (37 to 34 Ma) in the west. see Ducheˆne et al., 1997b). Therefore it cannot be The implications are as simple as they are funda- used to prove the Penninic (Briançonnais) origin of mental. Regarding possible HPÐLT metamorphism these rocks, which were buried to nearly 150 km. that some Briançonnais basement units could have A 49.2 Ma age obtained by the same authors on developed, this metamorphism should be necessarily the Monviso ophiolites proves that the Alpine Tethys younger than 45 to 49 Ma. Even if the toe of the oceanic crust was still being subducted around that Briançonnais margin entered the accretionary prism time. The middle and upper Penninic nappes must around 47Ð49 Ma, with a convergence rate of 2.0 have reached blueschist or eclogitic metamorphism cm=year and a Benioff zone at 35 to 45¼, it would during the subduction of the Alpine Tethys ocean, take 3 to 5 Ma for these rocks to reach HPÐLT con- not during the collisional phases (Fig. 15). Being of ditions, thus not before 45 Ma, or even 42Ð43 Ma in continental origin, their motherland could only be the case of UHP rocks. the southern Pie«mont margin (Adria or Austroalpine) Most HP=LT ages measured so far in the internal not the Briançonnais. massifs (Monte Rosa, Grand Paradis, Dora Maira) We hope that this will help conclude the ongoing are older than 45 Ma (see Dal Piaz and Lombardo, discussion about the existence or non-existence of 1986 and Hunziker et al., 1992 for a review of an eoalpine HPÐLT metamorphic phase (e.g. Tilton older ages, and Ducheˆne et al., 1997a for younger et al., 1989; Deville, 1993; Bowtell et al., 1994),

Fig. 15. (A) Reconstruction of the Alpine Tethys active margin in Aptian times: Ar D Arosa (Dent-Blanche); MR-GP D Monte Rosa, Grand Paradis; SA-Iv D southern AlpsÐIvrea; Va-Se D Valpelline (Dent-Blanche)-Sesia; Z-S D Zermatt-Saas ophiolite. (B) Thermal model in Campanian times with indication of depth reached by the eclogitic internal massifs. (C) Diagram of maximum PÐT conditions for the western Alps eclogitic internal massifs. Dora Maira: Hunziker et al. (1989), Michard et al. (1993), Sharp et al. (1993); Zermatt-Saas: Hunziker et al. (1989), Reinecke (1991), Balle`vre and Merle (1993). Sesia: Hunziker et al. (1989), Balle`vre and Merle (1993), Venturini (1995); Monte Rosa, Grand Paradis: Monie« (1985), Hunziker et al. (1989), Balle`vre and Merle (1993). 32

Fig. 16. Cross-section of the western Alps on a western Switzerland transect, and simplified palinspastic model, modified from Marchant (1993), Marchant and Stampfli (1997b). 33

or its interpretation as evidence of a Cretaceous rocks, recent (Hunziker et al., 1992) and ongoing collision between Adria and the Briançonnais (e.g. (J.C. Hunziker, pers. commun., 1998) measurements Deville, 1993). Somehow this eoalpine subduction of fission track ages in the internal massifs retrace phase is needed as the subduction of the Pie«mont the exhumation processes of these rocks, ages as old ocean lasted for approximately 80 Ma. Subduction as 49 Ma have been found in Dora Maira and would most likely started in the ValanginianÐHauterivian confirm the eoalpine age of the UHP metamorphism. due to the plate rearrangement at that time (Fig. 2) and subduction-related metamorphism just after that 5.2. Ongoing subduction and collision (e.g. HPÐLT ages of the Sesia massif at 129 Ma, Oberha¬nsli et al., 1985). It was accompanied by ma- The HPÐLT metamorphism lasted until the incor- jor flysch input starting in the CenomanoÐTuronian poration of the Briançonnais into the prism but most (Fig. 4) both in the Liguro-Pie«mont ocean (Caron likely after too, at least until the collision with the et al., 1989; Gasinski et al., 1997) and Lombardian Helvetic margin as shown by the ages for the HP rim basin (e.g. Bichsel and Ha¬ring, 1981; Bernoulli metamorphism in the Adula and Cima-Lunga nappes and Winkler, 1990), related to increasing underplat- (37Ð44 Ma, Becker, 1993; 35 Ma, Gebauer, 1996) ing. This implies that a subduction-related phase of pertaining to the distal Helvetic margin. The subduc- deformation and metamorphism has to be present tion of the remnant Valais ocean is constrained by in the Alps as well as subduction-related me«langes the ages of sedimentary rocks deposited in this re- where underplated elements of oceanic and conti- gion such as the Arblatsch flysch (PaleoceneÐEarly nental origin are mixed together, either on their way Eocene, Ziegler, 1956; Eiermann, 1988), the inner down or during exhumation phases. Such a me«lange Valais me«langes (Late EoceneÐ?Early Oligocene, M. is found in the upper Penninic nappes (Fig. 16) such Sartori, oral commun., 1998; Bagnoud et al., 1998) as the Antrona (ophiolites), Monte Rosa (continental and the Zone Subme«diane flysch, also of Paleocene basement), Zermatt-Saas (ophiolites) and Sesia (con- to EarlyÐMiddle Eocene age. Actually the latter de- tinental basement) juxtaposition; all of these units posits are poorly dated and, as is the case for other present HPÐLT assemblages and have ages between Helvetic flysch, reworking of large foraminifers and 52 (Bowtell et al., 1994) and 129 Ma (Oberha¬nsli et even plankton is quite systematic and ages based on al., 1985). nummulites are maximum ages. The Sardona (east Eclogitic eoalpine units also experienced more or Switzerland, Lihou, 1996) and Meilleret flysch (west less rapid uplift and changing PÐT conditions usually Switzerland, Homewood, 1974) of external Valais causing development of greenschist metamorphic fa- origin are as young as late-Middle Eocene (37 Ma). cies and obliterating previous HP assemblages. In These data point toward an ongoing process of ob- view of the changing geometry of the subduction duction of the Briançonnais terrane between 45 and zone (incorporation of exotic terranes, slab detach- 40 Ma, then of the Valais sequences, followed by the ment) it could also be envisaged that some units obduction of distal Helvetic margin elements since went into HPÐLT conditions in different stages, with 34Ð33 Ma. This is quite different from the Early only the last stage being recorded in terms of abso- Eocene subduction scenario developed by Pfiffner lute dating. A stepwise burial of underplated units (1986) and still supported by Schmid et al. (1997) is merely the result of coupling and decoupling of for the Valais and distal Helvetic units in eastern both plates and depends largely on the buoyancy of Switzerland. the lower plate. The subduction of a mid-oceanic As discussed above, closing of the Valais ocean ridge or of lithospheric roots of terranes (like the had already started in Late Cretaceous during the Austroalpine) increases this buoyancy and increases rotation of Iberia from Turonian to Campanian. At the coupling. A unit formerly scrapped off in the a certain stage (during the Paleocene) the closure of prism can then be buried deeper and this process the Valais ocean stopped and the Briançonnais was can be repeated several times. Tertiary ages do not severed from the Iberian plate and somehow attached exclude older eoalpine ages, both are compatible and to the subducting European plate. This rearrange- necessary. Finally and to close this discussion on HP ment of plates was accompanied by a relative lack 34

of convergence between Africa-Adria and Europe for continental subduction of the European plate at that time (Figs. 4 and 5). This is the Paleocene (Marchant and Stampfli, 1997a) at least until the restoration phase of Tru¬mpy (1980). One of the main Pliocene. The overfed prism developed large-scale outcomes is that the subducting European slab can backthrusting and the Adriatic indenter and Insubric be regarded since the Paleocene as a single slab line turned into an A-subduction zone with important carrying the lithospheric root of the Briançonnais southward thrusting of the southern Alps (Figs. 16 terrane (Fig. 17). and 18). Lateral westward escape of the over-thick- Collision processes proper started as soon as the ened prism became possible as it advanced into the unthinned Helvetic basement entered the subduction European plate leaving behind the Iberian plate. The zone. Decoupling of this basement left some room prism was then affected by severe unroofing and EÐ

Fig. 17. Evolution of the Pie«mont subducting slab from Eocene to Present. Location of cross-sections in Fig. 18. 35

W extension and the arc shape of the western Alps Ma) has been related to the closure of the western- was formed by lateral escape of the upper plate; most extension of the Meliata rift (Venturini, 1995). in this context HP units represent extensional core This closure in the Austroalpine domain produced complexes. These events correspond to the D2 to D4 HP rocks dated between 150 and 95 Ma (Tho¬ni deformations of the eastern Briançonnais basement, and Jagoutz, 1993) and HP detrital minerals are straddling the 30 Ma time boundary of the Bergell recycled in Turonian flysch in the eastern Alps (Win- granitic intrusion. kler and Bernoulli, 1986). Early Cretaceous (Albian) As soon as the Helvetic basement entered the sub- primitive alkaline volcanism found in the Northern duction zone, the increased buoyancy of the lower Calcareous Alps (Trommsdorff et al., 1990) preclude plate generated the detachment of the subducting the existence of any subduction zone in the Austrian slab (corresponding to the Pie«mont ocean, the Bri- Penninic ocean at least before that time. Subduction ançonnais lithospheric root and the Valais ocean; of the Penninic oceanic domain might have started Fig. 17). The temporary unflexing of the lower plate as late as the Campanian as shown by the rapid generated a rapid transgression of deep water fa- deepening of the Gosau basin at that time (Wagreich, cies over the Helvetic domain around 34Ð35 Ma 1993). (Fig. 14), followed by the emplacement of the peri- The Meliata slab produced a slab pull force into Adriatic intrusions and related volcanism as found Europe. The subducting slab buoyancy might have reworked in the Taveyannes flysch (Vuagnat, 1983). become quite high due to remnants of continental In this context of changing geometries of the oro- crust attached to the slab (i.e. the Austro-Carpathian gen, it is interesting to note the different structures of lithospheric roots); underplating was then the most the flexural bulge. During Eocene times, before the probable type of subduction=obduction mechanism. slab detachment, the flexural bulge was quite large The absence of an asthenospheric wedge between the (Fig. 18) and the elastic thickness around 50 km. two plates could then explain the absence of a mag- After slab detachment the bulge became smaller as matic arc along this subduction zone. Due to thick continental subduction took place. Modelling of the continental lithospheric roots, underplating of the development of the Molassic foreland basin shows subducting slab would have been more important in a decrease of the bulge and of the elastic thickness, the Austroalpine Carpathian domain. This relatively possibly down to 25 km (Sinclair, 1996; Burkhard high buoyancy and the absence of a separate sub- and Sommaruga, 1998). This changing behaviour duction zone north of the Austro-Carpathian blocks can be explained by the fracturing of the European could also explain the delayed slab break-off in slab and the opening of the Rhine, Bresse and Li- the Carpathians; a break-off which is still occurring magnes grabens within the former bulge. at the present. Tomographic images seem to prove such a changing geometry between the subduction 5.3. How many slabs, how many subduction zones zone around Adria and the Austro-Carpathian area and the arc problem (Spakman et al., 1993). High coupling of the two plates also explains the widespread inversion pro- Figs. 1 and 2 show the probable location of the in- cesses affecting most of western Europe since the cipient subduction zone in the Alpine domain in the Late Cretaceous, long before the final continentÐ Early to Late Cretaceous. This subduction was initi- continent collision. ated in the Early Jurassic by the eastward subduction In the Turonian, the Alpine Tethys oceanic floor of the Meliata ocean induced by the opening of the and its thin southern margin started subducting, in- Alpine TethysÐCentral Atlantic ocean (Fig. 1). The ducing the HP metamorphism of the future internal subduction zone followed the Meliata slab retreat, massifs. The opening of the North Atlantic and Bis- compensated eastward by the opening of the Vardar cay oceans at this time induced a rapid rotation back-arc ocean. In the Early Cretaceous the sub- of Africa. This corresponds to the separation of duction reached the western Austroalpine domain. the Austroalpine from Adria (Fig. 2) and the lat- The first phase of deformation affecting the southern eral escape of terranes (e.g. Trans-Danubian, Tizia Alpine Tethys margin in the Sesia area (130Ð120 terranes). The subduction zone first located in the 36

Fig. 18. PalaeogeographicÐpalaeotectonic maps of the AlpineÐPyrenean region in the Middle Eocene and Late Oligocene. 37

Meliata ocean moved into the Alpine Tethys and ration of basement slivers and sedimentary covers Valais ocean, then after the Paleocene, it cut between from the Briançonnais domain took place from the the Briançonnais and the Iberian plate, propagating Lutetian (in the east) to Priabonian (in the west). up to the Biscay ocean southern margin (Figs. 2 and Following this, elements from the Valais ocean 18). The plate limit westward was the mid-oceanic were mainly underplated, as were elements from ridge of the northern Atlantic (Ziegler, 1988). the distal Helvetic margin which were locally af- The Early to Middle Eocene Pyrenean collision fected by HPÐLT metamorphism, although most of induced a north-dipping subduction zone to develop the oceanic=continental substratum of these domains south of the Iberian plate, grading into the forma- was subducted. Collision processes started when tion of the Apenninic accretionary prism in Tertiary the unthinned Helvetic crust started subducting in times (Fig. 18). A similar collision, affecting the late-Early Oligocene. Detachment and obduction of south-dipping Alpine subduction zone, also occurred large parts of the Helvetic basement allowed fur- in the east where the Vardar ocean developed a ther subduction together with a reorganisation of the north-dipping subduction zone under the Moesian prism due to the Pie«mont slab detachment around the platform in the Late Cretaceous (Fig. 2), accompa- EoceneÐOligocene boundary. nied by an active volcanic arc in these regions and From Paleocene to Oligocene a single subduc- slab detachment in the EoceneÐOligocene (Yanev tion zone is envisaged as being responsible for and Bardintzeff, 1997). In a present-day configura- the formation of the Pyrenean, Alpine, Austroalpine tion (Fig. 6), the N-vergent Alpine-Carpathian belt is and Carpathian fold belt, a subduction zone which relatively isolated from the S-vergent MaghrebidesÐ jumped from one ocean to the other in time: the ApenninicÐDinaro-Hellenides younger segment, fol- Meliata ocean in the Jurassic, the Alpine Tethys lowing a rather complicated Tertiary history of clos- in the Early Cretaceous and the PyreneesÐBiscay ing and opening oceans in the Mediterranean region. ocean until the Middle Eocene. This mainly south- directed subduction was replaced then by a mainly north-directed subduction which gave birth to the 6. Conclusions Maghrebides, Apennines, Dinarides and Hellenides and which is still active today. Plate reconstruction as well as analysis of structural, stratigraphic and absolute age data sets for the Briançonnais and surrounding domains allows Acknowledgements the timing of subduction=obduction processes of the western and eastern Alps to be constrained. The in- We want to thank here our Alpine geologists corporation of the Briançonnais exotic terrane in the colleagues who shared with us their sometimes con- Pie«mont-Penninic accretionary prism did not take tradicting opinions and data, and more particularly place before 45 Ma. Older, more internal HPÐLT M. Sartori, M. Jaboyedoff, M. Cosca and J. Hun- metamorphic units (internal massifs: Monte Rosa, ziker. W. Frish and T. Dumont reviewed this paper Grand Paradis, Dora Maira) are therefore not of Bri- and we thank them for improving its readability. Part ançonnais origin, they belonged to the Alpine Tethys of this research was supported by the Swiss National southern margin. This margin became an active mar- Fund grant 20-49114.96. gin during the Cretaceous, the initial accretionary prism being represented by the lower Austroalpine units (internal massifs, Sesia, Dent-Blanche), some References of them developing eclogitic metamorphic facies, and ophiolite-rich me«langes (Tsate« nappe). Subse- Ackermann, A., 1986. Le flysch de la nappe du Niesen. Eclogae quently, the accretionary prism accreted mainly Cre- Geol. Helv. 79, 641Ð684. Ackermann, T., 1979. Etude se«dimentologique comparatrice du taceous oceanic sequences from the Alpine Tethys Me«sozoõ¬que ultrahelve«tique de la Tour d’Anzeinde. Unpubl. found nowadays in the Nappe Supe«rieure of the Diploˆme, Fribourg. Pre«alpes (Gets and Simme nappes). The incorpo- Allen, P.A., Crampton, S.L., Sinclair, H.D., 1991. The incep- 38

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