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Orléans University-Institute of and Geophysics Cooperation program GEOLOGICAL EXCURSION IN THE WESTERN

June 22 -July 2, 2018 Field guidebook

excursion leaders: M. Faure & Y. Chen Monviso from Agnel Pass Orléans University-Peking University Cooperation program 1

A GEOLOGICAL EXCURSION IN THE Field guide book 2018

M. Faure, Y. Chen

PART I: GEOLOGICAL OUTLINE OF THE FRENCH-ITALIAN ALPS

INTRODUCTION 1. The Alpine system in . The European continent was progressively edificated by several orogenic events since the Archean (Fig. 1). Paleoproterozoic belts are restricted to Scandinavia. A Neoproterozoic orogen, called the Cadomian Belt, from the name of the Caen city in Normandy, and formed around 600 Ma, is observed in the northern part of the Armoricain and also in Great Britain, in Spain, and East Europe. During the , three collisional belts are recognized, namely i) in western Scandinavia, , Ireland, Wales and Britain, the Caledonian Belt results of the collision between (or ) and Scandinavia (or ) that gave rise to the Laurussia continent in Silurian; ii) the Variscan (or Hercynian) Belt that develops in Middle Europe from SW Iberia to Poland, results of the collision between Laurussia and in Devonian and ; iii) the Urals formed by the collision between Laurussia and Siberia in Carboniferous. As the result of the Paleozoic , in , Europe and belonged to the Pangea megacontinent.

Fig. 1: Tectonic map of Europe

During the , several orogenic belts are recognized in (Fig. 1). The Pyrénées are due to the closure of a continental opened in between and Iberia (= Spain and Portugal). This system extends in SE France (Languedoc and ). Around the , many orogenic segments, collectively called “the Alpine system” result of the plate convergence between Europe and Africa, or some African-derived microcontinents. Orléans University-Peking University Cooperation program 2 - The Betics and belts form the northern and southern branches of the orocline, respectively; - The Maghrebides develop along the northern margin of Africa; - The Apenninic belt of Central and Southern is connected with the Maghrebides by the Ionian orocline; - The Dinarides and Hellenides develop on the eastern side of the ; - The Taurides of southern Turkey are the eastern continuation of the Hellenic Belt through the Aegean orocline; the Taurus Belt continues southeastward in the Zagros Belt of Iran which results of the collision between and Arabia; - The Pontides forms the northern part of the Anatolian plateau in Turkey; - The Carpathian belt of Romania and Slovaquia with its double orocline corresponds to the eastern extension of the Alps; - The Alpine belt stricto sensu develops from to the east, to the Mediterranean Sea. Southward,the Alpine belt extends to .

Indeed, the structural continuity of the entire peri-Mediterranean Alpine system has been destroyed by the to present geodynamics. In eastern Mediterranean Sea, the Europe-Africa convergence is still active, accommodated by the oceanic of the remnants of the Tethys along the , south of Crete island and the Ionian trench in southeastern Italy. Conversely, the western Mediterranean Sea is composed of newly created oceanic basins. The Tyrrhenian Sea is interpreted as a back- basin opened in the upper plate behind the Ionian subduction zone. The Algeria-Provence basin opened in Oligocene- within the Alpine- Corsica-Betics-Maghrebides belts.

Fig. 2: Tectonic map of the Alps (from Agard and Lemoine)

2. The subdivisions of the Alps

The Alpine Belt corresponds to the segment of the Alpine system that develops from the Mediterranean Sea (near the cities of and Genova) to Austria (Wien city). This belt can be subdivided in two ways (Fig. 2). i) Geographically, there are: the Western (or French-Italian) Alps, the Central (or Swiss- Italian) Alps and the eastern (or Austrian) Alps. Orléans University-Peking University Cooperation program 3 ii) Tectonically, there are: 1) the outer zones including the Mountains, 2) the inner zones, and 3) the austro-alpine zone. Moreover, the Alpine orogen is bounded to the north and south by two molassic troughs, the Swiss basin (that corresponds to a ), and the plain basin (that corresponds to the hinterland). In the following, we shall restrict this presentation to the Western Alps.

Fig. 3: Emile Argand’s cross-sections of the Alps

It is widely acknowledged that the Alps result of the between Europe, that forms the lower plate, and (presently Italy), which can be interpreted as a northern promontory of Africa. Such an idea of collisional is older than plate . The Swiss geologist Emile Argand already proposed this idea in the beginning of the XXth century (Fig. 3). At that time, the lithosphere was not recognized yet, but several concepts such as ophiolitic suture, austro-alpine overthrust, or backthrusting were already defined.

THE ZONATION OF THE ALPS As all orogens, the French-Italian Alps are divided into several tectonic zones. However, since the structural style depends also on the lithological composition, these tectonic zones partly superimpose upon paleogeographic ones. From West to East, the following zones are defined (Fig. 4). - The Dauphinois (or Dauphino-) zone. The easternmost part of this domain, called the ultra-dauphinois zone, disappears to the north in ; - The Briançonnais zone, including the sub-briançonnais zone that also disappears to the North and the Piemontais zone located eastward of the Briançonnais zone; - The Liguro-Piemontais zone, composed of and oceanic sedimentary rocks; - The Inner Crystalline Massifs; - The Austro-Alpine units subdivided into the and the ; - The zone; - The Prealps and the Helminthoid .

Among the numerous Alpine tectonic contacts, the Front corresponds to the boundary thrust between the Outer and the Inner zones, and the Canavese Line, also called the Insubrian Line that separates the Ivrea and Sesia zones. The Canavese Line can be interpreted as the Alpine suture, i.e. the boundary between Europe and Apulia plates but reworked by late events. For some authors, the Canavese line would represent the trace of the Liguro-Piemontais Ocean but for others, the suture zone should be located north of the Sesia zone. It is worth to note that the Liguro-Piemontais zone is not a suture, but a nappe of oceanic material overthrust upon the European continental . From the simplest and provocative point of view, the Alpes can be seen as a “tectonic sandwich” of oceanic material derived from the Liguro-Piemontais ocean inserted within an upper slice of Austro-Alpine derived from Apulia and a lower Orléans University-Peking University Cooperation program 4 slice of Paleozoic basement and its Mesozoic sedimentary cover corresponding to the European continent. As it is usually the case in collisionnal belts, the lower plate is more intensely deformed and metamorphosed than the upper plate. This excursion aims to present the French-Italian Alps through two cross sections. The first one, from the Dauphino-Helvetic zone, west of to the Po plain, west of Torino and the second one from Ivrea to Chambéry will allow us to observe the main lithological, structural and metamorphic features of this emblematic orogen.

Fig. 4: Zonation of the Western Alps (from Agard and Lemoine)

Orléans University-Peking University Cooperation program 5 The Dauphino-Helvetic zone This zone contains two lithologically contrasted elements: the “Outer Crystalline Massifs” and the sedimentary cover.

A) The Outer Crystalline Massifs consist of magmatic and metamorphic rocks formed during the Paleozoic Variscan orogeny. The Outer Crystalline Massifs that form the highest peaks of the Alps, like the , were weakly deformed and metamorphosed during the . B) The sedimentary cover is characterized by a thick sedimentary series of to Miocene age. In northern Alps, the Late () and late Early (Urgonian ) rocks consist of hundred metres thick reefal that form the white elevated cliffs (Fig. 5A). Structurally, the Dauphino-Helvetic cover is folded and involved into several thrust sheets (Fig. 5B). The deformation includes the Miocene and conglomerate that occupies the core of synclines. As a whole, the folds are overturned to the NW (Figs. 6A& 6B).

Fig. 5A: Panorama of the Chartreuse and Vercors massifs (W. of Grenoble)

Fig. 5B Cross section from to through the

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Fig. 6A: Map of the right bank of the Isère valley downstream from Grenoble

Fig. 6B: Cross section of the right bank of the Isère valley downstream from Grenoble

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Fig. 6C: Panorama of the N. Vercors massif with

Fig 7: cross section of the River showing the “Helvetic style” of recumbent folding

To the North, the structural style changes to the “helvetic style” characterized by kilometer-scale NW-verging recumbent folds (Fig. 7). At the crust scale, the Dauphino-Helvetic zone of the western Alps is an imbrication of thrust sheets (Fig. 8). The highly ductile rocks, such as gypsum or argilite localized several decollement layers. The pre-Triassic basement is also involved in the tectonics as revealed by the ECORS seismic line (cf below). The Outer Crystalline Massifs are partly transported to the NW. To the East of the Outer Crystalline Massifs, the deformation becomes stronger. This is for instance the case of the Bourg d’Oisans syncline in which the early Jurassic black shales are folded and cleaved. These features will be observed during the excursion. In spite of an intense Alpine deformation, pre-orogenic structures are locally well preserved. In particular, several kilometer scale synrift structures can be observed in the Dauphino-Helvetic zone. NNE-SSW trending east-facing normal faults define half-graben basins infilled by early to Orléans University-Peking University Cooperation program 8 Middle Jurassic sedimentary rocks and covered by Late Jurassic-Early Cretaceous post-rift series (Fig. 9).

Fig. 8: Crustal scale cross sections of the Dauphino-Helvetic zone showing how the External Crystalline Massif are involved in the tectonics

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Fig. 9: Jurassic rifting in the Dauphino-Helvetic zone, developed during the opening of the Liguro-Piemontais Ocean The Ultra-Dauphinois zone is not a major element of the Western Alps. However, as it can be clearly observed during our field trip, in particular on the Lautaret pass road, a short presentation is given here. The Ultra-Dauphinois zone consists of a folded Mesozoic series unconformably covered by a Late Eocene flysch. This unconformity attests for an early deformation locally called the “Arvinche chain” (Fig. 10). As a whole, the Ultra-Dauphinois zone is a nappe transported to the W-NW due to a thick ductile layer of gypsum at its base. Rocks similar to the Ultra-Dauphinois zone form the lower unit of the stack of the Prealps nappes, transported nearly 50km to the NW.

Fig. 10: Panorama of the Ultra-Dauphinois zone, viewed to the North-NE. A late Eocene flysch unconformably covers a folded Mesozoic series

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Fig. 11A&B: Panorama of the Lautaret pass showing the Penninic front

2. The Briançonnais zone The Penninic front separates the Outer and the Inner zones (Fig. 11). East of this major Alpine contact, the Sub-Briançonnais zone is a peculiar unit that crops out only in the Western Alps. It is interpreted as a sedimentary trough separating the Dauphino-Helvetic and the Briançonnais zone. As it will not be visited during the excursion, it is not presented here.

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Fig. 13: Examples of condensed Briançonnais series

Fig. 12: Tectonic map of the Briançonnais zone

Lithologically, the Briançonnais zone is subdivided into a western Briançonnais subzone (or zone houillère: Paleozoic coal bearing unit) and an eastern Briançonnais subzone (or zone Vanoise) where the Mesozoic series is folded and metamorphosed (Fig. 12). Conversely to the Dauphino-Helvetic zone, in the Briançonnais zone, the Mesozoic series is thin and corresponds to a condensed series. The Triassic rocks are widely exposed; in particular, Early Triassic and gypsum and Middle Triassic limestone and form most of the summits of this area. The Early Jurassic (Lias) is missing due to non-deposition. The Middle to Late Jurassic rocks are represented by a few meters-thick red or grey nodular (called “marbre de Guillestre”) deposited in shallow water conditions in a submarine topographic high with strong currents. The Early Cretaceous is also lacking, the Late Cretaceous and Early Eocene deposits consist of argillaceous black and reddish limestone (calcaires en plaquettes) and black mudstone (Fig. 13). In the Briançonnais zone, the sedimentation stops in Middle Eocene after the deposition of the so- called “black flysch”. The emplacement of the Helminthoid Flysch nappe took place immediately after. Structurally, the Briançonnais zone is characterized by a polyphase deformation. East verging folds deform the stack of nappes of the Briançonnais cover (Fig. 14A). The bulk structure of the Briançonnais is described as a fan structure, the famous “éventail briançonnais” (Fig. 14B). The cross section of the Guil valley, upstream of Guillestre, exposes one of the best examples of these refolded nappes that will be examined in detail (Figs. 14A, 15).

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Fig. 14A. Geological map of the Briançonnais area in the Guil Valley

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Fig. 14B: Cross section of the Briançonnais zone from Guillestre to Château Queyras showing the east verging (back folding) of the initially top-to-the-west stack of nappes

Fig. 15: Detail cross-section of the refolded nappes along the Guil Valley

3. The Liguro-Piemontais zone: the remnant of the Alpine ocean. The “schistes lustrés” (glimmering ) nappe consists of carbonated metapelites, deformed by several synmetamorphic events. Rare preserved foraminifera indicate a Late Cretaceous age for these rocks. In detail, several units of “schistes lustrés” can be distinguished. Some authors separate the “piemontais schistes lustrés“ from the “liguro-piemontais schistes lustrés. The former unit corresponds to the continental passive margin, it is devoided of any ophiolitic rock whereas the latter, that includes ophiolites, corresponds to the true deep oceanic domain. Nevertheless, the mapping of the boundary between those two units is not available (Figs. 16 & 17). The ophiolites form numerous meter-to-several kilometer sized blocks included into the schistes lustrés nappe. The Chenailllet ophiolitic massif, near Briançon, is one of the most famous places that exposes undeformed pillow , and gabbros, but the lower part of the ophiolitic sequence is poorly exposed (Fig. 18). The Middle Jurassic radiolarians, that have been recently discovered from cherts, indicate also the time of ophiolitic magmatism. The Monviso is also a well known area for ophiolites, but conversely to the Chenaillet massif, the Monviso ophiolites experienced a high pressure as shown by schists and . Orléans University-Peking University Cooperation program 14

Fig. 16: Tectonic map of the Briançonnais and Liguro-Piemontais zones and Inner Crystalline Massifs (from Agard and Lemoine)

Fig. 17: Synthetic lithostratigraphic columns of the schistes lustrés series (from Agard and Lemoine)

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Fig. 18: map and cross section of the Chenaillet ophiolitic massif

Fig. 19: Reconstruction of the Liguro-Piemontais Ocean from the study of the Queyras ophiolites

It is worth to note that, in the Alps, a complete ophiolitic sequence is never observed. The mafic, ultramafic and siliceous sedimentary rocks are scattered within the schistes lustrés. In some places, cherts, siliceous mudstone or limestone direcly cover serpentinites or hydrothermalized (called OC1 type ophicalcites, Fig. 19). OC2 type ophicalcites are sedimentary breccias with , gabbro, chert or serpentinite pebbles. Together with gabbroic , these rocks attest for an Orléans University-Peking University Cooperation program 16 ophiolitic detritism that took place in the Liguro-Piemontais ocean before its deformation and metamorphism. On the basis of their lithological and structural features, it appears that the Alpine ophiolites are not similar to the thick ophiolitic nappes as those observed for instance in the Oman belt. As discussed below, most of authors consider that the Liguro-Piemontais Ocean formed in a pull-apart basin that subsequently evolved in an Atlantic-type ocean with a mid-oceanic ridge.

4. The Inner Crystalline Massifs. The schistes lustrés nappe is tectonically underlain by para and ortho-derived continental metamorphic rocks that belong to the European basement. Conversely to the Outer Crystalline Massifs, the Inner Crystalline Massifs were strongly foliated and metamorphosed during the Alpine orogeny. Lithologically, the Inner Crystalline Massifs ( or Dora ) are formed by Paleozoic metamorphic or plutonic rocks that experienced the Variscan orogeny and by metasedimentary rocks corresponding to Permian and Mesozoic rocks that covered the Paleozoic basement. The Dora Maira massif became famous by the discovery of coesite eclogites derived from quartzites. These rocks are important since they demonstrate that continental rocks underwent subduction to more than 100km depth and were subsequently exhumed. In the Central Alps, the Inner Crystalline Massifs are involved into kilometer-scale recumbent folds called “penninic nappes” (e. g. Mt-Rose nappe). In the Western Alps, the surface geology shows a domal shape for the Inner Crystalline Massifs but their deep structure remains unknown (Fig. 20).

Fig. 20: Cross section of the Dora Maira massif showing the domal structure of the stack of nappes (from Agard and Lemoine)

5. The Austro-Alpine Unit : a piece of the Apulian crust. In the Western Alps, the Austro-Alpine Unit is represented by the , in which the Mt Cervin (or ) is one of the emblematic Alpine summits. The Dent Blanche nappe is formed by Paleozoic magmatic and metamorphic rocks emplaced above the Liguro-Piemontais Unit. The Austro-Alpine Unit develops widely in the Central and (Fig. 2) where it forms the largest part of Austrian Alps.

6. The Sesia zone: western apulian unit or easternmost European continental piece? The Sesia zone, or Sesia massif, consists of highly metamorphosed continental rocks, probably with Carboniferous and Permian terrigenous protoliths, and rare Permian plutonic intrusions, that crop out in Italy. Eclogitic rocks are widely distributed, however the tectonic and paleogeographic position of the Sesia zone remains disputed. Many authors consider this zone as the deepest and highly metamorphosed part of the Austro-Alpine Unit. In this interpretation, the Sesia zone would belong to the Apulian continental margin that experienced subduction and exhumation during the alpine orogeny. Another view is to interpret the Sesia zone as the easternmost Inner Crystalline Massif. In this case, it would belong to the European continent. These two interpretations will be illustrated below.

7. The : the lower crust of Apulia. Orléans University-Peking University Cooperation program 17 The Canavese Line separates the Sesia zone from the Ivrea zone (Fig. 3). This last zone of the western Alps orogen is remarkable as it exposes felsic and mafic Variscan that form the lower crust of Apulia. Due to Alpine tectonics, these high-temperature granulites were put to vertical along the Canavese line. From Ivrea to Milano, Paleozoic diorites, high temperature (called kinzigites) and micaschist form the basement of Apulia. Mesozoic sedimentary rocks, sometimes called the “ Lombard series”, overly the basement rocks. A small piece of the sedimentary series can be observed near Montalto. South of the Sesia massif, the Lanzo lherzolitic massif has been, since a long time, grouped with the Sesia massif. However, this interpretation is challenged. Many authors consider that the Lanzo massif is a piece of the infracontinental mantle of Apulia. Other small peridotitic massifs, such as the Baldissero massif, can be observed along the Canavese Line.

Fig. 21: Four cross-sections through the western Alps showing the similarities and differences along the belt. Cross-section 3 and 1 correspond approximately to the excursion routes

8. The Bio Unit: the possible suture zone. NW of Ivrea, the Bio Unit is formed by metapelites and rare mafic rocks that resemble the Schistes Lustés series. Some authors consider the Bio Unit as the suture zone between Apulia and Europe. For other authors, the Bio Unit is the remnant of a small basin developed within the northern margin of Apulia (Fig. 20b).

9. Conclusion The above presented zonation of the western Alps is summarized in four cross sections (Fig. 21). Cross sections 3 and 1 correspond approximately to the excursion routes from Grenoble to Torino and from Ivrea to , respectively. A synthetic cross section of the alpine belt from the Swiss Jura to the Po plain is also presented in Figure 22.

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Fig. 20b: The Biot unit. A: location map. B & C: cross sections (located in B). D: Interpretation of the Biot unit and the Canavese suture (Aubouin et al.1978).

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Fig. 22: Synthetic cross section of the Alps from the Swiss Jura to the Po plain (from Agard and Lemoine)

Fig. 23: Metamorphic map of the Western Alps (from Agard and Lemoine) THE METAMORPHISMS AND DEFORMATIONS

The Alpine metamorphic rocks have been extensively studied. As a whole, the P and T conditions increase from West to East, from facies to facies (Fig. 23). In the Dora Maira, the UHP metamorphism shows that the European continental crust experienced subduction to mantle depth (ca 100 km). The exhumation of the HP and UHP rocks is accommodated by Orléans University-Peking University Cooperation program 20 extensional tectonics with both top-to-the-West and top-to-the-East ductile shearing. Exhumation is coeval with retrogression of the high- rocks into facies (Fig. 24).

Fig. 24: P-T-t paths of various units (from Agard and Lemoine)

Fig. 25: Map of the synmetamorphic ductile shearing with corresponding ages

Orléans University-Peking University Cooperation program 21 The radiometric dates indicate that the climax metamorphic conditions were reached in Early Eocene times. The Schistes Lustrés and Briançonnais rocks record high pressure conditions followed by fast exhumation. It is worth to note that within a single unit (e.g. Dora Maira, Viso or Chenaillet), whatever their continental or oceanic origin, the PT path can be quite different. The Alpine metamorphism is coeval with ductile deformations that develop during nappe stacking (Fig. 25). The ductile shearing events comply globally with a top-to-the-NW displacement. However, in the SW part of the Alpine belt, top-to-the-SW shearing reflects the curvature of the belt. Moreover, top-to-the-east or SE shearing develops in the eastern part of the belt. This kinematics is in agreement with the backfolding and backthrusting observed in the Briançonnais and farther east.

Fig. 26: Metamorphic map of the dome in the Central Alps

The Alpine belt experienced a second metamorphic event restricted to the Central Alps, which is called the “Lepontine metamorphism”. Between the Aar massif and the Insubric Line, the high temperature metamorphic isogrades draw an elliptic pattern that defines a domal structure called the “Ticino dome” (Fig. 26). This HP/LT metamorphism that develops in Oligocene-Miocene times overprints the early Alpine metamorphism. The HT Lepontine metamorphism is a possible consequence of the increase in heat flow related to nappe stacking. Alpine are rare, the Adamello and Bergell plutons are the largest ones. The location of the Bergell pluton, along the Insubric Line, and within the Ticino dome, suggests a genetic relationship between emplacement, dextral shearing and doming (Fig. 26).

THE DEEP STRUCTURE OF THE ALPS 1. Gravity Orléans University-Peking University Cooperation program 22 Gravity studies reveal a large positive anomaly below the Dora Maira massif and the western edge of the Po molassic plain. This anomaly suggests that a dense mass, called the “Ivrea body” underlies the western Alps. Note that the “Ivrea body” must no be confused with the above described “Ivrea zone” (Fig. 27).

Fig. 27: Bouguer anomaly map showing the Ivrea Body heavy anomaly

2. The ECORS-CROP line This seismic refraction line from the to the Po plain documents several tectonic features of the Alpine Belt such as i) the layered lower crust formed by HT granulites, ii) the eastward deepening of the European Moho, iii) the east dipping reflectors in the Outer zone interpreted as intracrustal shear zones in the Outer Crystalline massifs; iv) the Penninic front (Fig. 28).

3. A crustal scale cross section of the Western Alps Orléans University-Peking University Cooperation program 23 The Ivrea body is often interpreted as a piece of the Apulian mantle that would have indented the European crust (Fig. 29). Such a view raises some difficulties. This indentation model implies that the plate boundary, represented in surface by the Canavese Line, is cut by the Apulian mantle wedge. From a rheological point of view, the strength of the Apulian mantle is probably too weak to deform the Alpine crust, therefore another interpretation is suggested (Fig. 30). In this view, the dense Ivrea Body might be considered as formed by eclogitized continental crust similar to that found in the Dora Maira Inner Crystalline Massif. As a result, the suture zone keeps its subvertical attitude throughout the entire lithosphere.

Fig. 28: Line drawing of the ECROS-CROP seismic line

Fig. 29: Interpretation of the deep structure of the Alps in the hypothesis of an “Apulian indenter”

PALEOGEOGRAPHIC RECONSTRUCTIONS AND GEODYNAMIC INTERPRETATIONS The Dauphino-Helvetic zone, the Briançonnais zone and the Inner Crystalline Massifs belong to the European continent whereas the Austro-Alpine Dent Blanche nappe and the Ivrea zone belong to the Apulian continent. As discussed above, the paleogeographic situation of the Sesia zone remains controversial. Several field observations allow us to conclude that, during the entire Alpine evolution, the European continent was a passive continental margin. Early Jurassic normal faults are identified in the Dauphino-Helvetic zone. On the basis of sedimentology, the Briançonnais zone is recognized as a submarine rise or Briançonnais High. On the eastern side of the Briançonnais High, in the Piemontese area, normal faults can also be reconstructed but most of these structures were inverted during the Alpine compression. Rifting evolved to extreme thinning of the continental crust followed by oceanisation in the Liguro-Piemontais zone. Mesozoic rifting Orléans University-Peking University Cooperation program 24 is also recognized in the Apulian side of the alpine ocean, however, as these structures are not observed during the excursion, they will not be presented here. The passive margin that developed in SE France during the Jurassic corresponds to the western side of a left-lateral pull apart controlled by the activity of the Azores-Gibraltar and faults to the south and north, respectively. Many authors consider that the opening of the Liguro-Piemontais ocean was controlled by the activity of a low angle normal responsible of the lithospheric mantle up to the surface. This model accounts well for the development of ophicalcites that represent fractured, hydrothermalised serpentinites and mafic-utramafic sedimentary breccias. During its exhumation, the decompressed mantle lherzolite underwent partial melting responsible for the genesis of mafic magmas emplaced as gabbros and pillow . In its later stages, a slow oceanic ridge accommodated the final opening of the Liguro- piemontais ocean. At the continental scale, the opening of the Liguro-Piemontais Ocean appears as a consequence of the Central opening (Fig. 33). As far as the Liguro-Piemontais Ocean can be reconstructed, it was a short lived basin. It closure probably started in Early Cretaceous.

Fig. 30: Another lithosphere-scale interpretation of the deep structure of the Alps

Fig. 31: Paleogeographic reconstruction of the European continental passive margin and Liguro- Piemontais Ocean (from Agard and Lemoine) Orléans University-Peking University Cooperation program 25

Several geodynamic models accounting for the evolution of the Western Alps have been put forward. In the following, only two models are presented. The first one proposed by Mattauer (Fig. 34), considers that the Sesia massif is a part of the European continent. This model also assumes a continental subduction and an ophiolitic of the Liguro-Piemontais Ocean upon the European basement in Early Cretaceous. These late Mesozoic events, called “eo-alpine tectonics”, are documented in Eastern Alps but not in Western Alps. Due to the abundance of Eocene radiometric ages, the idea of eo-alpine events in western Alps, that was popular in the 80’s is presently abandoned by most of authors. Nevertheless, it cannot be definitely ruled out. In this model, the Lanzo lherzolitic massif is interpreted as a piece of the Apulian infracontinental mantle. In a second model (fig. 35), the Sesia massif belongs to Apulia, an accretionary prism is assumed to develop in Late Cretaceous due to the oceanic subduction of the Liguro-Piemontais Ocean. This accretionary prism is possibly preserved in the Tsate Unit in Switzerland. In the Western Alps, the helminthoid flysch nappe might be also a part of this subduction complex. Note also, that the Ocean is restricted to the Central Alps, and does not exist in the Western Alps.

Fig. 32: A low angle detachment fault accounting for the lithological and structural features of the Liguro-Piemontais Ocean

Orléans University-Peking University Cooperation program 26 Fig. 33: Paleogeographic reconstruction map showing the connections between the opening of the Liguro-Piemontais pull-apart basin and the opening of the central Atlantic Ocean

Fig. 34: Geodynamic evolution model assuming a European origin of Sesia massif (Mattauer et al., 1986)

CONCLUSIONS Several features are in good agreement with the interpretation of the Alpine orogen as a collision belt. Namely, one can mention: - an ophiolitic suture; - a stack of nappes progressively younging from east to west; Orléans University-Peking University Cooperation program 27 - a high pressure metamorphism coeval with a polyphase ductile deformation in the underlying plate; - the development of foreland molassic troughs. However, some elements of collisional belts are not or poorly represented in the Alps. For instance: i) a magmatic arc is missing, even if some andesitic clasts are found in Eocene sandstone; ii) the thermal overprint due to crustal thinning is limited to the Ticino dome in the Central Alps; iii) the crustal melting responsible for the emplacement of peraluminous granitoids is limited since such plutons (e.g. Bergell and Adamello granites) are rare. Nevertheless, the Western Alps present some original features such as - an incomplete ophiolitic series - a well preserved passive margin in the lower plate - a huge basement nappe, the “lid” of some authors, covering the Schistes Lustrés nappe.

Fig. 35: Geodynamic model in which the Sesia massif belongs to Apulia (Lemoine et al., 2000)

Several questions remain disputed. For instance : i) what is the significance of the Ivrea body? ii) what is the paleogeographic position of the Sesia zone as European or Apulian crust? Conversely to the Himalayan collision, the two continents experienced the same Variscan orogeny as they both belonged to Pangea before being disrupted by the opening of the Liguro-Piemontais Ocean. Therefore, it is sometimes difficult to recognize to which plate belong the basement rocks involved in the Alpine Belt. The Alpine collision is not finished yet. As indicated by the seismicity, some faults are still active in the Alps. From the Jura front to the Mediterranean Sea, west directed thrust faults accommodate the present thickening of the Outer zone. Dextral strike-slip faults in the Belledonne Outer Crystalline Massif accommodated the southeastward extrusion of the southern part of the Western Orléans University-Peking University Cooperation program 28 Alps. East of the Penninic Front, focal mechanisms show that the Briançonnais and Liguro- Piemontais zones are presently rising up. The uplift is accommodated by normal faults.

Orléans University-Peking University Cooperation program 29 PART II: FIELD ROUTE AND OUTCROP DESCRIPTION

D1. Orléans-Clermont-Ferrand-St-Etienne-Tain l’Hermittage-Villard-de-Lans Aims: Alpine front, of the Dauphinois zone (Vercors massif), Molassic basin

Stop 1-1 (45.01382°N, 4.93467°E): Châteauneuf-sur-Isère. Middle Miocene (Langhian, ca 15 Ma). Littoral deposit (molasse)

Stop 1-2 (4506469.E, 5.35253°E): West of Pont-en-Royans. Miocene molasse W. limb of the Pont-en-Royans .

Stop 1-3 (45.08978°E, 5.46903): Balme-de-. Cross-section of the footwall of the Rencurel thrust.

Stop 1-4 (4507936°E, 5.46222°E): 2 Les Clots. Landscape of the Rencurel thrust: Urgonian facies of Lower Cretaceous (K1 ) on top of molasse.

Stop 1-5 (45.03829°E, 5.47729°E) : Along D103 road, contact of the Rencurel .

Overnight in Villard-de-Lans.

D2. Villard-de-Lans-Grenoble-Vizille-Romanchevalley-Bourg d’Oisans-Mizoën Aims: External crystalline massif of Belledonne/Grandes-Rousses, Bourg d’Oisans syncline, synsedimentary normal faults 1 Stop 2-1: Lans-en-Vercors. Miocene conglomerate in the core of the Villards-de-Lans syncline.

Stop 2-2 (45.21554°N, 5.66144°E): Sassenage. View on the famous Sassenage “fold-fault”, Urgonian facies of Lower Cretaceous 2 (K1 ) and Upper Cretaceous platy limestones.

Drive to Bourg d’Oisans

Stop 2-3 (45.07431°E, 6.01392°E): La Paute. Observation of the folds in the Early Jurassic (Liassic) limy mudstone due to Miocene folding. Intersection of S0 and S1.

Stop 2-4: La Chalp d’Ornon. Observation of the fault contact between the Taillefer crystalline basement and the Lias. Evolution of a Jurassic paleo-normal fault, initially due to Upper Triassic-Liassic rifting.

Stop 2-5 (45.08022°E, ): Bourg d’Oisans, road to Villars Notre-Dame. Variscan granite deformed by numerous alpine faults. View on the inherited Liassic paleo-faults, and the décollement of the Jurassic sedimentary cover.

Stop 2-5 (): Col de Sarenne. Alpine ductile shear zones in the Paleozoic rocks. Overnight in Mizoën.

Orléans University-Peking University Cooperation program 30 D3. Mizoën-- Lautaret pass -Briançon-Le Chenaillet-Briançon Aims: Structural style of thinner part of the Dauphinois zone; Penninic front; ultra-dauphinois flysch, Western Briançonnais-Liguro-Piemont Ophiolites

Stop 3-1: La Grave. View on the crystalline basement overthrusting the Early Jurassic rocks.

Stop 3-2: La Grave. Polyphase deformation in the Early Jurassic mudstone.

Stop 3-3: Lautaret Pass. Panorama of the Penninic Front, stack of Inner Zones nappes.

Stop 3-4: Old road Lautaret-Briançon. Exposure of the Late Eocene Ultra-Dauphinois flysch.

Stop 3-5: Chenaillet ophiolitic massif. Due to acces difficulty, the outcrop will not be visited.

Overnight in Briançon

D4: Briançon-Guillestre-St-Clément-Château-Queyras-Mt-Viso-Izoard pass-Briançon Aims : reduced sedimentation on the briançonnais high, briançonnais tectonic fan, stack of nappes, Schistes Lustés nappe and Liguro-Piemont ophiolites

Stop 4-1 (44.78819°E, 6.40016°E): Fournel valley. Late Eocene-Early Oligocene Ultra-Dauphinois flysch, contact with the Paleozoic basement

Stop 4-2 (44/70568°N, 6.60817°E): Saint Crépin. Reduced series of Jurassic-Cretaceous strata of Briançonnais Zone.

Stop 4-3 (44.65048°N, 6.58427°E): Saint-Clément. View on the Helminthoid Flysch nappe; St Clément fold.

Stop 4-4: Cross section of the Guil tectonic .

Stop 4-5: Château Queyras. Schistes Lustrés.

Stop 4-6 (optional, depending of weather): Agnel Pass. View on Mount Viso ophiolites.

Stop 4-7: Izoard Pass road, Casse Déserte. View on the Piemont thrust Zone, dolomitic landscape.

Overnight in Briançon

D5: Briançon-Mt-Genèvre-Sestrières--Ivrea Aims: Schistes Lustrés nappe, HP ophiolites, Dora Maira Inner crystalline massif, Baldissero

Stop 5-1 (44.94112°B, 6.80767°E): Champlas-Seguin, east of Cesana Torinese. Ophiolites and cherts within the Schistes Lustrés.

Stop 5-2 (45.02749°N; 7.05644°E): Fenestrelle, Chambons. HP meta-ophiolites in the Schistes Lustrés.

Stop 5-3 (44.97082°N, 7.17746°E):Entrance of Perosa. Quarry in a deformed granite (orthogneiss).

Stop 5-4 (): Ponte Battarello, road Perosa-Perrero. Meta-conglomerates of Pinerolo Complex. Orléans University-Peking University Cooperation program 31 Lunch at 44.89008°N, 7.256491°E

Stop 5-5 (45.41307°N, 7.74400°E): Baldissero. Baldissero peridotite.

Stop 5-6 (): Castellamonte. Triassic-Jurassic sedimentary (limestone) cover of Ivrea zone.

Stop 5-7 (45.51548°N, 7.87553°E): Andrate. Bio "schistes lustrés" HP micaschists

Stop 5-7 (45.50592°N, 7.89841°E): Apulia diorite on the road. Overnight in Ivrea

D6: Ivrea---Chamonix--St-Gervais Aims: Ivrea zone, Canavese fault, Sesia zone, Eastern Briançonnais, Blanfeng, Helvetic recumbent folds

Stop 6-1 (45.55508°N, 7.82747°E): Settimo Vittone. Gneiss of Sesia Zone, with -bearing basic boudins.

Stop 6-2: West of Donnas. Old Roman way cut within the Sesia HP gneiss.

Stop 6-3 : Valtournenche. View on Matterhorn (Cervin) klippe.

Stop 6-4: Courmayeur. Panorama of the eastern side of .

Stop 6-5: Arpenaz recumbent fold.

Back and overnight in Houches

D7: St-Gervais--Aravis pass-Thônes--Aix-les-Bains-Chambéry-Orléans Blanc; Dauphinois zone with Triassic unconformity upon Paleozoic External crystalline massif, Prealps klippe, fold and thrust belt, alpine front

Stop 7-1 (45.81848°N, 6.41549°E): Flumet. Angular unconformity of the Triassic strata on the Variscan Belledonne schists.

Stop 7-2: Aravis Pass. Panorama on the Belledonne massif and the Thônes syncline.

Stop 7-3 ()45.91929°N, 6.41549°E): St-Jean-de-Sixt. Eocene-Oligocene Dauphinois flysch.

Stop 7-4: St-Eustache, eastern limb of Semnoz anticline. Panorama on the eastern side of Annecy Lake. Folds and thrusts in the Bornes Dauphinois zone massif.

Stop 7-5: Molasse near Epersy. Alpine front and molassic basin (revision of the observation on the first day) Overnight near Chambery

Day 8 : conclsion and drive to Paris by highway. Taking the lunch on the road. Arrival in Parsi around 6 pm, depending on the departure time, taffic, and eventual stops.

1

Index Active margin: A continental margin where one plate is being subducted under another plate because of plate convergence. Also called a Pacific or convergent margin. Alps: a mountain system in Europe that extends in a curve from the coast of southeastern France through northwestern Italy, Switzerland, , southern , and Austria, into . The highest peak, Mont Blanc, rises to a height of 4,807 m. Austro-Alpine unit (or domain): A complex of several nappes of the Paleozoic (Variscan) magmatic and metamorphic rocks, and the Mesozoic sedimentary cover belonging to Apulia (Africa) continent, emplaced above the Liguro-Piemontais Unit corresponding to an oceanic domain. In the Western Alps, the Austro-alpine unit is represented by the Dent Blanche klippe, in which the Mt Cervin (or Matterhorn) is one of the emblematic Alpine summits. Back thrust: a thrust in which displacement is in an opposite direction to that of the main thrust propagation. Back thrusts are thought to form as a result of layer-parallel shortening in a late stage of thrust sequences. Bio Unit: a series of metapelites and rare mafic rocks exposed in NW of Ivrea (Italy) that resemble the Schistes Lustés series. It is a possible suture zone between Apulia and Sesia. Briançonnais zone – inner zone: the part of the Alps between the Outer zone and the Apulian (or Sesia) plate where the sedimentary cover ranging from Late Carboniferous to Middle Eocene is strongly deformed with often back-thrusts. The Paleozoic Variscan crystalline basement of the Briançonnais zone is never exposed. All authors agree that this basement is probably similar to the Inner Crystalline Massifs, but is involved in the Alp tectonics.. It belongs to the European plate. Brittle deformation: rocks respond to stress by discontinuous behaviour, producing structures such as joints, faults, tension gashes, breccias, cataclasites. Canavese Line: the Alpine suture boundary between Europe and Apulia (Africa) plates, but it is still debated. Chert: a fine-grained composed of microcrystalline or cryptocrystalline silica. Coesite: a form (polymorph) of dioxide SiO2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C), are applied to . Continental collision: a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together. continental margin: the shallow water area found in proximity to continent. There are two types of continental margins: active and passive margins. Crust: the outermost solid shell of the , which is chemically distinct from the underlying mantle. Continental crust has an average thickness of 30-40 km, whereas is ca 10 km thick. Dauphinois (or Dauphino-Helvetic) zone – outer zone: the westernmost part of the Alps orogen. The sedimentary cover is deformed by folding and thrusting, but metamorphism is low (anchizone). The crystalline basement of the Dauphinois zone, exposed in the Outer Crystalline massifs, was mainly involved in the Variscan orogeny. However, local ductile shear zones (eg in the Mont Blanc Massif) may form during the Alpine orogeny. Décollement: a gliding surface between two rock masses. Décollement is a deformation structure resulting in independent styles of deformation in the rocks above and below the fault. They are formed in both compressional and extensional settings. A detachment fault is a peculiar décollement formed during extensional tectonics. Detachment fault: a fault associated with large-scale extensional tectonics. Detachment faults often have very large displacements (tens of km) and juxtapose unmetamorphosed hanging walls against medium to high-grade metamorphic footwalls that are called metamorphic core complexes. Diabase : a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Diabase is the preferred name in North America, yet dolerite is the preferred name in most of the rest of the world, where sometimes the name diabase is applied to altered dolerites and basalts. Ductile deformation: rocks respond to stress by continuous behaviour producing structures such as: fold, cleavage, foliation, lineation. The most important physical parameters controlling the ductile deformation are temperature and strain-rate Eclogite (metamorphic) facies: all metamorphic rocks that experienced HP/HT metamorphic conditions similar to the eclogites. A gneiss may belongs to eclogite facies. Eclogite: a mafic , formed at pressures greater than 12 kb, and temperature higher than 500°C. These conditions may occur in the lower crust and of the Earth. Exhumation: the process by which a parcel of rock approaches Earth's surface. It differs from the related ideas of rock uplift and surface uplift in that it is explicitly measured relative to the surface of the Earth, rather than with reference to some absolute reference frame, such as the Earth's geoid. Fault: a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. It may be divided into thrust, normal and strike-slip faults depending on the relative displacement of the two sides. Flysch: a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves, the foreland basin becomes shallower and molasse is deposited on top of the flysch. It is therefore called a syn-orogenic Gabbro: a large group of dark, often phaneritic (coarse-grained), mafic intrusive igneous rocks chemically equivalent to basalt derived from the Earth's mantle,. Geodynamics: the dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to and geologic phenomena such as seafloor spreading, mountain building, volcanoes, earthquakes, faulting and so on. Gneiss: a common metamorphic rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous (ortho-gneiss) or sedimentary (para-gneiss) rocks. It is often foliated (composed of layers of sheet-like planar structures). Graben: a depressed block of the Earth's crust bordered by normal parallel faults. : a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate to high-pressure metamorphism. Half-graben: a geological structure bounded by a (normal) fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel (normal) faults. Helminthoid flysch: a peculiar alpine flysch of late Cretaceous age. The Heminthoid flysch is always allochthon (= thrusted). It is interpreted as turbiditic deposited in the upper part of a subduction trench. The HF was never subducted, and never metamorphosed, on the contrary, the Schistes Lustrés represent the lower part of the subduction trench, they underwent a HP/LT (Blue facies metamorphism). HP/LT: High pressure/low temperature. HT/LP: High temperature/low pressure. Inner Crystalline Massifs: Paleozoic metamorphic or plutonic rocks that experienced the Variscan orogeny and Permian to Mesozoic metasedimentary rocks corresponding to the sedimentary cover of the Paleozoic European basement. Ivrea zone: the lower crust of Apulia. The Canavese Line separates the Sesia zone from the Ivrea zone, where exposes felsic and mafic Variscan granulites. Kinematics: the motion of points, bodies, and systems of bodies without considering the mass of each or the forces that caused the motion. : the molten rock expelled by a volcano during an eruption. Liguro-Piemontais zone: a nappe of oceanic material (the remnant of the Alpine ocean) overthrust upon the European continental basement (not a suture). It is composed of ophiolites (oceanic crust) and oceanic sedimentary rocks also called "Schistes Lustrés". Lithosphere: the outermost shell of the Earth, which is composed of the crust and the portion of the upper mantle with rigid mechanical properties. Moho: the boundary between the Earth's crust and the mantle. Molasse: the term refers to sandstones, shales and conglomerates that form as terrestrial (fluviatile) or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flysch—like, for example, those that left from the rising Alps, or erosion in the Himalaya. Monvison: the highest mountain of the . It is located in Italy close to the French . It is also a well known area for ophiolites, but conversely to the Chenaillet massif, the Monviso ophiolites experienced a high pressure metamorphism as shown by glaucophane schists and eclogites. Nappe or thrust sheet: a large sheet-like body of rock that has been moved more than 2 km or 5 km above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Normal (reverse-thrust) fault: A normal (reverse) fault occurs when the crust is extended (shortened). Alternatively such a fault can be called an extensional (compressive) fault. The hanging wall moves downward (upward), relative to the footwall. Obduction: the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere. Subsequently, this definition has been broadened to mean the emplacement of continental lithosphere by oceanic lithosphere at a convergent plate boundary, such as closing of an ocean or a mountain building episode. Olistostrome: a sedimentary deposit composed of a chaotic mass of heterogeneous material, such as blocks and mud, known as olistoliths, that accumulates as a semifluid body by submarine gravity sliding or slumping of the unconsolidated sediments. Ophiolites: a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and emplaced onto continental crustal rocks. Orogeny: a process in which a section of the earth's crust is folded and deformed by lateral compression to form a . Outer Crystalline Massifs: magmatic, para and ortho-derived continental metamorphic rocks formed during the Paleozoic Variscan orogeny and belonging to the European basement. They werenot foliated and metamorphosed during the Alpine orogeny, except along localized shear zones. P-T-t path: Pressure-Temperature-time path. Passive margin: a passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. The transition between oceanic and continental lithosphere that is not an active plate margin. Penninic Front: the thrust boundary between the Outer and the Inner zones. The major tectonic thrust front in the . The thrust front moves over a developing decollement horizon, and separates the (internal) high grade metamorphic rocks of the Penninic nappes from the (external) sedimentary rocks and crystalline basement of the Helvetic or Dauphiné or Dauphinois nappes. Peridotite: a dense, coarse-grained igneous rock consisting mostly of and . Peridotite is ultramafic and derived from the Earth's mantle, Prealps or : any foothills at the base of the European Alps. Adiolaria:also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm.The elaborate mineral skeleton is usually made of silica. Recumbent fold: A fold whose hinge line and axial plane are horizontal or subhorizontal. Schistes lustrés: glimmering schists. Sedimentary rocks deposited on the Liguro- oceanic crust and in the subduction trench that experienced the HP/LT metamorphism. Serpentinite : a rock composed of one or more serpentine group . Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries. Sesia zone or Sesia massif (western Apulian unit or easternmost European continental piece?): a sequence of highly metamorphosed continental rocks, probably with Carboniferous and Permian terrigenous protoliths, and rare Permian plutonic intrusions, that crop out in Italy. Shearing a deformation of a material substance in which parallel internal surfaces slide past one another. Strike-slip fault: the fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion. It is divided into dextral (right- lateral) and sinistral (left-lateral) faults . Subduction: a geological process that takes place at convergent boundaries of lithospheric plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Tectonics: the process that controls the structure and properties of the Earth's crust and its evolution through time. In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as , and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Trench (oceanic) : a topographic of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other. Trough: a linear structural depression that extends laterally over a distance. Although it is less steep than a trench, a trough can be a narrow basin or a geologic rift. UHP metamorphism: metamorphism of ultra high pressure characterized by the apparition of coesite. Variscan or Hercynian orogeny: a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the of . Matterhorn 6-7 6-3 6-4 7-3 Mt Blanc D6 7-2 6-6 7-1 7-4 7-5 6-5 Clermont- Ferrand 6-2 6-1 5-6/7 D5

5-5

St-Etienne

Grenoble 2-2 2-4’ 2-5 5-2 1-3/5 2-1 3-4 2-4 3-1/2 D1 5-1 5-3 1-2 D2 3-3 1-1 2-32-3 5-0 3-5 5-4 D3 4-1 D4 4-7 4-2 710 4-5 4-3 4-6 Monviso 4-4