Internal Crystalline Massifs Research Articles

Exhumation of eclogitic ophiolitic nappes in the W. Alps: New age data and implications for crustal wedge dynamics

The W. Alps high pressure ophiolitic terranes (Monviso and Rocciavre areas) are a natural laboratory to study processes such as nappe-stacking and crustal exhumation in the deep regions of subduction margins. We sampled each of the main shear zones, representing thrust contacts later reactivated as detachments, for petrological and Rb-Sr multi-mineral geochronological analyses. Three generations of white mica are commonly found in mylonitized metasediments with crystal cores formed during the high pressure event, a broad rim formed during decompression in the epidote blueschist facies and texturally late muscovite flakes locally lining the main foliation. Semi-brittle discrete shear zones commonly crosscut previous structures witnessing deformation at temperatures lower than 300°C during exhumation. In spite of this apparent structural heterogeneity, homogeneous deformation ages mostly ranging between 38 and 35 Ma were obtained for all the shear zones bounding these major ophiolitic bodies. Pseudosection modeling confirms that the bulk of the shearing occurred in the epidote blueschist facies around 400–450°C and 1.0–1.5 GPa. These findings suggest that the shear zones between high pressure ophiolitic nappes were being actively mylonitized during exhumation in the lower epidote blueschist facies (25–40 km depth) between 38 and 35 Ma while some of the units forming the internal crystalline massifs (e.g. the Dora Maira Ultra High-Pressure unit) were still buried at more than 100 km depth (3–4 GPa and 36-34 Ma, according to most recent peak burial estimates). The growth of a crustal-scale duplex in the W. Alps is seen here as a consequence of basal accretion events that followed the exhumation of eclogitized coherent crustal slices in a serpentinized subduction channel. We conclude that in the study area (i) the buoyancy-driven exhumation of subducted continental crust has not been the only and decisive trigger for the exhumation of eclogitized oceanic lithosphere and (ii) continental subduction imprint on crustal wedge dynamics may have not been as instrumental as previously thought.

Jadeite in the Gran Paradiso massif: the high-pressure metamorphic evolution of upper continental crust

The formation and eventual preservation of high-pressure mineral assemblages in subducted continental crust are a long-standing issue with kinetic aspects and important geophysical implications in terms of buoyancy. The issue is especially acute in the internal crystalline massifs of the Western Alps, in which the dominant rock-type, metagranite, hardly bears evidence for high-pressure transformation whereas subordinate rock types clearly point to high or very high pressure conditions. The discovery of well-preserved jadeite-bearing rocks in orthogneiss of the western Gran Paradiso massif, reported here, revives the questioning and is the opportunity to describe uncommon mineral assemblages involving nepheline, preiswerkite, as well as a new trioctahedral mica.

The Alps 2: Controls on crustal subduction and (ultra)high‐pressure rock exhumation in Alpine‐type orogens

AbstractBuilding on our previous results, we use 2‐D upper mantle‐scale thermomechanical numerical models to explore key controls on the evolution of Alpine‐type orogens and the Alps per se, focusing on (ultra)high‐pressure ((U)HP) metamorphic rocks. The models show that UHP rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit. Here we test the sensitivity of the models to surface erosion rate, crustal heat production, plate convergence/divergence rates, geometry of the subducting continental margin, and strength of the retrocontinent. Surface erosion affects crustal exhumation but not early buoyant exhumation. Metamorphic temperatures increase with crustal radioactive heat production. Maximum burial depth prior to exhumation increases with plate convergence rates, but exhumation rates are only weakly dependent on subduction rates. Onset of absolute plate divergence does not trigger exhumation in these models. We conclude that contrasting peak pressures, exhumation rates, and volumes of (U)HP crust exhumed in the Alps orogen primarily reflect along‐strike contrasts in the geometry, thermal structure, and/or strength of the subducting microcontinent (Briançonnais) and continental (European margin) crust. The experiments also support the interpretation that the Western Alps (U)HP Internal Crystalline Massifs exhumed as composite, stacked plumes and that these plumes drove local crustal extension during orogen‐scale shortening. For weak upper plate retrocrusts, postexhumation retrothrusting forms a retrowedge. Overall, these results are consistent with predictions using the exhumation number (ratio of buoyancy to side traction forces in the conduit), which expresses the combined parameter control of the depth/volume of crustal subduction and the transition to buoyant exhumation.

Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps)

The Gran Paradiso Massif is one of the Internal Crystalline Massifs (Penninic Zone) in the Alps. This Massif comprises the widely investigated Gran Paradiso Unit and the underlying, much less-explored Money Unit, which is a well-exposed tectonic window below the Gran Paradiso Unit. This contribution provides a new detailed petro-structural map of the Money Unit. An integrated structural and metamorphic study can relate different structures to the evolving metamorphic stages. There is also recognition of evidence of Permo-Carboniferous sedimentation, contact metamorphism and polyphase Alpine evolution, mainly developed under blueschist to greenschist facies conditions.The 1:5000 scale map (and the cross sections) display evidence for the Alpine evolution of the Money Unit and for the structural and geological setting of this area. From these data, a 3D model of the Money Unit and its folded contact with the Gran Paradiso Units is constructed.

The Alps 1: A working geodynamic model for burial and exhumation of (ultra)high-pressure rocks in Alpine-type orogens

Eocene (ultra)high-pressure ((U)HP) rocks exposed in the Western Alps are generally interpreted to result from subduction of European continental crust beneath Adria and its subsequent exhumation. However, the roles of extension (either from plate divergence or internal to the orogen) and erosion during exhumation remain controversial. Here we use 2D numerical geodynamic models to explain the formation and exhumation of (U)HP rocks in an Alpine-type orogen and interpret the results in the conceptual Prowedge–Uplifted Plug–Retrowedge–Conduit (PURC) framework. (U)HP metamorphism of oceanic and microcontinent crust in the models results from burial and accretion to a subduction channel/conduit formed beneath an advancing retrocontinent. Rapid exhumation from (U)HP conditions is achieved by the buoyancy-driven transport of a composite plume of stacked (U)HP oceanic and microcontinent crust from the subduction conduit to the overlying orogenic prowedge, accommodated by coeval thrusting and normal-sense shearing. Subsequent ‘trans-crustal’ exhumation is achieved by a combination of doming/internal extension and later retrotransport of the (U)HP plume through the uplifted plug, during underthrusting of the thick continental margin crust, coupled with increased erosion. Our proposed mechanism implies that exhumation-related normal-sense shearing in the Western Alps, per se, was driven from below by the buoyancy of the ascending plume, and that extension owing to plate divergence is not required to explain (U)HP rock exhumation. The efficiency of the exhumation mechanism depends strongly on the buoyancy and strength of the (U)HP plume, suggesting that in order to exhume rapidly, it must achieve a critical size. By implication, the multiple small Eocene (U)HP complexes within the Internal Crystalline Massifs may have been exhumed as part of a single composite body comprising diverse units aggregated from different levels of the subduction conduit during burial or ascent, rather than as individual small bodies exhumed in separate pulses.

The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: A petrological and geochronological study of “silvery micaschists”

We present a petrological and geochronological study of “silvery micaschists” that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P–T conditions and age of the Alpine high-pressure metamorphism. The studied “silvery micaschists”, which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U–Th bearing accessory minerals: florencite → monazite → allanite. Thermobarometric calculations indicate that the Mg-chlorite + talc + chloritoid + allanite peak assemblage was stable at P = 1.9–2.7 GPa and T = 515–600 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7 ± 1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4 ± 0.9 Ma, implying a minimum duration of ∼ 2–4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ∼ 1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges.

Miocene lateral extrusion in the inner western Alps revealed by dynamic fault analysis

From early Miocene to the present‐day the core parts of the western European Alps experienced brittle extensional deformations, mostly in a strike‐parallel direction. Here we present new data constraining the brittle deformation of the Vanoise area (French Alps) and a synthesis of 312 paleostress tensors in the whole arc of the internal western Alps. The data show a continuous change in the direction of extension, from N065° (Simplon area), to N‐S (Vanoise area) and to NNW‐SSE (Briançon area). The abundance of orogen‐perpendicular σ3 axes increases from the north to the south. In the Briançonnais area, an extensional reactivation of the Basal Penninic Thrust seems to be the origin of the E‐W to NE‐SW oriented σ3. In light of these new data and the regional paleostress synthesis, we propose a predominant orogen‐parallel extension in the internal zone as a whole. This orogen‐parallel extension is related to the indentation/rotation of the Apulian microplate and to the opening of the Ligurian Sea during the lower‐middle Miocene. The locally observed orogen‐perpendicular extension is interpreted as an effect of the exhumation of the Internal Crystalline massifs, the uplift of the External Crystalline massifs and/or the present‐day geodynamics (postorogenic gravitational collapse). Some transcurrent tectonics, older than the extension in the Valais area and younger than the extension farther south is observed in the entire inner western Alps; strike‐slip movements are correlated with the Apulian rotation and local permutation of stress axes.

The Western Alps from the Jurassic to Oligocene: spatio-temporal constraints and evolutionary reconstructions

Despite extensive research in the last 150 years, the regional tectonic reconstruction of the Western Alps has remained controversial. The curved orogenic belt consists of several ribbon-like continental terranes (Sesia/Austroalpine, Internal Crystalline Massifs, Briançonnais), which are separated by two or more ophiolitic sutures (Piemonte, Valais, Antrona?, Lanzo/Canavese?). High-pressure (HP) metamorphism of each terrane occurred during distinct orogenic episodes: at ∼65 Ma in the Sesia/Austroalpine, at ∼45 Ma in the Piemonte zone and at ∼35 Ma in the Internal Crystalline Massifs. It is suggested that these events reflect individual accretionary episodes, which together with kinematic indicators and the speed and direction of plate motions, provide constraints for the discussed reconstruction model. The model involves a prolonged orogenic history that took place during relative convergence of Europe and Adria (here considered as a promontory of the African plate). The first accretionary event involved the Sesia/Austroalpine terrane. Final closure of the Piemonte Ocean occurred during the Eocene (∼45 Ma) and involved ultra-high-pressure (UHP) metamorphism of the Piemonte oceanic crust. Incorporation of the Briançonnais terrane in the accretionary wedge occurred thereafter, possibly during or after subduction of the Valais Ocean in the late Eocene (45–35 Ma). This subduction was terminated at ca. 35 Ma, when the Internal Crystalline Massifs (i.e. the assumed internal parts of the Briançonnais terrane) were buried into great depths and underwent HP and UHP metamorphism.

Brittle deformation between the Ambin and Vanoise domes in the frame of the structural evolution of the internal Alpine belt

New geophysical data and geological observations emphasize the importance of brittle tectonics in the Neogene to present day evolution of the Western Alps. The oldest Neogene stress state (F1) is characterized by extension direction parallel to the orogen and shortening direction vertical or perpendicular to the Alpine belt whereas the youngest tectonic event (F2) is characterized by brittle extension perpendicular to the mountain belt. According to new brittle microtectonics data from the High Maurienne valley near Modane and a synthesis of the available geochronological and microtectonics data at the scale of the internal Alps, we discuss a timing of the brittle tectonics phases and their role in the formation of basement domes (Internal Crystalline massifs and Brianconnais domes). We propose that the F1 event spans from 32-30 Ma to 22 Ma south of the Simplon fault and was probably active up to 5 Ma on the Simplon fault. The F2 event started at about 6-5 Ma and is still active. In this scheme, basement domes observed in the Western Alps are the results of interference between an early E-W extension along NE-SW ductile to brittle faults and the two F1-F2 brittle tectonic events documented here.

Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps

AbstractThe Schistes Lustrés (SL) suture zone occupies a key position in the Alpine chain between the high‐pressure (HP) Brianconnais domain and the ultrahigh‐pressure (UHP) Dora Maira massif, and reached subduction depths ranging from c. 40–65 km (Cottian Alps). In order to constrain the timing of HP metamorphism and subsequent exhumation, several phengite generations were differentiated, on the basis of habit, texture, paragenesis and chemistry, as belonging to the first or second exhumation episode, respectively, D2 or D3, or to earlier stages of the tectono‐metamorphic evolution. Ten carefully selected samples showing D2, D3 (D2 + D3), or earlier (mostly peak temperature) phengite population(s) were subjected to laser probe 40Ar/39Ar analysis. The data support the results of the petrostructural study with two distinct age groups (crystallization ages) for D2 and D3 phengite, at 51–45 and 38–35 Ma, respectively. The data also reveal a coherent age cluster, at 62–55 Ma, for peak temperature phengite associated with chloritoid which were preserved in low strain domains. The age of the D3 event in the SL complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs. Exhumation rates of the order of 1–2 mm yr−1 are obtained for the SL complex, which are compatible with velocities documented for accretionary wedge settings. Similarly, cooling velocities are only moderate (c.5 °C Myr−1), which is at variance with recent estimates in the nearby UHP massifs.

変成岩岩石学の現状と将来への展望 アルプスと西南日本 二つの代表的造山帯の年代学的特徴

There are two types of orogenic belts : the continent-continent collision type such as Alps and the subduction related (Cordilleran) type of southwestern Japan. Their metamorphic rocks have contrasting geochronological characteristics. Alpine metamorphic rocks have recorded three major metamorphic event : 1) the Cretaceous ultra-high and related high P/ T metamorphism in continental material such as the Sesia-Lanzo and the Pennitic Internal Crystalline Massifs, 2) the Eocene high P/T metamorphism in the ophiolites and calc-schists of the Mesozoic Tethys and 3) medium P/T metamorphism, in which both types of high P/ T metamorphic rocks were variably reset by Oligocene thermal events. However, almost all geochronological data reported from the Alpine metamorphic belt show mixed ages which are due to the mixture of minerals formed in the three metamorphic events, requiring a petrological and chronological re-scrutiny by both expert petrologist and geochronologist with a conscientious strategy. On the contrary, the southwest Japan has several metamorphic belts in which the rocks record generally mono-metamorphism, suggesting a simple history in comparison with Alpine rocks. However, recent study has revealed a complex history of the low P/ T metamorphic belt : the low P/ T metamorphism took place in the pre-dating accretionary complexes which have suffered the subduction related metamorphism or in the geologic unit consisting of the predating high grade metamorphic rocks. This gave us the discordant age relation similar to that in Alps. Thus, orogenic belts are so heterogeneous in geochronology, indicating that the belts have recorded the complicated history of earth crust. Conversely, the advanced geochronology and petrology will make it possible to peruse the complicated history of earth crust.

Radiometric ages of Alpine metamorphic rocks in the western and central Alps

Abstract The chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia‐Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U‐Pb monazite (23‐29 Ma), Rb‐Sr muscovite (15–40 Ma) and biotite (15–30 Ma), K‐Ar biotite (10‐30 Ma), muscovite (15–25 Ma) and hornblende (25‐35 Ma), and FT zircon (10‐20 Ma) and apatite (5‐15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U‐Pb zircon age for a coesite‐bearing whiteschist (metaquartzite) from the Dora‐Maira represents the peak of ultra‐high pressure metamorphism. Coesite‐free eclogites and blueschists related to ultra‐high pressure rocks in the Penninic crystalline massifs yield an 40Ar‐39Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high P/T metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra‐high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia‐Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium P/T metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid‐ocean ridge and Eocene continental collision triggered off the exhumation of the high pressure rocks.

A cross section of the lithosphere along the European Geotraverse Southern Segment (from the Alps to Tunisia)

The European Geotraverse Southern Segment (EGT-S) is a 200–300 km wide strip from the Alps to Tunisia, which crosses the youngest tectonic provinces of Europe where continued Alpine compression is accompanied by Recent extension, e.g., in the Ligurian Sea and Sicily Channel. The geophysical research along the EGT-S has been nearly continuous, both on land and sea, from 1982 to 1987 with deep seismic sounding (DSS) experiments and collection of heat-flow, gravity, magnetic, seismological and seismotectonic data. The new DSS and seismic reflection data testify to the asymmetry of the Alpine edifice. The deepest part of the crust-mantle boundary is found beneath the internal and external crystalline massifs where the European Moho disappears, after a steep flexure, in the seismic reflection data. The continuation of the Moho discontinuity into the southern Alpine domain is, however, evident in the seismic refraction data. Seismic evidence and heat flow data indicate incipient rifting in the Ligurian Sea which is underlain by a mantle with an anomalously low P n velocity of only 7.5 km/s. The Palaeozoic crust of the Sardinia-Corsica block exhibits a bowl shape with a rather undifferentiated internal structure (average P-wave velocity of 6.3–6.4 km/s). Crustal thickness increases from 20 km at its margins to more than 30 km beneath the centre of the islands. A thinned crust with thrusted blocks in its upper part characterizes the Sardinia Channel. Finally, the preliminary DSS results from Tunisia show an abrupt deepening of the Moho (from 20 to approximately 35 km in the Tell area), very low average crustal velocity, which requires a velocity inversion within the lower crust and subcrustal reflectors revealing a structured upper mantle.

A unique magnesiochloritoid-bearing, high-pressure assemblage from the Monte Rosa, Western Alps: petrologic and 40Ar-39Ar radiometric study

A phengite-talc-chloritoid-chlorite-kyanite-quartz assemblage is reported from a nearly undeformed quartz-rich metapelite found in the Monte Rosa massif (Western Alps). Chloritoid contains up to 74 mol % of the Mg end member and is the most magnesian ever reported. Textural relationships and mineral compositions suggest equilibrium and therefore a low-variance assemblage which represents the high-pressure stability limit of chlorite+quartz according to the terminal reaction $${\text{chlorite + quartz }} \rightleftarrows {\text{ talc + chloritoid + kyanite + H}}_{\text{2}} {\text{O}}{\text{.}}$$ Mineral compositions combined with new experimental data on the stability of the Mg-chloritoid end member lead, for a temperature close to 500° C, to a pressure estimate of 16 kbar and a water activity of 0.6 which is supported by fluid inclusions study. Chloritoid composition is in fact a fine metamorphic indicator which opens new ways for barometry in high-grade blueschists. It demonstrates here the existence of a high-pressure metamorphism in the Monte Rosa massif. The assemblage remained mineralogically unaffected during the subsequent lower-pressure evolution. Two size fractions of the single phengite generation were analysed by the 39Ar-40Ar incremental release method. Both spectra are identical with a plateau at 110±3 Ma representing over 96% of the 39Ar degassed. The ages of the first heating steps are discordant and increase with increasing temperature from values near 70 Ma to the plateau age. Isotope correlation diagrams show two 36Ar components, one released at high temperature and correlated with 40Ar and 39Ar, the other released at low temperature in a mixture of atmospheric argon and of a loosely held argon of 70 Ma apparent age. The 110 Ma plateau age may reflect the presence of homogeneously incorporated excess argon, the 70 Ma value might then be a true age. However we favour the alternative hypothesis that the 110 Ma plateau age is a true age, implying that the internal crystalline massifs of the Western Alps have endured high-pressure metamorphism as early as mid-Cretaceous. Whatever the interpretation chosen, the preserved high-pressure mineral assemblage remained isotopically unaffected during the low-pressure mid-Tertiary event which is recorded by the 37 Ma plateau age of phengite from a foliated, recrystallised quartzite collected in the same, westernmost part of the massif. The contrasting behaviour of the two samples shows that even at temperatures as high as 400–450° C deformation and recrystallisation are also major controlling factors of isotope mobility.