Shortening of the European Dauphinois margin (Oisans Massif, Western Alps): New insights from RSCM maximum temperature estimates and 40Ar/39Ar in situ dating

Abstract

International audienceThe Oisans Massif, located in the external zones of the western Alps, experienced significant shortening during the Alpine collision. While a series of major top-to-the west shear zones was recently described, the general low grade of the metamorphism has not attracted much petrological and geochronological studies. This paper provides combined temperature and age constraints on the evolution of the Oisans Massif. Temperature was estimated with the Raman Spectrometry of Carbonaceous Material (RSCM) method and chlorite geothermometry. Maximum temperature reached by the Mesozoic cover (Tmax) from Grenoble to the Galibier pass (E-W) and from Saint Jean de Maurienne to Embrun (N-S) yielded almost constant ca. 330 °C temperatures all through the Massif. Temperatures however strongly decrease either westward towards the top of the Vercors sedimentary sequence or eastward towards the Penninic Frontal Thrust. Age constraints were retrieved using 40Ar/39Ar in situ analyses performed on variously strained samples from Alpine shear zones. Over strain gradients, incipient Alpine recrystallizations progressively develop at the expense of former Variscan parageneses. A combined textural and EPMA approach permitted to identify newly formed chlorite and phengite that unequivocally grew in response to deformation or to low grade metamorphism. Chlorites recorded temperatures from ca. 350–150 °C during the activity of shear zones. In parallel, 40Ar/39Ar in situ experiments enabled dating deformation using both synkinematic phengites crystallized below the closure temperature of white-micas and former Variscan muscovite whose isotopic system have been, at least partially re-opened. Activity of top-to-the-west shear zones responsible for the shortening and thickening of the Oisans massif thus occurred between 34 and 33 and 25 Ma. Integrating these new age-constraints, Tmax estimates, and published geological data on the Oisans Massif and neighbouring areas allow proposing a new shortening scenario for the external zones. From ca. 34–33 Ma, the Oisans Massif was buried as a rigid block below more internal units and reached a temperature of ca. 330 °C. Shearing progressively localized along a series of top-to-the-W shear zones in the basement until 25 Ma. Deformation then localized along a major thrust at the base of the ECM massif and propagated along the basement-cover interface below the Vercors massif after 16 Ma

The 3‐D Thermal Structure of the Helvetic Nappes of the European Alps: Implications for Collisional Processes

“…The metamorphic peak of the ECM and the amount of shortening of the alpine External zone increase from south to north (e.g., Bellahsen et al, ). In the south of the arc, in the Oisans Massif (Figure , inset), the collisional P ‐ T peak conditions attained 350 °C ± 50 °C and 3.5 kbar ± 1 kbar (Bellanger et al, ) with a total shortening for the external zone of 28 km (internal deformation of the ECM, i.e., distributed shear zones, and thrusting along frontal ramp; Bellahsen et al, ). In the Oisans ECM, the thermal peak and the shortening lasted between 30 and 25 Ma (Bellanger et al, ).…”

“…In the south of the arc, in the Oisans Massif (Figure , inset), the collisional P ‐ T peak conditions attained 350 °C ± 50 °C and 3.5 kbar ± 1 kbar (Bellanger et al, ) with a total shortening for the external zone of 28 km (internal deformation of the ECM, i.e., distributed shear zones, and thrusting along frontal ramp; Bellahsen et al, ). In the Oisans ECM, the thermal peak and the shortening lasted between 30 and 25 Ma (Bellanger et al, ). For the Mont Blanc Massif (Figures and ), thermobarometric data from shear zone samples indicate a metamorphic peak at 400 °C ± 25 °C and 5 kbar ± 0.5 kbar (Rolland et al, ); shortening of the external zone there attained some 66 km (internal deformation including the Mont Blanc shear zone and frontal ramps; Bellahsen et al, ).…”

Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)

“…Starting from the latest Priabonian, the basin was filled by turbidites such as the Annot Sandstone, chiefly derived from the European basement exposed to the southwest [ Sinclair , ], whereas minor detrital supply from the axial belt is documented since the Oligocene [ Jourdan et al ., , ]. The Annot turbidites are unmetamorphosed and still lay on top of the European basement, which was thrust westward only after (U)HP exhumation was completed [e.g., Bellanger et al ., ] (Figure e). Such basement‐cover relationships exclude slab retreat along the Western Alps trench during exhumation of (U)HP rocks (see Figure c).…”

Advecting heating by hot fluids of an Alpine fissure in Lauzière Granite (Belledonne massif, Western Alps)

“…Related metamorphism was never evaluated in the Lauzière granite, and remains poorly constrained in the Belledonne massif (320°C-350°C; Nziengui, 1993). It is assumed to lie between the values of the Mont Blanc massif in the north (400°C, <5 kbar; Rolland et al, 2003;Rossi et al, 2005) and the Oisans massif to the south (270-360°C, 2-5 kbar; review in Bellanger et al, 2015). A complex geochronological dataset exists for the metamorphism recorded in shear zones and veins in the ECM, with three age-groups at ca.…”

“…For example in the external crystalline massifs (ECM) of the Alps, retrogression occurs at temperatures <400°C and is commonly localized in highly deformed domains (e.g. shear zone, mylonite; Bellanger et al, 2015;Rossi et al, 2005) with evidence of fluid circulation causing vein formation, metasomatism, or fluid assisted reactions (Rossi and Rolland, 2014). Ascending fluids can also modify the temperature gradient by advective heating, but the influence of fluid flow on thermochronological data is seldom considered in exhumation reconstructions (Derry et al, 2009;Whipp and 20 Ehlers, 2007), and remains to be quantified through numerical modelling.…”