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Evolution tectono-métamorphique du Briançonnais interne (Alpes Occidentales, massifs de Vanoise Sud et d'Ambin) : comportement du socle et de sa couverture dans un contexte de subduction continentale profonde

William Gerber
Abstract : The aim of this study is to characterize the main steps of high-pressure (HP) units, metamorphosed in a deep continental subduction context, during the subduction of European plate below Apulian plate. Selected targets are Vanoise Sud and Ambin massifs (Internal Briançonnais domain), where exposed basement and cover units underwent a HP event.

Field work led to a detailed map of structural patterns (schistosities, lineations, folds, shear bands), and allow us to understand the timing of deformations. Microstructures and petrological studies, suggest that a typical mineralogical assemblage developped during D1 and D2 deformation stages. Pressure and temperature estimates, linked with the first Ar-Ar in situ dating on phengites from these two massifs, led us to a synthetic P-T-t-deformation diagram where exhumation steps of basement and cover units are plotted.

- A first alpine stage (D1) occured in the blueschist facies around 50 My, and is related to the beginning of exhumation of subducted units. In the Vanoise Sud massif, we show a strong metamorphic contrast between basement (17kbar-480°C) and cover units (11kbar-300°C). This data is consistent with an early uncoupling, associated with the burial and related to a NW-verging shear in the blueschist facies.

- The D2 stage begins at 43 My in the lower blueschist facies, down to the greenschist facies. The main East-verging shear event (C2) starts at 37 My and reaches its paroxysm at 34 My. D2 is related to a heating event in the whole stack of units (+100°C), which leads to the thermal peak (530°C-7kbar in the basement and 350°C–6kbar in the cover).

- In the Vanoise Sud massif, basement and cover units have been set in contact around 30 My, thanks to East-verging shears (3-4kbar-350°C). The youngest limit for the latest ductile events is consequently shifted around 28 My (3kbar-300°C).

- The late exhumation of the units occured in the brittle domain, through 3 stages :

(i) between 28 and 20 My : high speeds, up to 1.5km/My.
(ii) between 20 and 5 My : units remain stable at 3km deep (period of thermal stability, cooling rate = 1°C/My).
(iii) since 5 My, increase of exhumation rate (0.6 km/My)
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Submitted on : Wednesday, November 19, 2008 - 5:44:07 PM
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William Gerber. Evolution tectono-métamorphique du Briançonnais interne (Alpes Occidentales, massifs de Vanoise Sud et d'Ambin) : comportement du socle et de sa couverture dans un contexte de subduction continentale profonde. Tectonique. Université Pierre et Marie Curie - Paris VI, 2008. Français. ⟨tel-00340057⟩

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