Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

Abstract

International audienceDetailed 40Ar/39Ar geochronology on single grains of muscovite was performed in the Variscan Tanneron Massif (SE France) to determine the precise timing of the post-collisional exhumation processes. Thirty-two plateau ages, obtained on metamorphic and magmatic rocks sampled along an east–west transect through the massif, vary from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation of the lower crust that lasted about 20 Ma during late Carboniferous. In the eastern part of the massif, the closure of the K–Ar isotopic system is at 311–315 Ma, whereas in the middle part of the massif it closes earlier at 317–321 Ma. These cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a major ductile fault, the La Moure fault that separates both domains. In the western part of the massif, the ages decrease from 318 to 303 Ma approaching the Rouet granite, which provides the youngest age at 303.6 ± 1.2 Ma. This age distribution can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex. These ages argue in favour of a cooling of the magmatic body at around 15 Ma after the country rocks in the western Tanneron. The emplacement of the Rouet granite in the core of an antiform is responsible for recrystallization and post-isotopic closure disturbances of the K–Ar chronometer in the muscovite from the host rocks. These new 40Ar/39Ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later stage of collision. During the first stage between 320 and 310 Ma, the differential motion of tectonic blocks limited by ductile shear zones controls the post-collisional exhumation. This event could be related to orogen parallel shearing associated with crustal-scale strike-slip faults and regional folding. The final exhumation stages at around 300 Ma take place within the tectonic doming associated to magmatic intrusions in the core of antiformal structures. Local ductile to brittle normal faulting is coeval to Upper Carboniferous intracontinental basins opening

Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects

“…Hermitan and Rouet) in the IZ are cored by syntectonics granitoids structurally and thermally concordant with the surrounding migmatites Corsini et al, 2010;Schneider et al, 2014). In contrast the EZ records only late HT/LP discordant overprinting (eg.…”

“…the Hermitan, Rouet and Cannes antiforms in the MTM, Fig. 1, Crevola & Pupin 1994;Corsini et al, 2010) and separated by NS transcurrent faults (Joyeuse-Grimaud and La…”

Paleo-European crust of the Italian Western Alps: Geological history of the Argentera Massif and comparison with Mont Blanc-Aiguilles Rouges and Maures-Tanneron Massifs

“…These cooling paths are interpreted as resulting from differential exhumation processes of distinct crustal blocks controlled by the La Moure fault that separates the two domains (Corsini et al, 2010). In the Western Domain, the ages decrease from ~ 318 to 304 Ma approaching the Rouet granite, which provided the youngest age at 303.6 ± 1.2 Ma (Corsini et al, 2010).…”

“…Ma, (Corsini et al, 2010). These cooling paths are interpreted as resulting from differential exhumation processes of distinct crustal blocks controlled by the La Moure fault that separates the two domains (Corsini et al, 2010).…”

“…Recent isotope dilution U-Pb monazite dating (Demoux et al, 2008) Carboniferous HT event at 317 ± 1 Ma (Demoux et al, 2008) In the eastern part, a migmatization event is recorded by monazites from a synkinematic leucogranite body and the Tanneron mylonitic orthogneiss, which yielded ages of 309 ± 5 and 310 ± 2 Ma, respectively (Demoux et al, 2008) Later post-collisional magmatism is recorded by the intrusion of the post-tectonic Rouet granite at 302 ± 4 Ma (Demoux et al, 2008) and of undeformed leucogranite dykes in the Eastern domain, which mark the final stage of the Variscan evolution at 297 ± 5 Ma (Demoux et al, 2008) Thirty-two 40 Ar/ 39 Ar plateau ages on single grains of muscovite from metamorphic and magmatic rocks sampled along an east-west transect through the Massif range from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation that lasted about 20 Ma during the Late Carboniferous (Corsini et al, 2010). In the Eastern Domain, closure of the K-Ar isotopic system is at ~ 311-315…”