Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

Abstract

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400°C near the dome, and 230°C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont colour alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. 
 The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300°C and 360°C, could be explained by the presence of granitic plutons under the foreland basin.

Deformation and temperature variation along thrust-sense shear zones in the hinterland-foreland transition zone of collisional settings: A case study from the Barbagia Thrust (Sardinia, Italy)

“…The Raman spectroscopy on carbonaceous material (RSCM; Beyssac et al, 2002Beyssac et al, , 2003Beyssac et al, , 2004) is based on the progressive transformation of carbonaceous material (CM) to graphite with increasing temperature. RSCM has generally been used as a geothermometer to determine the peak temperature reached during burial or tectonic thrust stacking (Chen et al, 2011;Scharf et al, 2013;Vitale Brovarone et al, 2013;Bellanger et al, 2015;Molli et al, 2018;Berger et al, 2020;Pérez-Cáceres et al, 2020;Montmartin et al, 2021;Nibourel et al, 2021), during contact metamorphism (Aoya et al, 2010;Mori et al, 2017;Beyssac et al, 2019;Skrzypek, 2021) or during frictional heating along fault planes after an earthquake (Fauconnier et al, 2014;Kaneki et al, 2016;Kuo et al, 2017Kuo et al, , 2018Nakamura et al, 2020;Muirhead et al, 2021). It is non-destructive and sensitive to thermal changes, enabling to constrain peak temperature in rocks from different geological contexts and metamorphic conditions (see Henry et al, 2019 for a complete review).…”

The Variscan Belt of Western Europe 2

“…In Albigeois and Viganais, a ductile top-to-the-south deformation was contemporary with a greenschist facies metamorphism (Guérangé-Lozes and Burg 1990). Raman spectrometry measurements of the graphite crystallinity indicate temperatures of approximately 300°C (Montmartin et al 2021). The fold-and-thrust belt underlying the Cévennes Para-autochthonous Unit is virtually free of metamorphism with only prehnite-pumpelliyte facies.…”

“…These planar-linear structures are at the origin of the "Himalayan model" (Mattauer and Etchécopar 1976). Kinematic criteria mainly indicate a shearing of the upper part towards the south (or towards the SW), even though opposite directions are also observed (Arnaud and Burg 1993;Faure et al 2001;Montmartin et al 2021). These syn-metamorphic ductile deformations were contemporaneous with the stacking of the litho-tectonic units and the repetition of the quartz-feldspar and micaschist series (Brouder 1968;Mattauer and Etchécopar 1976;Arthaud and Matte 1977;Faure et al 2001Faure et al , 2009Brouder et al 2009).…”

“…The fold-and-thrust belt underlying the Cévennes Para-autochthonous Unit is virtually free of metamorphism with only prehnite-pumpelliyte facies. In the Paleozoic series of the southern slope of the Montagne Noire, temperatures measured by the illite crystallinity, conodonts alteration color, fluid inclusions and the Raman spectrometry of carbonaceous matter crystallinity are around 250°C to 300°C (Engel et al 1981;Guiraud et al 1981;Wiederer et al 2002;Montmartin et al 2021). However, these anchizonal temperatures measured in the Paleozoic carbonaceous pelites of the southern slope of the Montagne Noire are related to the emplacement of the granitic and migmatitic dome of the Axial Zone that superimposed on the D 3 event.…”