Int J Earth Sci (Geol Rundsch) (2003) 92:338–347 DOI 10.1007/s00531-003-0321-3

ORIGINAL PAPER

O. Bruguier · J. F. Becq-Giraudon · N. Clauer · H. Maluski From late Visean to Stephanian: pinpointing a two-stage basinal evolution in the Variscan belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications

Received: 3 April 2002 / Accepted: 9 February 2003 / Published online: 17 April 2003 Springer-Verlag 2003

Abstract Post-convergence evolution of the Variscan Keywords Delamination · French Massif Central · belt is characterized by the development of intramontane Intramontane basins · Stephanian · Visean coal-bearing basins containing volcano-sedimentary suc- cessions. In the French Massif Central, K–Ar ages on clay particles from fine-grained sediments of the Bosmoreau Introduction basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a Extensional tectonics is preferentially located along siltstone underlying a volcanic layer previously dated at orogenic belts with a thickened crust and collapse of 332€4 Ma by the U–Pb method on zircon, yields a mountain belts represents an important feature of post- consistent K–Ar age of ca. 340 Ma. Upward in the collisional orogenic stages (e.g. Ratschbacher et al. 1989). sedimentary succession, illite yields Stephanian K–Ar Implicit to this is the creation of a pervasive series of ages, which can be combined to provide a mean continental basins accompanying extension in the upper deposition age of 296.5€3.5 Ma. The Bosmoreau basin, crust. Whereas most studies addressing evolution of albeit mainly filled with Stephanian deposits, was initi- orogenic belts usually deal with basement and deep ated during the late Visean, i.e. ca. 30 Ma earlier than crustal processes (Faure and Pons 1991), an alternative inferred from biostratigraphical constraints. During the approach, whenever possible, is to focus on sedimentary Stephanian, the same structure was reactivated and late records and to use sedimentary basins as tectonic markers, Visean deposits were eroded and subsequently blanketed considering that their formation mirrors deeper processes by thick clastic sediments. These results emphasise a two- (Zoback et al. 1993). Basin opening and sedimentary stage evolution for the Bosmoreau basin, which is closely infilling are often tectonically controlled (e.g. Bruguier et related to extensional tectonics identified on basement al. 1997). A key issue, therefore, is to determine precisely country rocks, and they are used to propose a geodynamic both the age of basin formation and the main stages of evolution of the studied area. infilling, and the relationships between basin initiation and tectonic structures such as major fault systems. These parameters potentially carry important information that have implications for understanding the tectonic control O. Bruguier ()) on the sedimentary record, and to punctuate the different Service ICP-MS, cc 056, ISTEEM, stages of extensional tectonics. The implied requirements Universit de Montpellier II, Place E. Bataillon, for such an approach to be fully operative are tightly 34095 Montpellier, France related to the possibility of selecting suitable sedimentary e-mail: [email protected] records and appropriate chronological methods. J. F. Becq-Giraudon The French Massif Central is one of the most BRGM, 3 Avenue C. Guillemin, BP 6009, important exposures of the Internal Zone of the Variscan 45060 Orlans, France Belt, which extends along ca. 3,000 km from the Iberian Massif in the West to the Bohemian Massif in the East N. Clauer (Fig. 1). In the whole belt, the Late Carboniferous–Early CGS-EOST, Universit Louis Pasteur-CNRS, 1 rue Blessig, 67084 Strasbourg, France Permian time interval is characterized by numerous coal- bearing intramontane basins corresponding to isolated H. Maluski troughs closely associated with fault-zones and filled with Laboratoire de Gophysique, Tectonique et Sdimentologie, coarse, clastic, non-marine sediments deposited uncon- CNRS-UMR 5573, ISTEEM, Universit de Montpellier II, formably on the metamorphic and igneous basement. The Place E. Bataillon, 34095 Montpellier, France 339 Fig. 1 Position of the French Massif Central within the Eu- ropean Variscides (after Matte 1986)

structural control of the opening and further infilling of Geological setting these basins is obvious from their association with faults, at least along one of their borders, and it has been widely The Bosmoreau coalfield is a small graben filled in by documented (e.g. Faure 1995, and references therein). continental Carboniferous deposits and located in the In this study, we have focused on the Bosmoreau basin north-western part of the French Massif Central (Limou- located in the northern part of the French Massif Central sin area). It is entirely bounded by normal brittle faults (Fig. 2). The sedimentary successions are well preserved and developed in the hanging-wall block of the northern in the basin, and they are known in detail. In addition, a end of the Argentat fault (Fig. 2). Sedimentary rocks biostratigraphic support provided by floral records allows unconformably overlie the Late Devonian Guret granitic age constraints to be placed (Becq-Giraudon 1985). This massif dated at 356€10 Ma by the Rb–Sr method on basin is located in the hanging-wall blocks of the Argentat whole rocks (Berthier et al. 1979). Basin fill is estimated fault, a major fault system of this part of the Variscan to be about 600 m thick and consists mainly of orogen (Fig. 2). Following our previous work on this siliciclastic fluvio-lacustrine to palustrine sediments, basin (Bruguier et al. 1998), our goal is herein to date the including coal seams. The lithostratigraphy of the basin sedimentary succession, by selecting illite particles for K– can be divided into four third-order sequences (Fig. 3). Ar analyses from fine-grained sedimentary rocks sampled Sequence 0 is only exposed in the south-western part of at different levels in the sedimentary pile. This approach the basin and consists of about 30 m of fine-grained was thought to help to pinpoint basin infilling and sediments, which include a 60-cm-thick volcanic ash relationships with nearby structuring faults, which may be layer. U–Pb zircon results from this volcanic ash layer viewed as a basin response to more global, crustal scale, yielded a Late Visean upper intercept age of 332€4 Ma, processes. interpreted as the age of eruption of the magma and, 340

Fig. 2.a Location of the main Stephanian–Autunian basins of the French Massif Central. b Simplified geologic sketch map of the Bosmoreau basin showing the main features (after Becq-Giraudon 1985) therefore, of deposition of the airborne ash in the radiogenic 40Ar for five determinations. The blank of the Bosmoreau basin (Bruguier et al. 1998). Sequence 0 is extraction lines and the mass spectrometers was also separated from sequence 1 by an erosional unconformity determined repetitively. During the course of the study, and the ages of sequence 1 to 3 are upper Stephanian the amount of residual 40Ar was always below 110-7 cm3 according to their palaeobotanical and palynological and the 40Ar/36Ar ratio of the atmospheric Ar averaged records (Becq-Giraudon 1985). 293€6 (2s). The usual decay constants were used for the age calculations (Steiger and Jger 1977), and the overall error of the K–Ar determinations was evaluated to be Analytical techniques systematically better than €2% (2s).

K–Ar isotopic determinations were made following a procedure close to that reported by Bonhomme et al. Results (1975). K was measured by flame spectrophotometry with a global accuracy of €1.5%, based on systematic controls Three fine-grained sediments were sampled at different of international standards. For Ar analysis, the samples levels in the sedimentary pile for K–Ar analyses (see were pre-heated under high vacuum at 100 C for at least Fig. 3). Sample ST1 belongs to sequence 0 and was taken 12 h to reduce the amount of atmospheric Ar adsorbed on from a micaceous siltstone underlying the volcanic ash the mineral surface during sample preparation and CI1. Sample ST4 is from the base of sequence 1 and was handling. The Ar isotopic results were periodically taken from a 0.5-m-thick micaceous siltstone located 3 m controlled by analysis of the international GL-O standard, above sequence 0. Sample ST7 is a brownish micaceous which averaged 24.40€0.15x10-6 cm3/g STP (2s)of siltstone located at the base of sequence 3. The clay 341 Fig. 3 Lithostratigraphic col- umn of the sedimentary se- quence accumulated in the Bosmoreau basin. A symbol denotes sample locations 342 Table 1 K–Ar isotopic results and mineralogical composition of Ar; the amounts in 40Ar* are given in the STP system. I/S Illite/ 40 -10 the investigated clay fractions. Constants used: l Kb=4.96210 smectite mixed layers; chl chlorite; sm smectite; fsp feldspar -1 40 -10 -1 40 -4 an , l Ke=0.58110 an , K/Ktot=1.16710 . Ar* radiogenic 40 Sample Grain size Mineralogy I/S Illite K2O Ar* Ar* Age (m) by XRDa (%) crystallinity (%) (%) (106 cm3/g) (Ma€2s) ST1 <0.2 I/S 100 9.5 4.06 87.24 48.84 342.4€8.7 duplicate – – – 35.70 49.40 345.9€10.9 ST4 <0.2 I/S, chl, fsp 85 11.4 6.14 95.09 63.41 295.0€6.7 0.2–0.4 I/S, chl, fsp 80 10.5 6.36 96.30 65.52 299.9€6.5 ST7 <0.2 I/S, chl, sm 7.20 96.50 75.34 298.6€6.5 0.2–0.4 I/S, chl, sm, fsp 7.88 98.28 80.63 292.5€6.2 a Ordered from highest to lowest concentrations separates provide K–Ar ages ranging from 293€6 to commonly observed when participation of detrital com- 346€11 Ma (2s), and can be divided in two groups. ponents occurs in clay concentrates (Clauer and Chaud- Sample ST1 yields the oldest age of 342€9 Ma (Table 1). huri 1995; Clauer et al. 1995; Schaltegger et al. 1995). This value was verified by a replicate analysis providing a value in close agreement (346€11 Ma) and giving an average value of 344€10 Ma. Although slightly older, Discussion and implications these Visean ages are similar within analytical uncertainty to the U–Pb zircon age of 332€4 Ma for the overlying Fine-grained sedimentary rocks preserved in the lower volcanic ash layer Ci1. A second group of ages was found part of the sedimentary pile that accumulated in the for illite of siltstones ST4 and ST7 sampled upward in the Bosmoreau basin have K–Ar ages of ca. 340 Ma, sedimentary pile. These two samples provide a narrow consistent with the 332€4 Ma U–Pb zircon age of a cluster of younger ages ranging from 293 to 300 Ma. volcanic ash layer (Bruguier et al. 1998). These results Individual error margins do not allow a distinction to be indicate that parts of the sedimentary succession are older made between the two samples and preclude any than Stephanian and date back to late Visean, a conclu- sedimentation rate to be calculated. However, the consis- sion that could not be drawn from palaeobotanical studies. tency between samples ST4 and ST7 makes it possible to Typical, Stephanian floral records were identified in the combine the data for an average upper Stephanian age of upper part of the basin and were used to attribute to 296.5€3.5 Ma (MSWD=1.1). Clauer and Chaudhuri successions accumulated in this basin an upper Stepha- (1995, 1998) and Dong et al. (1997) showed that thermal nian age (Becq-Giraudon 1985). These observations agree conditions necessary to reset the K–Ar systems of detrital with the mean K–Ar age of 296.5€3.5 Ma exhibited by and diagenetic micas correspond to the anchizone– clay particles recovered from siltstones sampled in epizone boundary and that diagenetic ages can be sequences 1 and 3 of the sedimentary pile. This age falls preserved for lower grade samples. A recent study on in the Gzelian stage of the Stephanian series (Odin 1994). coal maturation of the Bosmoreau coal indicates that the Radiometric dating results are scarce for Stephanian Carboniferous sediments were submitted to a palaeo- occurrences in the French Massif Central, including the geothermal gradient of 90 C/km during Permian times continental reference series of the St-Etienne basin, and (Copard 1998). As the sediments were never buried comparisons, thus, are limited. However, the ca. 297 Ma deeper than 1,500 m (Copard et al. 2000), the samples K–Ar age for sedimentation in the Bosmoreau basin is in were not heated above 150 C subsequent to deposition, close agreement with muscovite and biotite 40Ar/39Ar and the close agreement between the K–Ar results from ages (297€3 Ma) from the southern and northern part of sample ST1 and the U–Pb zircon age from the nearby ash the Montagne Noire area. These 40Ar/39Ar ages have been layer CI1, substantiates that the K–Ar ages can be interpreted as marking movement along an active detach- interpreted as corresponding to the depositional age of the ment (Maluski et al. 1991) and the contemporaneous sediments. The illite crystallinity value, which measures development of the Graissessac basin (see Fig. 2). Lastly, the diagenetic evolution of authigenic clay, is also a this age compares very well with those determined for valuable tool to distinguish between metamorphic and basins throughout the Variscan Belt (e.g. Breitkreuz and diagenetic ages. Diagenetic illites show crystallinity Kennedy 1999; Kninger et al. 2002). This is taken as values above 9, whereas metamorphic ones have lower evidence for a synchronous basin-forming event that values (Kbler 1984); this parameter is also grain-size occurred at orogenic belt scale, at the end of the dependent. From Table 1, it can be seen that all clay Carboniferous. separates analysed have illite crystallinity values above 9, The data presented here have general implications on again substantiating that the K–Ar values date a low- the evolution of the Internal Zone of the Variscan Belt, temperature crystallization. Finally, the K–Ar age deter- especially when compared with the available structural minations do not show any relationships of increasing and geochronologic data. The Variscan belt of Europe has ages with increasing grain-sizes, which is a feature been the site, from Visean to Permian, of repeated post- 343 collisional magmatic pulses that are characterized by The occurrence of the late Visean volcanic ash layer distinct geochemical signatures and geodynamic settings interbedded within sedimentation is also evidence that (Schaltegger 1997). In the French Massif Central, large magmatism in the deep crust and the associated surface volumes of granitoids were emplaced in the middle crust, volcanism were synchronous with basin formation. from 330 to 290 Ma (see review in Pin and Duthou 1990; Although volcanic sources have not been identified for Ledru et al. 1994), and basement studies have demon- this material, the similarity in age suggests a likely origin strated that extensional tectonics occurred during two from the neighbouring Late Visean ‘Tufs Anthracifres’ distinct periods (Faure 1995). The first, apparently located about 60 km east of the Bosmoreau basin (Scott et protracted, extensional event took place from late Visean al. 1984). Late Visean granitoids (340–325 Ma) are to Westphalian, while convergence prevailed at the scale numerous in the French Massif Central (Ledru et al. 1994; of the whole orogen (Van der Voo 1982; Matte 1986). It Faure et al. 2002), indicating widespread magmatism that propagated diachronously southward across the French is not restricted to this part of the Variscan belt, but is also Massif Central, starting in Visean times in the north to well documented in other segments of the Internal reach the southern margin during the Westphalian when Variscides, such as the northern Bohemian massif of the nappe stacking and ductile deformation were still pro- Saxothuringian domain (Wenzel et al. 1997; Krner et al. ceeding in the southern foreland of the Montagne Noire 1998), the Southern Vosges (Schaltegger et al. 1996) and area (Maluski et al. 1991). The second extensional event, the Central Alps (Schaltegger and Corfu 1992). The from late Stephanian to Autunian, has been related to widespread occurrence of this event suggests orogen-wide collapse during late stage evolution of the Variscan belt processes and was attributed to thermal relaxation during (Malavieille 1993; Becq-Giraudon and Van den Driessche episodic thinning of the lithosphere (Schaltegger 1997). 1994; Faure 1995); it is characterized by numerous half- In the French Massif Central, where the crystalline graben geometry basins. In the case of the Bosmoreau basement broadly consists of ca. 50% granitoids, the basin, U–Pb and K–Ar results substantiate that initiation ages available (Ledru et al. 1994; Faure 1995) suggest, in of opening occurred as early as the late Visean, contem- contrast, a continuum of magmatic activity from Visean poraneously with the first period of extensional tectonics. to Permian times, although large error margins often Earlier work (Becq-Giraudon 1985) and additional field result in dates straddling the magmatic pulses identified in observations collected during this study are summarized other parts of the Variscan belt (Schaltegger 1997). in the synthetic lithostratigraphic column of Fig. 2. The Therefore, this may hamper identification of distinct upper Visean ages are restricted to sequence 0 cut by the events occurring in a rather limited range of time. Results overlying conglomeratic sandstones of sequence 1. from the Bosmoreau basin point to two episodic periods Because of this erosional unconformity, it is unknown of basin evolution. The older, upper Visean, volcano- whether sequence 0 was initially restricted to the upper sedimentary sequence reflects basin opening and was Visean or extended through the Namurian or even coeval with explosive volcanism and extensional faulting Westphalian. Siltstone ST4 belonging to sequence 1, is along major crustal-scale faults. The second, Stephanian, located about 3 m above sequence 0 and gave a although not dated on volcanic material in the case of the Stephanian age (see Table 1) concordant with biostrati- Bosmoreau basin, is otherwise characterized by wide- graphical data. This indicates that the Stephanian sedi- spread pyroclastic tuffs (Bouroz 1966). This observation mentation started in the Bosmoreau basin with deposition indicates episodicity of uplift and basement erosion, of the thick conglomeratic sandstones separated from synchronous to magmatic activity and suggests a rela- sequence 0 by an erosional unconformity. Deciphering tionship between uplift and thermal pulses. whether sedimentation was continuous, from upper What caused these episodic events and this ‘accor- Visean to Stephanian is not possible in the case of the dion’-like evolution is probably one of the key issues Bosmoreau basin. However, Namuro-Westphalian sedi- challenging our understanding of the post-collision ments are not known in the French Massif Central and, Variscan evolution. Any proposed model should reconcile thus, we suggest that the two sedimentary cycles were the widespread and apparently pervasive post-collisional episodic and related to two distinct phases of uplift and magmatism in the inner part of the belt, together with the basement erosion. Identification of these two successions, occurrence of episodic magmatic/volcanic flare-up at moreover, indicates a long, although episodic, lifetime for distinct periods, associated with uplift and basin forma- the basin of over 35 Ma. The Argentat fault cuts across tion. These magmatic pulses clearly imply that significant the whole French Massif Central and borders the perturbations in the thermal regime have occurred. It is Bosmoreau basin on its eastern side. 40Ar/39Ar muscovite also probably noteworthy that the Carboniferous basins of ages (Roig et al. 1997) obtained on rocks located in the the French Massif Central are intramontane basins with shear zone developed along the Argentat fault at the high altitude sedimentological and geomorphological expense of metamorphic rocks, yield apparent age evidence (e.g. Becq-Giraudon et al. 1996). Because the patterns in the range of 335–337 Ma. The close agreement Stephanian sediments were unconformably deposited on between these 40Ar/39Ar ages and sedimentation in the metamorphic rocks of various grade, this indicates an basin, as inferred from K–Ar and U–Pb ages, is taken as important erosion of the reliefs sometime before the evidence for a genetic link between fault movement and Stephanian. Conceivably, long-lived magmatism in the basin opening. deep crust and episodic pulses can be combined. The 344 long-lived extensional tectonics observed in the present asthenospheric melting can occur) as can be expected in a outcropping late Visean to Westphalian mid-crustal collisional setting with a thickened crust (Davies and von granitoids (Faure 1995) suggests that the crust had been Blanckenburg 1995). Extensional tectonics experienced softened and had flowed over time, which, in turn, implies by mid crustal granitoids (Faure 1995) indicates thermal continuous heat advection. This is a phenomenon that can softening and a possible decoupling of the upper crust. be explained by lithospheric delamination (Nelson 1992) The above-envisioned mechanisms could explain the first and which has been already suggested to explain the post- Visean pulse and the long-lived magmatism observed in collision evolution of the Variscan belt (Pin and Duthou this part of the Variscan belt during the Namuro- 1990; Schaltegger 1997). In agreement with this model, Westphalian period because lithospheric delamination we envision that, after the continental collision, the has a characteristic time of ca. 60 Ma (Nelson 1992). A subducting lithosphere underwent extension related to second phenomenon, however, is needed to explain the buoyancy contrast between the continental and oceanic upper Stephanian (300–295 Ma) pulse, which is charac- part of the slab (see Fig. 4a). Due to opposite internal terized by instantaneous and widespread development of forces, detachment may have occurred at the weak point basins and intense volcanism in the whole Variscan belt of the system, i.e. at the junction between oceanic and (e.g. Breitkreuz and Kennedy 1999). As pointed out continental crust and initiated delamination of the litho- above, such a flare-up implies a strong perturbation of the spheric mantle (Fig. 4b). The detached continental crust thermal regime. Although this needs further substantia- then rotated upwards and started underplating to the tion, we suggest it may result from slab break-off and Armorica/Laurussia block. This was responsible for uplift detachment of the sinking mantle part of the lithosphere and the formation of extensional volcano-sedimentary (Fig. 4d). A large influx of asthenospheric material would basins such as those now found in the Limousin area (this then be allowed to replace the lithospheric roots, while the study), the north-eastern part of the French Massif Central orogen uplifted and extended due to gravitational insta- (Bertaux et al. 1993; Faure et al. 2002), the southern bilities. Because the lower crust was hot and already Vosges (Schaltegger et al. 1996), the southern Schwarz- softened by about 30 Ma of heat advection and/or wald (Schneider et al. 1989) and in some fragments of the production, it was able to flow rapidly. Mechanical Variscan basement preserved in the Central Alpine area extension was thus predominant over erosion as demon- (Schaltegger and Corfu 1995; Von Raumer 1998). The strated by the preservation of the Stephanian intramontane coincident thinning of the mantle part of the lithosphere basins. Slab break-off can be explained by gradual and rise of the asthenosphere triggered the first, late- thinning and final detachment of the sinking lithospheric Visean magmatic event, now represented by granitoids mantle, but also by the Late Carboniferous–Early Permian and associated volcanic expressions such as those found clockwise rotation of Gondwana (Matte 2001) and in the Bosmoreau basin or the so-called ‘Tufs An- resulting large strike-slip faults cutting across Central thracifres’ further east (Scott et al. 1984). This event was Europe and Northern Africa (Bard 1997). These structures synchronous to granulitic metamorphism identified in may have intersected parts of the sinking slab and, thus, some segments of the southern side of the Variscan belt facilitated slab break-off before the general N–S Permian (e.g. Krner et al. 1998, 2000) and suggests that collapse of the whole orogen. From this standpoint, this granulitization of the lower crust occurred at that time. two-stage model (delamination and slab break-off) may From late Visean to Stephanian (325–305 Ma), delami- explain the episodicity of volcanic and basin-forming nation proceeded by peeling away and underplating of the events along with the apparently long-lived magmatism. continental crust (Fig. 4c) resulting in uplift and the formation of high Namuro-Westphalian topographic reliefs whose erosional products accumulated in foreland Conclusions basins of the External Zones such as the Zone Houillre Brianonnaise (Brousmiche-Delcambre et al. 1995), the The Bosmoreau basin in the Limousin area of the French Montagne Noire area and the Ardennes (Matte 1986). As Massif Central records two main phases of basin infilling, the sinking slab looses its lightest part, it steepens, thus both coeval with extensional tectonics affecting the allowing a continuous thinning of the lithosphere and basement country rocks. uplift of asthenospheric material. The resulting mantle heat advection, along with the radiogenic heat accumu- – The first sedimentary cycle started in the upper Visean lated in the thickened crust, was responsible for melting with a continental succession deposited in troughs of the lower crust. Rising magmas were trapped in the developed in the hanging wall blocks of the Argentat middle crust to produce the numerous Namuro-West- fault. K–Ar analyses of fine-grained sedimentary rocks phalian granitoids or erupted on the surface. High erosion gave upper Visean ages (ca. 340 Ma), concurring with rates during this time interval suggest that volcanic the U–Pb zircon age of an overlying volcanic ash layer products were probably removed. As demonstrated by Sr previously dated at 332€4 Ma (Bruguier et al. 1998). and Nd isotope studies (Pin and Duthou 1990), the mantle These ages are similar to 40Ar/39Ar muscovites ages component involved in the source of these granitoids was (335–337 Ma) from the shear zone associated with rather limited. This is consistent with rise of the ductile deformation along the Argentat fault (Roig et asthenosphere at depths not shallower than 50 km (where al. 1997). Motion and related deformation along major 345 Fig. 4 Simplified ‘accordion’- like model illustrating the pro- posed post-collision (340– 290 Ma) evolution of the Var- iscan belt. Vertical scale exag- gerated

crustal-scale faults were coeval with basin opening, Erosional unconformity between the two successions suggesting it formed by accommodation of the move- suggests two distinct periods of basement uplift and ment and, thus, substantiating a genetic link. erosion. This two-stage behaviour closely mimics episod- – The second sedimentary cycle took place during the icity of magmatic/volcanic events in the basement and Stephanian when, at least in the case of the pre- concurs with emerging models that depict the late-stage existing Bosmoreau basin, inherited, structures were evolution of the Variscan belt as being related to reactivated. Previous deposits were eroded and subse- lithospheric thinning and delamination. In addition, we quently blanketed by thick detritus dated at propose that the Stephanian magmatic flare-up reflects 296.5€3.5 Ma by the K–Ar method on clay particles. slab break-off, possibly related to clockwise rotation of Gondwana and associated strike-slip faulting. 346 Acknowledgements We acknowledge funding of this study by the Copard Y, Disnar JR, Becq-Giraudon JF, Boussafir M (2000) BRGM-Geofrance 3D program. N.C. also thanks the technical Evidence and effects of fluid circulation on organic matter in assistance of D. Tisserant and R. Wendling (CGS). Helpful intramontane coalfields (Massif Central, France). Int J Coal comments by U. Schaltegger and J. Von Raumer on a previous Geol 44:49–68 version of the manuscript are greatly appreciated. Constructive Davies JH, von Blanckenburg F (1995) Slab break-off: a model of reviews by P. 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