The tectono‐metamorphic evolution of the very low‐grade hangingwall constrains two‐stage gneiss dome formation in the Montagne Noire (Southern France)

Abstract

The Montagne Noire in the southernmost French Massif Central is made of an ENE-elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low-grade petrology, K-Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faug eres units, which constitute part of the southern hangingwall of the dome. It is shown that southward-directed Variscan nappethrusting (D 1 ) and a related medium-P metamorphism (M 1 ) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono-metamorphic event D 2 /M 2 largely transposes D 1 structures, comprises a higher metamorphic thermal gradient than M 1 (transition low-P and medium-P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post-dating D 1 /M 1 by more than 20 Ma. D 2 -related fabrics are refolded by D 3 , which in its turn, is followed by dextral-normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faug eres units, D 2 /M 2 is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D 2 /M 2 also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low-P/high-T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D 2 fabrics and the M 2 metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone-epizone boundary to the diagenetic zone. The related zonation of the pre-D 3 metamorphic field gradients paralleled the dome. These observations indicate that D 2 / M 2 is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D 2 /M 2 . Based on integration of these findings with regional geological constraints, a two-stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull-apart-like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by northdipping detachments and hangingwall basin formation along the northeastern dome margin.

Permian exhumation of the Montagne Noire core complex recorded in the Graissessac‐Lodève Basin, France

“…The latter initiated Montagne Noire core complex exhumation along the footwall (Fig. ; Brun & Van Den Driessche, ; Pochat & Van Den Driessche, ; Pitra et al ., ; Doublier et al ., ), supported by data herein.…”

“…At present, the tectonic interpretation for the formation of the Montagne Noire gneiss dome remains controversial (see Van Den Driessche & Brun, and references herein, Demange, ; Soula et al ., ; Rey et al ., , ; Franke et al ., ; Van Den Driessche & Pitra, ; Doublier et al ., ). However, evidence for late Paleozoic post‐Variscan extensional tectonics, thermal relaxation and high‐temperature low‐pressure metamorphism (peak 700°C at 5 kb in the gneissic core; Fig.…”

“…Interpretations for the origin of these structures group into three types (see e.g. Van Den Driessche & Brun, ; Soula et al ., ; Franke et al ., 2011; Doublier et al ., ; Rabin et al ., and references herein): (1) compressive anticline, (2) diapirism and (3) extensional gneiss dome (core complex). For the first two, most of the dome exhumation predates basin development.…”

Strain partitioning along the anatectic front in the Variscan Montagne Noire massif (southern French Massif Central)

“…Among numerous gneiss domes documented around the world, some have been studied in much more detail, like the Thor‐Odin dome in the North American Cordillera [ Vanderhaeghe and Teyssier , ; Norlander et al ., ], the Naxos dome in the Hellenides [ Jansen and Schuiling , ; Kruckenberg et al ., ], the Aston‐Hospitalet in the French Pyrénées [ Denèle et al ., ], or the Montagne Noire gneiss dome in the French Massif Central [ Demange , ]. In this paper we focus on the Montagne Noire gneiss dome that has benefited from many studies since the early twentieth century [ Bergeron , ; Geze , ; Gèze , ; Marres , ; Denizot , ; Schuiling , ; Collomb and Ellenber , ; Debat , ; Demange , , , ; Hamet and Allegre , ; Bard , ; Vidal et al ., ; Thompson and Bard , ; Courtillot et al ., ; Faure and Cottereau , ; Echtler and Malavieille , ; Van Den Driessche and Brun , ; Cassard et al ., ; Aerden , ; Soula et al ., ; Laumonier et al ., ; Charles et al ., ; Faure et al ., ; Poilvet et al ., ; Rey et al ., ; Doublier et al ., ]. Large exposures of unmetamorphosed to weakly metamorphosed upper crust to migmatitic lower crust makes the Montagne Noire a valuable natural laboratory for studying the mechanisms responsible for crustal deformation and in particular the effect of partial melting on orogenic evolution.…”

Evidence for Deformation in the Cambrian‐Ordovician Warburton Basin and Implications for the Evolution of the Tasmanides (Eastern Australia)

“…In order to further characterize the samples and improve interpretation of geochronological results, the Kübler index (KI), K‐white mica (KWM) b cell dimension, and the mineral content of respective samples were determined using techniques described by Doublier et al (, ).…”