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

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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 Faugères units, which constitute part of the southern hangingwall of the dome. It is shown that southward-directed Variscan nappe-thrusting (D1) and a related medium-P metamorphism (M1) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono-metamorphic event D2/M2 largely transposes D1 structures, comprises a higher metamorphic thermal gradient than M1 (transition low-P and medium-P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post-dating D1/M1 by more than 20 Ma. D2-related fabrics are refolded by D3, 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 Faugères units, D2/M2 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. D2/M2 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 D2 fabrics and the M2 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-D3 metamorphic field gradients paralleled the dome. These observations indicate that D2/M2 is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D2/M2. 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 north-dipping detachments and hangingwall basin formation along the northeastern dome margin.

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