Episodic exhumation of HP rocks inferred from structural data and P-T paths from the southwestern Massif Central (Variscan belt, France)
Résumé
In the southwestern Massif Central (France), HP rocks are found in lower allochthonous units (i.e. HP units), sandwiched between upper allochthonous units (i.e. LP units) and the Parautochthonous unit, both devoid of HP rocks. HP rocks were formed at minimum conditions of 1.7 GPa/700 °C in response to the northward subduction of Gondwana beneath Armorica during the Silurian (430–390 Ma). Their exhumation results from an 80-Ma-long sequence of tectonic processes effective at various scales and active specifically at each main stage of the evolution of the Gondwana lithosphere. HP rocks were first exhumed by upward, southwestward extrusion during the Early Devonian (390–380 Ma), in relation to late stage of subduction. In the northern area, exhumation is achieved by rifting during the Upper Devonian (∼365 Ma). The most obvious event corresponds to the northwestward thrusting of LP units above HP units during early stages of continental collision (∼355 Ma). HP rocks were exhumed for a second time during orogen-parallel transpression in response to late stage of continental collision (350–345 Ma). Exhumation in internal zones is achieved by crustal-scale normal faulting and erosion during the Middle Carboniferous (335–315 Ma) synorogenic extension, while compression was still active in the southern external zones. Exhumation of the whole region is completed by regional uplift and erosional denudation during the Upper Carboniferous (310–290 Ma) postorogenic extension. Subduction and collision have played a key role for driving HP rocks to the surface, but their efficiency and the resulting strain pattern depend on the angle of convergence. Upper Devonian rifting and the subsequent Early Carboniferous crustal thickening prevents to consider exhumation of Variscan HP rocks as a single and continuous process and open new questions on the related Paleozoic lithosphere history.