In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

Low pressure-high temperature (LPHT) metamorphism, with geothermal gradients in the order of 50-100°C/km, is a common feature of the late evolution of collisional orogens. These abnormal thermal conditions may be the results of complex interactions between magmatism, metamorphism and deformation. The Agly massif, in the French Pyrenees, preserves the metamorphic footprints of the late Variscan thermal structure of an almost continuous section from the upper and middle continental crust. The upper crust is characterized by a very high geothermal gradient of ~55°C/ km, evolving from greenschist to amphibolite facies, while the middle crust, exposed in a gneissic core, exhibits granulite facies conditions with a near isothermal geothermal gradient (<8°C/km) between 740 and 790°C. The abnormal and discontinuous crustal geothermal gradient, dated at c. 305 Ma on syn-granulitic monazite by LA-ICP-MS, is interpreted to be the result of magmatic intrusions at different structural levels in the crust: the Ansignan charnockite (c. 305 Ma) in the deepest part of the gneissic core, the Tournefort granodiorite (c. 308 Ma) at the interface between the gneissic core and the upper crust and the Saint-Arnac granite (c. 304 Ma) in the upper section of the massif. The heat input from these magmas combined with the thermal buffering effect of the biotite dehydration-melting reaction resulted in the near isothermal geothermal gradient in the gneissic core (melt-enhanced geotherm). The higher geothermal gradient (>50°C/km) in the upper crust is only due to conduction between the hot middle crust and the Earth's surface. The estimated maximum finite pressure range suggests that ~10 to 12 km of crust are exposed in the Agly massif while the present-day thickness does not exceed 5-6 km. This pressure/depth gap is consistent with the presence of several normal mylonitic shear zones that could have contributed to the subtraction of ~5 km of the rock pile. Monazite U-Th-Pb ages carried out on monazite overgrowths from a highly mylonitized sample suggest that this vertical thinning of the massif occurred at c. 296-300 Ma. This later Variscan extension might have slightly perturbed the 305 Ma geothermal gradient, resulting in an apparent higher conductive geothermal gradient in the upper crust. Although the Agly massif has been affected by Cretaceous extension and Eocene 800 | SIRON et al.

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