Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

Abstract

<jats:p>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 Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work.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.</jats:p>

Contribution of magmatism, partial melting buffering and localized crustal thinning on the late Variscan thermal structure of the Agly massif (French Pyrenees)

“…In addition to the main foliation, the gneissic core is characterized by the development of low‐angle extensional mylonitic shear zones, mainly visible in the eastern part of the massif, dipping either towards the north or towards the south and associated with a strong stretching lineation that trends N20 (Delay, 1990; Olivier et al., 2004). The main foliation and the mylonitic shear zones show a similar normal and top‐to‐the‐north kinematics attributed to a same tectono‐metamorphic event responsible for the thinning of the Variscan crust (Vanardois et al, 2020). Olivier et al.…”

“…The metamorphic rocks of the upper Palaeozoic sequence and the lower gneissic core share a predominant strain fabric observed in the entire Agly massif that corresponds to a moderately to shallowly dipping LPHT foliation, holding a conspicuous N20 trending mineral and stretching lineation (e.g. Bouhallier et al., 1991; Olivier et al., 2004; Vanardois et al, 2020). Similarly, the Ansignan charnockite and the Cassagnes intrusives show a well‐developed magmatic foliation, with a minor component of solid‐state deformation (Althoff et al., 1994).…”

“…In contrast, Paquet and Mansy (1991) suggested that the thinning responsible for the loss of ~10 km of crustal material occurred during mid‐Cretaceous time (110–100 Ma). Recently, Vanardois et al (2020) proposed an alternative scenario in which a late Variscan transpressional event is responsible for the reorientation of foliations and extensional shear zones in the northern part of the massif. According to this new study, extensional and flat‐lying shear zones are post‐dated by dextral transpressional shear zones, all being Variscan in age.…”

“…Geological maps of (a) the Variscan Pyrenees and of (b) the Agly massif, modified from Vanardois et al (2020), with localization of the cross‐sections (a) and (b) (Figure 2). BAF, Bas‐Agly fault; PF, Planèzes fault; SPF, Saint‐Paul fault; TBF, Trilla‐Bélesta fault…”

Anatomy of an extensional shear zone leading to the exhumation of the middle crust within the Canigou dome (Eastern Pyrenees, Axial Zone)

“…Considering that the mid-lower crust was partially molten, recent studies suggest that domal structures were formed by folding after an initial stage of horizontal flow within the mid-lower crust under dextral transpression (Vilà et al, 2007;Denèle et al, 2007Denèle et al, , 2009Mezger, 2009). Alternatively, several models propose exhumation with local extension as a result of strain localization in regional shear zones (Bossòst dome: Mezger and Passchier, 2003;Cochelin et al, 2017;Hospitalet massif: Van den Eeckhout and Zwart, 1988;Basque and North Pyrenean massifs: de Saint Blanquat, 1993;Saspiturry et al, 2019;Vanardois et al, 2020).…”