Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

Abstract

Pre-kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre-kinematic alteration is challenging because of its usual obliteration by subsequent syn-kinematic metamorphism often occurring at conditions typical of the brittle-ductile transition. It is, however, to be expected that pre-kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma-old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid-rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low-T thermobarometry and in situ U-Th/Pb dating of anatase, titanite and monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270-350°C at 230-300 Ma. This event is followed by a second fluid-rock interaction stage marked by new crystallization of phyllosilicates at 200-280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U-Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40-70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post-Variscan time, at least 150-200 Ma before the main Alpine shortening while Alpine-age compression (c. 35-50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre-orogenic upper crustal softening controls the deformation patterns in granitic basement units through low-grade metamorphic reactions. K E Y W O R D SAxial Zone, Bielsa, chlorite-white mica thermobarometry, mylonites, U-Th/Pb anatase-titanitemonazite dating 184 | AIRAGHI et Al.

Central Pyrenees Mountain Building: Constraints From New LT Thermochronological Data From the Axial Zone

“…We also suggest that this southern margin was significantly heated during rifting: the geothermal gradient was probably as high as 40°C–50°C/km until Eocene times. Such a relatively high geothermal gradient most likely influenced the mode of shortening, as well as early softening of the crust (Airaghi et al., 2020; Bellahsen et al., 2019), distributed shortening during the early stages of convergence (until 50–40 Ma) as supported by our new thermochronological dataset.…”

“…As shown on Figure 14a, the models suggest a complex cooling sequence that may have been distributed through the whole AZ during Paleocene and Eocene times. Reverse top‐to‐the‐south shear zones have been dated to around 50 Ma such as Merens shear zone (A. M. McCaig & Miller, 1986), Neouvielle shear zones (Wayne & McCaig, 1998) and Bielsa shear zones (Airaghi et al., 2020) suggesting that their activity induced a distributed exhumation, hence cooling of the AZ as developed above. To sum up, in the AZ, the complex cooling sequence (Figure 14a) observed during this first orogenic stage (70–40 Ma) includes distributed slow cooling and exhumation which may be interpreted as due to a distributed shortening.…”

“…Such distributed shortening may be explained by different factors. Feldspars were altered early during the crust evolution (Airaghi et al., 2020; Bellahsen et al., 2019). Thus, at the onset of Alpine shortening, they were seriticized and most likely very weak.…”

“…This is also supported by several late Cretaceous to ca. mid‐Eocene shear zones within the AZ (Figure 3; Airaghi et al., 2020; A. M. McCaig & Miller, 1986; Wayne & McCaig, 1998).…”

Geodynamic evolution of a wide plate boundary in the Western Mediterranean, near-fieldversusfar-field interactions

“…This suggests a rather weak bulk rheology for the middle crust, opening the possibility of a decoupling during Pyrenean shortening. Bellahsen et al (2019) and Airaghi et al (2020) further show that fluid circulation during pre-orogenic rifting episodes since the Permian have significantly modified the mineralogical content of the upper crustal basement, with intense sericitization, and thus weakened the resistance of the upper crust, a possible explanation for the observed distributed deformation during Pyrenean shortening. The early Cretaceous rifting episode was locally associated with mantle exhumation and serpentinisation, but the intensity of this serpentinisation differs between peridotite bodies that were exhumed within the upper crust during rifting.…”