Yoann Denèle

Timing of granite emplacement, crustal flow and gneiss dome formation in the Variscan segment of the Pyrenees

Abstract The Variscan segment of the Pyrenees is well suited to study the timing of crustal-scale deformations as crustal flow and gneiss dome formation. This has been constrained from a synthesis of available structural and geochronological data of intrusive rocks, as well as new zircon U–Pb age determinations via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). After a stage of moderate thickening by fold–thrust belt development in the upper crust between 323 and 308 Ma, the Variscan segment of the Pyrenees experienced crustal flow at c. 306 Ma and then gneiss dome formation at c. 304 Ma. Localization of the deformation along reverse-dextral shear zones occurred at c. 300 Ma. The Variscan segment of the Pyrenees recorded a high-temperature regime, which allowed crustal flow of the middle crust, but with limited amounts of heat which induced rapid cooling. The development of this enigmatic orogenic segment of the Variscan belt is closely contemporaneous with the formation of the Cantabrian Orocline and could correspond to a lithospheric-scale shear zone that accommodated buckling of the orocline. Late Variscan lithospheric delamination and asthenospheric upwelling associated with buckling in the core of the Cantabrian Orocline could explain the short-period high-temperature regime in the Variscan segment of the Pyrenees. Supplementary material: Review of published U–Th–Pb, Ar–Ar and K–Ar ages of granites from the Axial Zone of the Pyrenees; LA-ICP-MS U–Pb dating methodology (Clermont-Ferrand, France); and zircon LA-ICP-MS U–Pb data for Ax-les-Thermes, Carançà, Mont-Louis and Cauterets granites are available at http://www.geolsoc.org.uk/SUP18729.

Structural inheritance in the Central Pyrenees: the Variscan to Alpine tectonometamorphic evolution of the Axial Zone

Estimating structural inheritance in orogens is critical to understanding the manner in which plate convergence is accommodated. The Pyrenean belt, which developed in Late Cretaceous to Paleogene times, was affected by Cretaceous rifting and Variscan orogeny. Here we combine a structural and petrological study of the Axial Zone in the Central Pyrenees to discuss structural inheritance. Low-grade Paleozoic metasedimentary rocks were affected by a Variscan transpressional event that produced successively: (1) regional-scale folds; (2) isoclinal folding, steep pervasive cleavage and vertical stretching, synchronous with peak metamorphism; (3) strain localization into ductile reverse shear zones. The persistence of a relatively flat envelope for the Paleozoic sedimentary pile and Variscan isograds, and the absence of Alpine crustal-scale faults in the core of the Axial Zone, suggests that the Axial Zone constitutes a large Variscan structural unit preserved during Pyrenean orogeny. This configuration seems to be inherited from Cretaceous rifting, which led to the individualization of a large continental block (future Axial Zone) against a hyper-extended domain along the North Pyrenean Fault zone. This study places the currently prevailing model of Pyrenean belt deformation in a new perspective and has important implications for crustal evolution and inheritance in mountain belts more generally. Supplementary materials: Raman spectroscopy of carbonaceous materials data and a figure illustrating peak-fitting of the Raman spectrum of carbonaceous material and Raman spectra from the various samples of the Pallaresa cross-section are available at https://doi.org/10.6084/m9.figshare.c.3906247

Strike-slip metamorphic core complexes: Gneiss domes emplaced in releasing bends

Thermobarometric calculations for paragneisses (samples MI11 and MI44) were performed using the version 6.6.8 (11_2013) of Perple_X (Connolly, 2009) and the internally consistent thermodynamic dataset “solution_model.dat” in the CaTiNKFMMnASH system for H2O content corresponding to the loss on ignition values obtained by ICP‐MS. Endmembers used for Mi‐11 pseudosection calculation are given in Table 1 (see documentation in Perple_X for references on solid solution models — http://www.perplex.ethz.ch). The reliability of our thermobarometric estimate is supported by a set of petrological data on metamorphic minerals (garnet, biotite, plagioclase, muscovite and oxides) provided in Table 2. Figure DR1 yields detail on the Perple_X‐elaborated P–T pseudosection built for sample Mi‐11. The pseudosection shows the invariant points (solid dots), univariant reactions (heavy solid lines), divariant fields (unshaded), trivariant fields (lighter‐grey shaded), to the extreme (n=6) variant fields (black). Bulk composition (weight percent) obtained by ICPMS of the metapelite Mi‐11 and details on the metamorphic phase assemblages present in all the fields are given at the top and bottom of the diagram, respectively. Red star: estimated peak P–T conditions; green star: retrograde P–T conditions.

Anisotropy of magnetic susceptibility of the Pyrenean granites

ABSTRACT In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator.