Maxime WALDNER

Kinematics and thermicity of the Pyrenean orogenic prism: New geo-thermochronological data from the Axial Zone

Supervisors:
Bellahsen Nicolas (Maître de Conference, Sorbonne Université, Paris)
Mouthereau Frédéric (Professeur, Université Paul Sabatier, Toulouse)

Major Results

The thermal modeling (QTQt) of thermochronological data and our new K-Ar dating protocol permitted to precise cooling/exhumation path in the Pyrenean axial zone and highlight a two-step deformation during Meso-Cenozoic times. A slow distributed cooling (5-10 °C/Myrs) is observed from 70 to 40 Ma, before a fast cooling (30 °C/Myrs) is localized on Maladeta until 20 Ma..

Abstract

The Pyrenean range is interpreted as an antiformal stack with a sequential thrust propagation. However, the early thermal and structural history in the Axial Zone is poorly constrained. The development of a new geochronological approach of in-situ K/Ar dating of small white micas was coupled to a textural control of samples and to thermobarometric estimates. The results from the western Axial Zone suggest a Variscan sericitization followed by the formation of anastomosed shear zones during early stages of Alpine collision. The modelling of new LT-thermochronological records (zircon and apatite fission track, (U-Th)/He on zircon) constrain the sequential restoration of the Pyrenean prism during the Alpine compression. The results along the ECORS profile suggests two main collisional cooling phases.

From ca. 70 to 40 Ma, an early distributed cooling/exhumation with low rates around 5-10 °C/Myrs. From 40 to 20 Ma, a faster cooling at ca. 30 °C/Myrs is only localized in the Maladeta massif (Figure). Much lower denudation rates are also recorded further North and South suggesting a rather symmetric exhumation of the Axial Zone and controlled by underplating of the Rialp thrust. This sequence of shortening from distributed to localized deformation appear to be characteristic in other collisional wedges too. The phase of distributed shortening may be controlled by the re-thickening of the passive margin, the high geothermal gradient recorded in the Axial Zone, and/or the presence of pre-orogenic softening phyllosilicates.

Thermal evolution and shortening sequence of the Pyrenean central Axial Zone from inverse thermal modelling.

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

Airaghi, L., Bellahsen, N., Dubacq, B., Chew, D., Rosenberg, C., Janots, E., Waldner, M. and Magnin, V. (2020). Pre-orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees. Journal of Metamorphic Geology, 38(2), 183-204.

keywords: Axial Zone, Bielsa, chlorite–white mica thermobarometry, mylonites, U–Th/Pb anatase–titanite–monazite dating

DOI: http://dx.doi.org/10.1111/jmg.12520

Shortening of the axial zone, pyrenees: Shortening sequence, upper crustal mylonites and crustal strength

Bellahsen, N., Bayet, L., Denele, Y., Waldner, M., Airaghi, L., Rosenberg, C.,… & Vacherat, A. (2019). Shortening of the axial zone, Pyrenees: Shortening sequence, upper crustal mylonites and crustal strength. Tectonophysics, 766, 433–452.

keywords: collision, rheology, Pyrenees, basement, greenschist

DOI: https://doi. org/10.1016/j.tecto.2019.06.002

PhD Thesis: Cinématique et thermicité du prisme orogénique des Pyrénées : nouvelles données géo-thermochronologiques de la Zone Axiale

Maxime Waldner. Cinématique et thermicité du prisme orogénique des Pyrénées : nouvelles données géo-thermochronologiques de la Zone Axiale. Sciences de la terre. Sorbonne université

keywords: Pyrenees, Thermal inverse modeling, Sequential restoration, Collision, Methodology, K-Ar in situ

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