Microstructure and paleotemperature record of the upper Cretaceous rocks from the Alpine collision in the western Pyrenean Axial Zone (Eaux-Chaudes fold nappe).

The Alpine collisional deformation of the upper Iberian margin in the western Axial Zone of the Pyrenees occurred under conditions of moderate paleotemperature (Caldera et al., 2021). Carbonates from the upper Cretaceous of the Eaux-Chaudes fold nappe (ECFN) preserve evidence of ductile deformation at both large- and microscale under greenschist facies. The ECFN provides evidence of high ductile strain achieved under considerable burial conditions (ca. 10 km), contrasting with the standard view of near-surface Alpine deformation of the upper part of the Iberian collided margin. The mylonites observed in the km-scale overturned limb of the ECFN feature strong shear deformation characterized by intrafolial folds, S-C fabrics, mineral lineation and boudinaged, asymmetric dolomite bodies.

Microscale observations by EBSD indicate that grain-shape and crystallographic preferred orientations (CPO) are well-developed in calcite aggregates. Deformation is mild in the normal fold limb, with spaced pressure solution seams. CPO and inverse pole figure (IPF) results from high-strain zones advocate for dislocation creep as the main deformation mechanism by basal-slip along the a- and m- axis. Dynamic recrystallization is also observed as well as local four-node.

In general, calcite-rich aggregates are characterised by fine recrystallized matrix, ranging in grain size between 12-30 µm, and therefore indicating relatively low-stress conditions (20-60 MPa) using the more common piezometers. The spatial distribution of the secondary phase content, such as dolomite and quartz, conditioned the strain partitioning in the polymineralic mylonites. On one hand, grain size reduction of the calcite phase was favoured in areas between small, spaced dolomite grains. On the other hand, calcite grain growth was favoured in shadow zones of large dolomite porphyroclasts. Dolomite phase shows dominant ductile-brittle behaviour expressed by low internal crystal plasticity developing weak CPO and featuring extensional fractures along grain boundaries. Quartz is the less common mineral phase and also affected by ductile-brittle deformation with occasional undulose extinction.

Complementing the microstructural results, we delve deeper into alpine paleotemperatures along the ECFN by Raman spectroscopy of carbonaceous material (RSCM) and microprobe analysis using Powell et al. (1984) and Covey-Crump (1989) methods. Results from all methods are consistent with the ductility observed. The higher values are obtained in the mylonites of the autochthon and overturned limb ranging 340-360ºC. On the other hand, the normal limb features paleotemperatures in a lower range of 300-330ºC. Those temperatures are in accordance with the deformation mechanisms observed.

The microfabrics and paleotemperature results here documented from the Eaux-Chaudes Massif show for the first-time ductile mechanisms of deformation in Alpine mylonites derived from post-Variscan sedimentary rocks during the Alpine orogeny in the Pyrenees.

 

Caldera, N., Teixell, A., Griera, A., Labaume, P. & Lahfid, A. (2021). https://doi.org/10.1111/ter.12517

Covey-Crump, S.J. & Rutter, E.H. (1989). https://doi.org/10.1007/BF00387202

Powell, R., Condliffe, D. M. & Condliffe, E. (1984). https://doi.org/10.1111/j.1525-1314.1984.tb00283.x