On the origin of cleavage in the Central Pyrenees: Structural and paleo-thermal study

- We determine the thermal conditions for cleavage development in the Central Pyrenees.
- New data about the geometry of the cleavage front and cleavage orientation
- The cleavage front is defined by vitrinite reflectance and clay mineralogy.
- A lower thermal boundary of 140-160 °C marks incipient cleavage development.
- Cleavage developed due to pressure-solution and it is thermally controlled.

The present work provides a combined structural and paleothermal (fluid inclusions, illite content in mixed layers illite-smectite, Kübler index and vitrinite reflectance) investigation of the Alpine cleavage domain affecting the western South Pyrenean Zone (Central Pyrenees). Integration of structural and paleothermal data allows us (i) to determine the position of the cleavage front in the sedimentary sequence and (ii) to define mechanisms and temperature conditions for cleavage development.Cleavage in the eastern part of the study area is approximately limited to the forelimb of the Gavarnie hanging wall anticline, but to the West it extends progressively southwards, affecting younger turbiditic units. At regional scale, the cleavage front is oblique to the strike of bedding and the overall trend of macrostructures. Pressure-solution is the main mechanism for cleavage development, and a strong thermal control on the position of the cleavage front is inferred. The transition between sedimentary and tectonic, compressional fabric is marked by vitrinite reflectance values of 1.12-1.14%, Kübler index values between 0.71 and 0.74°Δ2θ and the transition between R1 and R3 mixed layers illite-smectite. A lower thermal boundary of 140-160 °C is suggested for incipient cleavage development in the shaly beds of the turbiditic sequence, as well as an upper boundary constrained by a maximum measured value of about 215 °C.