Cleavage development within a foreland fold and thrust belt, southern Pyrenees, Spain

In the southern Pyrenees lithologically distinct cleavage fronts are each parallel to bedding and dip ∼20° towards the foreland. Pressure solution was the dominant mechanism of cleavage development. The mudstone cleavage front is coincident with the ∼195°C paleoisotherm and is associated with a pressure solution strain of ∼5%, a mechanical twin strain of ∼4%, and a deviatoric stress magnitude of ∼65 MPa. Illite crystallinity measurements define a geothermal gradient of 15°C km−1 and indicate that the paleoisotherms are bedding-parallel. Deviatoric stress magnitudes, from calcite twins, were regionally constant at ∼65 MPa and principal stress axes were perpendicular to cleavage. Temperature was the primary control on deformation micromechanisms and the position and orientation of the cleavage front within the foreland thrust wedge. Deformation below the cleavage front occurs predominantly by pressure solution, which in conjunction with mechanical twinning and microfracturing produces a quasi-plastic rheology. Stress magnitudes determined from mechanical twinning of carbonate grains and long-term (106–1076 y) strain rates determined for regional folds and faults suggest an apparent macroscopic viscosity of 9.8 × 1018 to 7.2 × 1019 Pa s for the lower thrust wedge. Above the cleavage front temperature, pressure solution strain, total strain, and mesoscale deformation diminish. The region of the thrust wedge above the ∼100°C paleoisotherm is characterized by large brittle faults with cataclastic fault zones and negligible grain-scale deformation indicating an elastico-frictional rheology.