Palaeostresses associated with thrust sheet emplacement and related folding in the southern central Pyrenees, Huesca, Spain

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Abstract
Brittle mesostructures have been studied in the detached Mesozoic and Lower Tertiary cover rocks of the south-central Pyrenees (Huesca province, Spain), in order to reconstruct the state of stress within the thrust sheets during Tertiary compression. The structure of the area studied consists of NW–SE- to E–W-striking, south-verging thrust-sheets, located west of the South-Pyrenean Central Unit. The palaeostresses obtained show dominant N–S to ENE–WSW compression directions for stress tensors with horizontal σ 1 axes, and E–W to NW–SE extension directions for stress tensors with vertical σ l axes. The values of the stress ellipsoid ratios for stress tensors with horizontal σ 1 are predominantly between 0 and 0.5 (indicating uniaxial compression to wrench regime). Regarding their relationship with macro-structures, the palaeostresses show transport-parallel compression in most sites, although thrust-parallel compression is also found. There are no differences between palaeostresses found in the hanging walls and the footwalls of thrusts. Stress tensors with vertical σ 1 do not show systematic relations with macro-structures, but transport-parallel extension and thrust-parallel extension are found in some sites. The analysis of palaeostresses in areas of fold-and-thrust belts shows a high degree of complexity and the results are not easy to interpret. The continuous variation from N–S to ENE–WSW in the orientation of the compression axes is here interpreted in two ways: (1) as the result of the clockwise rotation of the Mesozoic and Tertiary cover, which was pushed in the east by the South-Pyrenean Central Unit with a constant N to NNE regional compression, and (2) due to NE–SW compression resulting from a SW-directed local transport of minor thrusts. Palaeostress analysis can, therefore, reveal important information about thrust kinematics, at high structural levels where ductile deformation is absent.