PALAEOSTRESSES ASSOCIATED WITH THRUST SHEET EMPLACEMENT AND RELATED FOLDING IN THE SOUTHERN CENTRAL PYRENEES, HUESCA, SPAIN

Brittle mesostructures have been studied in the detached Mesozoic and Lower Tertiary cover rocks of the south-central Pyrenees (Huesca provi nce, Spain), in order to reconstruct the state of stress within the th rust sheets during Tertiary compression. The structure of the area stu died consists of NW-SE- to E-W-striking, south-verging thrust-sheets, located west of the South-Pyrenean Central Unit. The palaeostresses ob tained show dominant N-S to ENE-WSW compression directions for stress tensors with horizontal sigma(1) axes, and E-W to NW-SE extension dire ctions far stress tensors with vertical sigma(1) axes. The values of t he stress ellipsoid ratios for stress tensors with horizontal sigma(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 diff erences between palaeostresses found in the hanging walls and the foot walls of thrusts. Stress tensors with vertical sigma(1) do not show sy stematic relations with macro-structures, but transport-parallel exten sion and thrust-parallel extension are found in some sites. The analys is of pataeostresses in areas of fold-and-thrust belts shows a high de gree of complexity and the results are not easy to interpret. The cont inuous variation from N-S to ENE-WSW in the orientation of the compres sion axes is here interpreted in two ways: (1) as the result of the cl ockwise rotation of the Mesozoic and Tertiary cover, which was pushed in the east by the South-Pyrenean Central Unit with a constant N to NN E regional compression, and (2) due to NE-SW compression resulting fro m a SW-directed local transport of minor thrusts. Palaeostress analysi s can, therefore, reveal important information about thrust kinematics , at high structural levels where ductile deformation is absent.