CLEAVAGE DEVELOPMENT WITHIN A FORELAND FOLD-AND-THRUST BELT, SOUTHERNPYRENEES, SPAIN

In the southern Pyrenees lithologically distinct cleavage fronts are e ach parallel to bedding and dip similar to 20 degrees towards the fore land. Pressure solution was the dominant mechanism of cleavage develop ment. The mudstone cleavage front is coincident with the similar to 19 5 degrees C paleoisotherm and is associated with a pressure solution s train of similar to 5%, a mechanical twin strain of similar to 4%, and a deviatoric stress magnitude of similar to 65 MPa. Illite crystallin ity measurements define a geothermal gradient of 15 degrees C km(-1) a nd indicate that the paleoisotherms are bedding-parallel. Deviatoric s tress magnitudes, from calcite twins, were regionally constant at simi lar to 65 MPa and principal stress axes were perpendicular to cleavage . Temperature was the primary control on deformation micromechanisms a nd the position and orientation of the cleavage front within the forel and thrust wedge. Deformation below the cleavage front occurs predomin antly by pressure solution, which in conjunction with mechanical twinn ing and microfracturing produces a quasi-plastic rheology. Stress magn itudes determined from mechanical twinning of carbonate grains and lon g-term (10(6)-10(76) y) strain rates determined for regional folds and faults suggest an apparent macroscopic viscosity of 9.8 x 10(18) to 7 .2 x 10(19) Pa s for the lower thrust wedge. Above the cleavage front temperature, pressure solution strain, total strain, and mesoscale def ormation diminish. The region of the thrust wedge above the similar to 100 degrees C paleoisotherm is characterized by large brittle faults with cataclastic fault zones and negligible grain-scale deformation in dicating an elastico-frictional rheology.