Deformation pattern in a heterogeneous material: Folded and cleaved sedimentary cover immediately overlying a crystalline basement (Oisans, French Alps)

Particularly well exposed structures in folded and cleaved sedimentary cover immediately overlying a crystalline basement have been studied. Chemical analysis (X.R.F. and microprobe) reveal pressure solution process and give the possibilities of measurement of mass transfer. Study of fluid inclusion veins has determined the temperature pressure conditions: thermal effect of the basement and decrease of temperature and pressure with the age of various synkinematic veins.Characteristic examples of the behaviour of a heterogeneous material during coaxial and non-coaxial deformation are shown:

1. (1) Successive different asymmetrical folds, various cleavages and fractures appear in a shear zone parallel to the main fabric with variations of thickness and rock behaviour.

2. (2) Evolution of cleavage in such a shear zone (with or without slipping) is linked to the relations between the rotation of contraction direction and the rate of the cleavage process.

3. (3) Fold axes changed from the horizontal y direction to the vertical (or E—W transversal to the crystalline massif) X direction, with increase of the (X/Z) and (X/Y) ratios (obtained by fossils and reconstructed fold shape). This strain is always heterogeneous and the most deformed zone frequently evolves to discontinuities with slip.

4. (4) Indentation exists on all scale: from hard object (100 μ, with parenthesis form of pressure solution cleavage apparent on map distribution of various element) to basement block (with variation of strain value in the indented cover).

A model of the evolution of the deformation of sedimentary cover immediately overlying a crystalline basement is given in conclusion.