ROCK MATRIX STRUCTURES IN A ZONE INFLUENCED BY A STYLOLITE

A stylolitic structure is one of the visible manifestations of the pressure-solution process in a rock. The stylolitic 'process' zone (as defined in previous studies) on both sides of a stylolite, is studied by SEM. The chosen samples concern tectonic stylolites within a high porosity limestone (16%) without tension gashes. This zone corresponds to a local increase of porosity and comprises a large number of stylolitic joints showing a decreasing size from the macroscopic structure to simple contact between two grains. Two hypotheses are presented for the stylolite formation: (1) the macroscopic stylolite is the result of the coalescence of earlier minor pressure-solution structures; (2) small stylolites nucleate gradually from a major stylolite joint. The mass transfer can be realized either by diffusion between particles or by seepage through previously undescribed process zone fractures. The mass deposition can be localized in the porous volume and the microcracks, without requiring any additional tectonic openings including tension gashes. The rock deformation is therefore limited to shortening parallel to sigma1, without normal extension.