This volume is derived from the 13th meeting on Deformation Mechanisms, Rheology and Tectonics (DRT2003). The meeting was held in St Malo (Brittany, France) in April 2003, and organized by an informal group from Gdosciences Rennes (UMR 6118 CNRS, Rennes University), including Michel Ballbvre, St6phane Bonnet, Arlette Falaise, Olivier Galland, Fr6d6ric Gueydan, Charles Gumiaux, Benjamin Le Bayon, Alain-Herv6 Le Gall, Monique Le Moigne, Sylvie Schueller, and C61ine Tirel. It was sponsored by the Centre National de la Recherche Scientifique, Rennes University, the ity of Rennes, the Conseil G6n6ral d'Ille et Vilaine, and the R6gion Bretagne. Forty-eight reviewers have worked hard to improve the papers. We thank these persons and institutions for their contributions.

Grey and white carbonate mylonites were collected along thrust planes of the Helvetic Alps. They are characterised by very small grain sizes and non-random grain shape (SPO) and crystallographic preferred orientation (CPO). Presumably they deformed in the field of grain size sensitive flow by recrystallisation accommodated intracrystalline deformation in combination with granular flow. Both mylonites show a similar mean grain size, but in the grey mylonites the grain size range is larger, the grain shapes are more elongate and the dynamically recrystallised calcite grains are more often twinned. Grey mylonites have an oblique CPO, while the CPO in white mylonites is symmetric with respect to the shear plane. Combustion analysis and ТЕМ investigations revealed that grey mylonites contain a higher amount of highly structured kerogens with particle sizes of a few tens of nanometers, which are finely dispersed at the grain boundaries.

During deformation of the rock, nano-scale particles reduced the migration velocity of grain boundaries by Zener drag resulting in slower recrystallisation rates of the calcite aggregate. In the grey mylonites, more strain increments were accommodated by individual grains before they became refreshed by dynamic recrystallisation than in white mylonites, where grain boundary migration was less hindered and recrystallisation cycles were faster. Consequently, grey mylonites represent 'deformation' microfabrics while white mylonites are characterised by 'recrystallisation' microfabrics. Field geologists must utilise this different deformation behavior when applying the obliquity in CPO and SPO of the respective mylonites as reliable shear sense indicators