Incipient strain partitioning in a slate belt: Evidence from the early Variscan Monts d'Arrée slate belt (Brittany, France)

Partitioning of strain is a fundamental process during mountain building. It commonly causes a compartmentalisation of a bulk regional strain into deformational domains with contrasting strain characteristics and largely oriented parallel to the orogenic grain. The Monts d'Arrée slate belt (Brittany, France) offers an opportunity to study strain partitioning in a slate belt deformed in an overall transpressional regime. The slate belt consists of highly deformed, low-grade, siliciclastic metasediments of upper Silurian to lower Devonian age. The deformation occurred during an early Variscan nappe stacking event (‘Bretonian phase’). An extensive structural analysis has demonstrated that the slate belt reflects the initial stages of strain partitioning. The slate belt primarily reflects coaxial, contraction-dominated deformation. It resulted in NW-verging folding and a pervasive cleavage development, giving rise to a pronounced mechanical anisotropy. During the later stages of deformation, incipient strain partitioning lead to the development of punctuated strain heterogeneities, consistently reflecting dextral, belt-parallel, strike-slip strain. These structures are not organised in networks or domains. Incipient strain partitioning in the Monts d'Arrée slate belt did not reach the stage of compartmentalisation by the formation of an interlinked discontinuity network or wrench-dominated deformational domains.