Deformation – Progressive or multiphase?

Abstract

Abstract Complexly deformed rocks that exhibit refolded folds and overprinting planar fabrics are often interpreted as the result of polyphase deformation and described in a scheme of deformation phases (D 1 , D 2 … D n ). At the same time, a number of models have been presented over the last decades that explain how complex and overprinting structures can form by single-phase progressive deformation as strain accumulates. Strain generally accumulates heterogeneously in a deforming region, depending on strain intensity and geometry, flow parameters, rheology and lithology. The result is regions of different structural complexity, fabrics and fabric orientations that can be sorted into different structural facies. Such structural pattern should be mapped and presented before defining a scheme of deformation phases, and attempts should be made to explain them by a single structural-kinematic model, such as partitioned transpression. Only if this is not possible, or if independent geochronologic data reveals a more punctuated history, should we define an additional tectonic regime and deformation phase. Multiphase and progressive deformation histories are not mutually exclusive, and it is important that both possibilities are considered when describing the deformation history of any complexly deformed region. When the concept of deformation phases is employed, the number of phases should be minimized and be related to major tectonic event of external character. Uncritically defining deformation phases can easily generate a complicated discrete deformation history with no link to tectonic reality.