Rheologically induced structural anomalies in transpressive regimes

Local structural anomalies are sometimes found in transpressive regimes. They can be related to strain partitioning processes but in some cases other types of heterogeneities could affect the deformation style. This paper shows that rheological heterogeneities can radically influence the geometry of faults in transpression zones and cause them to be markedly non-planar. In the Mardo region of northern Portugal, a zone in which Variscan structures are SW-facing is found within the regional NE-facing geometry. This anomaly is adjacent to a complex shear zone (Mina/Ribeira das Cestas) where the fault plane is curved; however the surrounding material does not show any signs of a related folding event. Detailed field studies, mainly concerning the rheological contrasts between the deformed lithostratigraphic units, show a close relationship between the structural anomaly and the movement along rheological anisotropies. These observations, complemented by experimental deformation of multilayers composed of analogue materials, allow the construction of a generic model. Strain partitioning during the initial stage of the sinistral transpressive Variscan deformation produced the juxtaposition of lithostratigraphic units with strongly different rheological contrasts. Subsequent flattening deformation induced heterogeneous indentation phenomena, producing the non-planar geometry of the complex shear zone. The structural heterogeneities caused could have an important role in the evolution of a transpressive domain. (C) 2002 Elsevier Science Ltd. All rights reserved.