In orogenic belts, crustal thickening precedes rheological weakening of the middle and lower orogenic crust leading to lateral spreading and, ultimately, to collapse of an orogen. Recent studies have implied similar development in the middle crust during the Palaeoproterozoic composite Svecofennian orogen. Details of this spreading are, however, still not clear. The Svecofennian crust is imaged by the Finnish Reflection Experiment (FIRE) that was designed to image the major crustal structures in Finland. Here, we show an example of how seismic attributes, combined with the seismic facies interpretation technique, can be used to enhance the FIRE data. We focussed our study to the West Uusimaa complex WUC, where the middle crust of the Svecofennian
is exposed. We compare the interpreted, enhanced seismic data with field observations. The attribute analysis of the seismic data reveals unprecedentedly detailed information about the deformation fabrics within the WUC. The data show that the extension/lateral flow of the orogenic middle and lower crust was mainly accommodated by km-scale S-C’ structures. The structures form a penetrative deformation fabric that overprint the older, compressional structural grain. The extensional features observed in the seismic are correlated with outcrop observations. The successful enhancement of the seismic data confirms the ductile extension affecting hot orogenic crust, and gives new information about the strain distribution of the regional, syn- to late-orogenic deformation. The seismic attribute method and the seismic facies interpretational approach described here should be applicable to other seismic datasets from the crystalline basement.