The mode of extension in the orogenic mid-crust revealed by seismic attribute analysis

The processes and the structures within the middle and the lower crust are intimately related to the evolution of orogenies, but these deep crustal processes are still poorly understood. Seismic reflection data, chiefly displayed in amplitude, are commonly used to interpret deep crustal structures. The strongest amplitude events are probably related to lithological changes and do not necessarily correspond to structural boundaries or to deformation fabrics. Furthermore, the detailed structures and the strain distribution between the interpreted structural boundaries remain obscure. Here we show how seismic attributes can be used to enhance 2D seismic reflection data from the Palaeoproterozoic Svecofennian crust of southern Finland, to reveal unprecedentedly detailed information about the deformation fabrics within the deep crystalline basement. The images are plausibly interpreted to show that the extension of the orogenic middle and lower crust was mainly accommodated by mega-scale S-C’ structures. The structures form penetrative deformation fabrics which 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 deformation. The seismic attribute method described in this paper should be applicable to other deep seismic datasets.