7.1 Problems with Filtering and Interpreting 2D Data
 The 2D nature of most seismic reflection data acquired from the crystalline basement, together with the often complicated geological history and the resulting heterogeneity at various scales, creates some issues that need to be borne in mind when filtering and interpreting the data. The crustal heterogeneities and the often complex geometries cause reflection patterns that can be difficult to interpret. The crustal heterogeneity can also lead to significant scattering of the seismic signal, especially at greater depths, while processing and imaging techniques frequently assume weak/single scattering and uniform transmission throughout the crust [e.g., Wu and Aki, 1988; Levander et al., 1994]. As greater depths are approached (>4 seconds two-way-time), strong and multiple scattering progressively degrade the signal. The FIRE seismic data presented here is filtered and interpreted down to 15 km depth (<5 s TWT). The imaged section between 0 and 15 km lies mostly in the optical regime (Figure 4a) where conventional migration based on ray theory can be expected to work reasonably well [Wu and Aki, 1988].
 A well-known issue with 2D seismic reflection data is the so-called out-of-plane reflections (“sideswipe”). The input from out-of-plane reflections increases systematically toward greater depths, progressively producing a mixture of in-plane and out-of-plane reflections. To decrease this problem, a seismic profile should be shot at right angles to the structural (geological) strike, and the more the profile diverges from this geometry, the more likely it is to contain out-of-plane reflections. In reality, small variations occur both in the geological strike and in the strike of the seismic profile, resulting in out-of-plane reflections [e.g., Drummond et al., 2004]. Strong out-of-plane reflectors typically produce reflections in the migrated profile that cross-cut the in-plane reflections. The out-of-plane reflections do not seem to be a significant problem in the filtered FIRE data (Appendix III). The profile in the studied area is for most parts at right angles to the strike. Artifacts such as cross-cutting or “bow-tie” reflections were not encountered during picking of the filtered data. The interpretation technique used in this paper, based on the seismic facies concept, also reduces the risk of incorporating occasional out-of-plane reflections into the interpretation; the concept is based on the consideration of the general reflection patterns and packages, rather than individual reflections. Therefore, it separates the different 2D architectures reflecting the relative 3D heterogeneities between units. Finally, the structural pattern (the S-C′ structures) is not only observed close to the surface where out-of-plane reflections are less likely to interfere, but also at greater depths, indicating that the interpretation of the overall structural pattern is valid to the depth of 15 km.
7.2 The Seismic Reflection Patterns within the Orogenic Crust, and the Mode of Extension/Lateral Flow
 The extensional architecture dominates the interpreted FIRE seismic data from the middle and lower orogenic crust, currently exposed at the Earth surface. The apparent extensional geometries need not, however, imply an extensional overall tectonic setting or thinning of the lithosphere. The crust in southern Finland is still 45–50 km thick, after at least 15 km of erosion [Grad et al., 2009]. Several landmark studies have established the importance of syn- to late-convergent “extensional” and flow processes: continental plateau development and crustal coupling-decoupling [e.g., Kellett and Grujic, 2012; Sokoutis and Willingshofer, 2011], orogenic collapse [e.g., Dewey, 1988; Rey et al., 2001], and mid-crustal flow [e.g., Beaumont et al., 2001; Grujic et al., 2011; Culshaw et al., 2006; Vanderhaege and Teyssier, 2001]. To which orogenic process the S-C′ structures in the WUC belong to is beyond the scope of this study. The age determinations from the WUC anatectic granites, however, imply that the processes that formed the S-C′ structures were (early) synorogenic with respect to the c. 1.85–1.82 Ga Svecobaltic orogen [e.g., Kurhila et al., 2005; Skyttä and Mänttäri, 2008]. Therefore, it may be feasible to preliminarily speculate that the structures were formed as a result of syn-orogenic mid-crustal flow rather than late-orogenic collapse. The original subhorizontal to gently dipping foliations and other structures (including the recumbent folds) formed during the thickening phase of the Svecobaltic orogeny [Lahtinen et al., 2005; Skyttä and Mänttäri, 2008], and were subsequently extended by the mid-crustal flow. This scenario implies that the gently to moderately dipping shear zones along with the rest of the S-C′ structures would, therefore, have formed at this stage, i.e., at c. 1.84–1.80 Ga ago. Dating of the granulite shear zones is in process, and the results are expected to provide more information on the timing of the extension. The published P-T-time paths for southern Finland [e.g., Cagnard et al., 2007; Väisänen and Hölttä, 1999] and the present erosion level at which pervasive migmatization and, therefore, weakening of the crust occurred correspond well to the mid-crustal flow models presented in the literature [e.g., Beaumont et al., 2001] although they are not exclusive to mid-crustal flow. On the other hand, the interpreted palaeodepth of c. 15 km is shallower than the suggested depth of mid-crustal flow in the Himalayas [from c. 30 km downwards; e.g., Grujic et al., 2011]. If the structures in the FIRE profiles resulted from mid-crustal flow, implications for the geothermal gradient in the Palaeoproterozoic time may be significant.
 The strain distribution is more penetrative and smaller-scale than implied by the preliminary FIRE 2A interpretation [Nironen et al., 2006]. The pervasive nature of the deformation is expressed by the interference between the various structures. The early recumbent folds, formed during orogenic thickening, are deformed by the S-C′ structures. The structures interact with each other: the crustal shear zones (SF3) are not continuous, but strain transfer occurs both via distributed deformation (SF1) and along subhorizontal planes (SF4; Figures 8 and 9). Our results support recent models of crustal-scale strain transfer along both dipping and subhorizontal shear zones in extending/flowing orogenic crust [Chardon et al., 2011; Wang et al., 2011]. Specifically, Chardon et al.  suggest that, based on their field studies, the crustal extension in the Dharwar craton (southern India) was accommodated by a network of structures that very much resembles the kilometer-scale S-C′ structures presented in this study. A somewhat contrasting view is taken by Regenauer-Lieb et al. , who show that, based on numerical modelling, the strain in an extending lithospheric crust should partition into distinct, conjugate sets of extensional shear zones. However, an extensional tectonic setting and lithospheric thinning is not necessary for orogenic mid-crustal flow or orogenic collapse. It is true that a significant amount of the strain is accommodated by the shear zones in the WUC (SF3), but much of the strain is also taken up by the volumes between the shear zones (SF1) and the subhorizontal detachments (SF4), so that the overall pattern rather resembles a three-dimensional network than a set of conjugate shear zones (which is a natural consequence of general strain). The absence of a distinct pressure or temperature jump in and in the vicinity of the WUC can, therefore, be explained by the S-C′ structures: this mode of orogenic extension allows the accommodation of large regional strains without the need to develop crustal-scale detachment zones.
 A two to three orders of magnitude decrease in the overall rock strength, compared to unmolten rock (0% melt), is achieved with a few percents of melt within the rock volume [e.g., van der Molen and Paterson, 1979; Vanderhaege, 2009]. The most significant drop in the rheological strength occurs at a melt fraction of c. 0.07 [Rosenberg and Handy, 2005]. Importantly, at these low melt percentages, the rock can still deform as a solid, granular mass, meaning that strains can be taken up by the entire volume rather than just by the melt-bearing zones. The mechanical transition from solid to liquid occurs at about 30–40% melt fraction [e.g., Rutter and Neumann, 1995; van der Molen and Paterson, 1979], although even lower percentages of 20 ± 10% have been suggested by Arzi . A low amount of melts in the crust is important for the formation of the S-C′ pattern, as the crust needs to flow but simultaneously behave and deform as a solid mass rather than as a liquid. On the other hand, crustal deformation enhances melt migration and accumulation at certain crustal levels [e.g., Vanderhaege, 2009]. High melt percentages and melt accumulation result in strong strain partitioning [e.g., Holtzman et al., 2003; Vanderhaege, 2009]. In the study area, field data imply that some of the melts were removed from the deeper crust and accumulated into larger melt pockets within the crust (e.g., the c. 1.83 Ga microcline granite laccoliths at the present erosion level). However, the laccoliths themselves are not deformed to the extent to suggest strong strain partitioning, which implies that they did not accommodate the bulk of the strain. The melt transport might have been largely syn-deformational, but the final accumulation of the melts seems to have occurred later. As the melt accumulations grew the strain may, however, have progressively partitioned into the melt layers. Therefore, a temporal evolution of the strain partitioning is implied.
 The physical properties of the rock, such as lithological changes and mineral anisotropy, influence the obtained seismic reflectivity [e.g., Lloyd et al., 2009]. The unfiltered FIRE2A seismic data show few prominent reflective areas, while most of the interpreted section is non-reflective and noisy (e.g., Figure 5a). Therefore, it is unlikely that large, systematic lithological contrasts (suitable for generating significant acoustic impedance contrasts) exist in the crust within the interpreted section. However, the filtering reveals a penetrative fabric throughout the seismic section, even in the “white,” noisy areas (Figure 5). It has been demonstrated elsewhere that the seismic anisotropy, one mechanism for generating reflections from the crust in the seismic data, results from rock fabrics (i.e., deformation) rather than lithological layering [e.g., Weiss et al., 1999]. More specifically, the seismic anisotropy is usually attributed to the presence of regionally aligned mica and, in the lower crust, amphibole [e.g., Lloyd et al., 2009; Shapiro et al., 2004; Tatham et al., 2008]. Both aligned mica and amphiboles/pyroxenes are abundant in the studied area, and they follow and define the fabrics of the rocks. The structures observed at outcrop are generally gently to moderately dipping, which explains their efficient imaging at depth by the seismic data. Therefore, the observed seismic reflection patterns, revealed by the filtering, are here attributed to the internal mineral anisotropy, i.e., the structural fabric of the rocks, rather than lithological variation at the scale of the reflections.
7.3 Implications to Interpretation of Crustal Seismic Data
 Acoustic boundaries in the deep crust are usually due to lithological changes, and do not always correspond to structural boundaries. Structural interpretation of seismic reflection data should not, therefore, rely solely on the examination of (wiggle) amplitude images. Furthermore, distinguishing extensional structures (local or crustal-scale) from compressional ones can be difficult even when the exposures can be observed directly in three dimensions (Butler and Freeman, 1996; Wheeler and Butler, 1994). The significant uncertainties related to structural interpretation of seismic reflection data from the crystalline basement can be reduced by advanced seismic attribute analysis: the dip-steering median filter allows a more reliable and detailed detection of the deformation structures within the crust.
 In the studied seismic section, many seismic features in the mid-crust might generally be attributed to crustal thickening and/or thrusting. Fold structures mapped in the field are also commonly attributed to contractional deformation. However, after an examination of the seismic attributes and the structural trends, the reflections in the FIRE2A seismic data and the kilometer-scale gentle folds observable in the field are more plausibly interpreted to result from pervasive crustal extension. The extensional features overprint and obscure most of the earlier structures in the seismic data, including the terrane boundary (S-Kfz), although some of the extensional structures may follow and invert earlier (thrust) structures. This observation conforms to some previous works suggesting that the seismic fabrics from orogenic belts reflect the pervasive structural overprinting due to late orogenic processes [e.g., Oncken, 1998). The mode of the structural overprinting in hot orogenic roots is, however, extensional rather than compressional, resulting from syn-orogenic mid-crustal flow and/or syn- to late-orogenic collapse. It should be repeated, however, that this does not imply overall extensional tectonic setting and lithospheric thinning, e.g., Beaumont et al., 2001; Grujic et al., 2011).
 Modern seismic attribute methods should be utilized in and developed for crystalline basement studies, in order to facilitate increasingly detailed structural interpretation of crustal seismic datasets. However, the limitations caused by 2D data and the crustal heterogeneities have to be considered because the reliability of filtering techniques decreases at great depths. Therefore, the seismic facies interpretational approach should be utilized in interpreting filtered seismic data. The seismic attribute method described in this paper should be applicable to other deep seismic datasets, possibly even vintage data [e.g., INDEPTH, NFP20, LITHOPROBE; e.g., Choukroune and ECORS Team, 1989; Clowes et al., 1992; Klemperer and Hobbs, 1991; Nelson et al., 1996). The method can be utilized to study the mode and the geometries of sub-kilometer-scale, gently to moderately dipping structural fabrics of the last penetrative deformation phase in the studied dataset. The last penetrative deformation phase is recorded as patterns of reflections (seismic facies). Systematic discontinuities of the reflections, marking later (steep) fault zones and fractures, may also be more reliably detectable in filtered data. Further developments in the calibration of complex attribute methods to suit crustal studies could also permit (semi-)quantitative approaches to the geological processes within the crust.