Examples of Lateral Crustal Spreading from the Paleoproterozoic Svecofennian Orogen

Svecofennian Orogen is a Paleoproterozoic analogue of a laterally spreading accretionary orogen at a convergent margin setting. The exposed parts that are composed of granitoid intrusions as well as metamorphosed and highly deformed supracrustal units representing paleo-lower-upper and paleo-upper-middle crust. Both the exposed bedrock surface and the crust below carry frozen-in kinematic indicators of the spreading processes. We have been studying these processes using seismic reflection and tomography methods as well as structural observations and analogue modeling. The deep seismic reflection profiles (FIRE1-3) crossing the study area in two perpendicular directions suggest a three-fold layering of the thick crust (>55 km) that has developed after accretion. The crustal layers are separated by décollement zones on to which crustal reflection sole out and across which velocity steps occur. FIRE 1 and 3 profiles show complementary crustal scale structures of compression and extension in suborthogonal directions (N-S, WNW - ESE). The upper crust spread in a brittle to ductile regime along listric, low angle and transfer shear zones, and the middle crust moved via ductile flow and extrusion. The middle crust displays typical large scale flow structures: herringbone and anticlinal ramps, rooted onto large scale listric surfaces. Most of the major crustal structures described from the FIRE profiles could be reproduced in an analogue experiment testing convergence coupled with perpendicular extension. Another example of post-collisional movement of the bedrock blocks is the formation of persistent low angle lineations in most of the rock types. A new map of average lineations in Finland (78 000 measurements) displays consistent regional movement directions across the country. The lineation patterns suggest large-scale block-movements and deformation in the upper and middle crustal after collision interpreted to be related to lateral spreading of the Svecofennian orogeny. In those areas where the bedrock shows constant directions of lineations, the residual component of a tomographic velocity model hosts azimuthal velocity anisotropy (AVA). The anisotropy is subparallel to the direction of lineations observed at surface, suggesting that also the lower parts of the crust have moved laterally after collision. Granulites and migmatites, which are partially molten rocks of the middle crust, are well exposed in West Uusimaa Complex (WUC), where they can be studied for midcrustal flow mechanisms. Low to moderately E-dipping structures: schistosity, lineations and shear zones are bordered by steep E-W directed large-scale shear zones are compatible with a flow structure where the movement is in E-W direction. Large scale S-structures identified on FIRE2a seismic reflection profile suggest crustal scale shearing and extension in E-W direction as well. Both low angle and steeply dipping large scale shear zones play a central part in the mid-crustal flow architecture.