Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

Abstract

The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well‐defined shear zone systems. An anomalous transitional boundary has been investigated by integrating structural analysis and geochronology with published geophysical data. The nature of this boundary is interpreted to be a consequence of an apparent stacking in the lower and middle crust initiating 1.87–1.86 Ga dextral shear along the Gävle‐Rättvik Zone (GRZ) and adjacent shear zones, resulting in an arcuate northern boundary of the Bergslagen province. This boundary coincides with geophysical anomalies and temporal and metamorphic breaks. Owing to continuous convergence the pure‐shear overprint component increased on the GRZ and caused a shift of dextral shear to the Hagsta Gneiss Zone with recorded shear at 1809 ± 2 Ma. Most likely, both these structures are related to coeval shear zones farther to the east as a part of an ∼1500 km long crustal, or possibly terrane, boundary.

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

“…Similar results have been obtained from south-central Sweden where N-S shortening produced ductile shear zones until at least 1.80 Ga (e.g. Högdahl et al, 2009). We follow our previous results (Torvela & Ehlers, 2010) in concluding that the SJSZ forms a part of a terrane boundary (a 'Trans-Fennoscandian deformation zone') that compartmentalised the far-field orogenic stresses; in central Fennoscandia, the area to the north of the zone was affected by the Nordic orogeny and by the collision of Sarmatia from the SE, while the rocks to the south of the SJSZ responded to a convergence from the S-SSW (Fig.…”

“…1; e.g. Sjöström & Bergman, 1998;Högdahl, 2000;Högdahl et al, 2009). The continuation toward the E-SE is covered by the Baltic Sea, but the magnetic anomaly map (Korhonen et al, 2002) implies that the SJSZ and the NW-SE trending major shear structure in the Precambrian basement of Estonia, the Paldiski-Pskov Shear Zone, are related (PPZ; Fig.…”

“…1; e.g. Gaál & Gorbatschev, 1987;Sjöström & Bergman, 1998;Högdahl et al, 2009). Another such a postulated boundary is a several kilometres wide shear structure, the Sottunga-Jurmo shear zone (SJSZ) that extends in a NW-SE direction through the Åland and Turku archipelagos (Torvela & Ehlers, 2010;Figs.…”

From ductile to brittle deformation: structural development and strain distribution along a crustal-scale shear zone in SW Finland

“…1; e.g. Sjöström and Bergman 1998;Högdahl 2000;Högdahl et al 2009). The continuation toward the E-SE is covered by the Baltic Sea, but the current geophysical evidence imply that the SJSZ and the NW-SE trending major shear structure in the Precambrian basement of Estonia, the Paldiski-Pskov Shear Zone (PPZ; Fig.…”

“…1; e.g. Gaál and Gorbatschev 1987;Sjöström and Bergman 1998;Högdahl et al 2009). One such a postulated boundary is a several kilometres wide shear structure, Sottunga-Jurmo shear zone (SJSZ) that extends in a NW-SE direction through the Å land and Turku archipelagos (Fig.…”