Brittle tectonic evolution and paleostress field reconstruction in the southwestern part of the Fennoscandian Shield, Forsmark, Sweden

[1] Six hundred fault slip data have provided robust paleostress fields within an approximately 35 km3 volume of Paleoproterozoic (1.9 Ga) rocks in the southwestern Fennoscandian Shield, Forsmark, Sweden. These rocks were affected by penetrative ductile strain from 1.87 to 1.86 Ga, folding, ductile strain along discrete zones around 1.8 Ga, and semibrittle or brittle deformation around and after 1.8 Ga. Compatible paleostress fields have been identified using site-by-site and merged data sets from outcrops and oriented drill cores. Transpressive deformation with a regional NNW-SSE σ1 axis, associated with clockwise stress deviation inside a tectonic lens, resulted in dextral slip along regionally significant, steep WNW-ESE and NW-SE deformation zones. The semibrittle and most of the brittle structures, including specifically the epidote-bearing fractures, were established during this oldest regime around 1.8 Ga (latest Svecokarelian). A younger paleostress field with a NE-SW σ1 axis, which was also transpressive in character, is inferred to have been active at 1.7–1.6 Ga. The best defined paleostress field is transpressive in character with a WNW-ESE σ1 axis that resulted in sinistral reactivation along the WNW-ESE and NW-SE zones. The main set of laumontite-stepped faults developed at this stage at 1.1–0.9 Ga (Sveconorwegian). It is impossible to exclude fully the influence of reactivation during even younger Phanerozoic tectonic events. Subordinate extensional paleostress fields were related either to the latest Svecokarelian and Sveconorwegian transpressive regimes, due to σ1 and σ2 stress permutations, or to regional extensional tectonic regimes during the Meso- or Neoproterozoic or later during the Permo-Carboniferous and/or Mesozoic.