Lateral constraint rather than escape along a terrane boundary near Skelleftehamn

Abstract

Mylonitised greywackes with superposed pseudotachylites (fossil earthquakes) mark the eastern end of a W-E trending terrane boundary between the Skellefte supracrustal group of Svecokarelia to the north and the Bothnian gneisses of Svecofennia to the south. This region has been claimed to provide the clearest evidence that early Proterozoic plate tectonics (>1.85 Ga) involved the same processes as Phanerozoic continental suturing and convergence. Comparisons between the tectonic histories of greywackes in, and on either side, of this boundary demonstrate that the southern terrane already had major recumbent folds at upper amphibolite metamorphic facies before it met and converged on the northern terrane and eventually rose relative to it. These nappes were refolded by upright folds that are circumferential to a syn-Svecofennian granite south of the boundary but the first folds visible in the lower grade rocks to the north. The youngest Svecofennian structures in the region were kink bands that developed in the boundary while it was a steep seismic zone. These kink bands record lateral tectonic constraint rather than lateral escape. West of the study area, mylonites of the terrane boundary are intruded by sheets of post Svecofennian Skellefte granites and plutons of 1.75 Ga Revsund granites.