Age and structural constraints on the tectonic evolution of the Paleoproterozoic Saimaa orocline in Fennoscandia

Abstract

The Paleoproterozoic oroclines of Fennoscandia played a vital role in the making of continental equidimensional crust during the Svecofennian orogeny. We propose that the 1.90–1.88 Ga arc rocks in south-central Finland originally formed a single linear arc system that is now represented by the coupled Bothnian oroclines and their southern continuation, the Saimaa orocline, as defined in this study. A regional large-scale approach with robust geochronological age determinations obtained from metasedimentary rocks and granitoids, and field-based structural studies are used to produce a coherent view on the tectonic evolution of the Saimaa orocline.The pre-orocline tectonic evolution started with a NE-SW collision between the Karelia continent (lower plate) and Proto-Svecofennia (upper plate) at ca. 1.91 Ga (D1). This was followed by subduction reversal and arc magmatism at ca. 1.90–1.88 Ga. Collision or flat subduction at 1.885 Ga caused a widespread collisional event (D2). Our results indicate that the presently E-W oriented D2 structures are not, as is widely proposed, products of N-S contraction but instead originated as a N-S to NNW-SSE oriented belt that was subject to E-W contraction. Dorocline, recorded as the first deformation stage in post-D2 sediments, folded the pre-existing structures into their E-W and ENE-WSW orientation. Following orocline formation, the main shortening direction switched back to the NE-SW (D3) resulting in type-1 and type-2 refolded folds, long wavelength regional folds and bending of the hinge zone of the Saimaa orocline. D4-D5 deformation stages are localized intra-continental tectonic events at 1.83–1.79 Ga. The formation of the Saimaa orocline is bracketed at 1875–1865 Ma and formed by margin-parallel shortening. The Bothnian and Saimaa oroclines form continuous buckles, but whereas the Bothnian coupled oroclines are crustal-scale parallel folds due to more rigid crustal core, Keitele microcontinent, the Saimaa orocline is a crustal-scale isoclinal similar fold.