Age and structural constraints on the tectonic evolution of the Paleoproterozoic Saimaa orocline in Fennoscandia
Raimo Lahtinen, Paula E. Salminen, Mohammad Sayab, Hannu Huhma, Matti Kurhila, Stephen T. Johnston
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.