The Early Proterozoic Orijärvi triangle (southwest Finland): a key area on the tectonic evolution of the Svecofennides

Abstract

Abstract A consistent pattern of three successive deformation phases is present in the west Uusimaa area in the south Svecofennian schist belt of Finland, in which the Orijarvi triangle, bordered by shear zones, forms a low strain area. It is shown that the well-preserved early folds and foliation in the Orijarvi triangle correlate with the regionally oldest and completely transposed structures. A model is given for the tectonic development of the west Uusimaa area involving (1) an early phase of subhorizontal, probably thrust-related, folding, (2) a second phase of crustal shortening and (3) a late phase of folding and formation of vertical shear zones. The first two phases took place during prograde metamorphism, whereas the last one records retrogression. Shear zones were probably active during all phases: subhorizontally (thrusting) during D 1 , transversely (wrenching) during D 2 and vertically (uplift) during D 3 . It is suggested that the D 1 -D 2 sequence represents a progressive deformation in a plate-collision system, whereas D 3 is related to a 1533±23 Ma old post-Svecofennian, possibly Gothian, event. A model is presented in which the development and preservation of the Orijarvi triangle is explained by the local development of transversal shear zones along anatectic plutons during D 2 . These shear zones were then reactivated during D 3 . The model presented implies that the sigmoidal pattern present in southern Finnish geology was caused during a late stage of the D 2 crustal shortening event along three, broad, parallel sinistral shear zones. It is suggested that these shear zones represent deep-seated parts of wrench faults in a complex plate tectonic system involving several island arcs and that they were caused by differential movement of the south Svecofennian schist belt with respect to the Tampere schist belt.