The GGT/SVEKA transect: Structure and evolution of the continental crust in the Paleoproterozoic Svecofennian orogen in Finland

The Global Geoscience Transect SVEKA (GGT/SVEKA); a 160 km wide, 840 km lon g strip in the central part of the Fennoscandian shield, covers the western part of the Archean Karelian Province in the northeast, crosses the bounda ry zone between the Karelian Province and the Paleoproterozoic Svecofennian are complex, traverses the main tectonic units of the northern part of the Svecofennian complex, and ends in the Subjotnian region in the southwest. The transect area is well suited for investigation of the Svecofennian orog eny and its effects on the Archean Karelian crust as well as the later exte nsional events of the Svecofennian crust. The purpose of the GGT/SVEKA prog ram was the construction of a tectono-evolutionary model of the crust along the transect. By combining geophysical(seismic, electrical, gravimetric, t hermal, magnetic, and petrophysical) and geological (lithological, structur al, metamorphic, isotopic, and geochemical) data, the study focuses on solv ing the temporal and causal relationship between deformation, metamorphism, and magmatism and, in particular, on investigating the crustal thickness a nd density variations, the high metamorphic temperatures during the Svecofe nnian orogeny, and the significance of crustal conductors. The major stages of the crustal evolution include: (1) the prolonged riftin g of the cratonized Archean crust between 2.5 and 1.97 Ga; (2) the final br eak-up and ocean development in the southwest at 2.1 Ga; (3) the formation of the largely unknown 2.06 Ga protolith for Svecofennian are magmatism, ev idenced by geochemical data and detrital zircon ages; (il) the amalgamation of the Pyhasalmi primitive island are, the central Finland continental are , and the southern Finland sedimentary-volcanic complex in two major collis ions at 1.91 to 1.885 Ga and before 1.885 Ga; (5) the high temperature, low -pressure metamorphism between 1.885 and 1.800 Ga caused by magmatic underp lating and partial crustal thinning after subduction and crustal thickening ; (6) the Subjotnian extension associated with bimodal magmatism at 1.558 t o 1.540 Ga and the development of Jotnian sedimentary basins; and (7) diaba se dike magmatism at 1.27 to 1.26 Ga. The crustal thickness of the Precambrian crust in Fennoscandia exhibits rem arkable variations from 27 to 65 km. Most of the thickness variations, whic h are isostatically compensated within the crust by density variations, can be explained by variations in the thickness of the high-velocity lower cru st. The formation of the mafic, high-velocity deep crust, a cause for the h igh-temperature, low-pressure metamorphism in the Svecofennides, took place in several phases by subduction- and ex tension-related under- and intrapl atings. Thinner crust exists in the areas that have experienced anorogenic extensional events. Large parts of the Svecofennides have a much greater th ickness, indicating that the crust does not always reach the thickness of s imilar to 40 km, which is normally found in the Archean and Proterozoic cru st? but may remain much thicker. Orogenic collapse, in the sense of the pro duction of normal (thinned) crust, apparently has been prevented in these a reas. The collision of the Svecofennian are complex with the Archean continent ca used thickening, magmatic underplating, and metamorphic overprinting on the southwestern part of the Archean crust. Rifting episodes (2.5 to 1.95 Ga) and magmatic underplating at the Karelian-Svecofennian boundary zone (1.885 to 1.85 Ga) indicate thai not only was the Archean crust reworked during P aleoproterozoic time, but the entire Archean lithosphere was activated.