Seismic and geoelectric evidence for collisional and extensional events in the Fennoscandian Shield implications for Precambrian crustal evolution

Abstract

Extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles, normal-incidence reflection profiles, several hundred magnetotelluric soundings, and several magnetovariational array observations. Results are compiled into four maps: (1) Moho depth; (2) thickness of the lower crustal high-velocity layer ( V p > 7 km/ s); (3) depth to the upper interface of the high-velocity layer; and (4) distribution of upper and middle crustal electrical conductors. The Moho depth is approximately 40 km in the Archaean Domain and varies between 40 and 65 km in the Svecofennian Domain. Most of the thickness variation (0–24 km) is concentrated in the lower crustal high-velocity layer. Electrically, the crust is formed of several rather homogeneous, either resistive or conductive, blocks that are bounded by upper and middle crustal conductive zones. The cratonized Archaean crust experienced an extensional phase during which a thin, mafic, lower crustal high-velocity layer was produced in association with mafic dykes (2.2–2.1 Ga) and well-conducting volcanogenic and metasedimentary belts (2.5–2.0 Ga). The Moho of the Archaean Domain was, consequently, reformed in the Early Proterozoic. Major thickening of the crust (50–65 km) occurred when a Svecofennian island arc and the Archaean craton collided. The Svecofennian orogeny (1.76–1.9 Ga) resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc. The internal terrane boundaries are seen as good reflectors and/or inclined or vertical conductors that extend at least to the middle crust. The thick high-velocity (> 7 km/s) lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of Late Svecofennian subduction. The collisional boundary is also preserved as a discontinuous conductive zone between the Archaean and Svecofennian Domains. The thickened crust was subsequently thinned to 42 km along E—W-striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths, coeval mafic dykes, and gabbro-anorthosite bodies (1.6–1.5 Ga). The crust of the central part of the shield has since remained virtually intact.