Regional aeromagnetic and gravity studies in Scandinavia

Abstract

Attention is drawn to some large-scale structures in the Baltic Shield, interpreted from the regional magnetic and gravity measurements. One set of structures is of crustal scale and it is thought to be related to plate-tectonic processes while the others are smaller, expressing deformation in linear or circular patterns. The mid-Proterozoic Transscandinavian Granite-Porphyry Belt (1.6–1.7 Ga) is also a prominent magnetic feature related to high-magnetic granitoids and associated volcanics and the most important reference structure for tectonic interpretations. This belt traverses Scandinavia in a north-south direction and is partially covered by Caledonian thrust sheets. A 35 × 50 km area of low altitude aeromagnetic measurements in southern Scandinavia illustrates the magnetic and structural characteristics of this belt. These are high susceptibility intrusive rocks of great vertical extent and with rounded local structures, which give an overall high magnetic anomaly. The Transscandinavian Granite-Porphyry Belt is limited to the west by a shear zone up to 20 km wide, the Protogine Zone, which is characterized by distinct magnetic lows. This zone is examined in a 50 km-wide traverse of low altitude aeromagnetic measurements. To the west, a distinctly different magnetic pattern is observed, with dominating magnetic banding, low angle thrust faults, and highly magnetic sills. Contrasting to these large-scale linear structures, attention is drawn to two very large nearcircular structures characterized by regional gravity highs and aligned surrounding structures. The age of these structures is as yet unclear but they are younger than Archaean (in the north) or Svecokarelian (in the south). It is speculated that these structures formed as a consequence of large meteorite impacts.