Sven Aaro

Geophysical Investigations of the Siljan Impact Structure — A Short Review

Siljan in southwest Sweden is the largest impact structure in western Europe, with a present topographic diameter of ca. 75 km. Recent age determinations indicate an age of 377 Ma. The bedrock geology of the region has recently been re-mapped by the Geological Survey of Sweden in the scale of 1:50 000. There is now complete coverage with airborne geophysics. New maps of the geophysical data have been prepared for this review. In connection with the Deep Gas Project, further geophysical studies were made and two drill holes were sunk to over 6 km depth in the central uplift of the structure. The Deep Gas Project produced a large number of reports and publications, which are listed in the summary report of Juhlin (1991). Some of the results are compiled and shortly summarized here. Digital elevation data are available with 50 m spatial resolution, and a gray tone map has been prepared with the regional trend removed. A profile of these data shows that the peak ring of the structure is still visible in the morphology.

Structure and geophysics of the Gåsborn granite, central Sweden: an example of fracture-fed asymmetric pluton emplacement

Abstract The emplacement mechanisms of the Palaeoproterozoic Gåsborn granite, a satellite of the Transscandinavian Igneous Belt (TIB), are investigated using an integrated structural and geophysical approach. The pluton is discordant to c. 1.89 Ga folded supracrustal rocks that were deformed and metamorphosed at c. 1.85–1.80 Ga during the Svecokarelian Orogeny. Emplacement occurred at a depth of c. 10 km, within a regime of late Svecokarelian dextral transpression. Deformation of the pluton during cooling resulted in the formation of a variably developed foliation in the granite and deflection of less competent wall-rock units around its western and eastern contacts. Later E-W Sveconorwegian shortening resulted in the formation of shear zones that affect one of the pluton margins and may contribute to a component of the observed wall-rock distortion. The granite is situated above strong NNW-trending linear magnetic and negative gravity anomalies, which are interpreted to correspond to an important early Svecokarelian shear zone. The geophysical data indicate that the pluton is markedly asymmetric and modelling of the residual gravity field suggests that it consists of a deep root zone in the west and a thin sill-like body, which makes up most of the east and south parts of the body. Emplacement of the sill-like part occurred by lateral flow of magma from the root zone accommodated by downwarping of the underlying units. Intrusion of the thicker, discordant west part may have been accommodated by a combination of roof lifting and floor depression, aided by displacement on an active shear zone.

The Ljugaren granite massif, Dalarna, central Sweden

Abstract The circular, 13 km wide, c. 1700 Ma old Ljugaren granite massif intrudes and deforms 1890 to 1850 Ma old gneissose rocks and 1790 to 1740 Ma old late-orogenic granite sheets of the Svecofennian Orogenic Province. It is satellitic to the 1840 to 1680 Ma old Transscandinavian Granite-Porphyry Belt, and is partly overlain unconformably by Ordovician-Silurian sediments which are preserved around the Siljan Ring meteorite impact structure. Aeromagnetic and structural geological studies show that regional country-rock structure swings into concordance with the massifs margins 1–3 km from the contact. Gravity measurements indicate that the intrusion is shaped like an asymmetric funnel. Internally, the Ljugaren granite shows a weak, inwardly dipping, planar magmatic fabric concordant to its margins. Evidence to date indicates that the massif was emplaced into ductile crust as a magmatic diapir after the culmination of the Svecofennian orogeny c. 1700 Ma ago.