Reinhard. O. Greiling

Field data from recent geological mapping over a major part of the north-central Scandinavian Caledonides combined with published information give a detailed view of early Cambrian basin successions, comprising the Gärdsjön formation (Gf; Jämtland supergroup) in the Lower Allochthon and autochthonous equivalents (Dividal Group). The Gf comprises ten units of sandstone and siltstone or mudstone (Gf I—X, > 300 m thick). Green siltstones with red layers (Gf VI, c. 521 to 519 Ma) and green–grey siltstones at the top (Gf X, c. 516.5 to 513.5 Ma) are regional key horizons. Gf V, VI, VII, IX, and X deposition may be related to eustatic events. Restoration of Caledonian shortening reveals a major “Hornavan-Vattudal basin” (HVB; > 330 km NW–SE, > 400 km NE-SW) between the Grong–Olden culmination in the S and the Akkajaure–Tysfjord culmination in the N. Published zircon ages imply the latter separated the HVB from those shed from the Timan orogen in the N. The eastern basin margin straddles the present Caledonian erosional margin. Basement highs identified here within the Nasafjället, Bångonåive, and Børgefjellet “basement” windows define the western margin. They separate the HVB from the outer shelf towards the Iapetus Ocean in the W. The onset of sedimentation is time-related with E–W extension at c. 544–534 Ma. NNE–SSW-directed extension occurs after c. 518 Ma, perhaps related with Timan late-orogenic extension. The HVB is distinctly younger (c. 535–513.5 Ma) than Rodinia break-up and Iapetus ocean formation (> 550 Ma), comparable with post-rift basins in inner parts of modern passive margins. Graphical Abstract

This study reviews lithological and stratigraphic data, new geological mapping results and microscopic work from the lower and middle Caledonian nappes in central Scandinavia, and suggests a model of their tectonic evolution. The Jämtlandian and Blaik Nappes represent the lower nappes, and are overlain, successively, by the Stalon, Danabergen and Slussfors Nappes. The latter occupies the level immediately beneath the Seve Nappe Complex. Farther north, around Lake Gautojaure, the Åkerberget Nappe represents the level between the Stalon Nappe and the Ammarnäs Complex. Their hangingwall is the Sadenåive Nappe and the overlying Seve Nappe Complex. The Danabergen, Åkerberget, Sadenåive and Slussfors Nappes are formally defined here. In the Blaik Nappe, brittle deformation gave rise to composite thrust systems. The overlying nappes show a more ductile deformation. The arkose sequence of the Stalon Nappe shows fine-grained recrystallization of muscovite. In contrast, finely recrystallized mica minerals form a phyllitic foliation in the Åkerberget Nappe. The Danabergen Nappe contains small garnet porphyroblasts. The Ammarnäs Complex and the Sadenåive Nappes show garnet with a straight internal foliation (S_i), while the external foliation is refolded. This fabric suggests a time interval after early foliation development, in which garnet grew but no deformation occurred, and subsequent deformation. This may represent an episode of arrest of deformational activity and thrust movement. Spiral-shaped S_i in garnets of the Slussfors Nappe indicate a contrasting and more complex metamorphic and structural evolution, perhaps similar to that of the Seve Nappe Complex, with a well-known, polyphase tectonic evolution.

Basement-slices are frequent components in the lower nappes of the central Scandinavian Caledonides. New geochronological and geochemical data provide evidence that three of these basement-slices in the Middle Allochthon are derived from the Transscandinavian Igneous Belt. Dated samples in this study comprise a quartz monzonite from the Stalon Nappe Complex and a quartz monzodiorite and a monzonite from the Ammarnäs Nappe Complex. A Ce/Yb-Ta/Yb-plot of the mafic rocks from the Ammarnäs Nappe Complex suggests a calc-alkaline to shoshonitic geochemical character. Low Ni- and Cr-contents and low Mg# indicate that the mafic rocks originated from an already evolved magma. Enrichment of Fe-Ti oxides and V>200 ppm indicate presence of cumulus phases to various degrees. These basement-derived rocks in the Middle Allochthon yielded concordant U-Pb zircon SIMS ages of 1799 ± 10 Ma, 1787 ± 6 Ma and 1797 ± 5 Ma. They are therefore interpreted to represent rocks detached from the Palaeoproterozoic Transscandinavian Igneous Belt and incorporated in the Middle Allochthon during Scandian orogeny.

The study of shock pressure indicators can provide important clues for understanding the cratering process, though the estimation of shock pressures in weakly shocked rocks is commonly difficult. In this study, we selected a very young and well-preserved impact structure, the Lonar crater in India. The crater, devoid of any tectonic overprint, can be assumed as pristine. We used a combination of rock magnetic and microfracture studies to estimate shock pressure in the crater rim. On the basis of present results, the magnetic fabrics are interpreted to be of magmatic origin related to the Deccan basalt emplacement. The high-coercivity component of the natural remnant magnetization in the crater rim basalt is similar to that in the unshocked basalt. The lack of any shock-related magnetic overprint on the crater rim basalt is, therefore, evident in the Lonar crater.