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Geology in the north-central
Swedish Caledonides around Villa Heidelberg
Klimpfjäll (Vilhelmina, SW Västerbotten, S Lapland, Sweden)
A. Introduction
The Scandinavian Caledonides are one of the classic fold-and-thrust belts,
where large-scale, low angle thrusting has been recognized already more than
110 years ago (Törnebohm 1888). Since then, it has been shown that most
Caledonian rocks in Scandinavia are allochthonous and have been thrust east-
ore southeastwards onto the Baltoscandian platform (the terrane of Baltoscandia
or Baltica). Conventionally, the Caledonian successions are divided into five
structural tiers, in ascending order the Autochthon, the Lower Middle, Upper
(Seve and Köli Nappes) and Uppermost Allochtons (Kulling 1972, Gee et al.
1985; Fig 1). The lower units, up to and including the Seve, are interpreted
as part of the imbricated and shortened passive continental martin of Baltoscandia
(e.g. Stephens & Gee 1989). Rocks of the Lower Allochthon are dominated
by clastic cover sequences of late Proterozoic to early Palaeozoic age, related
to the continent Baltica. Minor Precambrian basement units were incorporated
during thrusting. The Middle Allochthon and the Seve units of the Upper Allochthon
were derived from similar quartz-rich sedimentary complexes of late Proterozoic-early
Palaeozoic (?) age, probably deposited along the western edge of the continent
Baltica, and metabasic rocks which represent intrusions and extrusions related
to the opening of a late Proterozoic-early Palaeozoic ocean (Proto-Atlantic).
These units, which derived from the continental terrane of Baltica and its passive
margin are distinct from overlying "exotic" oceanic, island-arc and
continental terranes building up the upper, Köli part of the Upper Allochthon
and the Uppermost Allochthon. The Köli rocks have been subdivided into
three major tectonic units, the Lower, Middle and Upper Köli (Stephens
1980a): The Lower Köli (Virisen - terrane) is interpreted to represent
intra-oceanic arc-basin complexes that probably developed in the vicinity of
the Baltoscandian margin of the ocean, whereas the Middle and Upper Köli
Nappes represent more exotic intra-oceanic arc-basin sequences which were probably
related to the Laurentian side of the ocean (Stephens and Gee 1985, 1989, Stephens
1988).
Whereas the late Proterozoic-early Palaeozoic plate configurations in the actual
area are still far from clear, there is general agreement that there was an
early compressional (collisional?) phase in late Cambrian-early Ordovician time
(Finnmarkian phase) and a final, continental collision between Baltica, Laurentia
and Avalonia in Silurian time (Scandian phase; see Fig. 2). The most prominent
traces of Finnmarkian tectonics are the high-grade metamorphic rocks in parts
of the Seve units of the Upper Allochthon. However, the best dated traces of
Finnmarkian activity are probably the Ordovician greywackes in the Autochthon
and Lower Allochthon of central and northern Scandinavia. During the Scandian
phase, continental collision led to the build-up of orogenic wedges, which moved
from the collision zone towards the foreland and onto the Baltic Shield. During
the orogenic procecces, the Baltica margin was "subducted" to high-pressure
dephts, as documented by the eclogites and other indicators such as diamond
at the present Norwegian coast (Western Gneiss Region). Movement of the orogenic
wedge eventually resulted in the stacking of nappe units, which are described
in detail below. Finally, further orogenic procecces (such as decoupling of
crust and mantle lithosphere), gravitational collapse and uplift of the HP rocks
or continued plate movement led to a modification of the orogenic structures
and to cooling and stabilization of the lithosphere. At the same time, "post-orogenic",
molasse-type, terrestrial sediments (Old Red) were deposited in intramontane
basins and shed from the orogen and towards the foreland (e.g. Baltica, Avalonia),
ranging in age from mid-Silurian to early Devonian.
The major part of this text is based essentially on a compilation of the map
descriptions of the 1:50.000 geological maps (SGU Serie A i), which cover a
traverse from the eastern Caledonian margin (Vilhelmina-Storuman) to the Norwegian
border in the west. However, it is intended as a more general review of the
geologic-tectonic evolution of the central Scandinavian Caledonides. Consequently,
the traverse in Sweden is complemented by an excursion into adjacent Norway
(Nord Tröndelag), in order to cover also the highest Caledonian thrust
units in the west. The text is divided into lithologic-structural succession
and structure, followed by references and a list of potential excursion stops.
Fig. 3 gives an overview over the central Scandinavian Caledonides, and the
excursion area.
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