The aftermath of the Caledonian continental collision in the North
Atlantic Region: A structural template for later events?
Torgeir B. Andersen1), Ebbe Hartz2), Trond H. Torsvik2), Per Terje Osmundsen2),
Arild Andresen1), Elizabeth A. Eide2) and Alvar Braathen2)
1)
2)
Dept. of Geology, Univ. Oslo, P.O.Box 1047 Blindern, 0316 Oslo, Norway.
Norwegian Geol. Survey., Leiv Eirikssons vei, 7941 Trondheim
Orogenic belts produced by continental collisions are traditionally viewed as the
ultimate products of Wilson-cycle tectonics. Modern studies of ancient as well as
semi-recent to recent examples have, however, demonstrated the importance of the
tectonic activity during the terminal stages of- and immediately after collision. The
Cenozoic collision between India and Eurasia and the associated intra-continental
deformation affecting vast areas in central and SE Asia is the best actualistic example
(Fig. 1). The Himalayan collision and the large-scale indentation of India was- and
still is accompanied by regionally distributed deformation involving crustal stacking
and thickening in the Himalays, around the Tibetan plateau and in Tien Shan, strikeslip and normal-faulting in the Tibetan Plateau and large scale strike-slip faulting with
associated pull-apart basins affecting vast areas in Central and SE Asia.
A
B
Fig 1. (A) Sketch-map showing the
intra-continental deformation of central
and S-E Asia associated with the
Cenozoic collision of India and Eurasia.
Notice that indentation of India is
accommodated by E-ward transport of
thickened and topographically elevated
crust (B) material above the free
subduction margin of SE-Asia.
(Modified from L. Jolivet 2001).
While the semi-recent to actualistic examples have precise records of the tectonic
processes involved and their kinematic boundary conditions, ancient examples
represent more evolved systems, which allow more complete examination of the
effects of these processes in crustal sections from the exhumed lower crust to the
surface. The Caledonian collision between Laurentia and Baltica-Avalonia in the
Silurian to Early Devonian represents one of the best ancient examples. The platetectonic boundary condition of the Caledonian collision is less well constrained,
nevertheless, it has been shown that a near orthogonal collision between Baltica and
Laurentia occurred when Baltica´s velocity relative to Laurentia was 8 to 10 cm/yr
(Torsvik 1998). This is comparable to the velocity of India, which was close to 9
cm/yr, during the initial stages of the Himalayan collision. Other important
similarities between the two belts are the size of continental Baltica and India, the
double polarity of the thrust stracking, evidence for extreeme crustal thickening and
the scales of the frontal thrust-belts. The exposed Scandinavian segment of the
Caledonides is approximately 1750 km, whereas the Himalayan thrust front on India
is approximately 2200 km between the eastern and the western syntaxes (Fig. 2)
Fig. 2. The Scandinavian and East Greenland Caledonides in a tight Late-Silurian to Early
Devonian reconstruction projected onto present day Himalay-Tibetean Plateau topography.
The most important difference appears to be the duration of continental convergence,
which in the case of the Caledonides was approximately 30 m.yr. as opposed to the
ca 55 m.yr. in case of the Himalayas.
Studies of the Caledonides of the North-Atlantic region have revealed that the
structure of the orogen only can be understood in light of tectonic processes which
outlasted the initial Scandian collision by as much as ca. 100 million years. Moreover,
the structure in the exposed parts of the hinterland on both sides of the North-Atlantic
and consequently also in the stretched and thinned basement of the shelf areas, are
dominated by the late to post-orogenic structures rather than those produced during
the collision itself. These include large exposed areas characterized by late to postcollisional high-grade ductile fabrics (cf. Western Gneiss Complex of SW Norway
and the allochthonous basement/cover nappes in the Fjord Region of E-Greenland),
localized ductile shear fabrics along major detachment- and strike-slip zones and
high-level brittle faults which in most cases have been reactivated repeatedly (c.f. the
Møre Trøndelag Fault Complex). These observations demonstrate the fundamental
nature of the processes related to late and early post-orogenic tectonics.
Depending on the plate-tectonic boundary conditions such processes include: [1]
Indentation and regional crustal thickening, [2] Large-scale strike-slip and escape
tectonics; [3] Major vertical movements accompanied by exhumation of high- and
ultra-high-pressure rocks; [4] Formation of penetrative fabrics related to late- and
post-collisional extension and accompanied by formation of sedimentary basins.
BALTICA
LAURENTIA
E-AVALONIA
Fig. 3. A schematic maximum tight-fit reconstruction for the Scandian collison taking into
consideration the crustal thickening as documented by high- and ultra-high metamorphic
complexes in Norway and Greenland. Palaeozoic sinistral strike-slip as well as post-orogenic
extension of the shelf areas have been removed. This has particular significance to the areas
outboard of the exposed high- and ultra-high pressure rocks shown with shaded ellipses in
Greenland and Norway, where the shelf areas presently are particularly wide (see Fig. 4)
In both the Scandinavian and Greenland Caledonides it has been demonstrated that
major re-structuring of the collisional infra-structure related to points [2]; [3] and [4]
occurred in the interior of the orogen throughout the Devonian. The main effects
were, exhumation of HP and UHP rocks, crustal thinning on major detachments and
normal faults; basin formation and a large-scale orogen-parallel strike-slip
(transtension?) tectonics. The sinistral strike-slip with releasing and restraining bends
apparently affected the entire orogenic belt from the British sector to Svalbard (Fig.
3). The Mid to Late Devonian extension and crustal thinning in parts of Greenland
was dramatic and resulted in a high heat-flow, magmatism and a thermally weakened
crust (Hartz et al. 2000). Evidence for indentation and regional crustal thickening of a
scale similar to that observed north of the Himalayan orogen in central Asia is
currently not documented. It is likely that indentation of Baltica came to a halt already
in the Lower Devonian because the trailing, free margins of Baltica developed into
active margins soon after Iapetus had closed; hence there was no driving force for
continued convergence (Fig. 4). The “free” Herzynian and Uralian margins of BalticaAvalonia entered subduction zones with polarity away from Baltica-Avalonia. It is
suggested that the free margins were important elements that permitted and perhaps
even drove large scale relative motions across the mountain belt.
Fig. 4. A) Pre-breakup reconstruction
modified from Mosar et al. (2002),
showing the large and highly extended
shelf-areas (magnetic anomaly map)
adjacent to Caledonian high-grade
areas onshore. Areas affected by late
Caledonian high- and ultra-high
pressures are shown as shaded areas.
The red arrows show the principal
directions of extension in the Hanging
walls of extensional detachments in
Greenland and Scandinavia.
B) A 400 Ma (Emsian) reconstruction
from Torsvik & Cox (2001). Notice
that the ”free” Uralian and Herzynian
margins of Baltica are subduction
margins with polarity away from the
continent. There is apparently no
plate-tectonic driving force that could
sustain convergence between Baltica
and Laurentia. The Lower to Middle
Devonian is consequently a time
during which extension and major
strike-slip movements dominated the
mountain belt.
In conclusion: (1) Late- to post-orogenic processes produce fundamental crustal
structures. (2) Collapse-related structures rather than those produced by collision
dominate the hinterland of orogenic belts and provide the structural template for later
events. (3) The late-to post-orogenic collapse structures are particularly important as a
template for renewed extension and rifting that affected the N-Atlantic region. For
these reasons, it is critical to establish the Late Paleozoic structural framework on the
East Greenland and Mid-Norway conjugate margins, in order to place both kinematic
and precise time constraints on subsequent, basin-controlling tectonic activity.