Devonian, orogen-parallel opposed extension in the Central Norwegian Caledonides:
Comment and Reply
COMMENT
A.H.N. Rice
Institut für Geologie, Geozentrum, University of Vienna,
Althanstrasse 14, 1090 Vienna, Austria
Braathen et al. (2000) described Early to Middle Devonian extensional faults around the central Norway basement window, one of
many tectonic windows exposing basement in the Scandinavian Caledonides (here these basement rocks are called tectonic window basement). Extension was orogen parallel, with top-to-west-southwest
(Høybakken) and top-to-east-northeast (Kollstraumen) detachments on
opposite sides of the central Norway basement window. From the
broad parallelism of the faults and the central Norway basement window margin, Braathen et al. (2000) inferred basement doming was
caused by bidirectional extensional nappe unloading (Fig. 1A). However, although the extensional structures are accepted, their role in basement doming is not. Two models for this have been proposed
previously.
Balanced cross sections through the Caledonian external imbricate
zone in north and central Scandinavia show that the Komagfjord and
Børgefjellet tectonic window basements were displaced 175 and at
least 45 km, respectively (Rice, 1998; Gayer and Greiling, 1989). Emplacement of these units by footwall shortcut thrusting through Iapetus
rifting faults formed ;5-km-thick basement pips, which domed the
overlying nappes.
Similarly, balanced sections (Morley, 1986) combined with lithostratigraphy (Gee, 1980) indicate a displacement at least 200 km for
the Western Gneiss Region tectonic window basement in south Norway
(Rice, 1999a). For this region, an ‘intrusion’ exhumation, of the type
described in Chemenda (1996), of the ultra-high pressure tectonic window basement under the nappes has been suggested (Rice, 1999a),
again causing nappe doming. Mykkeltveit et al. (1980) reported a sedimentary low-velocity layer ;14 km below the Western Gneiss
Region, roughly limiting its thickness.
The Rombak and Bångonåive tectonic window basements are also
allochthonous (Andresen and Rykkelid, 1989; Greiling et al., 1993), as
are the Mullfjället and Skadøra Antiform tectonic window basements
(Palm et al., 1991), but their thrust displacements are unconstrained.
The above is not meant to indicate that extension did not occur
in this region. Rather, it shows that where constrained, tectonic window
basement has been shown to be allochthonous, with thrust-related doming. As the tectonic window basement in the central Norway basement
window was originally part of the Western Gneiss Region, doming of
the nappes was most likely related to tectonic window basement
exhumation.
Allochthonous tectonic window basement in the central Norway
basement window permits a simple, unidirectional extension model, as
the tectonic window basement was not pinned to the Baltic Shield. In
the model proposed here, extension occurred subparallel to the thrusts
above and below the tectonic window basement, the latter reactivating
the earlier (exhumation) thrust, and is comparable to any detachment
fault system. A regional top-to-west-southwest extension direction is
assumed here, similar to the extension direction in southwest Norway.
However, the east-northeast–dipping nappes on the north side of the
central Norway basement window provided an easy-slip horizon, al-
374
lowing the tectonic window basement to move to the west-southwest
by pulling out from under the nappes, on the antithetic Kollstraumen
detachment (Fig. 1B). Thus, relative to the Baltic Shield, absolute
movement on this shear zone was not top-to-east-northeast, but bottomto-west-southwest. Movement on the Høybakken detachment and below the tectonic window basement (and thus the Kollstraumen detachment) may have been contemporary or consecutive.
A similar mechanism can explain the anomaly of sinistral movement on the Verran Fault linking with the proposed extensional detachment southwest of the Grong-Olden Culmination (Fig. 1 in Braathen et al., 2000). The nappes in the Trondheim area are underlain by
an extensional detachment that must crop out not only on the westsouthwest side of the Grong-Olden Culmination (Braathen et al., 2000),
but also antithetically east-northeast of the Western Gneiss region, as
the latter was pulled to the southwest from under the nappes. This
interpretation is consistent with the late downward facing, east-vergent
regional-scale sheath folds in this region (Vollmer, 1988). Different
amounts, or rates of displacement on the differing extensional blocks
were accommodated on the Verran and Hitra-Snåsa faults (extensional
lateral ramps).
In this model, the sense of ‘top- to-’ shear and of absolute motion
relative to the Baltic Shield are not necessarily the same. The latter is
critical for tectonic modeling, but difficult to establish. A similar model
was proposed for extensional structures east of the Børgefjellet basement window (Rice, 1999b).
Figure 1. Schematic, extension parallel sections
through the central Norway basement window. KoD,
HøD 5 Kollstraumen, Høybakken detachments. A: Autochthonous tectonic window basement (TWB) model,
with bidirectional nappe extension. B: Allochthonous
tectonic window basement model, with unidirectional
extension of nappes and the tectonic window basement. Note the Kollstraumen detachment is top-toeast-northeast, but the hanging wall is essentially fixed
relative to the Baltic Shield.
GEOLOGY, April 2001
REFERENCES CITED
Andresen, A., and Rykkelid, E., 1989, Basement shortening across the Caledonides in the Torneträsk-Ofoten area: Geologiska Föreningens i Stockholm Förhandlingar, v. 111, p. 381–383.
Braathen, A., Nordgulen, Ø., Osmundsen, P.-T., Andersen, T.B., Solli, A., and
Roberts, D., 2000, Devonian, orogen-parallel, opposed extension in the
Central Norwegian Caledonides: Geology, v. 28, p. 615–618.
Chemenda, A.I., Mattauer, M., and Bokun, A.N., 1996, Continental subduction
and a mechanism for exhumation of high-pressure metamorphic rocks:
New modelling and field data from Oman: Earth and Planetary Science
Letters, v. 143, p. 173–182.
Gayer, R.A., and Greiling, R.O., 1989, Caledonian nappe geometry in northcentral Sweden and basin evolution on the Baltoscandian margin: Geological Magazine, v. 126, p. 499–513.
Gee, D.G., 1980, Basement-cover relationships in the central Scandinavian Caledonides: Geologiska Föreningens i Stockholm Förhandlingar, v. 102,
p. 455–474.
Greiling, R.O., Gayer, R.A., and Stephens, M.B., 1993, A basement culmination
in the Scandinavian Caledonides formed by antiformal stacking (Bångonåive, northern Sweden): Geological Magazine, v. 130, p. 471–482.
Morley, C.K., 1986, The Caledonian thrust front and palinspastic restorations
in the southern Norwegian Caledonides: Journal of Structural Geology,
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Nature, v. 288, p. 473–475.
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seismic image of Caledonian structure in central Sweden: Sveriges Geologiska Undersökning, v. Ca75, p. 1–36.
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Kalak Nappe Complex (Middle Allochthon) in the RepparfjordFægfjord area, Finnmark, northern Norway: Norsk Geologisk Tidsskrift, v. 78, p. 277–289.
Rice, A.H.N., 1999a, An allochthonous Western Gneiss Region model for Caledonian normal-sense faulting and eclogite exhumation in the southwest
Norwegian Caledonides: Terra Nova, Abstract Supplement 1, v. 11, p. 41.
Rice, A.H.N., 1999b, The orogenic wedge in the central Scandinavian Caledonides: Scandian structural evolution and possible influence of the foreland basin—A discussion and alternative model: Geologiska Föreningens
i Stockholm Förhandlingar, v. 121, p. 73–74.
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shear in the Western Gneiss Region, Central Norwegian Caledonides:
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REPLY
Alvar Braathen
Øystein Nordgulen
Per-Terje Osmundsen
Geological Survey of Norway, N-7491 Trondheim, Norway
Torgeir B. Andersen
Institute of Geology, University of Oslo, N-0316 Oslo, Norway
Arne Solli
David Roberts
Geological Survey of Norway, N-7491 Trondheim, Norway
We thank Rice for his comment on our paper (Braathen et al.,
2000) and welcome the opportunity to respond to points raised. First,
however, we would like to disagree with the suggestion implicit in
Rice’s Figure 1A that we subscribe to the model of autochthonous
tectonic window basement. This is not the case. Several published papers, map-sheets, and interpreted seismic profiles from the mid-Norway
region show the central Norway basement window and other tectonic
window basements to be quite clearly allochthonous (e.g., Möller,
1988; Roberts, 1989; Hurich et al., 1989; Solli et al., 1997). Accepting
an allochthonous interpretation for tectonic window basements, however, does not mean that we concur with the model suggested by Rice.
In our paper, we specifically addressed deformation that occurred
along the margins of the central Norway basement window coeval with
GEOLOGY, April 2001
and following the intrusion of Early to Middle(?) Devonian granite
pegmatite dikes. The Høybakken detachment zone juxtaposes the central Norway basement window against Early to Middle(?) Devonian
coarse clastics preserved in overlying extensional basins (Séranne,
1992). The trace of the Kollstraumen detachment truncates several
nappe boundaries and largely juxtaposes rocks of the Uppermost Allochthon with the central Norway basement window (Braathen et al.,
2000). Thus, contrary to Rice’s model (his Fig. 1B), extensional detachments cut structurally upwards through the Caledonian nappe-stack
and most likely participated in the stripping of several kilometers of
structural section from above the central Norway basement window.
Therefore, in our view, a large component of the doming of the central
Norway basement window as viewed parallel to the maximum elongation direction (i.e., northeast-southwest) is probably due to nappe
unloading from the Early Devonian onwards.
In principle, we do not exclude models that explain exhumation
of deep crustal rocks by combinations of southeast thrusting and northwest-directed extension; such a model has recently been proposed to
explain exhumation of ultrahigh- and high-pressure rocks in the Western Gneiss Region southwest of the study area (Terry et al., 2000; Wain
et al., 2000). However, evidence for southeast-directed transport is
strongly overprinted by structures related to sinistral shearing, northeast-southwest extension, and lineation-parallel folding (Terry et al.,
2000; Wain et al., 2000). It is the last three types of deformation, and
associated retrogression metamorphic fabrics, that characterize the
shear zones flanking the central Norway basement window. Thus, in
reporting on our studies of these shear zones, we have limited ourselves
to models that are supported by the field observations.
Gneissic basement culminations are common features in the Scandinavian Caledonides (Roberts and Gee, 1985). Some domal structures,
particularly those in eastern districts and in parts of north Norway,
appear to preserve thrust-related geometries on both flanks, whereas
others, such as the Mullfjället and Skardøra Antiforms and the Rombak
and Nasafjället windows, are flanked by west-dipping extensional shear
zones and faults (Sjöström et al., 1991; Andersen, 1998). These faults
and shear zones are just as easily explained by postdoming extension
as by a model like the type described by Chemenda et al. (1996),
because they cut earlier, top-to-the-east shear zones and appear to be
rooted in the basal Caledonian thrust, reactivating it as a top-to-thewest extensional detachment (see review in Andersen, 1998). In westernmost central Norway, high-grade extensional fabrics have almost
completely erased the original, thrust-related, top-to-the-southeast kinematic indicators in the mylonites flanking the tectonic window basements (e.g., Roan window, central Norway basement window). Thus,
we maintain that culminations may either have different origins or are
formed by a combination of mechanisms, depending on where the
doming basement was located within the developing orogen, and also
on the actual time of formation in the protracted strain history. As
major Devonian detachments and normal faults in western Norway
have been shown to have been rejuvenated in the Late Paleozoic and
Mesozoic (Torsvik et al., 1992; Eide et al., 1997), the doming of basement tracts like the Western Gneiss Region may have developed over
a considerable period of time, irrespective of whether an allochthonous
or an autochthonous interpretation is favored.
We do not perceive the linking of ductile sinistral movement along
the Møre-Trøndelag Fault Complex with the top-to-the-west-southwest
extensional shear on the Høybakken detachment zone as an anomaly.
This pattern of Devonian sinistral shear is recorded widely in areas to
the southwest of our study area (Séranne, 1992; Robinson, 1995). Farther northeast, bulk lateral, sinistral offset along the multiply reactivated Møre-Trøndelag Fault Complex rapidly diminishes across the
Grong-Olden Culmination. There, the fault also displaces the top-to-
375
the-east-northeast shear fabrics on the eastward prolongation of the
Kollstraumen detachment and disperses into a horsetail splay (Roberts,
1998).
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GEOLOGY, April 2001