Caledonian sole thrust of central East Greenland:
A crustal-scale Devonian extensional detachment?
E. Hartz
A. Andresen
Department of Geology, University of Oslo, Box. 1047, 0316 Oslo, Norway
ABSTRACT
Structural observations of the basement-cover contact in the Central Fjord region of
the East Greenland Caledonides suggest Silurian to Devonian crustal thinning with topto-the-east displacement of the cover sequence. The east-dipping, low-angle shear zone
separating the Late Proterozoic (Eleonore Bay Group) to Lower Ordovician cover sequence
from the underlying high-grade gneisses has previously been interpreted as the Caledonian
sole thrust in the region, displacing the cover sequence toward the Caledonian foreland in
the west. However, emplacement of younger, low greenschist facies rocks on top of Archean
to Middle Proterozoic gneissic rocks across the shear zone instead favors an extensional
origin. This interpretation is supported by ductile and brittle shear-sense indicators in the
footwall formed under progressively lower temperature conditions, consistently showing
top-to-the-east displacement. The cover sequence in the hanging wall is cut by numerous
extensional faults. The inferred earliest fault set developed contemporaneously with the
deposition of the unconformably overlying Devonian deposits, whereas the youngest set
crosscut the entire Devonian stratigraphy. Neither of these late brittle extensional faults
are found in the basement, suggesting that they merge with the extensional shear zone at
depth. Collectively, these observations suggest that the Devonian basin formed as a supradetachment basin during collapse of the Caledonian orogen.
INTRODUCTION
Two distinct structural and metamorphic
regimes characterize the Caledonian fold
belt in central East Greenland (Fig. 1): (1) a
high-grade metamorphic, infracrustal complex with a complex structural history mantled by (2) a less deformed and variably metamorphosed sedimentary sequence ranging
in age from Upper Riphean to Ordovician
(Henriksen, 1985). Fundamental problems
in this part of the Caledonides have been the
structural and metamorphic relations between the infracrustal and supracrustal
rocks (Haller, 1971), and the timing and nature of the orogenesis. Haller (1971) considered the infracrustal and supracrustal
rocks to have been deformed and metamorphosed together during the Caledonian orogeny but at different crustal levels. More recent work (Henriksen and Higgins, 1976;
Rex et al., 1977), including radiometric dating of the infracrustal rocks giving Early and
Middle Proterozoic Rb-Sr and U-Pb ages,
does not confirm this model. In light of these
pre-Caledonian ages, the infracrustal rocks
have been interpreted to represent a preCaledonian basement complex with limited
Caledonian tectonothermal reworking, tectonically overlain by folded Late Proterozoic and Cambrian-Ordovician cover sedimentary strata (Henriksen and Higgins, 1976).
The basement gneisses are separated
from the cover rocks by a major east-dipping
Geology; July 1995; v. 23; no. 7; p. 637– 640; 3 figures.
Figure 1. Simplified geologic map of Central Fjord
region of East Greenland
showing structural relations between high-grade
Proterozoic gneisses in
west and overlying Caledonian supracrustal rocks
in east. Patterns: 1— highgrade metamorphic
rocks; 2—Late Proterozoic (Eleonore Bay Group) to
Ordovician sedimentary
rocks; 3—Devonian deposits; 4 —Caledonian
granites; 5—Post-Devonian deposits. SL—Strindberg Land; ORL—Ole
Rømer Land; KF—Kempes fjord; MF—Moskusokse fjord; KFJF—Kejser
Franz Joseph fjord; FF—
Forsblad fjord; AF—
Alpefjord; WFZ—Western
fault zone. Boxes mark areas where detailed mapping has been conducted.
637
A
B
C
D
shear zone, interpreted as a Caledonian
thrust in most recent publications (Larsen
and Bengård, 1991). Our mapping along the
basement-cover boundary in the Central
Fjord region (Fig. 1) suggests, however, that
the shear zone is a late Caledonian extensional detachment zone, with top-to-theeast displacement of the supracrustals of the
hanging wall. This has important implications for our interpretation of the central
East Greenland Caledonides, in particular
with respect to the origin of the Devonian
molasse basin.
GEOLOGIC SETTING OF THE
CENTRAL FJORD ZONE,
EAST GREENLAND
Caledonian rocks of the Central Fjord
region (Fig. 1) can be divided into three
major tectonostratigraphic units (Henriksen, 1985): (1) an infracrustal migmatitegneiss and schist complex of Archean to
Middle Proterozoic age, (2) a variably deformed, unmetamorphosed to weakly metamorphosed Late Proterozoic to Lower
Ordovician sedimentary sequence, unconformably overlain by (3) a thick sequence
(;8 km) of continental Devonian sedimentary rocks with minor volumes of felsic and basic volcanic rocks. The lower
Eleonore Bay Group is composed of up to
8 km of shallow-marine and fluviatile deposits (Caby, 1976); the most complete
succession is found in the Alpefjord region
(Fig. 1). The upper Eleonore Bay Group is
638
composed of 5 km of shallow-marine sedimentary sequences (Henriksen and Higgins, 1976). Unconformably overlying the
Eleonore Bay Group are 500 –700 m of
Vendian tillites (Tillite Group) overlain
by up to 3 km of Cambrian and Lower
Ordovician carbonates. A major shear
zone separates the Eleonore Bay Group
from the underlying Archean and Proterozoic gneisses and migmatites, except in
areas where the contact is obscured by synand postorogenic intrusive rocks (Alpefjord and Forsblad Fjord, Fig. 1). The intrusive rocks range in age from 560 to 380
Ma (Rex and Higgins, 1985). Gentle, upright folds with an overall north-south
trend characterize the contractional deformation in the cover sequence. Metamorphic grade increases stratigraphically
downward with growth of chloritoid porphyroblasts subparallel to the axial plane
cleavage in the lower part of the Eleonore
Bay Group. Superimposed on the contractional structures are several sets of extensional faults.
An unconformity, postdating contractional deformation in the cover sedimentary
rocks, generally separates the Devonian
deposits from the underlying units. Locally, however, the contact is disturbed by
younger, brittle faults. Unconformities
within the Devonian succession have traditionally been linked to phases of Caledonian deformation (Bütler, 1959). Haller
(1971) suggested that subsidence along
Figure 2. Diagrammatic
block diagram illustrating
structural relations between infracrustal rocks
in footwall, collapsed
hanging wall, and Devon i a n su prad etachm ent
basin. Note two fault sets
and their relations to Devonian basin fill. Numbers
refer to approximate locations of structures presented in Figure 3. Patterns as in Figure 1.
northeast-trending faults and filling of the
Devonian basin were repeatedly interrupted by Caledonian contractional deformation. Friend et al. (1983) argued that
movement on four north-trending fracture
zones controlled Devonian sedimentation,
whereas Larsen and Bengård (1991) argued that Devonian basin development
and filling were controlled by displacement along two left-lateral wrench faults,
located approximately along the presentday basin margins (Larsen and Bengård,
1991). Both McClay et al. (1986) and
Larsen and Bengård (1991) considered the
development of the Devonian basin to be
associated with the collapse of the Caledonides in the North Atlantic region.
However, neither of these studies presented structural or kinematic evidence
for a collapse origin.
Haller (1971) related folds and thrusts in
the Devonian basin to late Caledonian
spasms, whereas Larsen and Bengård (1991)
linked them to large-scale wrench faulting.
The deposition of the continental coarse
clastic material continued into the Carboniferous, where ;5 km of sediments were deposited upon the Devonian rocks.
LATE OROGENIC COLLAPSE
STRUCTURES
Several lines of evidence, including our
new field observations, reinterpretation of
previously published maps (Haller, 1970;
Bengård, 1991), cross sections (Bütler, 1957,
GEOLOGY, July 1995
Figure 3. A: Extensional detachment in Kejser
Franz Joseph fjord, separating Eleonore Bay
Group in hanging wall from Proterozoic
gneisses in footwall, viewed toward northnortheast. Detachment is located just below
white marble. Eleonore Bay Group to east is
cut by east-dipping extensional faults, which
merge with detachment. Height of cliff face is
800 m. B: Shear bands in muscovite schist just
below detachment, showing top to eastsoutheast (left) displacement. Length of bar 5
1 mm. C: Brittle extensional faults overprinted
mylonitic foliation in lower plate in Kajser
Franz Joseph fjord. Length of bar 5 1 mm. D:
Typical extensional faults in Eleonore Bay
Group of Forsblad fjord, antithetic to eastdipping detachment zone. View toward northeast. Height of cliff face is 1200 m.
1959), and sedimentological data (Olsen,
1993), suggest that the Devonian basin was
formed above a major east-dipping, extensional detachment zone (Figs. 2, 3A). This
detachment zone corresponds to the shear
zone between the infracrustal basement
complex and the overlying Eleonore Bay
Group. In areas where we have examined
this boundary (Fig. 1), it always juxtaposes
younger rocks (Eleonore Bay Group) of the
hanging wall against older basement
gneisses of the footwall. No depositional
contact between basement and cover has
been observed, nor have we observed repetition of strata caused by thrusting. Instead,
the lower part of the Eleonore Bay Group is
typically missing in the hanging wall. These
relations are consistent with an extensional
origin of the shear zone and with top-to-theeast displacement of its hanging wall. An extensional origin for the shear zone is substantiated by a variety of ductile shear-sense
indicators (lineations, folds, mica fish
[Fig. 3B]), formed under amphibolite conditions in the upper part of the basement
complex, all documenting a top-to-the-east
or southeast displacement of the hanging
wall (Figs. 1, 2). Superimposed semibrittle
to brittle shear-sense indicators (extensional
shear bands, faults) paralleling the earlier
ductile structures suggest that the footwall
of the detachment developed under progressively more brittle conditions (Fig. 3C),
GEOLOGY, July 1995
a feature characteristic of extensional shear
zones associated with metamorphic core
complexes (Coney, 1987).
Abundant, major extensional faults
(Fig. 3D) overprint the contractional structures in the hanging wall. In several areas,
three sets of oblique extensional faults are
recognized. The oldest predate the Devonian deposition. The intermediate set can be
traced structurally and stratigraphically upward into the lowermost Devonian basin fill
but does not offset the youngest Devonian
deposits. None of the extensional faults we
have mapped offset the detachment zone
(Fig. 2), a relation supported by published
maps from other parts of the Central Fjord
region (Bengård, 1991; Haller, 1970). Observations from Kejser Franz Joseph fjord
demonstrate that the normal faults in this
region are rooted and merge with the detachment downward (Fig. 3A).
No distinct detachment zone was found in
the Forsblad fjord and Alpefjord region
(Fig. 1). Instead, a gradual downward increase in metamorphic grade is observed
within the lower Eleonore Bay Group. This
increase appears to be associated with emplacement of granitic veins and stocks
(Fig. 2). West-dipping shear zones with a
variably well developed S-C fabric are
found locally in the granites. The displacement is consistently top to the west and
may represent the downward continuation
of the west-dipping brittle extensional
faults seen higher in the stratigraphy, antithetic to the detachment.
The steeply east dipping Western fault
zone generally marks the western margin of
the Devonian basin (Figs. 1, 2) (Larsen and
Bengård, 1991), separating it from late Precambrian to Ordovician sedimentary rocks.
At Strindberg Land the fault zone is a 2-kmwide zone of subparallel east-southeast-dipping major faults (Fig 2). These east-southeast-dipping faults are just as brittle and
intense in the Devonian rocks as in their
substratum. The total stratigraphic separation across the fault zone is 3– 4 km. Numerous east-striking faults are equally common on both sides of the Western fault
zone. At Ole Rømer Land (Fig. 1), where
the deeper parts of the Devonian basin are
exposed, fault zones similar to the Western
fault zone cut the Devonian basin as late
faults, showing several kilometres of normal displacement. Next to the faults, bedding in the Devonian basin fill is dragged
toward parallelism with the faults. Some
of the major north-trending faults making
up the Western fault zone have both dipslip and subhorizontal striations on the
fault surfaces.
DISCUSSION
The observations presented above support a tectonic model where the Devonian
basin developed in the collapsing hanging
wall of an east-dipping extensional shear
zone. Juxtaposition of upper-crustal, lowgrade Eleonore Bay Group rocks in the
hanging wall against gneisses and migmatites of the footwall with a superimposed
Caledonian amphibolite facies metamorphism suggest considerable crustal thinning.
The structural and metamorphic relations
between upper- and lower-crustal rocks described here show a clear correlation to relations displayed in metamorphic core complexes (e.g., Coney, 1987) and comparable
to the situation found in the Caledonian hinterland in western Norway (Andersen and
Jamtveit, 1990). An extensional rather than
contractional origin for the shear zone is
further supported by several reports arguing
for a late orogenic, eastward translation of
the Eleonore Bay Group (Talbot, 1979;
Caby, 1976; Henriksen, 1985).
Additional support for a Devonian detachment zone at depth is seen in seismicreflection data from Jameson Land, east of
the study area (Larsen and Marcussen,
1992). In Jameson Land, large-scale rotated
fault blocks, presumably containing Devonian strata, are found below the Carboniferous-Permian rift basin. If correctly interpreted, these fault blocks suggest that the
Devonian basin originally must have extended far east of its present outcrop area
and that it was not constricted by two
wrench faults in the Central Fjord region, as
previously suggested (Larsen and Bengård,
1991).
We consider the Western fault zone to be
a major normal fault within the hanging wall
of the extensional detachment. At a later
stage, the Western fault zone was possibly
reactivated as a strike-slip fault with minor
sinistral displacement, as indicated by subhorizontal slicken striations (Fig. 2). This interpretation is different from the model suggested by Larsen and Bengård (1991), who
interpreted the Western fault zone as a
major wrench fault controlling the basin
sedimentation.
The synorogenic igneous bodies along
parts of the detachment are inferred to be
synextensional, as the extensional faults
both cut and are cut by the intrusions. This
scenario is comparable to the situation encountered in many of the metamorphic core
complexes world wide. Granites in NorthEast Greenland, dated to Early Devonian
(Hansen et al., 1994), are also interpreted as
synextensional.
Soper and Higgins (1993) argued that the
mylonites separating infracrustal and supra639
crustal (Smallefjord sequence) rocks in the
Ardencaple fjord region (lat 758309N) represent an extensional shear zone generated
during opening of the Iapetus ocean in the
late Precambrian (Vendian) and later reactivated as a thrust in the Silurian–Early Devonian. Soper and Higgins proposed that
the extensional faults in the Eleonore Bay
Group in the Central Fjord region are related to this extensional event and that this
event predates Caledonian contractional
deformation. This interpretation is in direct
conflict with our own observations as well as
observations made by earlier workers— e.g.,
Haller (1971)—which demonstrate that the
extensional faults clearly postdate folding of
the Eleonore Bay Group in the Central
Fjord region. Furthermore, we observe no
difference in offset of pre-Vendian and Vendian sedimentary sequences, suggesting that
deposition of these sediments was not controlled by extensional faulting, as proposed
by Soper and Higgins (1993). Their model
has also been challenged by Strachan (1994)
who argued, on the basis of U-Pb dating of
zircons from synextensional granites (Hansen et al., 1994), that the extensional shear
zones in the Ardencaple fjord region are
Late Silurian to Early Devonian in age.
However, Strachan (1994) agreed with
Soper and Higgins (1993) that there was a
late phase of folding and thrusting that reactivated some of the marginal shear zones
and accounts for contradictory kinematic indicators in some places.
It is difficult to see how the 40Ar/39Ar ages
on amphibole and muscovite from the Ardencaple Fjord and Dronning Louise Land
regions, ranging from 438 to 370 Ma and
interpreted as dating the contractional deformation (Dallmeyer et al., 1994), fit into
this picture. On the basis of the recognition
of major extensional shear zones, structures
not reported by Dallmeyer et al. (1994), the
most straightforward interpretation would
be to relate at least the Late Silurian to Early-Middle Devonian cooling ages to uplift
and erosion controlled by late orogenic collapse and not to contractional deformation.
Such an interpretation would be in agreement with comparable cooling ages several
places elsewhere in the Caledonides, particularly the Scandinavian Caledonides (Berry
et al., 1993). This interpretation is also consistent with our observations structurally below and within the Devonian basin, indicating considerable crustal extension prior
to and contemporaneous with Devonian
sedimentation.
the high-grade Archean and Early Proterozoic terrane represents a metamorphic
core complex. The infracrustal-supracrustal
boundary separating the high-grade gneisses
and migmatites from the Eleonore Bay Group
is not a Caledonian thrust but an extensional
detachment of regional significance. The Devonian basin represents a supradetachment
basin within the collapsing Caledonian orogen. Thick accumulations of Carboniferous
and Early Permian continental deposits indicate that continental thinning continued far
into the late Paleozoic.
ACKNOWLEDGMENTS
Fieldwork was supported by grants from
VISTA, the Nansen foundation, and the Norwegian Research Council (grant 440.93/029). We
thank John Pedersen for initiating our interest in
the Caledonian geology of East Greenland, and
P. T. Osmundsen and David Rowley for comments on an earlier version of the paper.
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Manuscript accepted April 13, 1995
CONCLUSIONS
A preliminary conclusion of our ongoing
research in central East Greenland is that
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GEOLOGY, July 1995