Jökull - 01.01.2012, Page 181

Structural evolution of the 1890 Brúarjökull end moraine
a few high-angle normal faults at 1–3 m that dip 80-
85◦ S, a high-angle backthrust at ∼7 m dipping 75◦
NE, and a number of low-angle normal faults, partic-
ularly in the lower part (below 3 m) (Figure 7). The
most prominent structures in the central part are large
north verging, overturned polyclinal folds at 0–2 m
with upglacier dipping axial planes. The white Öræfa-
jökull AD 1362 tephra marker, LPT beds, and tephra
layers outline the fold axis which plunges 8◦ W. The
abovementioned high-angle normal faults cross-cut
the folds as do thrust faults in the upper part that corre-
late with the thrusts described from the section at 0–2
m (Figure 7). At ca. 1 m, the white tephra outlines an-
other prominent fold but its limbs are not easily iden-
tified due to intense faulting. From 4 to 6 m, there
are upglacier inclined beds of LPT and tephra that are
repeated upwards. These beds are thought to repre-
sent multiple fold limbs while the fold hinges have ei-
ther been eroded away (above) or are buried (below).
In the upper part at 6–9 m, there are a number of in-
clined, north-verging folds with shallow axial plunge
towards west. The limbs and hinges of these folds
are commonly cross-cut by high-angle normal faults
and backthrusts, some of which are slightly folded
and thus indicating fault formation before folding was
completed.
The distal slope, 9–12.7 m.
The most dominant structures in this part of the
section are fairly steeply inclined, south verging folds
with axes plunging 4◦ towards east. This is typified,
in particular, at around 12 m where black tephra and
LPT beds outline the folds. The fold vergence to
the south suggests an obstacle in front of the moraine
that created a counter-stress from the north during the
moraine-ridge formation.
The deformation in section 3 probably initiated in
a similar manner as in sections 0, 1, and 2, i.e. with
small-scale folding of the lower strata and subsequent
open folding and shearing of the upper strata across
the lower part. It is suggested that sections 3 rep-
resents the most developed stage in the end moraine
formation with an additional phases of multiple fold-
ing and faulting before the deformation ceased. The
main faulting phase occurred towards the end of
the moraine formation. This is evident from fre-
quent cross-cutting by faults through folds. However,
slightly folded fault planes indicate that faulting com-
menced shortly before folding ceased.
Steeply dipping normal and thrust faults are com-
mon in section 3. They show little consistency in
terms of orientation although the principal vector
plots as dipping towards WNW. Although the orien-
tation of only five fold axes could be measured di-
rectly, they show remarkably strong east-west orien-
tation, which corresponds to stress induced from the
south or north. Thus, the fold axes seem to be a more
reliable structural indicator of the direction of stress
than faults (Figure 7). The general steepness of both
folds and faults in the section indicates that the folds
are rooted and that the glaciotectonic stress was ab-
sorbed within a relatively narrow zone bounded by the
glacier to the south and permafrost to the north.
Due to the multiple folding and intense faulting,
in particular, it is very difficult to trace marker hori-
zons through the section. Therefore, it is impossible
to apply line and area balancing to the entire section
3. However, and just to get a glimpse of the total hor-
izontal shortening at this site, line balancing was ap-
plied to the polyclinal folds at 0–2 m, through which
the tephra marker horizons could be followed. This
revealed a horizontal shortening of 76%, which, be-
cause of the common refolding in the section, must be
considered to be fairly representative for the moraine
ridge at this site.
POLYPHASE STRUCTURAL
EVOLUTION OF THE 1980 END
MORAINE
Together with a marginal sediment wedge, the 1890
end moraine forms a dual, marginal end product of the
1890 surge (Benediktsson et al., 2008). Much of the
deformation observed in the end-moraine ridge, par-
ticularly in the proximal part, can be regarded as part
of the wedge formation, and the basal décollement
must therefore be common for these two ice-marginal
components (Benediktsson et al., 2008). The four
sections in the end moraine reveal different styles and
magnitudes of deformation and by studying the mor-
phology of the 1890 end moraine, Benediktsson et al.
JÖKULL No. 62, 2012 179