Jökull - 01.01.2012, Page 179

Structural evolution of the 1890 Brúarjökull end moraine
Section 2b, the distal part, 16–23 m.
This part of the section is dominated by an asym-
metric north-verging fold. This is indicated by anti-
clinal and synclinal structures observed on the surface
by the section foot (Figure 6). The hinge of the anti-
cline is visible in the section at around 16 m. At 16–17
m, near-vertical layers of LPT, sand, and tephra con-
stitute the distal limb of the anticline and the proximal
limb of the syncline in front, the hinge zone of which
is buried. At 20-23 m, the section is characterized by
gently dipping layers of LPT, sand, and tephra, indi-
cating the distal limb of the syncline. Repetition of
tephra layers at 22–23 m indicates an overturned fold,
the hinge of which can be inferred (Figure 6). The
east-west orientation and westward plunge of the fold
axes at 16–17 m indicates stress application from the
south.
Section 2 shows 3–4 separate phases of deforma-
tion. Initially, compressive deformation took place
producing small-scale folds which are visible in the
lower proximal part of the section. Continued pres-
sure from the ice forced the shearing of the upper
strata across the lower part to form an open anticline.
The shearing is implied by normal faults in the prox-
imal part. Simultaneously, a syncline formed in front
of this anticline and a second, overturned anticline in
the distal part. The final deformation phase included
thrust faulting within the proximal open anticline.
Line balancing of the tephra marker horizons
reveals different amounts of horizontal shortening
through the section and gives minimum numbers for
it. While layers in the upper part of the proximal core
(at ca. 2–11 m) have been shortened by about 9%, the
layers in the distal core have been shortened by 30%.
By ignoring the interbedded sand and silt at 11–16 m
and inferring the hinges of the syncline at 17–20 m
and the overturned fold at ca. 23 m, the minimum hor-
izontal shortening in the entire section is calculated as
20%. Calculations of the décollement depth require
data on the cross-section area. Due to wind erosion,
however, the section outline does not reflect an ideal
cross-section through the moraine ridge, and thus it
was not deemed relevant to calculate the section area
and the depth to the décollement at this site.
Section 3
Section 3 is situated in the easternmost part of the
central forefield in Kringilsárrani (Figure 1), along
an abandoned minor meltwater outlet channel of the
1890 surge. The end moraine is up to 15 m high and
30–50 m wide at this site, but is considerably higher
and wider further to the east and lower and narrower to
the west. The proximal slope is hummocky and steep
while the distal slope is smooth and gentle. In front
of the moraine at this site, the terrain is dominated by
circular rim ridges, indicating collapsed palsas, and
abruptly emerging channels that represent blow-out
of overpressurized water at the end of the 1890 surge
(Kjær et al., 2006; Benediktsson et al., 2008). Section
3 covers the core and the distal slope of the moraine
(Figure 7). The proximal part and the backslope are
not exposed.
Three sediment facies were identified in section 3:
interbedded sand and silt (F3), LPT (F4), and tephra
(F5) (Table 1). The section is characterized by pri-
mary multiple folding and secondary faulting. The
section is divided in three parts: (i) a section that sub-
parallels the moraine ridge (0–2 m), (ii) the core (0–9
m), and (iii) the distal slope (9–12.7 m).
Section sub-paralleling the moraine ridge, -0–2 m.
In this section, inclined folds are observed in
LPT beds and the white Öræfajökull AD 1362 tephra
marker. Repetition of the tephra marker probably in-
dicates the limbs of multiple folds, the hinge zones
of which can be identified in the central part of sec-
tion 3. The limbs of the folds dip towards the aban-
doned channel at the foot of the section and are fre-
quently sheared by both low- and high-angle faults,
two of which are normal faults that indicate syn-
tectonic slope failure during the moraine-ridge forma-
tion. A high-angle thrust fault in the upper part, dip-
ping 46◦ SE, correlates with a thrust fault in the upper
proximal part of the main section (Figure 7).
The central part, 0–9 m.
The central part of section 3 reveals the most in-
tense deformation with multiple folding and prevalent
faulting (Figure 7). The faulting is most intense be-
tween 0 and 4 m with both high-angle and low-angle
normal and thrust faults. This is exemplified by e.g.
JÖKULL No. 62, 2012 177