Jökull - 01.01.2012, Síða 182

Í. Ö. Benediktsson
(2008) identified a positive relationship between the
size of the end moraine and the magnitude of defor-
mation. Section 3 exposes the largest parts of the 1890
end moraine and shows by far most strain whereas
section 1 occurs in a low ridge and reveals the least
strain. These two can therefore be regarded as end
members in a structural continuum of the 1890 end
moraine. Sections 0 and 2 connect the two end mem-
bers with intermediate strains and deformation styles
that to some extent resemble those observed in sec-
tions 1 and 3.
As concluded by Benediktsson et al. (2008), high
porewater pressure, generated by the loading of the
ice and drainage retardation due to low permeabil-
ity of the overall fine-grained sediments, decreased
the shear strength and led to the original failure of
the submarginal sediment and the formation of the
marginal sedimentary wedge. The actual end-moraine
ridge formed at the last day of the 1890 surge when
the glacier became coupled to the bed and ploughed
into the reverse slope of the marginal sediment wedge.
The coupling was most likely caused by a drop in
submarginal porewater pressure associated with pore-
water blow-out in front of the ice, and took place a
few metres to a few tens of metres upglacier from
the terminal position of the surge, as carefully esti-
mated from the horizontal shortening within the dif-
ferent sections. New analysis of the structural ele-
ments within the end-moraine ridge shows that the
structural evolution of the moraine ridge was more
complex than previously described by Benediktsson
et al. (2008). It is proposed that the moraine ridge de-
veloped through polyphase deformation in the sense
that the phases of deformation are different (duc-
tile, brittle) and temporally separated; ductile defor-
mation dominated and occurred during high porewa-
ter pressures and preceded brittle deformation, which
was induced when porewater pressures decreased (cf.
Phillips et al., 2011; Ferguson et al., 2011). The for-
mation of the moraine ridge is considered to have
started with a small-scale folding and subsequent
thrusting of strata which later formed the central part
of the moraine ridge (Figure 8A). Subsequently, open
anticline-syncline pairs formed an upper unit of lower
strain as overlying strata were pushed and sheared
over the central part. This interpretation is based on
asymmetric box folds and sheath folds just below the
anticline-syncline pair in section 1 and normal faults
below the anticline in section 2. Simultaneously, the
central part of the moraine deformed further, possibly
obliterating some of the original deformation struc-
tures and resulting in tectonic foliation and homog-
enization (Figure 8B). As the ice continued to push
and porewater pressures remained high, the amplitude
of the original anticlines and synclines increased and
they started to overturn. Thrusting occurred in the
distal and proximal extremities of the moraine ridge,
possibly because of a local and temporary drop in
porewater pressure, as suggested by sections 1 and 2a
(Figure 8C). Further pressure caused the central anti-
cline to refold to form polyclinal overturned folds and
the original synclines became deeper and narrower.
New folds developed and overturned in the proximal
part accompanied by thrusting. Upglacier verging an-
ticlines began to develop in the distal extremity of
the ridge due to an obstruction in front of the ridge,
most likely a patch of permafrost, which generated a
counter stress towards the advancing ice (Benedikts-
son et al., 2008, 2010; Thomas and Chiverell, 2011;
Figure 8D). At the very end of the surge, the system
locked up as porewater pressure dropped and effec-
tive stress increased following porewater blow-out in
front of the moraine ridge. This stiffened the entire
wedge sequence and induced brittle deformation as
seen from faults overprinting folds in sections 2 and
3, in particular (Figure 8E).
The ductile and brittle structures in the sections
suggest deformation during high and low porewa-
ter pressure, respectively. Ductile deformation domi-
nated the construction of the moraine ridge, indicat-
ing that porewater pressure in the submarginal and
proglacial sediments was generally high. However,
faults and shears that formed during the earlier stages
of the moraine formation (e.g. section 1 and 2) sug-
gest that porewater pressure fell slightly and tempo-
rary and locally locked the system to induce brittle
deformation until porewater pressure rose again and
ductile deformation continued. The greatest drop in
porewater pressure occurred at the very end of the
surge following a serious porewater blow out in front
180 JÖKULL No. 62, 2012