Jökull - 01.12.1987, Page 77

affecting the Ásbakkar diamicton, is exposed (Fig. 17),
showing the lower uninverted limb, a streaked out
trough hinge zone and part of the inverted limb. The
exposure trends N-S, and as the fold axis trends 105°-
285° with an axial plane dipping about 18° towards 15°,
the exposure cuts the fold sub-perpendicularly to the
fold axis. Therefore, folded surfaces sub-parallell to the
axial plane are exposed, and examples of “slaty cleav-
age-like” axial plane foliations can be seen above the
streaked out trough hinge zone. The fold geometry in-
dicates a deforming push from approximately NNE.
Faults and joints
The Ásbakkar diamicton, south of about 3700 m, is
transected by a number of major normal and low angled
reverse (thrust) faults (Fig. 2). The thrust fault planes
are usually slightly concave upwards. Measurements of
10 fault planes revealed apparent dips between 28° and
42° towards NW-N. At 4975 m the Asbakkar diamicton
is transected by a thrust fault, running slightly concavely
upwards from the base of the sequence (Fig. 18). Imme-
diately below the thrust plane there is a body of stratified
sand, which has been subject to fault drag. The drag has
overturned the sand with a minimum displacement of
about 5 m along the thrust plane. The displacement
strike of the sand body measured 80°-260°, which is
perpendicular to the direction of the section and in-
dicates a displacement approximately normal to the out-
cropping trace of the fault, towards SSE-S. The thrust
faults belong to the same dislocation pattern as the
large-scale folds, showing a direction of maximal stress
from north.
The Ásbakkar diamicton and the glaciomarine facies
of the Látrar beds are at places heavily jointed. The
most apparent joints are sub-vertical to vertical joints,
usually spaced at 10-25 cm intervals. Sometimes the
joints can be traced over entire unit thicknesses, giving
the cliff face a columnar-like appearance. In connection
with folds and faults, straight to undulating, sub-parallel
fractures occur (Fig. 6A). They appear to strike parallel
to the associated sheared structure, and dip in opposite
directions to shear planes and axial planes. Similar joints
in sub-till sediments have been described as tension
fractures by Hicock and Dreimanis (1985).
Glaciodynamic deformations: a discussion
Although the study of glaciotectonic structures has a
long tradition (for short reviews, see Berthelsen 1978,
Aber 1982, Hicock and Dreimanis 1985), many funda-
mental problems of their development still remain to be
satisfactorily explained. There are two schools of
thought as to whether large scale dislocations take place
mainly in front of advancing glaciers (e.g. Berthelsen
1979), or beneath advancing glaciers via compressive
flow in the basal zone (e.g. Moran 1971). Another funda-
mental problem with regard to the conditions at the base
of the ice is whether glaciotectonic deformations devel-
op in frozen or unfrozen sediments, and consequently
the role of basal meltwater, pore-water pressure and
permeability of the substratum. Two basic concepts
have been presented regarding the connection between
the basal/frontal situation and the development of gla-
ciotectonic structures: A “permafrost concept” which
assumes that the effect of differential glacier load and
shearing stress is transmitted from the moving glacier to
its frozen pro- and substratum, causing the formation of
glaciotectonic structures (e.g. Moran et al. 1980, Aber
1982). The other concept calls upon the combined effect
of differential ice loading and hydrodynamic mecha-
nisms to account for the development of glaciotectonic
structures, and points out the potential importance of
Fig. 16. Deformation structures. (A) A thrust fault de-
veloped from an angular fold in the Ásbakkar diamicton
below the Melar diamicton at 1525 m. The inverted limb
of the fold has been replaced by a slide plane. Drawn
from a photograph; (B) A schematic field sketch of an
open, asymmetrical anticlinal structure, developed in
the Látrar glaciomarine facies beneath the Melar dia-
micton at 2400 m.
16. mynd. Dœmi um sköruð og kýtt jarðlög.
75