Jökull - 01.12.1989, Blaðsíða 11

Fig. 5. b) Seismic line 2, time section. ’a’ and ’b’ as
in Fig. 5a and ’c’, ’d’ and ’e’ are deeper reflections.
Mynd5. b) Endurkastsnið, mœlilína 2. ’a’ og ’b’
þau sömu og á mynd 5a og ’c’-’e’ eru endurköst frá
lagmótum undir vatnsbotninum.
Fig. 5. c) Seismic line 3, time section. ’a’ and ’b’ as
in Fig. 5a and ’c’, ’d’, and ’e’ are deeper reflections.
Mynd5. c) Endurkastsnið, mœlilína 3. ’a’-’e’ eins
og á mynd 5b.
MORPHOLOGY OF CALDERA FLOOR
The strength of the lake bottom refiection varies
along the seismic lines. Strong signals were
obtained in the northem half of line 2, the area were
the lake is present on line 3 and some central parts
of line 1. Steep or vertical steps are present at
4.7 km in line 2 and 2.6 km in line 3. A step is
present at 2.9 km in line 1 but is somewhat exag-
gerated on the time section, as the ice shelf is thick-
ening above the step. The fourth step can be seen at
4.2 km in line 1. The throw of the steps in lines 2
and 3 is similar, 25-30 ms, which corresponds to
about 20 m. At the westem step on line 1 the bottom
reflection splits in two and the continuation of the
lower reflection can be detected for a further 300 m
to the west. A possible explanation is that the step
cuts the line at a low angle.
The data suggest that the steps seen in the lake
bottom mark the divide between two seismic pro-
vinces. The areas on the downthrown side of the
steps give in general a stronger, more coherent sig-
nal than do the highstanding areas. This can be seen
clearly on the northem part of line 2, where the sig-
nal is very strong, and where deeper reflections
appear to be present. On line 1 the bottom reflection
is weak in the westemmost part of the caldera and in
the area where lines 1 and 2 cross. In these areas
steep steps with a bearing similar to line 1 are
known to be present. The focusing effects at the
ice-water interface would help to suppress
reflections and diflractions from the edges and the
northwardly dipping lakefloor north of the steps,
explaining the weak signals in these areas.
Magnetic profiles measured along the seismic
lines in 1987 and 1988 are shown above the cross
sections in Figs. 6a-c. As can be seen on Fig. 6b, a
clear anomaly is associated with the southern part of
the caldera floor. The decrease in the magnetic field
takes place directly above the step, clearly indicat-
ing that the rocks on the southem side of the step are
magnetic. In Fig. 6c, a high is also associated with
the southem part of the caldera floor but the main
anomaly is somewhat to the south of the step in line
3. No clear anomalies can be seen over the steps on
line 1 (Fig. 6a). However, the general east-west
magnetic field gradient along line 1 decreases
directly above the step at 2.8 km (Fig. 6a). A weak
positive anomaly could therefore be superimposed
on the general decrease in the magnetic field
towards west. This would suggest that the rocks
JÖKULL, No. 39, 1989 9