Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 47

CRUSTAL STRUCTURE OF ICELAND
47
This depth increases to 2.7—3.2 km towards the western end of the
profile.
In central western Iceland there are five unreversed profiles of
26.4 to 41.1 km length. The Borgarnes—Mýrar profile (54) and Borg-
ames-Nordurárdalur profile (53) run from a common shot point in
Borgarnes along the main roads to Mýrar and Nordurárdalur. The
Skorradalur—Thverárhlíd profile (51) runs from a shot point in lake
Skorradalsvatn in a northeast direction. The Thingvellir-Kaldidalur
profile (41) runs to the north along the Kaldidalur road from a shot
point at Thingvellir. The Hvítársída profile (52) runs to the west
from a shot point in river Nordlingafljót near Kalmanstimga.
In the neighbourhood of Borgames layer 1 is found at the surface
with a somewhat higher than normal velocity, ahout 4.5 km/sec. Its
thickness is ahout 0.7 km. The delay time for the P3-wave appears
to he about 0.30 sec in the neighbourhood of the shot point in Borgar-
nes, which gives a thicknesss of layer 2 of 1.6 km and a depth to
layer 3 of 2.3 km.
The apparent P3-wave velocity on the Borgarnes-Mýrar profile
is fairly low, 6.10 km/sec. This is due to increasing depth to layer 3
towards the northern end of the profile. This is confirmed hy profiles
in the eastern part of the Snaefellsnes peninsula, which will be dis-
cussed in section 6.5. The delay time for the P3-wave near the
northem end of the profile is found to be about 0.50 sec which is in
good agreement with the results from the Snaefellsnes peninsula.
On the Borgarnes—Nordurárdalur profile, which is about 41 km
long, the P3-wave delay time increases first and then reaches a con-
stant value of about 0.50 sec along the northernmost 10 km of the pro-
file. This corresponds to a depth to layer 3 of 3.5-3.8 km depending
on the relative thicknesses of layers 1 and 2.
It is clear from profiles 53 and 54 that the Borgarnes area has a
relatively small depth to layer 3, which increases to the north and
northeast. From the two offshore profiles L1 and L2, which will be
discussed in section 6.10, it appears that the Borgames delay time
anomaly extends somewhat to the northwest, at least to the end sta-
tion for the two profiles L1 and L2. Its extension to the west and
south is less clear but to the east its extension is less than about 10 km
from Borgames as evidenced hy the Skorradalur-Thverárhlíd profile
which will he discussed below.
On the Skorradalur—Thverárhlíd profile (51) there is a thin sur-
face layer with a P-wave velocity of about 3.4 km/sec and a thickness