Rit (Vísindafélag Íslendinga) - 01.06.1971, Qupperneq 103

CRUSTAL STRUCTURE OF ICELAND
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variations in the 3—4 boundary may be closely related to major geo-
logical features in Iceland. These depth variations should therefore
be mapped in greater detail.
Two cross sections of the crust in Iceland have been prepared on
the basis of the available data. Their locations are shown in Fig. 39,
where also locations of thermal gradient boreholes, which will he dis-
cussed in section 11.3, are shown. The section AA', which is shown
in Fig. 40, runs roughly perpendicular to the Reykjanes Ridge, but
section RB', which is shown in Fig. 41, runs in the direction of Báth’s
profile WP. The upper layers 0, 1 and 2 are here shown as a single
layer.
In SW-Iceland the crustal structxn-e appears to have a certain
symmetry ahout the Reykjanes Ridge, indicating that the ridge struc-
ture continues into SW-Iceland. Farther out on the Reykjanes Ridge
refraction measurements indicate a similar structure in the crestal
zone as given here (Ewing and Ewing, 1959; Talwani et al., 1968).
On section BB' the most conspicuous feature is the thickening of
layer 3 in N-Iceland, where the thickness reaches about 13 km,
whereas in SW-Iceland it is commonly 5-6 km. It should be kept in
mind, however, that this result is based on only one profile and
should be confirmed by additional data from N-Iceland, before it can
be considered a firmly established fact.
11. DISCUSSION OF THE SEISMIC STRUCTURE
IN RELATION TO OTHER GEOPHYSICAL
AND GEOLOGICAL DATA
11.1. Earthquake seismology.
Studies of earthquake wave travel times and surface wave dis-
persion have been used to obtain information on the structure of the
crust and upper mantle in Iceland. Tryggvason (1959) deduced from
P-waves of near earthquakes in Iceland a layer approximately 15 km
thick with P-wave velocity of 6.7 km/sec overlying a layer of un-
known thickness with a velocity of about 7.5 km/sec. Tryggvason
(1961) also deduced from the travel times of four earthquakes in the
Arctic-Atlantic Ocean an upper mantle P-wave velocity of 7.4 km/sec
for the area north of Iceland. At a depth of about 140 km a velocity
increase to 8.2 km/sec was indicated. Báth and Vogel (1958) de-