Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 108

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GUÐMUNDUR PÁLMASON
on the bowl-shaped central anomaly. This is suggested by the rela-
tively steep gradient in the Bouguer anomaly along the eastem
boundary of this area.
Einarsson (1954) has indicated a hght, thick layer at a depth of
the order of 5 km to explain the details in the gravity field. As shown
above, the present results indicate on the contrary that the details in
the gravity field are due to an increase in density at a variable depth
of a few km.
As will be discussed in section 14, the seismic data should permit
corrections to be applied to the gravity data for variations in cmstal
stmcture. The resulting gravity values should show clearer than the
Bouguer anomaly the low-density zone in the upper mantle be-
neath Iceland.
11.3. Subsurface temperature.
The subsurface temperature field is in many places in Iceland
heavily disturbed to a depth of at least several hundred meters by the
movement of hot or cold ground water (Bödvarsson and Pálmason,
1961; Pálmason, 1967b). Several boreholes exist, however, which are
believed to give temperature gradients that are indicative of condi-
tions at a depth of perhaps several kilometers. These holes are situated
well away from surface manifestations of thermal activity and are
characterized by fairly constant temperature gradients. The depth of
these holes is in most cases from ahout 100 meters to over 1000 meters.
It is of interest to look for a possible relationship hetween the
seismic boundaries and the temperature to be expected at the cor-
responding depths on the basis of the temperature gradient data in
Iceland. This has been done for the 2-3 boundary. The borehole
temperature data are mostly taken from Pálmason (1967b) and the
depth to the 2-3 boundary from Fig. 37. The location of the boreholes
is shown in Fig. 39. The pressure is assumed to he the lithostatic pres-
sure for a density of 2.7 g/cm3.
The results are shown in Table 11.1 and Fig. 43. It appears that
the temperature values in Fig. 43 tend to fall into two groups, one
corresponding to a temperature gradient of 40-60° C/km, and the
other corresponding to a more or less constant temperature of 350-
400°C.
The boreholes in the group of more or less constant temperature
are all in the coastal areas of southern and western Iceland which is
that part of Iceland which is most likely to resemble the Beykjanes