Jökull - 01.12.1989, Page 46

Fig. 4. The second order polynomial trend which is
subtracted from the Bouguer anomaly as regional
background. This trend is a part of the bowl-like
regional trend observed in the gravity map of Ice-
land. Contour lines are 2.5 milligals apart.
Mynd4. Bakgrunnssvið þyngdarmœlinganna. Frá
Bouguer gildunum er dregið bakgrunnssvið sem er
fundið með annarrar gráðufjölliðu. Bakgrunnssvið-
ið er hluti afþeirri skálarlaga þyngdarlægð sem ein-
kennir Bouguer þyngdarkortið af öllu Islandi. Milli
jafngildislína eru 2.5 milligal.
edges of the Langjökull glacier, but no data are
available from the glacier so their existence is not
well documented.
An altemative view of the particulars of the grav-
ity low has been offered by Sæmundsson (pers.
comm.). Rather than focusing on the minima, he
points out that they are separated by central volcanic
complexes or areas of maximum volcanic produc-
tion, which might be expected to give rise to posi-
tive gravity anomalies. Sæmundsson (in press) has
also suggested the existence of two separate rows of
volcanic complexes transverse to the rifting zone,
which would encompass these local gravity highs.
The second largest gravity anomaly in Figure 5 is
a gravity high in the Southern lowlands. This ano-
maly is separated from a somewhat lesser high in the
vicinity of Þjórsárdalur by a NW trending gravity
valley. The two highs could either be interpreted as
a plateau of high values between the westem and
eastem volcanic zones in South Iceland, or as two
local highs related to extinct central volcanoes. The
first interpretation requires an explanation of the
gravity low that runs NW from Hekla to the Hreppa-
fjöll mountains and possibly across the rifting zone.
The second interpretation requires an explanation of
the expansive gravity high in the Southem lowlands,
which seems far too large to be produced by a single
volcano.
East of this we see a small comer of a deep grav-
ity low, presumably associated with the eastem vol-
canic zone. The minimum of this low is located at
Hekla.
The northwest comer of the map is characterized
by relatively high gravity values, reflecting higher
density values in geologically older provinces.
Superimposed on this are a couple of roughly circu-
Iar positive anomalies, at Ferstikla by Hvalfjörður
and at Hafnarfjall by Borgarfjörður. These are both
related to extinct central volcanoes (Franzson,
1978). South of the Ferstikla anomaly there is a
lesser gravity high at Stardalur (Friðleifsson and
Kristjánsson, 1972), and southwest of it a part of an
anomaly with an offshore center. Both anomalies are
thought to reflect eroded volcanic centers.
DIRECTIONAL ANALYSIS OF
THE GRAVITY MAP
The directionality of the gravity map is analyzed
by calculating its two-dimensional power spectrum,
shown in Figure 61. The data are first windowed in
1. The gravity map consists of 64x64 grid points based on 290
gravity stations, which means it is heavily overgridded. The
spectrum shown in Figure 6, however, is limited to the lower
half of the frequency range (32x32 values). The wavelength
at the edge of the spectral plot is approximately 8 kilometers.
Thus, the contour lines drawn in Figure 6 generally
correspond to wavelengths exceeding the distance between
stations.
44 JÖKULL, No. 39, 1989