Jökull - 01.01.2017, Síða 53

Leó Kristjánsson and Jónsson
al. (1988), and one of these has been radiometri-
cally dated as being of early Brunhes age. Two other
southwest-trending negative lineations may be dis-
cerned south of the peninsula, but they are faint; the
field in the wider offshore area is fairly uniform, as
would for instance be the case over thick sediments.
Small-size lows seen within or close to some of
the major positive anomalies appear to be caused by
shallow structures. One such possibility is hyalo-
clastite hills buried in lava flows. Examples have also
been recorded where high-temperature chemical pro-
cesses have destroyed most of the magnetite in basalts
(cf. Námaskarð data in Fig. 3 of Kristjánsson and
Watkins, 1977). Geological and ground-level mag-
netic studies at these spots are needed to ascertain the
origin of these localized lows. It also cannot be ex-
cluded that some of these and other minor features
in the anomaly pattern may be due to artifacts which
went undetected during the surveys.
Negative residual fields, already seen in Sig-
urgeirsson’s (1970b) map, dominate over most of the
north coast of the Reykjanes peninsula. It is inter-
esting that some volcanically inactive fissured areas
as shown for instance in Fig. 3 of Jakobsson et al.
(1978) do not seem to influence the magnetic pat-
tern. Around Reykjavík and farther northeast the neg-
ative anomalies, with steep lows in the Kollafjörður
inlet discovered in a 1993 survey (Jónsson and Krist-
jánsson, 2002), are clearly caused by thick series of
Matuyama age rocks. This lineation is pierced by the
strong positive Stardalur anomaly.
The nature of the sources of many of the magnetic
anomalies shown in Figure 3 hence remains rather un-
certain. However, it is hoped that this map will aid
in future deciphering of the unique volcanic, tectonic
and geothermal situation of the Reykjanes peninsula,
and its history.
Acknowledgements
Stefán Sæmundsson and Úlfar Henningsson owned
and piloted the airplanes used in our magnetic sur-
veys. The late Marteinn Sverrisson of the Science
Institute was a key person in the 1985–1986 survey.
Comments by Páll Einarsson, Kristján Sæmundsson
and editor Bryndís Brandsdóttir helped to greatly im-
prove the paper. Rósa Ólafsdóttir re-drafted Figure
1 and Ásta Rut Hjartardóttir made the map in Figure
3, using data from Clifton and Kattenhorn (2006) and
the National Land Survey of Iceland.
NÝTT KORT AF SEGULSVIÐI YFIR REYKJA-
NESSKAGA
Þorbjörn Sigurgeirsson mældi styrk jarðsegulsviðs-
ins á nokkrum fluglínum yfir Reykjanesskaga 1965,
og yfir öllu Suðvesturlandi í um 900 m hæð 1968.
Segulsviðið reyndist tiltölulega sterkt víða á suður-
og miðhluta skagans, en veikt yfir norðurhlutanum.
Orsakir hinna jákvæðu frávika frá styrk megin-
jarðsegulsviðsins virtust samkvæmt reiknilíkönum
liggja djúpt í jörðu, sem kom heim við mælingar á
seguleiginleikum bergsvarfs úr borholum niður fyrir
1.5 km dýpi. Á áttunda áratugnum var bent á, að
hinum fjölmörgu bergsprungum og misgengjum á
nesinu mátti skipta upp í fáeina nokkuð afmarkaða
SW-NE „sveima“. Þeir hafa verið skilgreindir á tals-
vert mismunandi hátt af ýmsum fræðimönnum undan-
farna áratugi, en eru ljóslega að hluta tengdir bæði ný-
legri eldvirkni, þekktum háhitasvæðum, og jákvæðu
segulsviðsfrávikunum. Frekari segulsviðsmælingar
á Faxaflóa og suður fyrir Reykjanesskaga fóru fram
1973, 1985–1986 og 1991–1992. Hefur nú verið unn-
ið úr þeim nýtt marglita kort af segulsviði yfir skagan-
um og næsta umhverfi hans. Mest áberandi í kortinu
eru aflöng segulsviðsfrávik sem skarast að nokkru við
eldvirka hluta sprungusveimanna.
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48 JÖKULL No. 67, 2017