Jökull - 01.12.1992, Síða 16

Figure 1. Generalized geological and tectonic map of
the northem volcanic zone, the Tjömes fracture zone,
and the Kolbeinsey ridge. Adapted from McMaster et
al. (1977), Sæmundsson (1974) and Einarsson (1991).
— Einfaldað jarðfrœðikort afTjörnesbrotabeltinu.
long and 80 km wide right-lateral, transfomi zone be-
came active 6-7 Ma ago after the volcanic rift zone
had jumped about 150 km to the east, to its present
location (Sæmundsson 1974,1979). The TFZ is char-
acterized by high seismicity, mainshocks as large as
magnitude 7 and frequent earthquake swarms. The
current transform motion appears to be taken up by se-
ries of parallel WNW-NW faults which are expressed
as seismic lineaments (Einarsson 1976,1991).
The Grímsey lineament and the Húsavík-Flatey
lineament are the two main seismic lineaments within
the TFZ. A third seismic lineament, the Dalvík lin-
eament, is inferred from diffuse seismicity which is
mainly confined to its western part (Einarsson 1976,
1991). There is, however, no structural evidence for
this lineament on land. The 80-90 km long Grímsey
lineament is cut by a series of N-S en echelon troughs
and extends from the southem tip of the Kolbeinsey
ridge and Skjálfandadjúp trough into the Axarfjarðar-
djúp trough, where it connects with the northem vol-
canic zone (McMaster et al. 1977; Sæmundsson 1974,
1979; Einarsson 1991). The offshore segment of the
Húsavík-Flatey lineament runs from the southem tip
of the Eyjafjarðaráll trough towards Húsavík, from
where it continues on-land into the northern volcanic
zone(McMasteretal. 1977; Sæmundsson 1974,1979;
Young et al. 1985; Guðmundsson et al. in press).
The Húsavík-Flatey lineament is marked by two ma-
jor NW-striking faults with opposite dips. Drill core
data at the Húsavík faults, indicate a vertical displace-
ment of up to 1400 m, with the south side subsid-
ing (Tryggvason 1973) whereas the northem block of
the Flatey fault has a minimum vertical displacement
of 1100 m (Thors 1982). The transform motion on
the Húsavflc faults was greatly reduced about 1 Ma
ago when the northern volcanic zone extended north-
wards, into Axarfjarðardjúp (Sæmundsson 1979). A
pronounced negative gravity anomaly along Flatey in-
dicates the existence of thick sediments (Pálmason
1974). A multichannel seismic reflection survey re-
vealed up to 4 km thick sediments in Eyjafjarðaráll
whereas the sediment thickness is at least 1 km thick
at the coast of Axarfjörður (Flóvenz and Gunnarsson
1991).
Tlie crustal structure of Iceland has been studied
by seismic refraction since 1959. The first results
were published by Báth in 1960, by Tryggvason and
Báth (1962), and by Pálmason (1963, 1971). In their
models the crust was interpreted in terms of several
layers, each with a constant seismic velocity. Pálma-
son (1971) compiled an average crustal velocity model
for Iceland which consisted of five crustal layers with
sharp discontinuities overlying an anomalously slow
upper mantle with a P-velocity of 7.2 km/s. The ve-
locities of the crustal layers correspond to velocities
of the oceanic crust (Raitt 1963; White et al. 1990)
but the crustal thickness of each layer is greater in
Iceland. The velocity structure of the upper crust of
Iceland depends on the state of alteration of the basaltic
crust. This alteration leads to a steady increase in ve-
locity with depth as secondary minerals fill the free
pore space and cracks in the rocks (Flóvenz 1980;
Flóvenz and Gunnarsson 1991). Flóvenz (1980) rein-
terpreted Pálmason’s data using continuous velocity-
depth profiles and divided the crust into 2 parts, the
upper and lower crust. The upper crust is characterized
by continuously increasing velocity with depth, from
14 JÖKULL, No. 42, 1992