Jökull - 01.12.1992, Page 21

Table 2. The currently used crustal model for northern
Iceland (NOR) and Raitt’s (1963) model for the stan-
dardoceanic crust. Layer 1 ofRaitt’smodelisomitted
due to its great variation in sedimentary thickness.
Jarðlagagerð Norðurlands (NOR) í samanburði við
meðaltals-jarðlagasnið úthafsskorpu, (Raitt 1963).
NOR-model Raitt’s-model
P-wave vel. km/s depth to top of 1. km P-wave vel. km/s thickn. km
Layer 0 2.4 0.0
Layer 1 3.8 0.5
Layer 2 5.2 2.5 5.04 ±0.69 1.39 ±0.50
Layer 3 6.5 5.0 6.73 ±0.19 4.97 ± 1.25
Mantle 7.0 11.0 8.15 ±0.31
crustal models deduced from individual stations along
the profile.
A velocity of 1.49 km/s was used to correct for
water-wave travel time. The range in water depth
along the prohle is 250-290 m. P-velocities lower
than 3.8 km/s were considered to represent sediments.
The sediments overlie a 4.3 km/s isovelocity basement
which most likely marks the transition from sediments
t0 flood basalts. The transition is sharp and is repre-
sented by a steep velocity gradient at a depth of 1.9-
2.3 km (Figure 4, left). The approximately 2 km thick
sedimentary sequence seems to be divided into two
parts. The inter-sedimentary interface has a constant
depth range of about 1 km and is indicated along most
of the profile as a change in the velocity gradient.
However, our data resolution is not good enough to
firmly confirm the existence of this interface and no
reflection is observed in the data.
A prominent low-velocity zone is inferred from
the data recorded at OBS 12. Fast P-arrivals recorded
at OBS 12 from shots E31 and E32 can be explained
hy a high-velocity layer in the top of the sedimentary
sequence. A low-velocity zone was needed below
the high-velocity layer in order to fit the calculated
velocity curves to the data. By using a low-velocity
zone, the delay of the shots at a greater distance than
E31 can be explained. A fault could also create the
same delay but the low-velocity zone is preferred.
Crustal velocities below the sediment/basalt inter-
face range from 4.3-6.8 km/s. These velocities are
analogous to P-velocities down to Layer 3 in Iceland
(Table 2). No marked change occurs in the veloc-
ity gradient down to a depth of 6 km. A velocity of
6.8 km/s at a depth of 5-6.5 km best fits the data (Fig-
ure 3). This velocity is slightly higher than the charac-
teristic (6.5 km/s) velocity at the Layer 2-3 boundary
(Flóvenz and Gunnarsson 1991; Bjarnasonetal. 1993)
but within the range observed in Iceland. The sharp
decrease in velocity gradient at the Layer 2-3 bound-
ary which is generally observed in Iceland is also seen
in our data as the deepest rays observed probably sam-
pletheupperpartof Layer3. Noindicationof amantle
phase or a Moho reflection is detected in the data.
DISCUSSION
The Axarfjarðardjúp trough is approximately
80 km long and 20 km wide and lies in a direct contin-
uation of the Krafla fissure swarm. The trough strikes
25°to the west of the N10°E Krafla fissure swarm.
The junction between the Krafia fissure swarm and the
Grímsey seismic lineament was seismically activated
during the recent rifting episode in the Krafla volcanic
system (Björnsson et al. 1977). The Axarfjarðardjúp
depression is bordered in the east and west by re-
cent volcanics, but volcanism is unknown within the
trough itself (Sæmundsson 1974). The Axarfjarðar-
djúp crustal structure consists of two major units, sed-
iments and flood basalts, with a range in P-velocities
analogous to those generally found in crustal layers 1
and 2 (Figure 5). A refraction seismic survey re-
vealed up to 1 km thick volcaniclastic sediments with
a velocity of 3.4-3.6 km/s at a depth around 0.4 km
in Kelduhverfi and a velocity of 4.3 was reached at
a depth less than 1 km on the coast of Axarfjörður
(Flóvenz and Gunnarsson 1991; Flóvenz pers. comm.
1993). This study showed a fairly uniform sedimen-
tary thickness of about 2 km within the Axarfjarð-
ardjúp trough. The different sedimentary thickness
observed indicates that the sedimentary layers thicken
northward into Axarfjarðardjúp where the thickness is
fairly constant.
There is, however, a slight deviation in the upper-
JÖKULL, No. 42, 1992 19