Jökull - 01.12.1999, Side 23

On the pattern of faults and dykes in Borgarfjörður, W-Iceland
Maryam Khodayar
Science Institute, University of Iceland, Hofsvallagötu 53, 107 Reykjavík, Iceland
Also at: National Energy Authority of Iceland-Orkustofnun (visiting scientist),
Grensásvegur 9, 108 Reykjavík, Iceland. E-mail: mak@os.is
Abstract — Eroded Tertiary and Quatemary lava piles in the Borgarfjörður area are transected by numer-
ousfaults and dykes with variable strike, forming a complex tectonicpattern. These structures were observed
at different erosional levels in the volcanic crust built up during the activities of the Snœfellsnes rift zone,
SRZ, (15 to 6 Ma), the Reykjanes-Langjökull rift zone, RLRZ, (6 Ma to present), and the ojf-rift Snœfellsnes
volcanic zone, SVZ, since 2 Ma. Two overlapping rift zones, one propagating (RLRZ) and the other receding
(SRZ) seem to have been active before 2 Ma. Preliminary field observations on faults, dykes, and striated
planes indicate that normalfaulting predominates on bothflanks ofthe Borgarnes anticline. Strike-slip and
oblique-slip movements have also occurred. Thefault pattern is dominated by WNW and NNE faults; both
populations show normal and strike-slip movements with similar lengths, displacements andfrequency. The
dyke pattern is dominated by N-S and NNE trends showing similar thickness andfrequency. The fracture-dip
analysis shows that NNE faults are not rotated by the tilting towards the rift axes, implying reactivation or
continuous activity of the faults through time. NNE andN-S dykes show, by contrast, indications of the two
tilting histories. The dip and displacement of the faults, the frequency and dip-directions ofthe dykes, and
cross-cuttings indicate that thefracture pattern is not monogenetic but results from polyphase tectonics. Al-
though tectonic phases have not been dated, cross-cuttings betweenfaults and dykes suggest that N-S, NNE,
andNW to WNWfractures are inheritedfrom early stages ofrifting in adjacent regions. Strike-slip faulting
occurred after normalfaulting, but both modes have been repeated through time. Mixture and simultaneous
activity along variable trends offaults and dykes imply tlie coexistence ofrifting and non-rifting conditions.
Tlie complex tectonics result from the instability ofthe plate boundaries, as the Borgarfjörður blockwas
caught between propagating and receding rift zones, and then affected by the presently active SVZ.
INTRODU CTION
rift propagation is supported by magnetic anomalies
(Kristjánsson and Jónsson, 1998). A part of Borgar-
fjörður was caught between two overlapping rift seg-
ments, one propagating (RLRZ) and the other reced-
ing (SRZ). Since 6 Ma, the area west of the RLRZ
evolved as a block flanking the new rift zone. Finally,
a part of this block became an active transverse vol-
canic zone (SVZ), considered by some authors to be
generated by dextral shearing along a WNW transcur-
rent fracture zone during the past 2 Ma (Sigurðsson,
1970; Scháfer, 1972). Recent intraplate earthquakes
up to magnitude 6 in the easternmost part of the Borg-
arfjörður near its junction with the rift zone (Einars-
son et al., 1977) show that the area is still tectonically
Borgarfjörður in western Iceland (Figure la) is
presently a part of an intraplate block located west of
the RLRZ, and is frequently referred to as a flank zone
(Jakobsson, 1972). The area is densely fractured (Fig-
ure 2) and has a complicated tectonic history since it
began to form at the plate boundary at least 15 m.y.
ago. The main steps of this history are as follows: An
early rift (Snæfellsnes rift zone) was active between
15 and 6 Ma. Due to the instability of the plate bound-
ary around 6-7 Ma, the active rift zone jumped from
Snæfellsnes to Reykjanes-Langjökull rift zone, which
then propagated southwestwards as a new rift zone
(Jóhannesson, 1980). This model of ridge jurnp and
JÖKULL, No. 47 21