Jökull - 01.01.2014, Side 73

Earthquake Sequence 1973–1996 in Bárðarbunga volcano
mal σ1 for reactivation would be in a plane perpendic-
ular to the fault plane with dip 50–57◦. For the same
dipping of a weak fault, the dip range of σ1 would be
38–46◦.
Today most authors accept Anderson’s derivation
(1936) that cone sheets are mode I fractures formed
by upward pressure of magma. He showed that they
propagate in the direction of σ1 and open up in direc-
tion of σ3. Sheets in the dip range 38–46◦ or 50–57◦
would therefore signify a paleo-stress field that was
optimal to reactivate near vertical inward dipping (80–
85◦) weak or normal strength faults, respectively, ac-
cording to the relation of Sibson (1985). Cone sheets
are commonly observed within eroded central volca-
noes in Iceland and are closely related to calderas.
They have a wide range of dip. However, it seems
common among all observations carried out, that
peaks in distributions of cone sheet dips are within
the range 25–45◦ (Annells, 1968; Sigurdsson, 1970;
Johannesson, 1975; Franzson, 1978; Gudmundsson,
1998a; 2002; Siler and Karson, 2009; Burchardt et al.,
2011). Most authors also find steeper dipping sheets
within the range ∼60–90◦ (Annells, 1968; Johannes-
son, 1975; Franzson, 1978; Gudmundsson, 1998a,
2002; Siler and Karson, 2009; Burchardt et al., 2011).
The steep dipping sheets tend to be less numerous
than the shallow ones, with one or two exceptions
(Gudmundsson, 1998a; Franzson, 1978). Gudmunds-
son (1998b) has modelled the formation of normal
fault calderas numerically. He predicts that during the
time of doming of a magma chamber, σ1 has inter-
mediate dips (∼30–45◦) in the vicinity of the lower
(deeper) half of the caldera fault, but in the upper half
steepening (∼40–75◦) is indicated.
Distribution of cone sheets in Iceland therefore in-
dicates paleo-stress field within extinct central volca-
noes that may have been commonly favourably ori-
ented to reactivate weak steeply dipping (80–85◦)
caldera faults, during periods of cone sheets forma-
tions. The assumed large ratio of slip to fault length
of mature calderas in the world, and observations of
rapid subsidence of caldera floors (e.g. Hartley and
Thordarson, 2012; Sigmundsson et al., 2015), sug-
gests that caldera faults are commonly weak faults.
As cone sheets dips in the range ∼50–60◦ are by
no means uncommon in Iceland, the requirement of
weak faults may not be necessary in order to reacti-
vate steeply dipping normal caldera faults during pe-
riods of cone sheet formations. However, when the
least effective principal stress is tensile, reactivation
of regular strength high angle normal faults becomes
easier (Sibson, 1985). This should be the situation ex-
pected in plate spreading environment like Iceland.
Studies on calderas in Iceland indicate a major
caldera ring fault, with relatively regular circular or
oval geometry, but the caldera floor has often con-
siderable faulting and flexing (H. Jóhannesson, pers.
comm., Jan. 2015). However, some of the central vol-
canoes have more complex structure, e.g. couple of
calderas within their domain (Jóhannesson and Sæ-
mundsson, 2009). Therefore, Icelandic calderas are
usually neither a pure end member piston collapse nor
a chaotic piecemeal collapse on random faults, but
comprise probably components of both. It is thought
that caldera formations in Iceland take thousands of
years to develop (Fridleifsson, 1973; Jóhannesson,
1975; Torfason, 1979; Franzson, 1978; Fridleifsson,
1983). However, there are observations, which in-
dicate that incremental caldera collapse can be rapid
(Hartley and Thordarson, 2012; Sigmundsson, 2015),
the final adjustment of an incremental collapse taking
half a century (Hartley and Thordarson, 2012).
Dynamics of the 1973–1996 Bárðarbunga earth-
quake sequence
If the caldera fault dip towards the centre of Bárð-
arbunga, which is the only evidence available from
the geological record as of today, then thrust earth-
quakes on the caldera fault were caused by uplift
movement, and the driving force within Bárðarbunga
was likely to be increased pressure within the volcano
(Figure 5; Bjarnason and Þorbjarnardóttir, 1996). The
other explanation, with outward dipping caldera fault,
would be decreased pressure with subsidence (Einars-
son, 1991). Reactivated normal faults have been ob-
served in Iceland (Gudmundsson et al., 2008), and in
the present work it is proposed that the observation of
Fridleifsson (1983) should be interpreted as a reacti-
vated inward dipping ring fault. Nettles and Ekström
(1998) assume downward movement on outward dip-
ping cone (ring) fault structure below an expanding
JÖKULL No. 64, 2014 73