Jökull - 01.01.2014, Side 72

Ingi Þ. Bjarnason
main ring fault of calderas, or even on what is com-
monly observed in the field (e.g. Walker, 1984; Gud-
mundsson, 1998b; Roche et al., 2000; Gudmunds-
son, 2007). Anderson (1936) predicted, theoretically,
that calderas ring faults should dip steeply outward,
with reverse sense of motion above an underpressured
magma chamber.
The Anderson model has been favoured for a long
time and has in general been confirmed by analogue
experiments on caldera structures and development in
the laboratory (e.g. Roche et al., 2000; Burchardt and
Walter, 2009). Acocella (2007) has reviewed the sub-
ject. Most of the work carried out in this field seems to
comply with the Anderson-type ring fault in the first
stages of caldera collapse (Acocella, 2007). In later
collapse stages of caldera formation, the laboratory
experiments show a second set of ring faults develop,
with inward dip and normal sense of motion. The sec-
ond set of ring faults was not a part of Anderson’s
prediction, perhaps because his analytical theory was
describing the initial stress stages in caldera forma-
tion, and because his theory explained well observa-
tions of calderas in Scotland in his time. The second
major ring fault is concentric with the initial major
ring fault but lies further outside, which increases the
diameter of the caldera. The analogue experiments in-
dicate that inward dipping major fault has steeper dip
than the initial major ring fault at shallow depth, but
the two join at greater depth (Acocella, 2007). Bur-
chardt and Walter (2009) have shown with analogue
experiments that the drop of the caldera floor in later
stages of a caldera development is increasingly taken
up with normal faulting on the outer lying ring fault.
In studies on deeply eroded (1–2 km) calderas of
extinct central volcanoes in Iceland, usually only one
set of near vertical ring fault patches is reported (e.g.
Sigurdsson, 1970; Fridleifsson, 1973; Jóhannesson,
1975; Torfason, 1979; Franzson, 1978). An excep-
tion is perhaps Geitafell, an extinct central volcano in
Southeast Iceland (Figure 1), where patches of con-
centric sets of ring faults are observed inside and out-
side the main ring fault (Fridleifsson, 1983). None
of the reported studies mentioned did directly mea-
sure the degree of the dip angle of the caldera fault.
They usually infer inward dip with normal sense of
motion, based on a sharp change in the dip of the
strata with steeply inward dipping layers, just inside
the caldera fault. Recently, however, a measurement
has been carried out on a 300 m long segment of
caldera fault in Southwest Iceland that has an average
85◦ inward dip with normal sense of motion (Brown-
ing and Gudmundsson, 2015). Jóhannesson and Sae-
mundsson (2009) have mapped ∼15 eroded calderas
in Iceland. All of them have inward dipping ring
fault patches, interpreted to be a part of the main ring
fault of the calderas (H. Jóhannesson, pers. comm.,
Jan. 2015, and several other geologists). So far the
only exception found to this comes from Steffi Bur-
chardt (pers. comm., Feb. 2015). She observed curved
outward dipping antithetic fault patches in the extinct
Geitafell volcano, in the same outcrop as the main in-
ward dipping ring fault. However, an antithetic fault
may not be a good candidate for a major second set of
a ring fault.
Fridleifsson (1983) mapped curved patches faults
close (∼1.0 km) to the Geitafell’s main caldera fault.
One of these, which can be traced a considerable dis-
tance, has inward dip with reverse sense of motion,
and appears to join the caldera fault. Fridleifsson
(1983) speculates that this fault first acted as a reverse
fault, but later as caldera fault. In the present com-
munication it is, however, proposed that the opposite
may have happened. The reverse fault is reactivated
caldera fault from the time of caldera resurgence.
For a normal fault, originally close to vertical, to
be reactivated into a reverse fault, a major change in
stress field is required. The question arises if there are
more signs in the geological record of Iceland than al-
ready reported (Fridleifsson, 1983; Gudmundsson et
al., 2008), to support that normal faults have been re-
activated. Sibson (1985) presents an expression for
the optimal angle (θ?) between a fault plane and the
maximum principal stress (σ1) for reactivation, that
depends on the coefficient of friction (f). For faults
of normal strength (f=0.6–0.7), θ? is in the range 28–
30◦, but for weak faults (f=0.1–0.2) (Carpinteri and
Paggi, 2004) the range is 39–42◦. It should be noted
that faults can get reactivated in a wider range around
the optimal angle. In the case of a caldera fault with
normal strength and dipping inwards 80–85◦, the opti-
72 JÖKULL No. 64, 2014