Page 387 - Caldera Volcanism Analysis, Modelling and Response

362 Thomas R. Walter

for outwardly dipping ring-faults. However, a more complex shape of the ring-
fault or magma chamber or a variably dipping ring-fault may yield variable
amounts of opening, thus also affecting the geometry and completeness of a ring-
dike.

3.2.2. Deflating sill-shaped magma chamber
Because magma chambers are typically flattened rather than spherical (Marsh,
2000), a sill-shaped (oblate spheroid) geometry must be considered, as it may
significantly affect the distribution of ring-fault opening. In this model, a deflating
sill-shaped magma chamber is encircled by a ring-fault, and the setup of the model
is similar to the scenario described above with the only exception that the magma
chamber is flat: 10 km wide and 5 km high (Figure 6). Depressurizing the
sill-shaped magma chamber causes the most pronounced ring-fault opening along
a narrow band around the magma chamber (Figure 6C). However, the amount of
opening is smaller than for the spherical magma chamber model (note the different
color scale in Figures 5 and 6). The most likely upward propagation path of a
dike cannot be determined in this scenario, because the pressure source is perfectly
radially symmetric; therefore, any location around the ring-fault may be used.
This pattern significantly changes if the magma chamber is elliptical, as shown
below.

3.2.3. Deflating an elliptical magma chamber
Most actual calderas and their associated ring-faults are not circular, but elliptical
(Holohan et al., 2005). As a consequence, the pattern of ring-fault opening and
intrusion differs significantly from the radially symmetric scenario. In this model,
a sill-shaped deflating magma chamber elongated in the east-west direction is used
(ellipsoid shaped). The magma chamber is 20 km long, 10 km wide, and 5 km high
(elongated in the x-direction), and is encircled by a ring-fault (Figure 7A). The
magma chamber center is 10 km below the surface and is subject to a pressure drop
of 10 MPa (Figure 7B). The model predicts a maximum in ring-fault opening at
magma chamber depths. The x–z and y–z side views onto the ring-fault show the
regions subject to opening. The largest amount of opening occurs at the short-axis
side of the elliptical ring-fault (Figure 7C left) and drops to zero along the long-axis
side of the elliptical ring-fault (Figure 7C right). The model results can thus be
interpreted in such a way that dike intrusions are more likely to occur and to
propagate upward along the short-axis side of the elliptical ring-fault. It may also be
possible that ring-dike intrusions change their intrusion direction and migrate
laterally into the short-axis side, as indicated in Figure 7C. The caldera that formed
the Bishop Tuff eruption at Long Valley, for instance, is thought to have initiated at
the short-axis side, as suggested also by recent analog experiments (Holohan et al.,
in press).