Page 392 - Caldera Volcanism Analysis, Modelling and Response

Facilitating Dike Intrusions into Ring-Faults 367

views show that maximum opening occurs at the part of the ring-fault that is closest
to the dike (Figure 9C). This suggests that ring-dike intrusion might occur at the
segment of the ring-fault that is closest to the radial dike intrusion in the periphery
of the caldera. Accordingly, a dipping dike would lead to ring-dike formation that
obliquely propagates upwards.

4. Discussion

This paper uses numerical models to (i) describe deformation around a
depressurized magma chamber and test how local structures can affect this
displacement, (ii) describe the potential of opening a ring-fault in order to examine
the location at which potential dike intrusions occur, and (iii) describe how
processes external to the caldera, such as an earthquake or radial intrusion, affect the
location of a ring-dike intrusion.
The models are simplified and assume a homogeneous elastic material. Volcanoes,
especially caldera volcanoes that experience a high degree of fracturing and
hydrothermal weakening, may have a time-dependent viscoelastic rheology (New-
man et al., 2001) and rock-strength values that vary by as much as several orders of
magnitude (Wattersetal.,2000). The variations in material may be lateral (due to
fracturing, dikes, alteration) or vertical (due to lithologic layering, increasing
confining pressure). Thus, the development of a stress field and the deformation
pattern may be influenced by such heterogeneities (e.g., Manconi et al., 2007;
Gudmundsson, 2006). One may speculate, for instance, that the amount of ground
displacement in a deflating or inflating caldera basin can be amplified by rock types
that have a lower modulus of elasticity. Likewise, the effect of external processes, such
as earthquakes or dike intrusion in the periphery of the volcano, may have an effect on
caldera systems. The models used in this study assume that the intrusion of ring-dikes
as subvertical extensional fractures (mode I) is governed by opening perpendicular
to a ring-fault. This generally agrees with historical caldera activity, where
displacement often occurs along near-vertical ring-faults (Newhall and Dzurisin,
1988). However, it must be noted that this is a simplification of more complex
geometries, as some historical caldera ring-faults have shear fractures that dip slightly
inward (Darwin caldera) or outward (Rabaul caldera), the latter also approximating
the findings of previous seminal work (Anderson, 1936, 1937). Some (probably most)
dike intrusions intrude in a mixed-mode mechanism, including opening (mode I)
but also strike-slip shear (mode II) and dip-slip shear (mode III) dislocation, which is
not considered in the models presented here. Nevertheless, important conclusions can
be drawn from these models, as shown by comparisons to natural caldera systems.
The models imply that a ring-dike forms in a uniform circular way only when
the magma chamber is uniform, perfectly spherical, and not influenced by regional
or external structures or deformation fields. Most other numerical models suggest
that a potential ring-dike is more likely to intrude only partially into a ring-fault.
Likewise, typical ring-dikes found in nature show such complexities, where in
combination with the models summarized here, external influences can be assessed.