Page 384 - Caldera Volcanism Analysis, Modelling and Response

Facilitating Dike Intrusions into Ring-Faults 359

(contours in Figure 2B) is influenced by the reactivated fault, with larger
displacement vectors near the fault. The cross-sections show that evacuating the
magma chamber results in higher horizontal (U x ) and vertical (U z ) displacements
near the fault. The vertical displacement field U z shows a maximum downward
displacement centered above the chamber. Similarly, previous researchers have
discussed the effect of freely slipping faults near magma chambers subject to
pressure changes (Gargani et al., 2006). Some other studies considered circular
faults around a magma chamber, as detailed below.

3.1.3. Deflating spherical magma chamber enclosed by a reactivated ring-
fault
A caldera ring-fault can be considered a zone of weakness that may accumulate
volcano-tectonic strain. In the end-member scenario, caldera ring-faults are free
to slip, considerably affecting deformation at the surface. Figure 4 shows a circular
fault at a radius of 10 km circumscribing the deflating magma chamber. The fault
can be considered a ring-fault peripheral to the active magma system, and can be
reactivated in dip-slip and strike-slip. All other model parameters are the same as
described above. The horizontal displacement vectors show material convergence
towards the reservoir (Figure 4B). In cross-section, the displacement U x shows
slightly smaller values near the surface (Figure 4C). In contrast, vertical
displacement U z shows a broad area of subsidence (Figure 4D). The bell-shaped
area of peak subsidence is less expressed, meaning that subsidence would most likely
occur as a uniform block, limited by the reactivated ring-fault. There is more
subsidence than in models lacking such a ring-fault, although the pressure drop
within the magma chamber is the same (cf. Figure 2). Several previous studies have
considered the effect of reactivated ring-faults surrounding a magma chamber
subject to pressure changes, for instance, at the caldera of Campi Flegrei, Italy.
These studies analyze the extent of deformation due to magmatic or hydrothermal
activity influenced by fault reactivation (Troise et al., 1997, 2003, 2004; Petrazzuoli
et al., 1999; Beauducel et al., 2004). It must be noted that the pattern of
displacement fields can vary depending on the type and geometry of the reactivated
ring-fault.

3.2. Predicting the location of ring-dike intrusions

The following models consider an evacuating magma chamber and a passively
opening ring-fault. Results are shown in map view and in side views, displaying the
contoured amount of ring-fault opening. In these calculations, the displacement
perpendicular to each element of the ring-fault is determined; a positive dis-
placement means that ring-dike intrusion is facilitated, while a negative
displacement signifies areas where ring-dike intrusions are hindered. These models
are designed to predict the most likely location and direction of a ring-dike
intrusion, based on the assumption that a ring-dike intrusion may preferentially
occur where the ring-fault is opened.