Page 281 - Caldera Volcanism Analysis, Modelling and Response

256 J. Martı ´ et al.

Figure 10 Sketch of the thermo-mechanical model showing initial and boundary conditions
(modi¢ed after Guillou-Frottier et al., 2000).

models (e.g. Gray and Monaghan, 2004) is that one can track the nucleation
and growth of fractures. Alternatively, the influence of thermal effects on the
stress field can be explored (Burov and Guillou-Frottier, 1999; Guillou-
Frottier et al., 2000), since these effects can alter crustal rheology and hence
influence the formation and subsequent development of fractures. In general,
non-elastic models dealing with overpressure load condition divide the
collapse process into two different stages. First, the chamber’s overpressure
triggers ground uplift, roof bending, fracturing, and magma extrusion, and
second, erupted materials accumulate. When the sum of chamber excess
pressure and roof strength can no longer balance the load of the erupted
products, the chamber roof starts to flex down and subsides. Some models
cover both stages of the collapse (e.g. Burov and Guillou-Frottier, 1999)
while others simply focus on the development of fractures during the second
phase (e.g. Gray and Monaghan, 2004). The calculations provide stress and
thermal regimes versus time around the magma chamber and predict fault
location. For example, Burov and Guillou-Frottier (1999) find that during
uplift, overpressure results in flexural uplift of the roof causing bending and
eventually failure at the borders and initiation of normal inclined border
faults. The area affected by posterior subsidence of the roof is thus limited to
the inward-dipping cover. A later snapping of the roof triggers a piston-like
subsidence. Consequently, this model predicts two groups of faults: inclined
primary (initiated during the bending stage) and subvertical secondary
(initiated during overloading and subsidence). Inclined normal faults may be
initialised either at the surface during the subsidence phase or at depth during
the possible uplift phase (in this case, they first appear as inverse faults) and
propagate upward to the surface. Again, magma chamber geometry is a key
parameter because the number and location of faults depend on the chamber
aspect ratio. For large aspect ratio chambers (W3), the flexural stress
concentrates at the upper corners of the magma chamber resulting in the
formation of inverse inclined border faults with an inclination controlled by
the friction angle. More eccentric geometries can, in addition, create internal
embedded faults.