Page 362 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 337

Figure 16 Sill-like magma chamber subject to an underpressure of 5 MPa.The chamber is
8 km wide and 2 km thick; it is located at 5 km depth in a homogeneous, isotropic crustal
segment, 20 km thick and 40 km wide, with a sti¡ness of 40 GPa. (A) Con¢guration of the
model; (B) contours of tensile stress s 3 around the magma chamber; (C) maximum principal
tensile stress s 3 and von Mises shear stress s at the free surface; (D) contours of shear stress s
around the magma chamber and (E) tensile s 3 and shear s stress at the upper boundary of the
magma chamber.

I know of no well-documented field observations of such double-fault systems being
associated with collapse calderas; however, if such a system is observed, then it might
be explained in terms of this model or a similar one.
To show the effects of mechanical layering on the local stress fields and the
likelihood of ring-fault formation, I present several models of a sill-like chamber in
a layered crustal segment. In all the models, the chamber is 8 km wide (horizontal
diameter), 2 km thick and with a top at 3 km below the free surface. The crustal
segment hosting the chamber is 20 km thick and 40 km wide. The segment’s upper
part is composed of 30 layers, each 100 m thick and alternating in stiffness between
1 and 100 GPa, whereas the layer hosting the chamber, and the remainder of the
crustal segment, has a stiffness of 40 GPa. Underlying the crustal segment is a