Page 341 - Caldera Volcanism Analysis, Modelling and Response

316 Agust Gudmundsson

Figure 2 Caldera faults are commonly referred to as ring faults, as is done in this paper. Most
caldera faults, however, are not perfect circles but rather ellipses where the radius or semi-major
axis a is larger than the semi-minor axis b. Most measured caldera faults appear to dip close to
vertical, as is indicated here.

Branney, 1995; Lipman, 1997; Gudmundsson, 1998a; Burov and Guillou-Frottier,
1999; Roche et al., 2000; Acocella et al., 2000, 2003, 2004; Newman et al., 2001;
Walter and Troll, 2001; Saunders, 2001, 2004; Jellinek and DePaolo, 2003; Folch
and Marti, 2004; Gray and Monaghan, 2004; Cole et al., 2005). All these models
agree that the displacement along a ring fault is somehow related to the existence of
a crustal magma chamber into which the caldera block, sometimes referred to as a
‘piston’, subsides. Before the subsidence takes place, however, the ring fault must
form, and at present there is no consensus as to its mechanics of formation.
One principal aim of this paper is to put constraints on the mechanical
conditions and processes that are likely to take place during ring-fault initiation and
subsequent caldera collapse. These conditions and processes include the shape and
local stress field of the associated magma chamber as well as the fluid transport out
of the chamber prior to and during collapse. The focus is on ring-fault formation
on Earth because this has been best studied and actually observed. Furthermore,
ring faults on Earth are exposed not only at the surface but also in deeply eroded
sections, thereby providing a three-dimensional view of the faults. Data on calderas
on some other planets and satellites, however, are mentioned to emphasise the point
that ring-fault formation is a universal feature of volcanism. The implication is that
general models of caldera formation on Earth should, with suitable modifications,
be equally applicable to calderas on other planetary bodies. In the section on caldera
structure, there is thus a brief review of the geometries of extraterrestrial calderas.
Any general discussion of calderas, however, refers to calderas on Earth.
A second principal aim is to present conceptual and numerical models of the
most likely conditions for ring-fault formation and slip. The conceptual models aim
at clarifying the issues of general ring-fault dip and caldera stability. Thus, the
purpose of these models is to help clarify, first, whether ring faults are generally
outward dipping or inward dipping and, second, how the dip relates to the process