Page 21 - Caldera Volcanism Analysis, Modelling and Response

xx Preface

contribution to the understanding of the relationship among ignimbrites, graben-
caldera vents and post-caldera fills for economic prospecting.
Turning to Archean times and highlighting the economic importance of
geological caldera investigations, Mueller et al. report on subaqueous calderas in
Canada, which are primary sites for volcanogenic massive sulfide (VMS) deposits.
Based on detailed volcanic facies mapping, they distinguish caldera subsettings in
the ancient deposits of the Hunter Mine and Normetal calderas, two little-known
effusive-dominated edifices and the explosive-dominated Sturgeon Lake caldera.
The authors propose a new hydrothermal alteration model that helps in explaining
the formation of chert–Fe-carbonate assemblages in Archean sequences and in pin-
pointing highly favourable sites for VMS exploration.
With a review on collapse caldera modelling, Martı ´ et al. summarise recent
development in investigating caldera formation via analogue and numerical
modelling and cross-correlate key findings with results from geophysical imaging
of the sub-surface architecture at calderas. They conclude that the combination of
field studies with experimental, theoretical and geophysical modelling enables
identifying and quantifying the main controlling factors for the formation of
collapse calderas. These include magma chamber size and shape, magma chamber
depth, host rock rheology, previous history of deformation, topography, regional
tectonics, temperature field around the magma chamber and magma composition
and rheology.
Acocella explores the structural development of calderas as investigated by
analogue experiments. He finds that despite differences in the instrumental
setup and imposed boundary conditions, a complete collapse can be summarised
through four main stages, proportional to the amount of subsidence and
progressively characterised by a downsag, reverse ring fault, peripheral downsag
and peripheral normal ring fault. This proposed evolutionary scheme incorporates
not only the geometric features of calderas, but more importantly, also their genetic
features.
Focussing on rock mechanical conditions, Gudmundsson discusses the initiation
of ring faults to trigger caldera-collapse as opposed to mere dyke emplacement.
He finds that underpressure and excess pressure in a shallow, crustal chamber
normally results in dyke injection rather than caldera formation. Dyke injection is
also favourable over ring-fault initiation for doming or tension above a spherical
magma chamber. Numerical results indicate that the local stresses in composite
volcanoes most likely to initiate caldera faults are associated with sill-like chambers
subject to tension, doming or both.
Via a systematic set of numerical models, Walter summarises a variety of types,
mechanisms and patterns of caldera ring-dykes that can be observed in nature. The
findings suggest that caldera deformation may be affected by pre-existing and
reactivated tectonic faults and ring fractures, and that sites of ring-dyke intrusions
are controlled by various tectonic and magmatic loading processes. The models
suggest that ring-dykes commonly form incompletely, i.e. only part of a ring can be
intruded, as a result of a non-uniform stress field around the ring-fault.
The following three papers focus on the analysis and interpretation of
geophysical time series collected during unrest periods at caldera volcanoes