High-resolution simulations of fluid flow in active hydrothermal systems : applications to the Tjornes Fracture Zone and Askja Volcanic complex in Iceland
Abstract
This study provides new insights into the transient uid ow dynamics that
characterise high temperature hydrothermal systems. So far, these kind of
studies have used process models that tend to over-simplify the geology and
focus instead on the physical processes, hence not revealing the hidden behaviour
that depends on the complex geological structures often encountered
in hydrothermal systems. This study is one of the rst examples where high
resolution numerical simulations in two and three dimensions have been applied
to hydrothermal systems in order to preserve geological detail in the
models explicitly. Two prime examples in Iceland for seismicity-induced uid
ow and groundwater ow during volcanic eruptions, respectively, have been
selected for this purpose. The rst example is the Tjörnes Fracture Zone, a
heavily faulted transform zone o shore in North Iceland where most of the
Icelandic earthquakes occur. The work demonstrated that a moderate permeability
contrast between the shallow sedimentary basins and deep crustal
basement causes two distinct uid ow regimes which are only connected
during a seismic event. When such an event occurs, faults in the Tjörnes
Fracture Zone in ate and connect the two uid ow systems, causing hot
uids migrating from the basement into the basins at extreme ow rates.
This explains key geochemical observations made in the Tjörnes Fracture
Zone before and after a seismic event. The second study investigated the
1874 to 1875 volcano-tectonic episode at the Askja volcano, which is the
third largest silicic eruption since settlement in Iceland. Here it was demonstrated
how syn-eruptive groundwater ow inside the Askja caldera changed
the eruptive style of the March 1875 eruption, causing a well-documented
change from a wet to dry eruptive style. The results of this work provide
some fundamental new insights into the transient dynamics of uid ow in
active high temperature hydrothermal systems and suggest that these kind
of simulations may be used to complement studies assessing the risk and
hazard of future volcanic eruptions and seismic events.