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.