We use high-resolution simulations to analyze fluid flow, pore pressure, and fault permeability evolution in the seismically active Tjrnes Fracture Zone (TFZ), a major transform fault zone in the North of Iceland. Our results show that the TFZ is characterized by four distinct areas where pore pressures are above hydrostatic, consistent with geophysical observations. Basement and faults, which are assumed to have low permeabilities, often display pore pressures close to lithostatic. Fault permeabilities are allowed to vary freely as a function of the effective fault normal stress. They hence inflate periodically to release excess pore pressure in a few minutes. This is accompanied by an increase in permeability of over seven orders of magnitude and causes short-lived fluid fluxes of more than 0.01 m s^-1. After pore pressures have dissipated, fault permeabilities decay back to their original values in 2 to 3 years as the effective fault normal stress increases. This behavior is consistent with a toggle switch mechanism and could have two important implications for fluid flow in seismically and hydrothermally active oceanic crust. First, the rapid changes in fault permeability and pore pressure provide an explanation for distinct cyclical geochemical changes observed on a similar timescale in thermal waters near the town of Hsavik in the TFZ before and after a magnitude 5.8 M_w earthquake. Second, our results provide another line of evidence in the growing number of observations that crustal permeabilities are constantly evolving and geological processes in hydrothermal systems can be dominated by short-lived and extreme flow events.

We investigate the regional fluid flow dynamics in a highly faulted transform area, the Tjörnes Fracture Zone in northern Iceland which is characterized by steep geothermal gradients, hydrothermal activity, and strong seismicity. We simulate fluid flow within the Tjörnes Fracture Zone using a high-resolution model that was based on the available geological and geophysical data and has the aim to represent the complex geological structures and the thermodynamical processes that drive the regional fluid flow in a physically realistic way. Our results show that convective heat flow and mixing of cold and saline seawater with deep hydrothermal fluids controls the large-scale fluid flow. The distribution of faults has a strong influence on the local hydrodynamics by focusing flow around clusters of faults. This explains the nature of isolated upflow zones of hot hydrothermal fluids which are observed in the Tjörnes Fracture Zone. An important emergent characteristic of the regional fluid flow in the Tjörnes Fracture Zone are two separate flow systems: one in the sedimentary basins, comprising more vigorous convection, and one in the crystalline basement, which is dominated by conduction. These two flow systems yield fundamental insight into the connection between regional hydrothermal fluid flow and seismicity because they form the basis of a toggle switch mechanism that is thought to have caused the hydrogeochemical anomalies recorded at Húsavik before and after the 5.8 M earthquake in September 2002.