3D structural mapping of the Theistareykir geothermal field, NE-Iceland: Fault and fracture connectivity within a volcanically active rift zone.

We present a new structural model of the Theistareykir high-temperature geothermal field, located in NE Iceland within a volcanically active rift zone. Various interdisciplinary geoscientific methods are applied and cross-analyzed. We use remote sensing data, structural and geological surface and subsurface mapping, drone surveys, borehole images, seismicity, potential field, and CO₂ surface emissions data. This composite data modelling approach aims to highlight primary fault zones and fractured intervals at the surface by assigning fault types and their spatial orientation. The compiled surface and subsurface datasets were used for 3D fault projections and the delineation of fault-block compartments. Fault plane solutions from earthquakes supported fault property assignments, slip direction, and stress-field orientations. Surface fault segments and fracture intensity maps highlight areas of relay ramping, fault damage, and accommodation zones in between the NW-SE, NE-SW to N-S striking fault systems of the Theistareykir rift segment. The fault systems are intersected by WNW-ESE striking embryonic transfer zones that form boundaries between rift valley graben segments south and within the Theistareykir geothermal field area. These WNW-ESE transfer zones accommodate the differential and oblique opening of the rift zone, which overall follows the right-lateral opening direction of the region south of the Husavik-Flatey transform fault. Our structural model of the Theistareykir geothermal field area is subdivided into six structural domains that form fault block compartments, with varying degrees of faulting and fracturing, reflecting the different quality of hydraulic connectivity across the field.