Structural and geothermal field assessment at Ytri-Vik, North Iceland

Abstract

Structural and geological fieldwork, drone surveying tied to borehole image data, high-resolution magnetic anomaly, geothermal water prospecting, and both regional and local seismicity data have provided a new understanding of the Ytri-Vík low-temperature geothermal field in Eyjafjördur, North Iceland. The field is located within a dike swarm system fully exposed along the coastline that is covered by younger sediments and lavas onshore and below sea level into the fjord. Delineating the extent of the geothermal field at Ytri-Vík requires a composite data analysis approach of surface and subsurface data. DJI Matrice 200 drone surveying resulted in detailed topographic and image quality of 2.8 cm resolution using the Zenmuse X4S camera that supported structural field mapping for outcrop sites along the shore at the exposed dike systems. Structural field measurements obtained for the dike systems, fracture zones, faults, fractures, joints, mineral veins, and slickenside were summarized and compared to subsurface structural analysis data based on acoustic image logging. The primary dike systems are near-vertical, strike N-S to NNE-SSW, and are linked to an active geothermal system. A second dike swarm was investigated that showed a weaker total magnetic field strength and little to no indications of surface geothermal activity and consists of NNE-SSW to NW-SE striking segments. The two dike systems correlate well with the mapped structural trends and fault zones in good agreement with magnetic anomaly trends. The primary N-S to NNE-SSW striking dikes and associated fault-fracture trends is parallel to elevated surface water conductivity, indicating increased mineral contents. Associated fracturing indicates a slight left-lateral motion along fracture surfaces and NW-SE-oriented stress-field orientation for ?Hmax. that is the same as indicated by direct offset earthquake locations and their fault-plane solutions. The primary dike system and the connected fracture zone are considered to be related to the main feed zone within the geothermal system at Ytri-Vík, thus enabling the up-flow of geothermal water in this area. This study underlines the necessity of combining high-resolution magnetic and drone survey data with detailed structural geological, geophysical and geochemical field studies to increase our understanding of low-temperature geothermal systems in Eyjafjördur and within low-enthalpy areas in Iceland in general.