Fracture pattern of Thjَrsلrdalur central volcano with respect to rift-jump and a migrating transform zone in South Iceland

An analysis of dykes, mineral veins, and tectonic joints in the Matuyama-age Thjórsárdalur volcano provides insights into the relative development of rift and transform zones at slow-spreading centres. Rifting resulted in the formation of NNE fractures. Transform faulting gave rise to shear fractures striking predominantly N–S and ENE, parallel to the main earthquake fractures of the South Iceland Seismic Zone (SISZ), but also WNW, E–W, and NNW. All fractures were filled with the same geothermal fluid and magma. The strikes, motions, and fracture cross-cuttings show that the transform fault was active early in the history of the volcano, coeval with the rift(s). The shear fractures are consistent with a Riedel shear model of an E–W transform zone, and with a minimum principal stress, σ3, striking N135°E to N150°E. These values are close to the direction of extension across the Eastern Rift Zone (ERZ), supporting an interpretation that the E–W shear zone has migrated from the latitude of Thjórsárdalur to the SISZ with the southward propagation of the ERZ. Migration of synchronous transform and rift systems at a slow-spreading centre yields a distinctive fracture pattern, but lacks some of the elements found at similar migration sites for fast-spreading centres.