The Reykjafjall N-S source fault ruptured during the M-w 6.3 earthquake doublet on May 29, 2008, in the South Iceland Seismic Zone (SISZ). The northern part of the fault intersects the easternmost high-temperature geothermal field of Hengill in an extinct volcano that began shifting away from the Western Rift Zone 120,000 years ago. Geophysical data indicate that the N-S fault ruptured over 20 km length at depth. Our detailed field mapping along the fault length shows that the southern part of the fault did not rupture the surface mostly due to the greater depth of seismicity (down to 9 km) there. The surface ruptures were dominantly found along the northern half of the fault in the highland within the high-temperature field where aftershocks were shallower than 5 km. There, the surface ruptured on portions of six parallel N-S fault segments and not on a single fault plane. Hot springs are aligned on the two westernmost fault segments within the area of increased geothermal activity after the earthquake. Other Riedel shears (ENE, E-W, WNW and NNW) hosting geothermal activity also ruptured. On the N-S faults, surface ruptures and hot springs are organised in left-stepping arrays indicating dextral motion with offsets between 0.5 and 2 m. On other Riedel shear sets ruptures have a right-stepping arrangement indicating sinistral motion with a maximum horizontal offset of a metre. All fresh ruptures had centimetre-scale normal-slip and openings <1 m but up to 1.5 m along atypical N-S structures. Our results fit with the lineations in the aftershocks and with fault plane solutions. They also show that the 2008 source fault ruptured within a 2-km wide deformation zone where previous earthquakes and changes to geothermal activity have occurred at least since 1706. This implies that major earthquakes in this area may have a common source fault at depth and a wider deformation zone at the surface. Our study demonstrates how transform faulting at the junction of a rift segment causes recurring medium-large earthquakes that control fault behavior and permeability in fractured geothermal fields. (C) 2013 Elsevier Ltd. All rights reserved.