| Abstract |
High temperature fields in Iceland are within active rifts where heat flow is at maximum. Producing reservoirs are generally between 1 and 3.5 km depth, with 200°C to 350°C temperature. These reservoirs are fracture controlled as faulting and intrusions provide magmatic heat source, permeability and flow paths among other processes. Therefore, in-depth knowledge of tectonic settings is crucial for exploration, production, re-injection, and modelling. Icelandic rift fissure swarms of normal faults, open fractures, dykes, and volcanic centres strike N-S in Northern Rift Zone (NRZ), and NNE to NE in Western Rift Zone and Eastern Rift Zone. The older transform zone of Tjörnes Fracture Zone strikes WNW, has a dextral motion and three segments, one of which is the GrÃmsey Oblique Rift. The younger South Iceland Seismic Zone is a general E-W sinistral zone, extending westward to the Reykjanes Peninsula (RP) as an ENE oblique rift, influenced by both rifting and transform faulting there. Within transform segments and oblique rifts, fracturing occurs as strike- and oblique-slip Riedel shears, with frequent earthquakes dominantly on sets of dextral and sinistral conjugate faults. With conspicuous vertical displacements of normal faults and prominent topography of eruptive fissures, geothermal activity has been traditionally attributed to rift-parallel fractures rather than to the Riedel shears. Recent multidisciplinary structural analyses of two high temperature systems, Theistareykir in the NRZ and Reykjanes on RP, revealed the role of transform structures combined with rift from regional to local scales, and from surface to the reservoirs and deeper: (a) The mapped fracture populations can be up to 75% Riedel shears and 25% rift-parallel. (b) Magma injects into rift structures and equally into strike- and oblique-slip faults, some of which are also reactivated during earthquakes. (c) While subsidence occurs mostly on rift structures, the structural compartmentalisation of the reservoirs, alteration, seepage of steam and gases, formation temperature provinces, structural carriers and barriers to flow are dominantly controlled by the Riedel shears. Rotations of resistivity anomalies on the same Riedel shears indicate segmentation of these strike- and oblique-slip faults down to 8 km crustal depth. Selected examples of the above processes from Reykjanes and Theistareykir are presented here. |