Maryam Khodayar

Shift of a rift by a transform zone: a case from the Northern Rift Zone and Tjörnes Fracture Zone of Iceland

Transform zones present fascinating geometry and tectonic deformation, some of which can be confusing, especially, when rift and transform zones interact, and rift structures are displaced by transform fractures. A case study from north Iceland includes the transform zone of Tjörnes (the Tjörnes Fracture Zone) and the Theistareykir Fissure Swarm of the Northern Rift Zone. This fissure swarm contains two north-trending central graben that are apparently shifted in a sinistral motion across a WNW-trending fault of the transform zone (the Stórihver Fault). The present analysis takes into account the number of faults and their relative displacement as criteria, and demonstrates that the configuration of the north-trending rift structures is compatible with the dextral slip on this WNW blind leaky fault of the transform zone. The dextral motion along the WNW fault explains more coherently the greater number and greater throw of the north-trending normal faults in two of the blocks on the shoulder of the graben, as well as with the formation of the central graben themselves. A dextral motion is also compatible with the regional tectonics and the mechanism of transform faulting. The apparent displacement of tectonic structures at diverging plate boundaries can be misleading. Therefore, identifying the correct sense of motion is essential for exploration and structural drilling targets where rift and transform of any age interact.

Tectonic Control of the Reykjanes Geothermal Field in the Oblique Rift of SW Iceland: From Regional to Reservoir Scales

This paper presents a multidisciplinary structural analysis of the Reykjanes Peninsula where Holocene deformation of a young oblique rift controls the geothermal processes in presence of a transform segment. The new structural map from aerial images and outcrops is correlated with selected surface and subsurface data and shows a complex pattern: NNE extensional rift structures, N-S dextral and ENE sinistral oblique-slip Riedel shears of the transform zone, and WNW and NW dextral oblique-slip faults. Shear fractures are more common, and along with the NNE fractures, they compartmentalise the crustal blocks at any scale. The fractures are within two ENE Riedel shear zones, indicating a minimum 7.5 km wide transform zone. The greatly deformed Southern Riedel Shear Zone is bounded to the north and the south by the 1972 and the 2013 earthquake swarms. This shear zone contains the geothermal field in a highly fractured block to the west of a major NW structure. Some of the deformations are: a) clockwise rotation of rift structures by the 1972 earthquake zone, inducing local compression; b) magma injection into extensional and oblique-slip shear fractures; c) reactivation of rift structures by transform zone earthquakes; d) tectonic control of reservoir boundaries by WNW and ENE shear fractures, and the distribution of surface alteration, fumaroles, CO2 flux, reservoir fluid flow and the overall shape of pressure drawdown by N-S, ENE, WNW/NW and NNE fractures. Results demonstrate the role of seismo-tectonic boundaries beyond which fault types and density change, with implications for permeability.