Within PRENLAB-TWO there has been a focus on the analysis of fault slip data and the reconstruction of paleostress tensors on the Tjörnes fracture zone.
Detailed field observations carried out by Á. Guðmundsson in the Tjörnes fracture zone and elsewhere show that major fault zones consist of two main structural units: a fault core and a fault damage zone. The core, where most of the fault displacement is accommodated, consists mostly of breccia and cataclastite; for major zones, it is commonly up to a few tens of metres thick. On either side of the core is a damage zone, as much as several hundred meter thick. The damage zone includes numerous faults and fractures, many of which are filled with secondary minerals, but lacks large volumes of cataclastic rocks and breccia. A model is being developed where during the interseismic period of an active fault zone, the fault core normally behaves as a porous medium with a very low hydraulic conductivity whereas the damage zone behaves as a parallel-fractured medium with a normal hydraulic conductivity many orders of a magnitude higher than that of the interseismic core. A field work has been carried out in the Flateyjarskagi peninsula (Figure 17). This peninsula has been chosen because the main fault of the Tjörnes fracture zone: the Húsavík-Flatey fault, is well expressed along the northern coast of this peninsula by a large zone of crushed rocks. Collection of more than 1300 fault slip data in 20 sites and determination of paleostress tensors in Flateyjarskagi allowed us to identify eight major brittle tectonic regimes arbitrarily named S1, S2, S3 and S4 (strike slip in type) and N1, N2, N3 and N4 (normal in type). Frequent contradictions in this relative chronology data suggest that these tectonic states alternated in a complex manner. These eight regimes can be arranged two by two (normal and strike-slip regimes with a same direction of S3) They have not the same importance in terms of numbers of sites and data. The tectonic regimes S3, S4, N3 and N4 are widespread. The main couple, S3-N3, indicates a N95E trending extension on average. The second couple, S4-N4, indicates a N35E trending extension on average. The two other couples, less important, S1-N1 and S2-N2 are related to N130E and N175E trending extensions on average, respectively. For each couple, the relationships Si-Ni involves simple permutation s1/s2 (Figure 15).
Considering the geometrical relationships between the directions of two major groups. Each group countains dominating S-type and N-type regimes, consistent with right-lateral transform motion, but also minor S-type and N-type compatible with left-lateral motion. One group is constituted by S3-N3 and S2-N2, the other one by S4-N4 and S1-N1. The dominating couple S3-N3 shows an angle of 25 between the trend of extension (s3) and of the Flatey segment of the transform zone. This behaviour of the transform zone reflects moderate mechanical coupling. In contrast, the couple S4-N4 shows an angle of 85 between extension and the transform direction (Figure 16).
This behaviour of the transform zone indicates very low mechanical coupling. The reversed regimes (S1-N1 and S2-N2) have little expression except near the transform, where large deformation occured. The drastic reversal s1/s3 relative to the dominating stress regimes S3 and S4 and the minor ones S2 and S1, respectively, probably results from elastic rebound, fault block accommodation and magmatic injection phenomena. Variations in mechanical coupling accross the Tjörnes fracture zone are the major source of the variations in the nature and orientation of tectonic regimes. Evidences for intermediate situations are few, suggesting that changes in coupling were abrupt rather than progressive.