Task 1.2: The Tjörnes fracture zone (TFZ)

The inversion of about 1000 fault-slip data collected in 20 sites in the Flateyjarskagi peninsula allows reconstruction of eight normal and strike-slip regimes (Angelier et al. 2000b; Bergerat et al. 2000) related to the general behaviour of the Húsavík-Flatey fault (HFF), a major structure of the Tjörnes fracture zone connecting the Kolbeinsey ridge and the North Icelandic rift. The two most important regimes (E-W and NE-SW extensions), consistent with the right-lateral motion along the Húsavík-Flatey fault, constitute the main tectonic group. The two others (NW-SE and N-S extensions), forming the subordinate tectonic group, are incompatible and result from drastic stress permutations (as a probable result of stress drop, elastic rebound and dyke injection). Figure 33 summarizes the local paleostress determinations made in Flateyjarskagi.

 

Figure: Summary of local paleostress determinations made in Flateyjarskagi using Angelier's inversion (1990) (after Angelier et al. 2000b). Solid bars: trends of minimum compressive stress, $\sigma _{3}$. Bar size increases with quality of paleostress tensor determination. Nature of brittle tectonic regime: S-type for nearly vertical $\sigma _{2}$, N-type for nearly vertical $\sigma _{1}$.

The relationships between these stress regimes imply not only $\sigma _{1}$/ $\sigma _{2}$ and $\sigma _{2}$/ $\sigma _{3}$ stress permutations, but also $\sigma _{1}$/ $\sigma _{3}$ reversals. An important factor controlling the transform mechanism is the variation of coupling along the HFF. The obliquity between the direction of transform motion and the trend of extension for the two main regimes may vary between $25^\circ$ and $85^\circ$, reflecting repeated changes of the coefficient of friction along the HFF (Figure 34). Such changes from very low mechanical coupling (weak fault) to moderate friction may occur very rapidly since it takes place several times in a few years, as shown by focal mechanisms of earthquakes analysis. Thus, the tectonic regimes need not be interpreted in terms of numerous tectonic episodes, but rather as a consequence of variable coupling across the transform zone.

 

Figure 34: Interpretation of tectonic regimes in terms of variable coupling near transform zone (after Angelier et al. 2000b). (a) Groups of tectonic regimes. Angles between transform direction (dotted lines) and averaged trends of extension (black arrows) indicated. (b) Schematic pattern of minimum compressive stress trajectories (thin lines) due to tensile loading parallel to spreading vector in North Iceland around oblique dextral transform fault zone (thick line).