Transform deformation of tertiary rocks along the Tjornes Fracture Zone, north central Iceland

Abstract

Significant fracture zone deformation has been imparted to Tertiary lavas on the peninsula (Flateyjarskagi) located between Eyjafjördur and Skjálfandi in north central Iceland. The region lies immediately south of the Flatey fault, one of a series of WNW trending, left‐stepping faults comprising the right‐lateral, oblique slip Húsavík‐Flatey fault system (HFFS). The HFFS defines the present southern margin of the 70 to 80‐km‐wide Tjörnes Fracture Zone (TFZ). Lava bedding, dikes, and faults in the western half of Flateyjarskagi display a progressive 0°–110° clockwise change of trend and steepening of lava dip angles to 15°–45° over an 11‐km‐wide zone nearest the Flatey fault. Accompanying this structural curvature is an increase in fault and dike frequency and amygdule/vein mineralization. Northeast striking faults develop predominant downthrows to the east approaching the TFZ, accommodating the steep lava dips via repetition of stratigraphic section and normal fault‐bounded block rotations. Although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations, the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear. Maximum on‐land shear strain has been crudely estimated at γ = 3.5; estimates of shear displacement suggest that the TFZ was active for at least 2 m.y. sometime prior to 6–7 Ma, at a strain rate of approximately 5.5×10−14 s−1. Antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished. WNW extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear. Paleomagnetic and structural evidence suggest that rocks of Tertiary age on Tjörnes peninsula immediately to the east also have been tectonically rotated, implying an original shear zone width of at least 25 km.

Fault interaction and stresses along broad oceanic transform zone: Tjörnes Fracture Zone, north Iceland

“…Regarding the slip amount along each of these three faults, the HFF, which is generally regarded as the main plate boundary structure of the TFZ, is marked onland as a geomorphological feature over a distance of 25 km across the Tjörnes peninsula and by a 3–5 km wide zone of intense deformation [e.g., Fjäder et al , 1994; Garcia and Dhont , 2005]. The minimum cumulative right‐lateral slip along the HFF is 5–10 km or 20 km and may be as large as 60 km [ Saemundsson , 1974; Young et al , 1985] whereas its vertical displacement is larger than 200 m and may reach 1400 m [ Gudmundsson et al , 1993]. The DF is thought to have absorbed little displacement because of its discrete onland expression.…”

“…(a and b) Maximal horizontal stress S Hmax inferred from inversion of fault‐slip data. Trends of dikes and trace of normal faults in the Flateyjarskagi peninsula [ Young et al , 1985] are also shown. (c) (S Hmax ) from focal earthquakes inversion.…”

“…In order to identify the stress states that reflect the dextral shear along the TFZ, we compared the paleostress states with the major structures of the TFZ. A detailed study of these structures in the southern part of the TFZ has been conducted by Young et al [1985]. These authors provided a detailed map of the dikes and normal faults within the eastern part of the block between the DF and HFF, in the Flateyjarskagi peninsula (Figure 2a and 2b).…”

Interaction between Transform Faults and Rift Systems: A Combined Field and Experimental Approach

“…In addition, Tryggvason (1973) suggested that such fault can reach an overall vertical offset of as much as 1400 m, although the location of this onshore measurement remains undefined. Saemundsson (1974), based on local geology, proposed a cumulated total component of rightlateral displacement ranging from 5 to 10 km near the town of Husavik whereas, according to Young et al (1985) it may amount to 20 km.…”

Infrastructure and evolution of ocean-ridge discontinuities in Iceland

“…The HFF has its main on-land exposure on the north coast of the Flateyjarskagi Peninsula (Young et al, 1985;Gudmundsson, 1993;Fjäder et al, 1994;Garcia et al, 2002;Garcia and Dhonit, 2005). There the fault is marked by Fig.…”