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

Significant fracture zone deformation has been imparted to Tertiary lavas on the peninsula (Flateyjarskagi) located between Eyjafjordur and Skjalfandi 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 Husavik-Flatey fault system (HFFS). The HFFS defines the present southern margin of the 70 to 80-km-wide Tjornes 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 Tjornes peninsula immediately to the east also have been tectonically rotated, implying an original shear zone width of at least 25 km.