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Lithostratigraphy, magnetostratigraphy, and new K/Ar ages from the region between Eyjafjordur and Skjalfandi, north central Iceland, reflect tectonic reorganization of crustal accretion zone activity ∼7 Ma. Two flood basalt piles are in unconformable contact along this western flank of the Northeast Iceland Axial Rift Zone (NEIARZ). The older ranges in age from ∼9.5 to 13 Ma and is largely composed of tholeiite flows. In eastern Dalsmynni the upper to middle portions of this older basaltic pile define a 15°–35° SE dipping, monoclinal flexure developed ∼6–7 Ma during early development of the present NEIARZ. Lavas of a flood basalt group younger than about 6.5 Ma were deposited unconformably on the older, flexured basalt pile. These postflexure age flows mostly consist of compound doleritic basalts and olivine tholeiites; possible tillites are found in the upper portion of this sequence. The unconformity between the older and younger basalt piles represents a major structural, temporal, and lithological boundary in north central Iceland. Zeolite zones appear subhorizontal through the Dalsmynni flexure zone and have been largely reset, subsequent to flexuring, by westward onlap of the younger basalt sequence onto the older subsided and flexured flows. Radiometrie ages, regional isochron patterns and timing constraints for Tjornes Fracture Zone-related shear deformation indicate the model of a singular, ∼130 km eastward “jump” of the axial rift zone in north Iceland, ∼6–7 Ma, is too simple. Crustal accretion older than 12 Ma along a “proto-NEIARZ” is apparently required, indicating that northern Iceland has had multiple-branched crustal accretion zones up until about 7 Ma, at which time a single spreading zone along the present NEIARZ developed. An eastward shift of axial rift zone activity ∼6–7 Ma in southwestern Iceland is time correlative with reorganization of spreading activity in northern Iceland. Our proposed 6–7 Ma reorganization of axial rift zone activity on land is time correlative with an independently proposed increase in discharge from the Iceland mantle plume.

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.