Stratigraphy and K/AR ages across the west flank of the northeast Iceland Axial Rift Zone, in relation to the 7 MA volcano‐tectonic reorganization of Iceland

Abstract

Lithostratigraphy, magnetostratigraphy, and new K/Ar ages from the region between Eyjafjördur and Skjálfandi, 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.