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.

Crustal modeling of the North Atlantic from spectrally correlated free-air and terrain gravity

“…While the Mid-Atlantic Ridge apparently overrode the Iceland hotspot from the east, the thermally active Iceland Plume accommodated eastward shifts in rifting as the lithospheric plates continued to move westward (Oskarsson et al, 1985). Stratigraphic analysis and radiometric dating of the active Eastern and Western Rift Zones indicate a recent overall eastward shift of rifting in Iceland at about 6-7 Ma (Jancin et al, 1985). Aeromagnetic results also show that rifting centered in western Iceland at ca.…”

“…The Iceland Plateau was created by ridge-centered and near-ridge hotspot magmatism and volcanism with maximum volcanic ages of 16 Ma that generally decrease towards the rift zones (Jancin et al, 1985). While the Mid-Atlantic Ridge apparently overrode the Iceland hotspot from the east, the thermally active Iceland Plume accommodated eastward shifts in rifting as the lithospheric plates continued to move westward (Oskarsson et al, 1985).…”

Glaciation, erosion, and landscape evolution of Iceland

“…The lack of continuous records elsewhere in Iceland through the Quaternary may be due to a lack of systematic research and radiometric dates; old glacial and interglacial deposits of unknown age have been reported from various sites all around Iceland. Tillites from the Middle Pleistocene have been reported from North Iceland (Jancin et al, 1985), and on the Snaefellsnes Peninsula in West Iceland (Fig. 3, Einarsson, 1994;Leifsdóttir, 1999), where paleomagnetic correlations suggests that glacial deposits were probably formed between 2 and 0.7 Ma.…”

Subvolcanic subsidence and caldera formation during subaerial seafloor spreading in Iceland

“…7-9 Ma basaltic lava fl ows cut by sparse NWtrending dikes. Generally, lava fl ows in the Skagi region dip gently west, toward the proposed Húnafl ói-Skagi rift zone, an abandoned rift zone created by hotspot-driven eastward migration of the spreading axis (Hardarson and Fitton, 1997;Jancin et al, 1985;Jóhannesson and Saemundsson, 1998b;Kristjánsson et al, 1992;Kristjánsson and Jónsson, 1998;Saemundsson, 1979;Fig. 2), rather than east toward the Neovolcanic zone.…”