Volcano-tectonic evolution of Flateyjarskagi, north central Iceland

Lithostratigraphy, magnetostratigraphy, and K/Ar ages from Flateyjarskagi, north central Iceland, reflect tectonic reorganization of crustal accretion zone activity &sim7 Ma. Two flood basalt piles are in unconformable contact along this western flank of the Northern Volcanic Zone (NVZ). The older ranges in age from &sim9.5 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 &sim6--7 Ma during early development of the present NVZ. Lavas of a flood basalt group younger than about 6.5 Ma were deposited uncomformably 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.We provide the first direct confirmation of large distributed rotational shear along an active transform within autochthonous oceanic crust, based on work along the Tjornes Fracture Zone (TFZ). Over an 11-km broad region, dike and bedding composite trends show progressive northward clockwise curvature up to 110°, but a tectonic shear interpretation for this curvature has been controversial. Paleomagnetic analyses of three lava sequences in the zone of pronounced structural curvature indicate large clockwise rotations about apparent vertical axes have occurred, 98°--113° at two sites, and 149°--164° at the most deformed site, depending on the chosen reference declination. Comparisons of mesofracture orientations (n=2041), from the southern and northern parts of the study area, are consistent with and permissive of the shear zone hypothesis.Much of the tectonic literature on Iceland states that spreading in north Iceland has always occurred along a single rift zone that migrated or jumped to the east and that the NVZ developed &sim7 Ma when activity ceased along the western Snaefellsnes-Skagi Zone (SSZ). However, this is not compatible with observations multiple lines of evidence show that spreading about a proto-NVZ has occurred since &sim13+ Ma. Such proto-NVZ spreading may have started as early as &sim26 Ma. Both on-land and marine evidence suggest that prior to &sim7 Ma, north Iceland experienced spreading along parallel (or dual) volcanic rift zones, as presently occurs in south Iceland along the Western Volcanic Zone and Eastern Volcanic Zone.Flateyjarskagi shear zone deformation started &sim9.5 Ma, which apparently formed in conjunction with spreading along a forerunner of the NVZ. Along the latitude of Flateyjarskagi, this proto-NVZ was apparently largely amagmatic from &sim9.5 to 7 Ma, after which buildup of the major younger basalt pile in this area started. In northern Flateyjarskagi, this amagmatic period apparently coincided with (1) large tectonic rotations about subvertical axes due to TFZ shearing, and (2) the imposition of the steep NW lava dips that are associated with large rotational normal faulting that reflects regional extensional strain of &sim13% along a WNW-ESE direction.We suggest that over much of north central Iceland, a broad E-W trending region between the SSZ and proto-NVZ underwent diffuse, non-zonal crustal accretion. Lava isochrons and dips indicate that this broad region of volcanism retreated to the S over a time period from > 11 Ma to &sim7 Ma. The duration of the hiatuses flanking the NVZ generally decreases from north to south, and they apparently disappear given the existence of several very long, continuous stratigraphic sections which lack significant hiatuses. This suggests that volcanic production along central Iceland has been quite continuous during the last 13 Ma, and that the NVZ hiatuses were developed by a combination of (1) southward retreat of the zonal volcanism along a proto-NVZ from &sim9.5 to 6.5--7 Ma (after which renewed zonal volcanism along the re-established NVZ occurred as far north as Flateyjarskagi, resulting in the accumulation of the younger basalt pile), and (2) the age distribution of the flexured, older basalts that were buried by the renewed NVZ volcanism. Volcanic rift zone activity at a given location along most of the NVZ has been relatively transient, while more southerly ("hotspot"?) volcanism in central Iceland has persisted for at least 13 Ma. (Abstract shortened by UMI.)...