Volcano-Tectonic Evolution of Flateyjarskagi, North Central Iceland

• Main Author: Jancin, Mark D.
• Other Authors: Barry Voight, Terry Engelder, Derek Elsworth, Donald Fisher
• Format: Text
• Language: English
• Published: Penn State 2010
• Subjects: Geoscience; Dalsmynni; Tjörnes; Iceland
• Online Access: http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-5126/index.html

Lithostratigraphy, magnetostratigraphy, and K/Ar ages from Flateyjarskagi, 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 Northern Volcanic Zone (NVZ). 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 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. 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. Radiometric ages, regional isochron patterns and timing constraints for Tjörnes 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-NVZ 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 NVZ developed. We provide the first direct confirmation of large distributed rotational shear along an active transform within autochthonous oceanic crust, based on work along the Tjörnes 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. Regionally distributed shear deformation has been reported from many continental transcurrent and transform settings, and from allochthonous oceanic crust in some ophiolites, but there are few submarine analogs. Rotational normal faulting in north Flateyjarskagi has caused NW lava dips of 30°-45°. Kinematic stereonet modeling suggests that the idealized, incremental combination of clockwise rotations about vertical axes, with normal fault rotations about horizontal axes, can lead to the apparent over rotation of the paleomagnetic declinations by ~ 40°-80°. This potential issue is geometrically tied to the steep inclinations (e.g., 75°) of the paleomagnetic reference field directions associated with Icelands latitude. 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 ~ 7 Ma when activity ceased along the western Snæfellsnes-Skagi Zone (SSZ). However, this is not compatible with observations; multiple lines of evidence show that spreading about a proto-NVZ has occurred since ~ 13+ Ma. Such proto-NVZ spreading may have started as early as ~ 26 Ma. Both on-land and marine evidence suggest that prior to ~ 7 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. Robust conceptual models of crustal accretion in north central Iceland need to incorporate two unusual geologic characteristics that are spatially associated over a width of ~ 125 km: (1) the regional presence of high obliquity angles in flows from ~ 7-11+ Ma, which imply offlap of volcanic production to the south over that time period; and (2) the inferred presence of submerged oceanic crust that may be as old as 37 Ma. Flateyjarskagi shear zone deformation started ~ 9.5 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 ~ 9.5 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 ~ 13% along a WNW-ESE direction. The TFZ may be a very long-lived transform zone that started ~ 26 Ma, sharing characteristics with a highly oblique rift zone. A large area of approximately E-W trending lava isochrons in the Tertiary series of north central Iceland, in flows from > 11 Ma to ~ 7 Ma, has a N-S extent of ~ 150 km. Such isochrons are subparallel to presumed plate flow lines; in other areas of Iceland, as along the crust flanking most submarine spreading ridges, isochrons are approximately perpendicular to plate flow lines. 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 ~ 7 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 ~ 9.5 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.