Middle Miocene rifting and volcanic history of the Berufjordor- Breiddalur region, eastern Iceland revealed by 40Ar/39Ar geochronology

Abstract:

Eighteen new 40Ar/39r incremental heating analyses of groundmass concentrates from fresh holocrystalline interiors of basalt lavas and dikes collected in an E-W, 35-km-long transect across the Berufjordor- Breiddalur region, eastern Iceland shed important new light on the Miocene spreading history and age of the Breiddalur central volcano. Despite fine-grain sizes and low K contents, most samples yield high quality ages (either simple plateaus or spectra indicative of modest recoil ± low T argon loss) with estimated uncertainties of ±0.1 to 0.2 Ma. Ages decrease monotonically westward in the eastern half of transect from 12.1 to 10.0 Ma over an 18 km distance, in excellent agreement with the estimated half spreading rate of 0.9 cm/yr. In contrast, the western 15 km of the transect (and ~ 1 km of “apparent” vertical succession), including lavas below, above and within the Breiddalur volcano shows no systematic westward younging - all lavas in this region range from 9.1 to 9.8 Ma. Ages from diabase dikes similarly range from 9.1 to 9.8 Ma, except one distinctly younger at 7.8 Ma.

The simplest interpretation of the new age determinations is that up until ~9 Ma, a ≥15 km-wide rift zone/plate boundary was situated in the eastern half of the transect. The entire rift zone (including Breiddalur volcano) was then accreted to the Eurasian plate during a westward rift jump of ≥ 15 km. These types of minor jumps in the plate boundary and accretion of entire rift zones to one plate or the other may help explain the “excess width” of Iceland. Also, the data raise questions about the strict applicability of the Palmeson (1973) model, as in this case, a paleo rift zone is not reflected by a dip reversal, and “proximal” or within rift volcanic and plutonic rocks are well exposed at the present surface and are not buried by younger flows as the spreading center moves away.