The architecture of two Neogene olivine basalt lava groups in eastern Iceland

The architecture of the Hólmar and Grjótá olivine basalt groups within the Neogene flood basalts of eastern Iceland is assessed. The Hólmar group can be traced over 45 km north-south, encircling and partly covering the Reydarfjördur central volcano, thickest at Nordfjördur with >20 lava flows and decreasing to ~9 flows where thinnest. The approximate minimum volume of the group is 60 km3. Four scoria deposits with several olivine basalt dikes were identified within Hólmar and scattered around the Reydarfjördur central volcano. Grjótá olivine basalt group is stratigraphically located above Hólmar, separated by few tholeiite flows. The group is thickest at Reydarfjördur, with ~30 lava flows also thinning down dip to ~10 flows, and with minimum volume of 100 km3. The total volume for both groups might exceed well over twice as large as the calculated minimum. Two thick olivine dolerite sills cross cut Hólmar, within the Reydarfjördur central volcano, thus candidates for having fed the Grjótá olivine basalt group. The structure and lava morphology of both groups are identical, with the exception that Grjótá has embedded a series of very thick (15-20 m) and extensive (> 1 km long) lava flows. In general, the lava flows within the groups are highly compound, with lobes stacked horizontally and vertically, varying from 1-15 m thick and 2-~200 m long, often directly emplaced or welded, but also interbedded with thin redbeds and occasionally thicker tuff deposits. Several lava tubes were located, all having the characteristic of being filled, and with a general trend perpendicular to the regional dike swarm. Tree molds were occasionally identified between flow units. The internal structure of the studied lava lobes is characteristic for lava lobe morphology in general, with an upper vesicular crust forming half to one third of the total thickness, a massive core with abundant vesicle cylinders and a thin lower vesicular crust. Flow tops are of pahoehoe type, seldom with scoriaceous rubble or clinker. Inflation structures as tumuli and inflation clefts were identified in few lobes but absent in others. General surface and internal features of the lava flows in both groups, suggest the olivine basalts flowed with low viscosity from vents, likely along fissures, feeding lobes that developed internal insulated pathways. These lava lobes advanced out of the vents controlled by the pre-existing topography, many inflating to invert the lobate topography. The parasitic and monogenetic nature of the scoria cones and the location of the olivine dolerites suggest that the magmatic system feeding these groups pertains to or intersects the Reydarfjördur volcanic center. It is also evident that the large and dense, uniform spatial distribution of these groups, compassing numerous thick lava units, suggest high magma production rates, although each fissure system and vent might have developed differing eruption dynamics, varying from high to low effusion rates.