Three-dimensional geometry of concentric intrusive sheet swarms in the Geitafell and the Dyrfjöll volcanoes, eastern Iceland

Abstract

[1] Sheet intrusions (inclined sheets and dykes) in the deeply eroded volcanoes of Geitafell and Dyrfjöll, eastern Iceland, were studied at the surface to identify the location, depth, and size of their magmatic source(s). For this purpose, the measured orientations of inclined sheets were projected in three dimensions to produce models of sheet swarm geometries. For the Geitafell Volcano, the majority of sheets converge toward a common focal area with a diameter of at least 4 to 7 km, the location of which coincides with several gabbro bodies exposed at the surface. Assuming that these gabbros represent part of the magma chamber feeding the inclined sheets, a source depth of 2 to 4 km below the paleoland surface is derived. A second, younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm. The source of this second swarm is estimated to be located to the SE of the source of Swarm 1, below the present-day level of exposure and deeper than the source of the first swarm. For the Dyrfjöll Volcano, we show that the sheets can be divided into seven different subsets, three of which can be interpreted as swarms. The most prominent swarm, the Njardvik Sheet Swarm, converges toward a several kilometers wide area in the Njardvik Valley at a depth of 1.5 to 4 km below the paleoland surface. Two additional magmatic sources are postulated to be located to the northeast and southwest of the main source. Crosscutting relationships indicate contemporaneous, as well as successive activity of different magma chambers, but without a resolvable spatial trend. The Dyrfjöll Volcano is thus part of a complex volcanic cluster that extends far beyond the study area and can serve as fossil analog for nested volcanoes such as Askja, whereas in Geitafell, the sheet swarms seem to have originated from a single focus at one time, thus defining a single central volcanic complex, such as Krafla Volcano.

Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber

“…A test for the existence of multiple centres would be to analyse systematic cross-cutting relationships between cone-sheets of the different centres (cf7.). Ardnamurchan cone-sheets assigned to different centres generally occur in geographically distinct areas3, but where cone-sheets of Centres 1 and 2 should be adjacent, exposure is poor and no intersection has been observed.…”

“…Down-dip projection for a distance of 5000 m along a linear trajectory was then performed. The assumption of a linear trajectory is a necessary simplification as the subsurface geometry of cone-sheets in Ardnamurchan is not known (cf7. ), but is expected to broadly follow the outcrop measurements (cf3.…”

“…Anderson (1936)4 based his pioneering conceptual model of the emplacement mechanics of cone-sheets and ring-dykes largely on the Ardnamurchan example. Cone-sheets have recently gained renewed attention for their fundamental role in building the interior of long-lived volcanoes, for feeding surface eruptions, and because cone-sheet swarms may be used to infer depth, size, and shape of underlying magmatic source(s) (cf567. ).…”

Laboratory Modelling of Volcano Plumbing Systems: A Review

“…Other good examples are found in the Canary Islands (Ancochea et al 2003), Galápagos Islands (Chadwick and Howard 1991;Chadwick and Dieterich 1995), and Iceland (e.g. Schirnick et al 1999;Walker 1999;Klausen 2004;Burchardt et al 2011). Despite the prominence of cone sheets, little is known about their much-debated emplacement mechanisms (e.g.…”

Seismic velocity structure of a fossilized Icelandic geothermal system: A combined laboratory and field study

“…1), the geology of which has been extensively studied in the past (e.g. Fridleifsson, 1983;Burchardt et al, 2011). Due to the effects of rifting and subsequent erosion, the once deeply buried igneous formations are now exposed at the surface, enabling the collection of rock samples for laboratory velocity determinations and the conduct of shallow seismic tomography experiments.…”