The Sandfell Laccolith and ‘Dome of Elevation’

Abstract

. Introduction The eastern part of Iceland is built mainly of a great series of plateau basalts dipping at low angles to the west, and the structure is magnificently exhibited in the sides of the deep valleys and fjords which incise the 3000–4000-foot plateau. The traveller is impressed by the constancy of the almost flat-lying trap featuring, which persists in the region of the ‘stock’ intrusions to the south (Cargill et alii , 1928), so that Sandfell, a hill of light-coloured rock on the south side of Faskrudsfjord, at once attracts attention because the basalts are disposed at high angles in proximity to it—a circumstance which was noted by Thoroddsen (1906, pp. 268, 277). The hill is a splendid example of a laccolith similar in type to those of the Henry Mountains, Utah, and equally impressive in its exposure. The combination of conditions which made the intrusions of the Henry Mountains the type of their class is present here—approximately level strata, intense erosion, and absence of vegetation (Gilbert, 1877, p. 97). The main hindrance to the complete elucidation of the structure is the extensive screes—to which Sandfell owes its name. This paper is based upon field investigations made on four visits between 1914 and 1932. In the earlier stages of the work snow-drifts hid some of the higher parts, but there has been a marked amelioration of climate in latter years, and in August 1932 all the drifts had disappeared. The district is unmapped, and in 1928 the second author made

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

“…host-rock that unconformably underlies younger lavas and pyroclastic rocks unaffected by the laccolith (Figure 2F; Hawkes and Hawkes, 1933). The up-domed host-rock consists primarily of basalt lava flows, but also a ∼30 m thick rhyolitic andesite lava flow and a ∼10 m thick felsic tuff (T3) (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966).…”

“…The 11.7 Ma Sandfell laccolith is situated in Fáskrúðsfjörður ( Figure 1A) and composed of a feldspar-and quartz-phyric rhyolite (Martin and Sigmarsson, 2010;de Jesus Padilla, 2015). The Sandfell laccolith intruded along, and inflated within, an ∼30 m thick ignimbrite layer (named T2) in the Neogene lava-pile (Figure 2) (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966). The intrusion depth was determined to ∼540 m below the paleo-surface by measuring the thickness of the up-domed FIGURE 1 | (A) Map of locations of exposed fissure swarms and central volcanoes in the East fjords of Iceland (based on data from National Land Survey of Iceland, http://www.lmi.is/wp-content/uploads/2013/10/License-for-use-of-free-NLSI-data-General-Terms.pdf).…”

“…The up-domed host-rock consists primarily of basalt lava flows, but also a ∼30 m thick rhyolitic andesite lava flow and a ∼10 m thick felsic tuff (T3) (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966). The Sandfell laccolith may also be the source of a felsic tuff that crops out between the domed and younger undeformed lavas, the so-called Sandfell tuff (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966).…”

“…The up-domed host-rock consists primarily of basalt lava flows, but also a ∼30 m thick rhyolitic andesite lava flow and a ∼10 m thick felsic tuff (T3) (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966). The Sandfell laccolith may also be the source of a felsic tuff that crops out between the domed and younger undeformed lavas, the so-called Sandfell tuff (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966). The absence of compositional heterogeneities, internal contacts, and mafic enclaves within the Sandfell laccolith (Hawkes and Hawkes, 1933) indicates that the Sandfell laccolith formed by the intrusion of a single pulse or by the rapid succession of several continuous pulses of magma.…”

“…However, relict cryptodomes display extensive concentric internal deformation, including brecciated carapace and strong flow-banding and partially brecciated rims (Snyder and Fraser, 1963;Goto and McPhie, 1998;Goto et al, 2000Goto et al, , 2004Stewart and McPhie, 2003). The Miocene rhyolitic Sandfell laccolith intruded the flank lava-succession of the Reydarfjörður central volcano at a depth of ∼500 m (Hawkes and Hawkes, 1933;Gibson, 1963;Gibson et al, 1966;Martin et al, 2011;de Jesus Padilla, 2015), and can therefore be considered a cryptodome. Cryptodomes and laccoliths are sub-surface, dome-shaped magma intrusions that uplift and deform their host rocks (Gilbert, 1877;Minakami et al, 1951;Lipman et al, 1981).…”