Cracking the crust: Rifting and lateral magma transport at the early Holocene Sveinar-Randarhólar Fissure, NE Iceland

The ~11 ka Sveinar-Randarhólar Fissure (SRF), located in the neovolcanic Northern Volcanic Rift Zone (NRZ; Central East Iceland), is a ~ 75 km long, north-south trending fissure comprising of a discontinuous chain of scoria cones and basaltic lava flows distal to any central volcanoes. We report petrography, mineral-glass compositions, whole-rock major and trace elements, Pb-isotopes, and olivine O-isotopes, for 39 scoria and lava samples collected along the fissure, together with analyses of the rhyolitic Askja-Skolli (Askja-S) tephra previously linked to the same eruptive event. The geochemical uniformity indicates the SRF magma was derived from a compositionally homogeneous reservoir that experienced minimal modification during transport. The geochemical similarity between the SRF and the sub-glacial and early-Holocene eruptive products of the Askja central volcano, combined with their shared rift-axis alignment, supports the magma system beneath Askja as the source. The SRF eruption was likely contemporaneous with the Plinian Askja Skolli event that deposited the Askja-S tephra across distal reaches of the fissure. We suggest that rift initiation released accumulated tectonic stress, driving magma withdrawal from a mid-crustal (~8–10 km) reservoir beneath Askja. The resulting release of tectonic stress triggered eruption and caldera collapse, which in turn further facilitated lateral magma transport into the SRF. The eruption of the SRF into ice-free and lower-elevation terrain suggests that topographic gradient likely influenced vertical and horizontal magma migration. The SRF represents one of the longest Holocene fissure eruptions in the NRZ, with lateral magma transport extending ≥135 km. Its timing coincides with rapid deglaciation (~10–12 ka) in northeast Iceland, when ice unloading and lithospheric rebound enhanced melt production and eruption rates.