Off-rift volcanism in rift zones determined by crustal unloading

Abstract

When continents are stretched over a long period of time, deep elongated rift valleys form at Earth's surface and zones of ponded magma, centred beneath the rift, form at the crust-mantle boundary 1,2 . Ascending magma sometimes erupts within the rift valley 3,4 or, counterintuitively, at volcanic fields away from the rift valley that are o set by tens of kilometres from the source of magma at depth 5-8 . The controls on the distribution of this o -rift volcanism are unclear. Here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley, whereas others are confined to the valley. We find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure, caused by crustal thinning and faulting along the rift borders. When gravitational unloading dominates over tectonic stretching forces, dykes ascending from the ponded magma are steered towards the rift sides, eventually causing o -rift eruptions. Our model also predicts the formation of stacked magma sills in the lower crust above the magma-ponding zone, as well as the along-rift propagation of shallow dykes during rifting events, consistent with observations of magmatism and volcanism in rift zones globally. We conclude that rift topography-induced stress changes provide a fundamental control on the transfer of magma from depth to the surface.Continental rifts are commonly flanked by magmatism during their early stages 5,6 . In the Miocene-Recent Main Ethiopian Rift (MER), for example, many Pliocene volcanoes lie outside the Miocene rift border faults 9,10 ( Fig. 1). As the cumulative extension has increased, magmatism has become focused within the rift axis 3 . A similar pattern is observed during the evolution of the Red Sea Rift, where ∼25-Myr-old rift-parallel dykes focus near the ∼30-Myr-old rift margins and the youngest volcanoes focus along the ridge axis 11 . Even in far less magmatically active settings, such as the Baikal Rift (Siberia) or the Chaine des Puys (CdP) in the Cenozoic rift system of France, off-rift volcanism occurred after the onset of rifting 7,8 .During rift initiation, the lithosphere thins by ductile stretching beneath a rift valley formed from normal faulting 12 . The upwelling asthenosphere melts by adiabatic decompression, with the greatest degree of partial melting beneath the most thinned rift valley 1,2 . Although most studies focus on in-rift magmatism 3,4 , a model is needed to explain how off-rift volcanoes are fed. Progressive rift spreading results in the oldest volcanoes being transported away from the rift axis. However, such a process does not explain off-rift volcanism, with volcanoes located outside the rift border faults. Previous models for off-rift volcanoes include low-angle detachments tapping the asthenosphere to the sides of highly asymmetric rifts 13 , or the flexural response at the base of the footwall of the active rift border faults 5 . These models are unsatisfac...

Historical Volcanism and the State of Stress in the East African Rift System

“…Multiple factors combining to generate such composite fields have been advocated and analyzed: loading due to the edifice (e.g., Dahm, 2000;Pinel and Jaupart, 2000;Maccaferri et al, 2011) and unloading (e.g., Maccaferri et al, 2014), the effects of volcano morphology (e.g., Tibaldi et al, 2014;Corbi et al, 2015), the generation of magma reservoirs and calderas (e.g., Tibaldi, 2015) and the anisotropy of host rocks (Gudmundsson, 2011a). Many dykes do not propagate all the way to the surface, but may be arrested by layers with variable associated stress (Gudmundsson and Philipp, 2006).…”

“…Recent examples in the EARS include the 100-km long Dabbahu dyke intrusion in Afar (Wright et al, 2006) and the 2007 Lake Natron dyke intrusion in Tanzania (Calais et al, 2008;Biggs et al, 2009), which were both aligned perpendicular to the plate motion. However, superimposed upon the large-scale stress regime are local stresses related to topography, seismic and magmatic processes (e.g., Biggs et al, 2013b;Maccaferri et al, 2014;Pagli et al, 2014) and which are also seen to control the orientation of magmatic features, such as the Jebel al Tair eruption in the Red Sea (Xu and Jonsson, 2014) and the orientation of fissures around Oldoinyo Lengai in Tanzania (Muirhead et al, 2015).…”

How caldera collapse shapes the shallow emplacement and transfer of magma in active volcanoes

“…Topographic effects have been previously considered as a notch in the morphology. Such notches concentrate stresses around them and are found to influence dike propagation only at very shallow levels (few tens of m to few hundreds m; e.g., Gudmundsson, 1998;Gudmundsson, 2011), while gravitational unloading due to the removal of mass from the surface may lead to significant rotation of the principal stresses and affect the dynamics of magma propagation also at deeper levels (Hooper et al, 2011;Maccaferri et al, 2014). We test this possibility using numerical Finite Element (FE) models to calculate the stress field within a volcanic edifice decompressed during the formation of a caldera and investigate the expected orientation of the magma intrusions.…”

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

“…Moreover, dike pathways through the crust may not be vertical and may not coincide with pre-existing weaknesses in the host rock as is often assumed . Dike pathways are often tortuous as evidenced by recent studies [Bagnardi et al 2013] and strongly influenced by the distribution of surface loads [Maccaferri et al 2014, Corbi et al 2015, layering [Rivalta et al 2005, Maccaferri et al 2010 and faulting [Passarelli et al 2015. In addition to this, there are problems with the terminology used to define magma chambers: there is confusion when we refer to what different techniques are detecting at different time scales.…”