Structure and dynamics of surface uplift induced by incremental sill emplacement Craig Magee1, Ian D. Bastow1, Benjamin van Wyk de Vries2, Christopher A.-L. Jackson1, Rachel Hetherington3, Miruts Hagos4, and Murray Hoggett5 1Department of Earth Science and Engineering, Imperial College, London SW7 2BP, UK 2Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France 3Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931, USA 4Department of Earth Sciences, Mekelle University, P.O. Box 231, Mekelle, Tigray, Ethiopia 5School of Geography, Earth and Environmental Science, University of Birmingham, Birmingham B15 2TT, UK ABSTRACT are simple and, critically, non-unique (Galland, Shallow-level sill emplacement can uplift Earth’s surface via forced folding, providing 2012). To address these problems, analyses of insight into the location and size of potential volcanic eruptions. Linking the structure and actively deforming forced folds for which the dynamics of ground deformation to sill intrusion is thus critical in volcanic hazard assess- geological history can be reconstructed are ment. This is challenging, however, because (1) active intrusions cannot be directly observed, required. The Erta’Ale volcanic segment in meaning that we rely on transient host-rock deformation patterns to model their structure; the Danakil depression, a rift basin in northern and (2) where ancient sill-fold structure can be observed, magmatism and deformation has Ethiopia (Fig. 1A), is a superb natural labora- long since ceased. To address this problem, we combine structural and dynamic analyses of tory for studying the surface expression of active the Alu dome, Ethiopia, a 3.5-km-long, 346-m-high, elliptical dome of outward-dipping, tilted magmatism. Geodetic data from several volca- lava flows cross-cut by a series of normal faults. Vents distributed around Alu feed lava flows noes within the Erta’Ale volcanic segment have of different ages that radiate out from or deflect around its periphery. These observations, linked ground deformation to the growth of sill- coupled with the absence of bounding faults or a central vent, imply that Alu is not a horst like reservoirs (Amelung et al., 2000; Nobile or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic et al., 2012; Pagli et al., 2012). In particular, aperture radar data captured a dynamic growth phase of Alu during a nearby eruption in Pagli et al. (2012) suggested that uplift and sub- A.D. 2008, with periods of uplift and subsidence previously attributed to intrusion of a tabu- sidence (of up to 1.9 m) during the A.D. 2008 lar sill at 1 km depth. To localize volcanism beyond its periphery, we contend that Alu is the eruption at the Alu-Dalafilla volcanic center first forced fold to be recognized to be developing above an incrementally emplaced saucer- (Fig. 1B) occurred in response to inflation and shaped sill, as opposed to a tabular sill or laccolith. deflation of a tabular sill, which correlates spa- tially with the Alu and Alu South domes. Here, INTRODUCTION underpins the application of geodetic data to we combine structural mapping and dynamic Emplacement of shallow-level magma res- volcanology. analyses of ground deformation and extrusions ervoirs is commonly accommodated by uplift Recent field-, modeling-, and seismic reflec- at Alu to constrain how magmatism has been of the overlying rock and free surface. Ground tion–based studies have questioned the accuracy expressed at the surface through time. deformation monitoring, such as by interfero- of intrusion geometries inverted from ground metric synthetic aperture radar (InSAR), can deformation. For example, geodetically derived STRUCTURE OF THE ALU DOME AND thus be used to track magma movement and intrusion shapes are too simple compared to the SURROUNDING LAVA FIELD accumulation (e.g., Pagli et al., 2012). A salient complex geometries seen in natural examples The actively extending Danakil depression assumption when inverting geodetic data for (e.g., Galland, 2012). In addition to elastic is a NNW-trending, 120-km-long, 50-km- intrusion shape is that the location, geometry, bending, inelastic space-making mechanisms wide half-graben that contains a thick (>1 km) and volume of surface uplift and/or subsid- can also accommodate magma, meaning that sequence of siliciclastics, carbonates, and evapo- ence equal those of the magma body (Galland, the volume of surface uplift may differ from rites (Hutchinson and Engels, 1970). The Alu 2012). This assumption is partly based on field that of the underlying intrusion (e.g., Galland dome, located ~30 km north of Erta’Ale and analyses of sills and laccoliths, which show that and Scheibert, 2013; Wilson et al., 2016). To covered by basaltic lavas (Figs. 1 and 2A), has space can be generated by folding of overlying fully utilize geodetic data when tracking magma previously been interpreted as either a horst (Bar- rock (e.g., Pollard and Johnson, 1973; van Wyk within and characterizing the plumbing system beri and Varet, 1970) or a volcano (Pagli et al., de Vries et al., 2014). These folds are termed of active volcanoes, it is critical to discern how 2012; Hagos et al., 2016). In addition to Alu, the forced folds, because their overall geometry intrusions are expressed at the Earth’s surface. Alu-Dalafilla volcanic center includes a 3-km- mimics and is controlled by the shape of an Constraining geometric and temporal rela- long basaltic ridge (i.e., the Alu South dome; underlying sill or laccolith (Stearns, 1978; van tionships between intrusion and forced fold- Pagli et al., 2012) and the Dalafilla stratovolcano Wyk de Vries et al., 2014). Forced fold growth ing is, however, challenging because (1) the (Fig. 1B) (Barberi and Varet, 1970). can influence the distribution, structure, and sta- dynamic, short-time-scale (<1 m.y.) evolution Satellite images reveal that Alu is elliptical bility of volcanoes (e.g., Magee et al., 2013a; of ancient forced folds, where magmatism has (3.5 × 2 km), with a NNW-trending long axis van Wyk de Vries et al., 2014). Understanding long since ceased, is difficult to elucidate (e.g., subparallel to the local rift axis (Figs. 1 and 2A). structural and dynamic interactions between Magee et al., 2014); and (2) geodetically derived Alu has a peak elevation of 397 m above sea intrusions, forced folding, and volcanism thus modeling solutions for transient intrusion events level and an amplitude of ~346 m, assuming a GEOLOGY, May 2017; v. 45; no. 5; p. 431–434 | Data Repository item 2017129 | doi:10.1130/G38839.1 | Published online 15 March 2017 ©GEOLOGY 2017 The Authors.| Volume Gold 45 |Open Number Access: 5 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 431 A B C N 2 km Sill contraction (m) 0 -2 -4 -6 Figure 2. A: Uninterpreted and interpreted Google Earth™ satellite images of Alu dome (Ethiopia; see Fig. 1B for key and location). Only major fractures and faults are interpreted. B: Topographic profile across Alu (see A for location) and position of sill modeled from InSAR data (Pagli et al., 2012). C: Uninterpreted Google Earth™ satellite image of Alu and Alu South and the same image overlain by a map of sill contraction facilitating subsidence recorded by InSAR data in 2008 (see Fig. 1B for location) (redrawn from Pagli et al., 2012). discrete lava flow channels, cross-cut by the array southwest and southeast of Alu are not associ- of fractures/faults, are observed on the slopes ated with clear fissures (Fig. 2A). Some of the and summit of Alu; pressure ridges, pahoehoe spatter cones to the southwest of Alu are aligned lobes, channel levees, and intra-flow islands sug- approximately northwest-southeast (Fig. 2A). gest that most lava flow directions are oriented Figure 1. A: Topography of Erta’Ale volcanic downslope with one oriented obliquely upslope ORIGIN AND GROWTH OF THE ALU segment (EVS), Danakil depression, Ethiopia. (Figs. 2A and 3). The only major vent is a spat- DOME AND SURROUNDING LAVA FIELD Ale Bagu (Al), Hayli Gabbi (H), Erta’Ale (E), ter cone, offset from the summit, which does not Previous interpretations that the Alu dome is Borale’Ale (B), Alu (A), Dalafilla (D), Gada’Ale (G), and Dallol (Da) volcanic centers are high- appear to feed dome lavas and is faulted (Fig. 2A). a horst (Barberi and Varet, 1970) or a volcano lighted. B: Uninterpreted and interpreted Cross-cutting relationships between lava Google Earth™ satellite image of Alu-Dalafilla flows surrounding Alu reveal a broadly basal- volcanic center and surrounding lavas (based tic to rhyolitic evolutionary sequence, with the on Barberi and Varet, 1970; Pagli et al., 2012). exception of a basalt eruption in 2008 (Fig. 1B) (Barberi and Varet, 1970; Pagli et al., 2012). Ropey flow textures and lobe terminations within horizontal pre-dome datum (Fig. 2B); the aspect the lavas indicate that extrusion sites are typi- ratio (L/A) of its length (L) to its amplitude (A) cally situated <2 km from Alu; most lavas appear is 10.1. The lateral limits of Alu, Alu South, and to emanate from point sources (e.g., the trachyte an underlying sill modeled using InSAR data are lava to the southwest of Alu) but others can be co-located (Fig. 2C); the inferred tabular sill is traced back to fissures that are either radial to 10 km long and located at a depth of 1 km (e.g., Alu (e.g., along its eastern base) or northwest Fig. 2B) (Pagli et al., 2012). An array of NNW- trending (e.g., to the west of Alu) (Figs. 1B and Figure 3. Levee-bound lava channel that and ENE-striking normal faults and tensional 2A).