Mapping Hydrothermal Alteration at the Fentale-Dofan Magmatic Segment of the Main Ethiopian Rift

Abstract

The Main Ethiopian Rift (MER) is characterized by extensional tectonics and volcanism, associated with active hydrothermal systems, hydrothermal alteration and fumarolic deposits. The spatial distribution of these hydrothermal products and their link with faults and rock types provides important clues to what controls fluid flow in the subsurface. However, little is known about this in the East African Rift. We address this issue with a multidisciplinary approach in the Fentale-Dofan magmatic segment of the MER, an area characterized by intense volcanic and tectonic activity and a geothermal prospect. Primarily we conduct mapping of hydrothermal alteration and fumarolic deposits, and rock lithologies using a surface feature classification technique of multispectral satellite images. Then we interpret the map using a new database of faults and active hydrothermal manifestations such as hot-springs and fumaroles. We find that the surface hydrothermal alteration and deposits are mainly focused near Fentale and the Dofan Volcanic Complex (DVC). At DVC the hydrothermal products are focused on rhyolites on the western side of the volcano, in an area of intense NNE striking, rift parallel faults. At Fentale volcano the hydrothermal products are mainly associated with ignimbrite and show a circular pattern around the volcanic edifice, but also in places follow the NNE striking faults. At Fentale, the more complex association of hydrothermal products and active manifestations around the edge of the ignimbrite suggests formation contacts may also localize fluid flow in places. At both volcanoes the association between hydrothermal products with either the rhyolites and ignimbrites is likely due to them being relatively easily altered (in comparison to basalt), and also their brittle nature allows for fracturing through which localized fluid flow can occur (as opposed to the sediments). The general pattern of hydrothermal products suggests a stronger structural influence at the DVC with respect to Fentale. The presence of hydrothermal products and active hydrothermal manifestations, along with other lines of evidence such as locus of subsurface dike intrusion at the volcanic centres, suggest that discrete and localized magma reservoirs beneath Fentale and the DVC are the heat source for hydrothermal circulation. Our study also demonstrates that geology, including hydrothermal deposits, can be successfully mapped using automated remote sensing based classification.

Evidence of Fluid Induced Earthquake Swarms From High Resolution Earthquake Relocation in the Main Ethiopian Rift

“…The depth range of most of the seismicity (5–15 km) suggests that the source of the fluid is not linked to the shallow (<5 km‐deep) meteoric driven hydrothermal system. This is supported by the lack of spatial correlation between our seismic catalog and the surface distribution of active and fossil hydrothermal features (Raggiunti et al., 2021). The depth of our fluid induced seismicity is instead more likely to be related to fluid migration from below, as has been interpreted in a number of other continental rifts.…”

“…The FD magmatic segment is a half‐graben bounded by faults with 60–80 m high scarps on the eastern flank (Casey et al., 2006). It is characterized by intense seismic activity (Keir et al., 2006), as well as the presence of hydrothermal and fumarolic activity and numerous hot springs (Mohr & Gouin, 1976; Pürschel et al., 2013; Raggiunti et al., 2021; Wolfenden et al., 2004). The segment includes two main Quaternary‐Recent volcanoes: the Dofan volcanic complex (DVC) and the Fentale volcano (Wolfenden et al., 2004).…”

“…The morphology of the DVC is characterized by a series of subparallel, ∼10 km‐long, NNE − SSW striking faults with scarps that range from few tens to ∼300 m that cut the volcanic complex. In the central sector of the complex the faults create a ∼NNE − SSW striking graben while across the rift there is a WNW − ESE alignment of craters and cones (Raggiunti et al., 2021). The second volcano in the magmatic segment is Fentale volcano.…”

“…The second volcano in the magmatic segment is Fentale volcano. The morphology of Fentale is dominated by a summit caldera with an elliptical shape ∼3.5 km wide, 300 m deep and elongated WNW − ESE (Maestrelli et al., 2021; Raggiunti et al., 2021). The southern portion of the Fentale caldera is cut by a series of NNE − SSW striking extensional fissures, associated with, and parallel to, the major fault system, the Wonji Fault Belt (Williams et al., 2004).…”

Temporal clustering of fissural eruption across multiple segments within the Ethiopian Rift

“…Frontiers in Earth Science frontiersin.org segments are typically 40-70 km long, ~20 km wide and include the majority of the Holocene volcanic centers, monogenetic cone fields, and fissure swarms (Ebinger and Casey, 2001) (Figure 1). The magmatic segments are also the locus of modern day strain, with seismicity, geodetically detected volcano deformation and dike intrusion, and hydrothermal activity focused within them (Keir et al, 2006;Hunt et al, 2019;Temtime et al, 2020;Raggiunti et al, 2021;Albino et al, 2022). Subsurface, wide-angle seismic reflection/refraction profiles (Mackenzie et al, 2005;Maguire et al, 2006) as well as controlled source P-wave tomography (Keranen et al, 2004) image high P-wave velocities in the upper and mid crust beneath the magmatic segments.…”