Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex

Abstract

In volcanically and seismically active rift systems, preexisting faults may control the rise and eruption of magma, and direct the flow of hydrothermal fluids and gas in the subsurface. Using high-resolution airborne imagery, field observations, and CO 2 degassing data on Aluto, a typical young silicic volcano in the Main Ethiopian Rift, we explore how preexisting tectonic and volcanic structures control fluid pathways and spatial patterns of volcanism, hydrothermal alteration and degassing. A new light detection and ranging (lidar) digital elevation model and evidence from deep geothermal wells show that the Aluto volcanic complex is dissected by rift-related extensional faults with throws of 50-100 m. Mapping of volcanic vent distributions reveals a structural control by either rift-aligned faults or an elliptical caldera ring fracture. Soil-gas CO 2 degassing surveys show elevated fluxes (>>100 g m -2 d -1 ) along major faults and volcanic structures, but significant variations in CO 2 flux along the fault zones reflect differences in near-surface permeability caused by changes in topography and surface lithology. The CO 2 emission from an active geothermal area adjacent to the major fault scarp of Aluto amounted to ~60 t d -1 ; we estimate the total CO 2 emission from Aluto to be 250-500 t d -1 . Preexisting vol canic and tectonic structures have played a key role in the development of the Aluto volcanic complex and continue to facilitate the expulsion of gases and geothermal fluids. This case study emphasizes the importance of structural mapping on active rift volcanoes to understand the geothermal field as well as potential volcanic hazards.

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

“…Despite an extensive geological record of explosive volcanism in EARS, in the form of large calderas and widespread tephra layers (e.g., Hutchison et al, 2015), there have only been two historical eruptions with VEI ≥ 4: at Dubbi in 1861 and Nabro in 2011. Both were explosive in their initial stages, generating large, but unmeasured silicic tephra deposits, followed by large volume basaltic lava flows, suggesting that prior to eruption, batches of basaltic magma intersected highlevel bodies of trachyte magma.…”

“…Biggs et al (2016) used observations from the Kenyan Rift to show that even small changes in strain associated with minor unrest can affect multiple reservoirs beneath individual volcanoes, but typically do not extend to neighboring volcanoes at distances >10 km. The hypothesis could be tested by (1) improving the historical record by dating the numerous small-volume lava flows found at volcanoes in the EARS (e.g., Hutchison et al, 2015) and (2) constructing 3-D velocity fields from InSAR and GPS (e.g., Pagli et al, 2014).…”

“…This applies both to the pre-rifting basement rocks, mainly Proterozoic in age, whose inherited properties, for example crustal fault systems, may have become re-activated during rifting (Coblentz and Sandiford, 1994;Corti, 2009; Figure 2B) and to recent structures, including active rift faults and caldera ring faults, which have been shown to act as pathways for both magmatic and hydrothermal fluids (Hutchison et al, 2015). The most obvious heterogeneity is the presence of the Tanzanian Craton (Koptev et al, 2015) which effectively guides the rift as it splits into two arms around a deep keel of Proterozoic rocks.…”

Diffuse carbon dioxide emissions from hidden subsurface structures at Asama volcano, Japan

“…High CO 2 emissions were found in narrow areas along crater rims or old flank eruption fissures at Vulcano Island (Chiodini et al 1996) and Stromboli (Carapezza et al 2009). Recently, Hutchison et al (2015) presented evidence that, at Aluto volcanic complex, Main Ethiopian Rift, the fluid pathways are controlled not only by pre-and post-caldera structures at large scale but also by topography and surface lithology at small scale using soil CO 2 flux measurements and geomorphological analyses. This idea is based on recent thermal infrared imagery studies focusing on the fluid ascent in volcanoes (Schöpa et al 2011;Peltier et al 2012;Pantaleo and Walter 2014).…”

Soil CO2 Degassing Path along Volcano-Tectonic Structures in the Pico-Faial-São Jorge Islands (Azores Archipelago, Portugal)

“…Mantle-derived CO 2 can also be released away from active volcanoes through deep tectonic structures, as demonstrated by several studies (Chiodini et al, 1999(Chiodini et al, , 2010bJung et al, 2014;Lee et al, 2016). In fact, in the last 30 years, CO 2 spatial distribution has been used worldwide to identify hidden tectonic structures, since faults/fractures act as preferential pathways (high permeability zones) for the escape of gases from the deep crust or mantle to the surface (Giammanco et al, 1999(Giammanco et al, , 2006Baubron et al, 2002;Hutchison et al, 2015;Liuzzo et al, 2015). Based on the release of soil gases in confined areas, Chiodini et al (2001) named the anomalous CO 2 degassing areas, where hydrothermal/volcanic CO 2 is released, as diffuse degassing structures (DDS), whose shape depends on morphological, geological, and structural factors, such as the topography, existence of lithological heterogeneities, and presence of faults/fractures (Schöpa et al, 2011;Peltier et al, 2012;Pantaleo and Walter, 2014).…”