Category: Newsletter articles

Erica Emry¹, Andrew Nyblade², and Yang Shen³

¹New Mexico Tech, ²Penn State University, ³University of Rhode Island

The East African Rift System (EARS) was one of the GeoPRISMS primary sites within the theme of Rift Initiation and Evolution, because of the variety of rifting stages and styles exhibited along this margin and because of the number of science questions that can be addressed there. Along this margin and in neighboring regions of Africa, Europe, and the Middle East, many broadband seismic instruments have been previously deployed, and numerous studies have explored the subsurface structure over a broad range of scales. However, there is often a disjoint between features that had been previously imaged through smaller-scale, regional tomographic inversions and those imaged by larger-scale inversions. In a recent tomographic study of the upper mantle beneath Africa, we used a full-waveform tomography method, constrained by long-period signal from ambient seismic noise to image the upper mantle beneath Africa to the top of the mantle transition zone (Emry et al., 2019). We found good agreement with prior models, at both large and regional scales, and we imaged new features in higher detail beneath more poorly resolved segments of the EARS. Here, we highlight the overall patterns along the EARS and focus on the complexity observed beneath the Turkana region.

What did we do?

We gathered continuous seismic data for more than 800 seismic stations and extracted Rayleigh waves from ambient seismic noise at periods as high as 340 seconds (Shen et al., 2012). Long period signal is valuable, because it is sensitive to structure deeper in the upper mantle and allows us to resolve down to about 350 to 400 km. Of the more than 800 seismic stations, we identified stations that provided clear signal at 40-340 seconds and used them to constrain our inversion (Fig. 1). This was a new set of data that had not yet been used to image the deeper lithosphere and asthenosphere beneath Africa.

Figure 1. Station map modified from Emry et al. (2019). Cratons are outlined in thick black lines. Blue triangles denote stations for which ambient noise data were collected and red triangles show stations that were used to invert for tomography. Abbreviations are as follows: AF-Afar, AP-Arabian Peninsula, DB–Damara Belt, KpC–Kaapvaal Craton, LR–Luangwa Rift, MER–Main Ethiopian Rift, MR–Malawi Rift, OR–Okavango Rift, RVP–Rugwe Volcanic Province, SS–South Sudan, TC–Tanzania Craton, TD–Turkana Depression, VVP–Virunga Volcanic Province, ZC–Zimbabwe Craton.

Figure 2. Two depth slices showing shear velocity at a) 165 km and b) 424 km, modified from Emry et al. (2019). For each depth, the color scale (m/s) is centered around the shear velocity in AK135 for that depth. Coastlines are shown as thin black lines, gray and blue lines indicate velocities that are 1.7% and 5% greater than AK135 model. Gray triangles show stations that inform the inversion. Abbreviations are as in Figure 1.

Although other seismic phases are often used to constrain full-waveform tomographic models, we used Rayleigh waves, as it is the principal phase extracted from seismic ambient noise. We used high-performance computing (HPC) clusters at the University of Rhode Island Graduate School of Oceanography to simulate waves propagating through a laterally variable Earth structure. Once synthetic waveforms were calculated for each seismic source in the model, we measured misfit between synthetic Rayleigh waves and those extracted from the data, determined the volume of Earth that influences the traveling wave, and inverted to identify a better-fitting model. For each new model, these steps were repeated until minimal change was made to the model. Our final results provide the absolute, isotropic, shear wave velocity (Fig. 2).

New results from the East African Rift System

There were many similarities between our results and prior studies of the EARS in regions where dense seismic or magnetotelluric arrays have been previously located (Benoit et al., 2006; Bastow et al., 2008; Adams et al., 2012; Mulibo and Nyblade, 2013; O’Donnell et al., 2013; Civiero et al., 2015; Gallacher et al., 2016; Accardo et al., 2017; Yu et al., 2017; Sarafian et al., 2018). As in prior models, we saw abundant indications for mantle upwellings or plumes as well as a pattern of lower velocities at shallow upper mantle depths in the northern EARS and higher velocities at shallow depths in the southern EARS. However, in our results, the patterns of low-velocities at middle upper mantle depths were laterally discontinuous along the full length of the EARS, and we imaged variable lithospheric topography that may influence the shallow flow of mantle upwellings.

Segmented upwellings beneath East Africa

Beneath the EARS, we imaged low-velocities at mantle transition zone (MTZ) depths, but at middle upper mantle depths, we imaged persistent patterns of separation between low-velocity features. While we have confidence in the pattern of separation within the upper mantle, we cannot resolve small features at deep depths and therefore cannot be certain whether the separation at shallower depths continues into the MTZ. At the shallowest upper mantle depths, the low-velocities appear to be overall more connected than at the middle upper mantle and are located mostly beneath the rift axis. In many regions, at shallow and middle upper mantle depths, the low-velocity anomalies are located adjacent to or between high-velocity features.

This pattern provides an overall sense that distinct buoyant upwellings, presumably of a thermal or thermochemical nature, are rising through the upper mantle and that their paths are likely influenced at shallow depths by rigid, presumably lithospheric, structures. Ultimately, it appears that these upwellings are sourced from MTZ depths. Such a pattern of secondary upwellings could be sourced by a deeper, ponded anomaly at or beneath the mantle transition zone, as has been previously suggested for the EARS from seismic and geochemical observations (Kieffer et al., 2004; Furman et al., 2006; Bastow et al., 2008; Huerta et al., 2009; Mulibo and Nyblade, 2013; Civiero et al., 2015). This pattern of buoyant upper mantle upwellings appears to be occurring along much of the EARS, and also in some other regions of Africa, however we note that fewer upwellings were imaged beneath the less evolved southern and western segments. In our view, this may be due to the history of upwellings or to the generally thicker lithosphere in the south and west that may act to divert upwellings.

Complex upper mantle beneath Turkana

One region that is most suggestive of a complex upwelling and diversion process is beneath the Turkana and South Sudan region. Here, the upper mantle has been difficult to image due to a lack of broadband seismic instrumentation. The Turkana segment is part of the primary EARS focus site and is particularly unique with regards to other segments of the EARS, because of the broad, diffuse rifting pattern and history of previous rifting oblique to the current-day trend (Brune et al., 2017; Ebinger et al., 2017).

Beneath this region, the indication of a low-velocity anomaly at deep upper mantle and mantle transition zone depths was most prominent (Fig. 3c). Directly above this, at middle upper mantle depths, a high-velocity feature was identified in the west beneath South Sudan, and the lowest velocities at these depths were located immediately adjacent to the high-velocity feature, to the north and to the southeast and southwest (Fig. 3b). Above this, at the shallowest upper mantle depths, the lowest velocities were imaged to the east beneath Lake Turkana. At these shallowest depths to the west beneath South Sudan, slightly slow to average upper mantle velocity was observed, while the fastest structure was located to the south and southwest beneath the laterally continuous Uganda and Bomu-Kibalan Cratons (Fig. 3a).

Figure 3. Three depth slices from the Turkana-South Sudan and Ethiopian Plateau regions showing shear velocity at a) 123 km, b) 260 km, and c) 424 km, modified from Emry et al. (2019). Cross-sections correspond to lines plotted on a. Abbreviations explained in Figure 1.

This pattern may suggest that rising asthenosphere is being diverted north and south around a lithospheric structure within the middle upper mantle beneath South Sudan. However, it is difficult to be certain of the spatial relationship and possible connection between this high-velocity feature and the Uganda and Bomu-Kibalan Cratonic roots located at shallower depths to the south and southwest. At this this point we can only speculate whether the structure is part of a stable, deep lithospheric root or whether it is sinking or foundering lithosphere (see discussion in Emry et al., 2019). However, we expect that this feature may affect the style of rifting, patterns of magma-rich vs. magma-poor extension, and connections between the Main Ethiopian Rift and the Eastern and Western Branches.

Summary

Overall, the EARS shows variability in lithospheric topography and reveals regions where the lithospheric structure may be affecting the path of upwellings at shallow and middle-upper mantle depths. However, there is also a clear sense of distinct upwellings within the upper mantle that might be sourced from a common, deeper anomaly. Our results of the upper mantle and mantle transition zone are useful in understanding the spatial relationships and possible connections between different segments and we hope that they will aid the overall goal of synthesis.

Acknowledgments

AfricaArray seismic stations in Dodoma, Tanzania. The photo was taken looking down into the 4-m deep vault at the Geological Survey in Dodoma, Tanzania. Photo credit: Andy Nyblade.

AfricaArray seismic stations in Zomba, Malawi. The photo was taken looking down into the 3-m deep seismic vault shown in photo at left at the Geological Survey in Zomba, Malawi. Photo credit: Andy Nyblade.

We thank the NSF Earth Sciences Postdoctoral Fellowship program for supporting this research (EAR-1349684). The shear velocity model from Emry et al. (2019) is available through the IRIS-EMC and through the GeoMapApp tool. We thank Manochehr Bahavar from the IRIS-EMC and Andrew Goodwillie from IEDA-GeoMapApp for helping to format the model and make it available.

Because our data came from ambient seismic noise, it was necessary that stations had temporally overlapping records. In this regard, the sparsely distributed long-term seismic deployments, such as the GSN, GEOSCOPE, AfricaArray (see photos), and MedNet were irreplaceable, and allowed us to also incorporate several 1-2 year (‘PASSCAL-type’) seismic deployments throughout Africa and the surrounding regions. ■

Reference information

Rebecca Bendick¹, Mike Floyd², Elias Lewi³, Gladys Kianji⁴, Robert King², El Knappe¹

¹University of Montana, ²MIT, ³Addis Ababa University, ⁴University of Nairobi

The East African Rift System is a complicated set of extensional structures reaching from Malawi in the south to Eritrea and Djibouti in the north (Fig. 1)(e.g. Ebinger, 2005). These structures are broadly interpreted as the expression of the ongoing breakup of the African continent into a “Somali” block moving east or northeastward relative to a “Nubia” block, with perhaps additional smaller blocks (e.g. Saria et al., 2014) also involved. The details of the kinematics, the presence or importance of entrained microplates, and even the components of the force balance exciting relative block motions and extensional strains are all the subject of ongoing research and incompletely resolved scientific debates.

Figure 1. Overview of the EARS, with shaded topography in gray, major faults in red, recorded seismicity of Mw>5 as blue circles, and generalized kinematic velocities from Saria et al. (2014).

Several decades of geophysical and geologic research have contributed a large body of observational data related to the timing (Bosworth, 1992; Bosworth and Strecker, 1997; George et al., 1998; Wichura et al., 2010), chemistry (Aulbach et al., 2008; Bianchini et al., 2014; Chesley et al., 1999; Kaeser et al., 2009; Pik et al., 2006), mechanics (Buck, 2004; Calais et al., 2008; Corti et al., 2003; Weinstein et al., 2017), kinematics (Birhanu et al., 2015; Calais et al., 2008; Modisi et al., 2000; Saria et al., 2014), mantle involvement (Adams et al., 2012; Bastow et al., 2005; Bastow et al., 2008; Chang and Van der Lee, 2011; Fishwick, 2010; Hansen and Nyblade, 2013), magmatism (Bastow et al., 2010; Kendall et al., 2005) and natural hazards (Ayele, 2017) of continental extension in Africa. However, most of these studies are focused on a single “segment” of the larger rift system, hence on a distinct structural unit. Some work has been done to compare segments as a means of exploring the relative importance of contributing factors, such as the availability of fluids in magma-rich and magma-poor segments (Bialas et al., 2010; Hayward and Ebinger, 1996; Roecker et al., 2017; Rooney et al., 2011), the influence of total finite strain (Ebinger, 2005) on rift morphology, or the importance of sublithospheric plume impingement on the force balance (Ebinger and Sleep, 1998; Lin et al., 2005; Nyblade and Robinson, 1994). However, fully synoptic studies for the whole East African Rift System (EARS) are few in number.

A GeoPRISMS-supported collaboration between MIT and the University of Montana targeted the development of a comprehensive, consistent geodetic surface velocity solution for the entire EARS focus area (Fig. 2). This effort included several components:

1. Collection of all publically available raw GPS observations from East Africa from 1992 to 2015;
2. Negotiation for the release and inclusion of several additional restricted GPS observation data sets from European and African sources;
3. Compilation and verification of all related metadata;
4. Systematic assessment and quality control on all available data sets; and
5. Processing of the merged data sets with a consistent processing strategy and reference frame.

Figure 2. The most recent community geodetic solution, using all available raw data from the EARS region, processed using GAMIT/GLOBK with a consistent quality standard and editing approach, in a single common reference frame. This solution, data sources, and relevant metadata are available from the GeoPRISMS data portal at http://www.marine-geo.org with doi:10.1594/IEDA/321764

The supported work addresses the GeoPRISMS Rift Initiation and Evolution (RIE) goal of synthesis, especially in the context of multiscale mechanics and controls on deformation and localization of strain.

During the period of support for this experiment, we also leveraged the NSF funding to invest in permanent geodetic instrumentation in Ethiopia and add new observations in the Turkana Depression of Ethiopia and Kenya, the part of the EARS with the fewest prior geodetic observations. In the first case, we extended operations of a previously-funded Ethiopian Highlands continuous GPS network for an additional year. That year allowed Addis Ababa University and the University of Montana to negotiate with several different stakeholders in the U.S. and Africa, with the end result that the Institute of Geophysics, Space Science, and Astronomy of Addis Ababa University adopted a fully operational, scientific-grade geodetic network of ten sites for permanent ongoing observations (Fig. 3). The network became the largest entirely African owned and operated geophysical system, and maintains operations and a fully open data policy to the present. In the second case, we added an additional epoch of campaign observations on six campaign GPS sites (Fig. 4) and added two continuous GPS systems in the Turkana Depression (Fig. 5). The continuous sites are located on either side of Lake Turkana and are hosted by the Turkana Basin Institute, a nonprofit entity supporting research through the region.

The primary purposes of the project were scientific and infrastructural capacity-building. The synoptic geodetic velocity field is intended for use by a wide range of researchers in many different disciplines within the rifting initiative and the EARS focus area. Many users will likely leverage the kinematic framework as boundary conditions, a priori constraints, or tectonic context for more focused studies without having to address data collection, standardization, quality control, metadata management, or processing strategies. We hope that the solution will inform other work and serve as an example of the value of a community commitment to open sharing of high-quality observations. In addition, the successful adoption of the instrumental array by African scientists sets a precedent for negotiated transfers of other instruments and capabilities throughout the region. African researchers and institutions can and should use such combinations of infrastructure and technical skills to pursue their own novel scientific targets and build indigenous training capabilities. Finally, the new Turkana Basin continuous sites are approaching a full year of operation, and will begin to yield usable scientific constraints on the most enigmatic part of the EARS very soon. ■

Figure 3. Birhanu Bekele (IGSSA MS student in geodesy) and El Knappe service continuous station SHIS (near Lalibela) supervised by children from a nearby farm. Photo Credit: Shannon Hall

Figure 4. Campaign GPS installation (SEOL) near Sereolipi, Kenya guarded by local wildlife rangers.

Figure 5. Welding the antenna mount for the Turkwel continuous site (XTBT) at the Turkana Basin Institute.

Sunset in Nekemt, Ethiopia after servicing the Highlands GPS network. Photo credit: Ellen Knappe

Central island in Lake Turkana, which has been active in the Holocene and currently has fumarolic activity. This photo was taken in February 2018 while flying between the Turkana Basin Institute field stations, which now host two new GPS stations. Photo credit: Ellen Knappe

Reference information

James D. Muirhead¹, Tobias P. Fischer², Amani Laizer³, Sarah J. Oliva⁴, Emily J. Judd¹, Hyunwoo Lee⁵, Emmanuel Kazimoto³, Gladys Kianji⁶, Cynthia J. Ebinger⁴, Zachary D. Sharp², Josef Dufek⁷

¹Syracuse University, ²University of New Mexico, ³University of Dar es Salaam, ⁴Tulane University, ⁵Seoul National University, ⁶University of Nairobi, ⁷University of Oregon

Figure 1. Annotated SRTM map showing the extent of the rift basins in the current study. Filled circles show the location of sampling regions within each basin, and the dashed brown line delineates the eastward-dipping surface boundary between the Tanzanian craton and Proterozoic mobile belt rocks (from the geological map of Thiéblemont et al. (2016)). Also included is the mean flux of magmatic CO₂ from sampling sites in each basin. Inset in the top left shows the location of the DEM map on the African continent. Red lines show the extent of the Eastern (EB) and Western (WB) branches of the EARS.

In May and June 2018, our team of researchers completed the longest along-strike magmatic CO₂ degassing survey in the East African Rift System (EARS) to date. Our CO₂ flux data now extend over four rift basins, from the Magadi basin (Kenya) southward to the Balangida basin (Tanzania) (Fig. 1). During the 25-day field campaign, we collected over one thousand diffuse soil degassing flux measurements, and sampled hydrothermal spring systems along major fault zones to analyze the sources and fluxes of different volatile species. Here we present preliminary results of diffuse CO₂ flux in zones within one hundred meters of observed spring discharge and use these values to examine variations in magmatic CO₂ discharges between basins. The spatial variability of these data reveal how mantle CO₂ fluxes in the EARS may evolve over the course of rift basin development, and are impacted by the initial composition and structure of the East African lithosphere.
Continental rifts are sites of lithospheric thinning and heating, which is commonly accompanied by magmatism and volatile transfer from Earth’s mantle to the lithosphere and atmosphere (White and McKenzie, 1989; Ebinger, 2005; Rooney, 2010; Lee et al., 2017; Foley and Fischer, 2017). They represent a key tectonic setting for natural CO₂ emissions and possibly modulate Earth’s climate on geological timescales (Brune et al., 2017; Foley and Fischer, 2017). However, the total volume of mantle CO₂ emitted at rift settings is poorly constrained, as are the mechanisms that control variations in CO₂ flux over the lifetime of rifting.

The original carbon content of cratonic lithosphere is expected to be relatively low (~0.25 Mt C km^–3 for 2-3 Ga lithosphere; Foley and Fischer, 2017). However, abundant carbon may be sequestered in the mantle lithosphere during the infiltration of both plume melts (e.g., Thompson et al., 2015) and carbon-rich hydrous-silicate melts generated during subduction (Foley and Fischer, 2017; Malusà et al., 2018).

These processes can potentially enrich carbon contents in the mantle lithosphere up to a hundred times above background values (Foley and Fischer, 2017). The resulting carbon accumulated during these events may be released during the generation and ascent of magma at continental rift settings (Malusà et al., 2018) (Fig. 2).

Although continental rifts represent potentially key sites of CO₂ release, measuring the flux of CO₂ from these settings is challenging and requires direct measurements and observations of CO₂ discharge from zones of active rifting. The magma-rich Eastern branch of the East African Rift System (EARS) represents an ideal location to investigate these processes. Earlier degassing studies focused on direct measurements of volcanic plumes emitted from active volcanoes, such as Nyiragongo (Sawyer et al., 2008) and Oldoinyo Lengai (Brantley and Koepenick, 1995). In addition to these plume sources, EARS volcanoes release mantle volatiles to the atmosphere via springs, fumaroles, and zones of diffuse soil degassing, as well as during eruptive episodes (Darling et al., 1995; Fischer et al., 2009; Barry et al., 2013; de Moor et al., 2013; Hutchison et al., 2015; Lee et al., 2017). More recent studies in the EARS have shown that large volumes of mantle carbon are also released to the atmosphere along extensional fault systems situated away from volcanoes (Lee et al., 2016, 2017; Hunt et al., 2017). During this process, termed “tectonic degassing” (Burton et al., 2013; Lee et al., 2016), mantle carbon ascends to the surface along permeable fault zones and exits via springs, diffuse soil degassing zones, and gas vents (Muirhead et al., 2016; Lee et al., 2016, 2017; Hunt et al., 2017). This mantle carbon is primarily sourced from an enriched sub-continental lithospheric mantle and released into the crust and atmosphere by magmas emplaced at lower crustal depths (Lee et al., 2017; Roecker et al., 2017).

Figure 2. Production and transport of magmatic CO₂ at continental rift settings modified from Hunt et al. (2017). White arrows represent zones of CO₂ fluid flow, yellow stars are hydrothermal springs, and orange stars are deep earthquakes. The CO₂ depicted exsolves from cooling upper and lower crustal magmas. The distribution of crustal magma (red polygons) is based on seismicity from Weinstein et al., (2017) and the seismic tomography model of Roecker et al. (2017).

Given the large aerial extent, pervasive faulting, and widespread magma emplacement occurring at depth in the EARS (e.g., Keranen et al., 2004; Roecker et al., 2017; Plasman et al., 2017), quantifying the volumes of CO₂ released requires observations from a wide variety of structural settings along the rift system. Results of diffuse soil degassing surveys have thus far been reported from the northern and central Main Ethiopian Rift (Hunt et al., 2017) and Magadi-Natron basin (Lee et al., 2016), with estimates of 0.52-4.36 Mt yr^-1 and 2.15-5.95 Mt yr^-1 for each rift sector, respectively.

Extrapolation of these estimates point to potential CO₂ fluxes on the order of 10-100 Mt yr^-1, particularly when accounting for dissolved CO₂ volumes transported in springs (Lee et al., 2017). However, these estimates do not consider the spatial and temporal variations of mantle CO₂ discharge expected along any active rift system. The flux of CO₂ within any rift basin should depend on a number of critical factors, such as the volume of carbon trapped within the underlying mantle lithosphere, rates of magma production, and the dissolved CO₂ contents of ascending rift magmas (Foley and Fischer, 2017; Hunt et al., 2017). These variables are expected to vary both spatially and temporally within any continental rift setting, and quantifying their importance for mantle CO₂ release requires extensive along-strike sampling of zones of volatile discharge.

Our recent field campaign was specifically designed to fill in these critical gaps in our understanding of rift CO₂ fluxes, through an investigation of four segments of the Eastern branch of the EARS: the Magadi, Natron, Manyara, and Balangida basins (Fig. 1). These basins encompass a ~350 km-long stretch of continental rifting and range in age between 1 and 7 Ma, and are thus currently at different stages of development. Furthermore, these basins exhibit varying volcanic/magmatic fluxes and histories, and even cross the boundary between Proterozoic mobile belt rocks and the Archean Tanzania Craton (Fig. 1). Therefore, from these data we can assess:

1. How mantle CO₂ fluxes may evolve over the course of basin development; and
2. How CO₂ fluxes are impacted by the initial lithospheric composition and structure of the East African lithosphere.

Given the inherent variability of CO₂ flux within individual rift basins (e.g., Hunt et al., 2017), when comparing CO₂ discharges between basins it is critical to compare data from sites exhibiting similar structure, substrate, and hydrology. Therefore, we present here a subset of our collected data, focusing specifically on flux data (1) from rift-graben sediments, (2) in the vicinity of faults, and (3) in areas within 100 m of observed spring discharge.

The sources for diffuse soil CO₂ discharges in volcano-tectonic settings are typically characterized as either biogenic or magmatic, with flux data in each population exhibiting a log-normal distribution and the highest mean flux observed in the magmatic population (e.g., Chiodini et al., 1998, 2008). Data from each study site, presented as probability plots in Figure 3, were sub-divided into two distinct populations by adapting the methodology of Sinclair (1974) into a newly designed MATLAB^® code. This code iteratively fits biogenic and magmatic regression lines to the log-transformed data. Based on these functions, synthetic data sets are generated for each population and plotted against observed data, with the final solution being that which produces the highest R-squared and smallest root-mean-squared error values between the compared datasets. Outputs from this procedure provide an estimate of the percent contribution of biogenic and magmatic sources and their mean flux values.

Figure 3. Probability plots of diffuse soil CO₂ fluxes for each rift basin in the study. Note that the overall CO₂ flux values decrease from north (Magadi) to south (Balangida). Flux values below the equipment detection limit (<0.24 g m^-2 d^-1) cannot be presented on the plots, but still affect the probability distribution of flux values above the detection limit.

Comparing data between basins, we observe a north to south decrease in both the percent contribution of the magmatic flux population and the mean magmatic flux value (see mean flux values in Figure 1). Lower magmatic CO₂ flux values also correspond with younger rift basins (e.g., the Manyara and Balangida basins). These younger basins also exhibit lower volcanic/magmatic inputs (Le Gall et al., 2008; Albaric et al., 2014), which may relate to the low degree stretching and related decompression melting during this earlier stage of rifting, or to the relatively dry nature of thick Archean mantle that enables its preservation (e.g., Currie and van Wijk, 2016). Finally, as the locus of rifting gradually transitions from the Proterozoic mobile belt in the Natron basin, to the Tanzanian craton in the Balangida basin, we observe a significant reduction in the mean magmatic CO₂ flux.

These preliminary results suggest that the volume of mantle CO₂ discharge in the Eastern branch of the EARS is strongly dependent on the degree of lithospheric thinning, mantle hydration state, and related magmatism. The greatest mantle CO₂ discharges in the EARS likely occur in more evolved systems outside the Archaean craton, such as the Kenya Rift (Lee et al., 2016) and Main Ethiopian Rift (Hunt et al., 2017). Furthermore, basins in their earliest rift stages (the ~1 Ma Manyara and Balangida basins) within Proterozoic mobile belt rocks exhibit higher CO₂ fluxes than those in the Archean craton. This observation suggests that the Proterozoic lithosphere in East Africa may contain greater volumes of sequestered carbon, with its structure and composition suited for volumetrically significant CO₂ discharges compared to the thick and probably dehydrated cratonic lithosphere. ■

Measuring diffusely degassing soil CO2 in the Balangida basin using a CO2 accumulation chamber. Photos credit: James Muirhead

Graduate students Amani Laizer (University of Dar es Salaam) and Sarah Oliva (Tulane University) sampling gas from an actively degassing vent observed along a fault in the Natron basin. Photo credit: James Muirhead

The field team enjoying the view of Oldoinyo Lengai volcano from the top of the border fault to the Natron basin (from left to right: James Muirhead, Tobias Fischer, Amani Laizer, Sara Oliva). Photo credit: James Muirhead

Tobias Fischer samples an actively degassing vent on the western border fault to the Natron basin. The world’s only active carbonatite volcano, Oldoinyo Lengai, can be seen in the background. Photo credit: James Muirhead

Reference information