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Geothermal energy, aluto volcano, and Ethiopia's rift valley

Geological Society of America

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IMAGE: This is a conceptual model summarizing the evolution of the major structures on Aluto volcano and their controls on surface volcanism, geothermal fluids, and degassing. view more

Credit: Hutchison et al. and Geosphere

Boulder, Colo., USA - In their open access paper published in Geosphere this month, William Hutchison and colleagues present new data from Ethiopia's Rift Valley and Aluto volcano, a major volcano in the region. Aluto is Ethiopia's main source of geothermal energy, a low-carbon resource that is expected to grow considerably in the near future. Preexisting volcanic 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.

Using high-resolution airborne imagery, field observations, and CO2 degassing data, the authors explore in great detail how these preexisting structures control fluid pathways and spatial patterns of volcanism, hydrothermal alteration, and degassing. Understanding these preexisting structures, they write, "Is a major task toward defining the evolution of rift zones and also has important implications for geothermal exploration, mineralization, and the assessment of volcanic hazard."

In concluding their paper, Hutchison and colleagues write, "The new model for the structural development and volcanic edifice growth at Aluto opens up a number of avenues for future work. A major challenge is to determine how geothermal and magmatic fluids are distributed and stored in the subsurface of Aluto and how they ascend along the mapped fault zones." These future studies, they note, "should focus on generating high-spatial-resolution maps of off-rift tectonic structures and should be complemented by detailed field work to constrain the stress field orientations during the development of the Aluto magma reservoir."

FEATURED ARTICLE

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

William Hutchison et al., COMET, University of Oxford, Oxford, UK. Published online on 2 Apr. 2015; http://dx.doi.org/10.1130/GES01119.1. Themed issue: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms. This article is OPEN ACCESS.

Other GEOSPHERE articles (see below) cover such topics as

1. A second open-access study focusing on continental rifting in main Ethiopian Rift Valley and the Afar Depression in East Africa;

2. How sediment provenance in subduction zones may help geoscientists predict the intensity of earthquake hazards;

3. Ancient footprints of carbon dioxide and methane in the Navajo Sandstone, USA; and

4. Knickzones in the Grand Canyon, USA; and

All GEOSPHERE articles available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Magma-induced axial subsidence during final-stage rifting: Implications for the development of seaward-dipping reflectors Giacomo Corti et al., Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Florence, Italy. Published online on 22 Apr. 2015; http://dx.doi.org/10.1130/GES01076.1. Special issue: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms. This article is OPEN ACCESS.

Continental rifting is one of the most important geodynamic processes that shape our planet: During its evolution, lithospheric plates are torn apart and broken, and -- eventually -- a new oceanic basin bordered by a pair of passive rifted margins is formed in between. A consensus is now emerging from studies of continental rifts and rifted margins worldwide that significant extension can be accommodated by magma intrusion prior to the development of a new ocean basin. Despite this, the influence of loading from magma intrusion, lava extrusion and sedimentation on plate flexure and resultant subsidence of the basin is poorly understood. In this paper, Giacomo Corti and colleagues address this issue by using 3-D flexural models constrained by geological/geophysical data from the Main Ethiopian Rift and the Afar Depression in East Africa. Model results show that axial mafic intrusions in the crust are able to cause significant downward flexure of the opening rift and that the amount of subsidence increases with decreasing plate strength accompanying progressive plate thinning and heating during continental breakup. This process contributes to the tilting of basaltic flows toward the magma injection axis, forming the typical wedge-shaped seaward dipping reflector sequences on either side of the eventual rupture site, as the new ocean basin forms.

Sediment provenance and controls on slip propagation: Lessons learned from the 2011 Tohoku and other great earthquakes of the subducting northwest Pacific plate J. Casey Moore et al., University of California, Santa Cruz, California, USA. Published online on 22 Apr. 2015; http://dx.doi.org/10.1130/GES01099.1. Themed issue: Exploring the Deep Sea and Beyond, volume 2.

The great M 9.0 Tohoku earthquake and its associated tsunami off Northern Japan occurred along a very weak, fine-grained red-brown clay horizon in the Japan Trench. This clay correlates with similar pelagic clay recovered seaward of the trench. Scientific Ocean Drilling cores suggest that this extensive weak clay may have enabled at least five other earthquakes with shallow slip that generated large tsunamis. South of the Tohoku earthquake, the extent of the weak clay layer on the Pacific Plate is interrupted by numerous seamounts comprised of volcanic rocks and limestones. Failure of these seamounts produces earthquakes up to the M 7 range, but no large tsunamis. J. Casey Moore and colleagues believe that these seamount failures relieve stress in the area and thus may prevent extensive seismic rupture along the weak clay layers interspersed between the seamounts. Thus, knowledge of the types of sediments and rocks incoming to subduction zones can help predict earthquake hazards.

The footprints of ancient CO2-driven flow systems: Ferrous carbonate concretions below bleached sandstone David B. Loope and Richard M. Kettler, University of Nebraska, Lincoln, Nebraska, USA. Published online 22 Apr. 2015; http://dx.doi.org/10.1130/GES01094.1.

Loope and Kettler write, "Our work is concerned with the movement of iron in sandstone aquifers. In modern deserts, many sand grains have reddish iron-oxide coatings; apparent evidence for the color of many ancient sandstones. In the southwestern U.S., the lower part of the highly porous Navajo Sandstone is red (the Vermillion Cliffs of Utah's Grand Staircase, north of the Grand Canyon), but the upper part is nearly white (the White Cliffs). In the red part of the formation, just below the white, there is an abundance of very dense, spherical masses of iron-rich sandstone. We explain the present distribution of iron in the Navajo by calling on the trapping of two buoyant fluids -- carbon dioxide and methane -- in the pore spaces of the upper part of the formation tens of millions of years ago. The methane reduced iron on the sand grains facilitating its aqueous dissolution. Meanwhile, the more abundant CO2 dissolved in the underlying water, increasing that solution's density. This solution, devoid of oxygen, carried the dissolved iron downward into the lower part of the formation, leaving the upper part 'bleached.' An iron carbonate mineral (siderite) crystallized in the lower rock, forming solidly cemented spheres (concretions or 'Moki marbles'). As the region was uplifted and the Grand Canyon was cut, water carrying oxygen started to enter the Navajo Sandstone. Microbes colonized the perimeters of the cemented spheres, and used the iron carbonate as a source of both energy and carbon. When the microbes were done, all of the iron carbonate was gone and distinctive thick rinds composed of iron oxide had formed on the perimeters of the spheres surrounding their friable, bleached, cores. Our work shows the grand scales at which, methane, carbon dioxide, and microbes -- working together -- can redistribute the iron in porous rocks. Our work may eventually help not only explain the movement of iron in aquifers, but also the movement of heavy metals, including radioactive elements like radium. The iron-oxide coatings on sand grains act as sponges, adsorbing any heavy metals moving in the water. When the sandstone is bleached by methane, however, the heavy metals become dispersed in the water, contaminating it."

Rates of river incision and scarp retreat in eastern and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone

Lon D. Abbott et al., University of Colorado, Boulder, Colorado, USA. Published online on 2 Apr. 2015; http://dx.doi.org/10.1130/GES00978.1. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II.

Scientists have long suspected that the upstream migration of a knickzone -- an anomalously steep section of the river -- has been largely responsible for cutting Grand Canyon. Such knickzones naturally migrate upstream because the water at the knick has high energy, which leads to rapid erosion there. Researchers are keen to determine when such a knickzone passed through Grand Canyon. Previous work has shown that the incision rates for eastern and central Grand Canyon during the past 400,000 years have been steady, meaning that no such knick passed during that interval. Our study extends that incision rate history back to 500,000 years ago. We document a period of very rapid erosion (1-4 kilometers per million years) in both eastern and central Grand Canyon between 500,000 - 400,000 years ago that subsequently slowed to the same steady rate (less than 200 meters per million years) observed by previous workers. We interpret this period of rapid incision to mark the passage of a migrating knickzone about a half million years ago. Our methods also allow us to document the rate at which the Redwall Limestone escarpment has been retreating away from the river in response to undercutting and erosion during the last half-million years. That scarp retreat rate is 600-800 meters per million years.

Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

Peter W. Lipman, U.S. Geological Survey, Menlo Park, California, USA, and Olivier Bachmann, Institute of Geochemistry and Petrology, ETH Zurich, Zürich, Switzerland. Published online 2 Apr. 2015; http://dx.doi.org/10/1130/GES01091.1.

Explosive eruptions of large-volume continental-margin ignimbrites (Cordilleran ash-flow sheets), the most catastrophic geologic events that have been generated within the Earth, provide direct records of the growth of large silicic magma bodies at depth in the crust. In conjunction with geologic, geophysical, and geochronologic data for the caldera sources and associated shallow granitoid intrusions, ignimbrites provide insights into the development of vertically extensive (greater than 20 km) subvolcanic batholiths that are constructed incrementally over million-year time spans. Processes of ignimbrite-associated batholith assembly have caused drastic chemical and physical reconstruction of the entire lithosphere, with important societal implications for volcanic-hazard evaluation, generation of mineral and geothermal resources, and landscape and climate evolution.

Relations between thermal history and secondary structures of ignimbrites exclusive of rheomorphism

J.R. Riehle, Hayden Lake, Idaho, USA. Published online 2 Apr. 2015; http://dx.doi.org/10.1130/GES01089.1. Themed issue: Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain-Yellowstone Region and Adjacent Areas.

Ignimbrites -- ash-flow tuffs -- have long been known to show devitrification of glass shards, vertical and horizontal jointing, and lithophysal cavities. Each of these kinds of secondary structures are developed to differing degrees from locality to locality, and some (devitrification, cavities) tend to be stratabound, that is, occur in certain horizons at different depths in the deposit. The variables governing development of these structures have only generally been understood, for example, thicker deposits tend to be more devitrified and have more cavities. This paper uses a previously published model of ash compaction to provide model cooling histories of different ignimbrite deposits. The cooling histories can then be empirically related to observed secondary structures. Devitrification appears to require temperatures in excess of 600 °C that last for at least 2 years. Once devitrification begins, both heat and dissolved water are released, which in turn inflate the lithophysal cavities; inflation is more effective deeper in a deposit and in horizons having intermediate porosity. Some horizontal joints form approximately contemporaneously with lithophysal cavities, suggesting that the brief period of pressurization accompanying devitrification causes horizontal jointing as well as cavity formation.

Low-temperature thermochronologic constraints on the kinematic histories of the Castle Cliffs, Tule Springs, and Mormon Peak detachments, southwestern Utah and southeastern Nevada

Tandis S. Bidgoli et al., University of Kansas, Lawrence, Kansas, USA. Published online on 22 Apr. 2015; http://dx.doi.org/10.1130/GES01083.1.

The Castle Cliffs, Tule Springs, and Mormon Peaks detachments in the eastern Basin and Range are among the best cited examples of large-displacement normal faults that initiated and slipped at low dips. However, the origin of these structures and magnitude of extension across them is controversial. The study presents new low-temperature thermochronologic data that provide much needed constraints on the deformational histories of these faults and an independent check on published cross-sections and extension estimates for the region. The results are significant because they limit potential interpretations for these low-angle structures and reduce the total extension across this system of faults by ~25%.

Behaviors mapped by new geographies: Ichnonetwork analysis of the Val Dolce Formation (lower Permian; Italy-Austria)

Andrea Baucon et al., Universitá; di Milano, Milano, Italy. Published online on 22 Apr. 2015; http://dx.doi.org/10.1130/GES00994.1.

From the abstract: The Pramollo Basin (Italy-Austria) is one of the richest body and trace fossil sites of the Alps, and exhibits a well-preserved Permian-Carboniferous fluvio-deltaic to marginal-marine sedimentary succession. Despite the exceptionally abundant and well-preserved ichnological heritage, the trace fossils of the Pramollo Basin are not well studied, particularly those of Permian units. This study focuses on the ichnofauna of the Val Dolce Formation (Permian; partly Asselian to partly Sakmarian), with the goal of documenting its ichnological heritage and reconstructing its paleoenvironment.

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Contact: Kea Giles
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