Tigistu Haile

New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of a ∼2 Ma reference pole for stable Africa

[1] Joint French–Ethiopian field trips in 1995–1996 yield new geochronologic and paleomagnetic data, which significantly expand our knowledge of the recent magmatic and tectonic history of the Afar depression. Twenty-four new K-Ar ages range from 0.6 to 3.3 Ma. There is quite good agreement between magnetic polarities and Geomagnetic Polarity Timescale (GPTS). Eight age determinations with uncertainty less than 50 kyr can be used in future reassessments of the GPTS (upper and lower Olduvai/Matuyama reversals and Reunion and Mammoth subchrons). Paleomagnetic analysis of 865 cores from 133 sites confirms that low-Ti magnetites are the main carrier of the Characteristic Remanent Magnetization (ChRM). A positive tilt test (based on two subgroups with 63 and 23 sites, respectively) confirms that this ChRM is likely the primary magnetization. The main paleomagnetic results can be summarized as follows. A

Paleomagnetic and geochronological identification of the Reunion subchron in Ethiopian Afar

2 Ma reference pole for stable Africa is determined based on 26 sites located on either side of the northern termination of the East African rift. It is located at l = 87.2°N, f = 217.1°E (A 95 = 4°). A 4.6 ± 1.8° (2s) inclination shallowing is identified within a population of 231 stratoid lava flows, consistent with a global axial quadrupole of 6 ± 2% of the axial dipole. Combined with earlier data of Acton et al. [2000], our new data allow mean paleomagnetic field directions to be determined for five individual, fault-bounded blocks previously identified by tectonic analysis within central Afar. These all have suffered negligible rotations about vertical axes since emplacement of the lava. This contrasts with the significant rotations previously uncovered to the east in Djiboutian Afar for three major individual blocks. Taken altogether, the declination differences with respect to reference directions are 2 ± 4° for central Afar and 13 ± 4° for eastern Afar, consistent with the model of Manighetti et al. [2001a]. It appears that in the last

The interference of a deep thermal system with a shallow aquifer: the case of Sodere and Gergedi thermal springs, Main Ethiopian Rift, Ethiopia

3 Ma the Afar depression was extensively floored by trap-like basalts, which were deformed by a single but complex physical (tectonic) process, combining diffuse extension, rift localization, propagation, jumps and overlap, and bookshelf faulting.

Environmental impact and vulnerability of the surface and ground water system from municipal solid waste disposal site: Koshe, Addis Ababa

This paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault-related scarp of Pliocene age in the central part of the Afar depression (Ethiopia). Paleomagnetic work uncovers a clear characteristic remanent magnetization carried by (titano-) magnetites, with high unblocking temperatures, close to the Curie temperature of pure magnetite. The characteristic direction, based on 27 non transitional flows, is D=8.1 o, I= 10.1 o (• 5=4.1 o). This is consistent with earlier determinations of the clockwise tectonic rotation, due to rift propagation and overlap, of the block to which the section belongs. Magnetic stratigraphy consists of a succession from bottom to top of 16 reversed, 7 normal, and 4 reversed flows. One flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field (AF) demagnetization, which is due to remagnetization by the overlying flow. K/Ar dating of five samples provides consistent determinations, averaging 2.07 + 0.05 Ma. All ages agree with this average, given their rather large individual uncertainties (from 0.04 to 0.08 Ma at the lc • level). Comparison with recent reference geomagnetic polarity timescales (GPTS) indicates that the normal subchron recorded in the Gamarri section must be linked with (one of) the R6union event(s). The normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double R6union event. Although uncertainties in ages are too large to allow unequivocal inferences, our age determination for the R6union event is in better agreement with earlier determinations of McDougall and Watkins [ 1973] than with a more recent value proposed by Baksi et al. [1993] and used in the GPTS.

Geophysical exploration of the Boku geothermal area, Central Ethiopian Rift

An integrated survey program involving geological, hydrogeological and geophysical techniques has been employed to characterize the aquifer geometry, recharge and circulation dynamics of thermal springs within a shallow aquifer system in Ethiopia. The selected springs for the case study are Sodere and Gergedi, which are situated within the tectonically active Main Ethiopian Rift (MER). Geologically, the studied springs are located on Plio-Quaternary volcanic rocks. The geophysical results indicate the presence of subsurface weak zones represented by extensional tectonics and weathering zones which are responsible for thermal water circulation and facilitate recharge from the adjacent surface-water bodies. The structures inferred by the resistivity survey, both sounding and electrical tomography, present contrasts in rock resistivity response. The anomalous zones in the magnetic data are in good agreement with the zones that are revealed by geological mapping and surface manifestation of the thermal water discharge zones. The shallow aquifer of the central MER is under the influence of thermal water, which increases the groundwater temperature and mineral content.RésuméUn programme d’investigation intégrée comprenant géologie, hydrogéologie et géophysique a été mis en œuvre afin de caractériser la géométrie de l’aquifère, la recharge et les circulations thermales au sein d’un aquifère de surface en Ethiopie. Les sources retenues pour ce cas d’étude sont les sources de Sodere et de Gergedi, qui sont localisées au sein du rift éthiopien tectoniquement actif (RE). Du point de vue géologique, les sources étudiées émergent au niveau des formations volcaniques du Plio-Quaternaire. Les résultats géophysiques indiquent la présence de zones souterraines « faibles » correspondant aux zones d’extension tectonique et d’altération, responsables de la circulation des eaux thermales et facilitant la recharge des eaux de surface. Les structures mises en évidence à partir des sondages et des tomographies électriques, sont associées à des réponses contrastées en termes de résistivité. Les zones d’anomalies magnétiques sont cohérentes avec les zones identifiées lors de la cartographie géologique et par les zones de décharge des eaux thermales. L’aquifère de surface de la partie centrale du RE est influencé par les circulations des eaux thermales, ayant pour conséquence une augmentation de la température des eaux souterraines et de la minéralisation.ResumenSe empleó un programa integrado de relevamiento que involucra técnicas geológicas, hidrogeológicas y geofísicas para caracterizar la geometría del acuífero, la recarga y la dinámica de circulación de los manantiales termales dentro de un sistema acuífero somero en Etiopía. Los manantiales seleccionados para el caso de estudio son Sodere y Gergedi, los cuales están situados dentro del Rift Principal Etíope (MER) tectónicamente activo. Geológicamente los manantiales están localizados en rocas volcánicas Plio-Cuaternarias. Los resultados geofísicos indican la presencia de zonas de debilidad subsuperficial representadas por una tectónica extensional y zonas meteorizadas las cuales son responsables de la circulación de agua termal y facilitan la recarga desde los cuerpos de aguas superficiales adyacentes. Las estructuras inferidas por los relevamientos de resistividad, tanto sondeos como tomografía eléctrica, presentan contrastes en la respuesta de la resistividad de la roca. Las zonas anómalas en los datos magnéticos están en buen acuerdo con las zonas que son revelados por el mapeo geológico y las manifestaciones en superficie de las zonas de descarga de aguas termales. El acuífero somero del MER central está bajo la influencia de agua termal, la cual incrementa la temperatura y el contenido mineral del agua subterránea.摘要为刻画埃塞俄比亚境内一个浅部含水层系统的含水层形态特征以及温泉补给与循环机理,开展了一个包括地质、水文地质和地球物理技术的综合性调查。本文以位于构造活跃的Main Ethiopian Rift (MER)上的Sodere和Gergedi温泉为例。地质上,本文所研究的温泉位于上新世-第四纪火山岩上。地球物理探测结果表明存在有以伸展构造和风化带为代表的地下薄弱地带,是热水循环的原因,并促进了附近地表水体的补给。根据电阻率测量结果(包括测深与电子成像)推断的构造,在岩性电阻率信号上形成对照。基于磁场数据确定的异常区与基于地质填图和排泄区地表热显示所圈定的范围一致。MER中央的浅部含水层受热水影响,提高了地下水温度和矿物质含量。RiassuntoAl fine di definire le dinamiche di ricarica e di circolazione di sorgenti termali profonde e di delimitare la geometria dell’acquifero superficiale alimentante è stata realizzata un’indagine integrata idrogeologica e geofisica. Le sorgenti Sodere e Gergedi, ubicate all’interno del Main Ethiopian Rift (MER) (Etiopia centrale), sono state selezionate come rappresentative. Nell’area in studio sono presenti rocce vulcaniche riferibili al Plio-Quaternario. Le interpretazioni dei dati geofisici indicano la presenza nel sottosuolo di zone fratturate legate a tettonica estensionale e alterazione, che sono responsabili della circolazione dei fluidi termali e che facilitano la ricarica dai limitrofi corpi idrici superficiali. Le strutture dedotte dalle indagini di tomografia elettrica ed acustica mostrano un contrasto di resistività nelle rocce. Le zone ad anomalia magnetica sono in accordo con quelle evidenziate dalla geologia di terreno e dalle manifestazioni superficiali delle emergenze delle acque termali. L’acquifero superficiale del MER centrale è influenzato dalle acque termali profonde, che fanno aumentare la temperatura e la mineralizzazione delle acque sotterranee.ResumoFoi desenvolvido um programa integrado de pesquisa que envolveu técnicas geológicas, hidrogeológicas e geofísicas, para caraterizar a geometria do aquífero e a recarga e a dinâmica da circulação de nascentes termais dentro de um sistema aquífero superficial na Etiópia. As nascentes selecionadas para o estudo de caso foram as de Sodere e Gergedi, situadas dentro do Rift Principal Etíope (Main Ethiopian Rift – MER), que se encontra tectonicamente ativo. Geologicamente, as nascentes estudadas situam-se em rochas vulcânicas de idade Plio-Quaternária. Os resultados geofísicos indicam a presença de zonas de fraqueza na subsuperfície, representadas por tectónica extensional, e zonas de meteorização, as quais são responsáveis pela circulação de águas termais e facilitam a recarga a partir de massas de água superficiais adjacentes. As estruturas inferidas pelos ensaios de resistividade, tanto em sondagem como em tomografia elétrica, apresentam contrastes na resposta da resistividade da rocha. As zonas anómalas nos dados magnéticos estão de acordo com as zonas delimitadas por mapeamento geológico e com as zonas de manifestação superficial da descarga de águas termais. O aquífero superficial do MER encontra-se sob influência de águas termais, aumentando a temperatura e o conteúdo mineral da água subterrânea.

TEM-fast small-loop soundings to map underground tunnels and galleries connecting the rock-hewn churches of Lalibela, Ethiopia

Geo-environmental assessment and geophysical investigations were carried out over the only functional municipal solid waste disposal site of the city of Addis Ababa, Ethiopia, known locally as Koshe. The accumulated wastes from Koshe have impact on the surrounding human and physical environment since the disposal site was not designed. The study deserves emphasis because the city of Addis Ababa currently obtains a considerable portion of its domestic water supply from a well field developed not much farther from and along a groundwater flow direction in relation to the waste disposal site. It was found out that the leachates from the site contain high concentration of biological oxygen demand, chemical oxygen demand, chloride and sulphate besides high concentration of cobalt, nickel and zinc in the surrounding soils. The geophysical results have mapped weak zones and near-vertical discontinuities that could potentially be conduits for the leachate from the wastes into the deep groundwater system. Further, a zone of potential leachate migration from the landfill was identified from the electrical models; the location of this zone is consistent with the predicted direction of groundwater flow across the site. The results further suggested that the open dump site tends to cause increasing amount of pollution on the surrounding soil, surface and ground waters. Furthermore, it was observed that the Koshe waste disposal site has grown beyond its capacity and the poor management of the open dump landfill has reduced the aesthetic value of the surrounding environments. The need to change/relocate the existing waste disposal site to a more suitable and technologically appropriate site is emphasized.