Multidisciplinary exploration of the Tendaho Graben geothermal fields

The NW-SE trending Tendaho Graben is the major extensional feature of the Afar, Ethiopia. Rifting and volcanic activity within the graben occurred mostly between 1.8 and 0.6 Ma, but extended to at least 0.2 Ma. Very recent (0.22\u2013 0.03 Ma) activity is focused along the southern part of the younger and active Manda Hararo Rift, which is included in the north-western part of the graben. Extension gave rise to about 1600 m of vertical displacement (verified by drilling) of the basaltic Afar Stratoid sequence, over a crust with a mean thickness of about 23 km. The infill of graben, overlying the Stratoids, consists of volcanic and sedimentary deposits that have been drilled by six exploratory wells. Within the graben, two main geothermal fields have been explored by intensive geological, geochemical and geo- physical surveys over an area that approximately covers a square sector of 40x40 km. Both new and existing data sets have been integrated. The Dubti-Ayrobera system is located along the central axis of the graben. Available data, acquired in the last three decades, comprise more than two thousands gravity and magnetic stations, 229 magnetotelluric stations and structural-geological and geochemical observations. The Alalobeda system is located along the SW flank of the graben, at about 25 km from the Dubti-Ayrobera system and has been very recently stud- ied by means of gravimetric (300 stations), magnetotelluric and TDEM (140 stations) geological and geochemical surveys. The new residual magnetic anomaly map has been used to map the younger normal polarity basalt distribution and infer the location of the unknown main rift axis. The bedrock surface resulting by the 3D inversion of the new residual Bouguer anomaly enlightens the main normal faults hindered by sediments and the secondary structures represented by horsts and grabens. The three-dimensional resistivity models allow mapping the sedimentary infill of the graben, fracture zones in the Afar Stradoids bedrock and the dome-shape structure of the clay cap layer. The 2D and 3D gravimetric, magnetic and resistivity models have been integrated with the structural, geological and geochemical outcomings in order to get an updated conceptual model of the geothermal systems

The ARGeo geophysical model of the Tendaho geothermal field, Ethiopia

The Dubti and Ayrobera geothermal systems are located in the Tendaho Graben (Afar region, Ethiopia). As part of the ARGeo 2014 program, we have integrated the available geophysical data set with new measurements. The new residual magnetic anomaly map has been used to map the younger (<0.78 Ma) normal polarity basalt distribution and infer the location of the main rift axis hindered by sediments. The bedrock surface obtained by 3D inversion of the gravity residual anomaly gives a first order insight on the geometry of the graben. The graben sediments are marked by a shallow electrical conductor, with exceptionally low resistivities <1 Ohm m, mainly ascribed to low temperature clay alteration. Local resistivity variations are due to basalt interlayerings and coarser components. Resistivity 2D inversion models show a deep conductive anomaly that has been previously mapped and interpreted as an upper crustal fracture zone that lies above a 15 km wide area of partial melt . The new magnetotelluric data acquired to the South of Dubti manifestations indicate that the conductive anomaly is shallower and more intense to the South and approximately centred below the anomalous stripe of recent dikes/intrusions

Integration and Modeling of Geoscience Data from the Tendaho Geothermal Area, Afar Rift, Ethiopia

The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the Tendaho geothermal area, Ethiopia. The geothermal area is located within the NW-SE trending Tendaho Graben (TG), which includes the southern part of the younger and active Manda Hararo Rift (MHR) in the Afar region. Rifting and volcanic activity within TG occurred mostly between 1.8 and 0.6 Ma but extended to at least 0.2 Ma. Extension gave rise to about 1600 m of vertical displacement verified by drilling of the Afar Stratoid sequence, over a crust with a mean thickness of ~23 km. The main emphasis of our study was on the Dubti and Ayrobera geothermal areas, near the central axis of the TG, although we also reviewed data for Allalobeda, located near its western bounding faults. New data from these areas include magnetotelluric (MT) and time domain (TDEM) surveys completed in 2013-2014, as well as radon and shallow temperature surveys. The digital enhancement and modeling of existing potential field data revealed previously unrecognized structural lineaments trending mainly NW, NE, and WNW. New MT and TDEM data have been integrated with the existing electromagnetic data set, obtaining >200 MT stations covering the study area. 1D and 2D modeling of the data set has revealed local updoming of a widespread shallow conductor associated with thermal areas at Dubti and Ayrobera, and a possible relation to intrusive heat sources. Updoming is attributed to conversion of smectite clay to higher-temperature mixed-layer clays and chlorite due to hydrothermal fluid circulation. A review of the linkages between faulting and volcanism was also undertaken. Mapping the main axis of young volcanism and the base of the shallow electrical conductor provides input to a 3D temperature model constructed from well data (6 geothermal, 9 shallow gradient and multiple shallow water), near surface temperature measurements, liquid and gas geothermometry, and assumptions regarding conductive heat flow from young intrusion. We have compiled the most relevant data for Tendaho and organized it in a way that it can be viewed and interpreted in 3D using Leapfrog\ua9 Geothermal Software. We integrated all the key data to visualize patterns and develop internally consistent subsurface temperature and conceptual models. The model for Dubti indicates deep upflow to the SE of the drilled wells that migrates up NW-trending structures and then spreads laterally in shallow permeable horizons. At Ayrobera, deep upflow is likely localized by NW-trending faults on the eastern flank of the main axis of youngest volcanism. The temperature model, along with conceptual considerations, was used to define risked areas of interest for further exploration activities including targeting and drilling of deep wells. A key uncertainty is to what extent large regions of deep (>2 km) low resistivity are real, and how to model them in terms of temperature and permeability