[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 $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°f or central Afar and 13 ± 4°for eastern Afar, consistent with the model of Manighetti et al. [2001a]. It appears that in the last $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.
As one of the first attempts to utilize the technique for intrasite archaeological prospection, a series of coincident square-loop TEM-fast geophysical surveys were carried out over the compounds of the largest of the rock-hewn churches of Lalibela-Bete Maryam and Bete Amannuelin North Ethiopia. Archaeologists have long believed that the different churches within each group were connected by underground tunnels. The aim of this survey was to identify, delineate, and map these underground channels and galleries. A total of 33 sounding surveys were conducted, the majority of which used a 3-m-side square loop. The survey traverses explored around the sides of the churches where underground connections to the other churches are possible. The results of the surveys, which are presented in terms of resistivity and depth pseudosections, clearly depict the presence of anomalies that could be associated with cavities, whose orientation suggests the presence of connecting galleries between the different churches. Whether these cavities were actually connecting galleries with religious implications or designed to be used as drainage paths remains a subject requiring further study involving additional geophysical investigations and physical excavation.
The paper highlights the potential drawback of mapping a single geophysical property for subsurface characterization in potential engineering sites. As an exemplary case study, we present the geophysical survey conducted along the surface projection of a tunnel in the quaternary volcanic terrain of the Main Ethiopia Rift. Initially, geoelectrical mapping involving 12 Vertical Electrical Sounding (VES) and a short Electrical Resistivity Imaging (ERI) line, was carried out. The 1D geoelectric model indicates that the formation resistivity at tunnel zone varies from 50 to 500 Ω∙m. The corresponding value on 2D model, (>350 Ω∙m), is also compatible. Based on limited available geological information, the geoelectric horizon was attributed to weathered and variably saturated ignimbrite. Following unexpected encounter during excavation, refraction seismic and core drilling were carried out for additional insights. Tomographic analysis of the seismic arrival times revealed that below a depth of 45 m, (tunnel zone), the velocity substratum is marked by a range, (1200–1800 m/s). Such low velocity range is typical of unconsolidated materials and, thus, cannot rationalize the geoelectrical attribution (ignimbrite). In a joint interpretation, the likely formation that may justify the observed range of the electrical resistivity and low P-wave velocity appears to be unwelded pyroclastic deposit (volcanic ash). Eventually, core samples from the tunnel zone confirmed the presence of thick ash flow. However, the unexpected ground conditions encountered at the early phase, due to insufficient information derived from a single geophysical parameter, caused extra cost and considerable delay.
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