The Gravity Field, Magnetic Field and Regional Tectonic Setting of the Corlletti Caldera and its Adjacent Areas

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Addis Ababa Universty


In this thesis work analysis of about 2220 gi'aVity' alia 75 liuigileli6 data' colieCted ih the 'Lakes District of the Main Ethiopia Rift (MER), between latitudes 6.88° - 7.75° and longitudes 38.00° - 39.00° consisting of the Corbetti caldera is made. A gravity survey covering about an area of 12100 sq. kms with obselvation points of about 2220 stations are discussed (about 50 wavity and 75 magnetic stations, were occupied during field work by the author of tlus thesis and Dr. Abera Alemu, advisor to tlus thesis work, wIllie the rest of Wavity data were obtained from the Ethiopian Institute of Geological SUlveys and the Geophysical Observatory of Addis Ababa University). Both the magnetic and gravity data sets are of senu-regional type, with station separations ranging on the average from about 1-10 kill. The analysis includes compilation in a standard fonnat of the recent and the previous gravity data sets, wluch required homogelusation to IGSN-71 datum from the previous surveys that was found out not available in this datum. The theoretical (nornml) gravity values for each stations were calculated by means of the international gravity fonnula (GRS-67) and all Wavity field stations are tied to the IGNS-71 datum of the Geophysical Observatory of Addis Ababa Uluversity (whose gravity value is 977452. 16 mOaI). Similarly, the magnetic daia an! alr,o d.r,ffi cqll'e4t~:(or tl¥ di\lfiial v~llations: Results of both the gravity and magnetic surveys'll'ave' revealed sig'nwcailf achievetllehtsin mapping several zones of hydrothennal alterations, weak stmctural indications such as faults, lineaments, fi'actures, joints, etc. within the study area. The delineation of weak stmctural features and hydrothennal manifestations i.e. altered grounds with fumarolic activities that are associated with fissures, craters, fanlts and caldera rims, where tectonic stmctures give major access for hydrothennal fluids which in tum are favourable for economic mineral potentials like sulphides, epitennal gold deposits, etc. In addition, to these invaluable natural resources these data are also important in the mapping and location of potential geothennal zones. Both the gravity anomaly maps and the magnetic anomalies are found in agreement in mapping of the geological and stmctural trends. The distinction between the lift and the adjacent plateaus and the regional stmcture features were depicted fi'om the anomaly maps on the basis of the general shape and wavelengths ofthe anomalies.



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