The Gravity Field and Tectonic Setting of Tile Gedamssa Caldera and its Adjacent Areas

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Addis Ababa Universty


In this thesis work analysis of587 gravity data collected in the central patt of the Main Ethiopian Rift (MER), between latitudes 8.00- 8.75N & longitudes 38.75- 39.5E ( consisting of Gedamssa caldera is made). The data sets incorporate 50 gravity obselvations that are recently collected by the author with his principal advisor & the remaining 537 gravity observations obtained from previous stllveys carried out by EIGS & Geophysical Obselvatory of AAU. The analysis include compilation in a standard fonnat of the recent & and the previous gravity data sets, which require homogenisation to IGSN71 Datum of the data set fi'om the previous sUlveys that is found out not available to tIns datum. All available data, about 587 stations, obtained li-Oln the EIGS and partly collected by the author, were reduced to sea level with a Ulnfonn crustal density of2.67 gm/cnl Effects Bouguer masses were calculated applying the simple Bouguer correction. Terrain cOlTection of majority of the datas was not applied in which case its effect was treated as systematic error in computing the over all mean square error of the simple Bonguer anomalies at each station. Theoretical gravity field was computed by means of the intematiollal gravity fomlUla of 1967 (GRS67) and tied to the IGSN71. The accuracy of the Bouguer anomaly at each station is calcnlated to about ± 2.8 mGal. The regional and residual gravity field were calculated from the Bouguer anomaly map by means of a software that uses Hanning filter. The compiled Bouguer anomaly map shows a strong con'elation between the regional pattem and topograplnc feature of the sUlvey area. Owing to Ingher elevations, the adjacent plateaus are marked by negative Bouguer anomalies, the minimum of magnitude -245 mGals being located around Areb gebya. The maximum gravity values occur in the NE cOlTesponding to the relatively lower elevations. The main Ethiopian rift including Afar is characterised by a relatively positive anomaly as compared to the plateaus. The high on the rill accounts for the excess Illass at depth on which the emst lies relative to the plateaus where a mass deficiency is assumed. Gravity values on the main Ethiopian rift are generally less as compared to the north pmt of the Ethiopian rift. TIns accounts for the thinning of clusl material towards the Afar along the rift, or it can equally be explained as by a thickelnng of a denser material beneath the cl1lst(probably the anomalous mantle). The margins separating the rift 11'0111 the plateaus are marked by steep gravity gradients which their magnihlde represent the slope of the escarpment. Steep gradients indicllte the transition of gravity field fi'om the platellus to the rift floor.