MAPPING CRUSTAL THICKNESS VARIATIONS BENEATH THE ETHIOPIAN PLATEAUS AND RIFTS USING AIRBORNE GRAVITY AND ELEVATION DATA: IMPLICATIONS FOR THE EVOLUTION OF RIFTING IN ETHIOPIA

Crustal thickness reflects growth, delamination and deformation of the crust as well as its isostasy. It also defines the depth of Moho, a first-order discontinuity of seismic velocities, density, composition and rheology inside the Earth. In order to map the crustal thickness variations (gravity Moho depths) of the Ethiopian plateaus and adjoining rifts, we used an empirical equation developed from a regional airborne gravity survey constrained by Moho depths obtained from previous receiver function studies. Isostatic Moho depths were also computed by regressing the gravity Moho depth estimates with low-pass filtered elevation anomalies. Vertical tectonic stress and percentage of compensation maps were used to explain the variations of crustal thinning and indicate possible causes of observed crustal deformations. The study delivered new Moho depth estimates across the entire study area including regions with limited previous seismic Moho depth estimates or totally unmapped. The crust thins both westwards and eastwards beginning from the center of the plateaus (39⁰ E) due to extensional stress and hot spot volcanism in the region. The thickest crust (43 km) is observed beneath the northwestern plateau and the thinnest crust (15 km) is observed beneath the Afar depression. Within the Main Ethiopian rift, a relatively thicker crust is observed beneath the central rift segment and a thinner crust beneath the northern rift segment, inconsistent with previous results that suggest a steady crustal thinning from south to north along the rift axis. The vertical tectonic stress map shows that the crustal thinning observed particularly in Afar is dominantly due to extensional stress fields. The map also shows that some part of the Afar region has experienced magma compensated crustal thinning. Part of Afar that underwent magma compensated crustal thinning strike north-south and is in good agreement with results of some previous seismic studies. Magmatic processes including underplating and intrusion are inferred to have caused local uplifts leading to crustal thickening in some parts of the study area, particularly the north western plateaus and the central main Ethiopian rift.