Geophysical studies of Anza Graben N.E. Kenya

Abstract

Anza Graben is situated in the north and northeastern Kenya. It is about 120km wide and approximately 600 km long. The area of the present study is located mainly in NE Kenya and constitutes a strike length of about 300 km. The graben is almost entirely sediment filled and is estimated to be of Cretaceous age. It is characterized by a prominent negative Bouguer anomaly reaching -40 mGal. Several studies were made in an effort to determine its subsurface structure. These involved interpretation of a number of seismic reflection profiles, 3D interactive gravity modelling using fixed density contrasts between the sediment in fill and the basement, and the analysis by filtering and modelling of the aeromagnetic data over the graben. A crustal modelling of refraction data along the western shoulder of the Anza graben using 2D ray tracing was also performed. Two major basins are recognizable from the results of the studies, namely the southern basin which has a northeasterly dip and the northern basin having a southwesterly one. This alternating asymmetry is also indicated for an adjoining smaller basin located to the west of the graben. The density contrast used for the final gravity model is -300 kg/m3 which indicates a maximum sediment thickness of about 8 km. The modelling of aeromagnetic data however favour a maximum thickness of at least 10km. A 2D refraction crustal P wave model of the western shoulder indicates that the crust thins northwestwards with the Moho depth decreasing from 35 km under Chanler's Falls to approximately 29 km under the southeastern shores of Lake Turkana. The abrupt shallowing of the Moho in the Lake Turkana area is attributed to the effects of the Kenya Rift Valley. However the gentle variation in the Moho depth along the remaining section of the shoulder of Anza Graben appears to depict a regional phenomenon also attributable to the formation of the Kenya Rift Valley.It is proposed from the study that sections of the graben shoulder are underlain by slightly denser rocks and that the crustal thinning associated with Anza Graben is not Significant.