Abstract

Jombo alkaline complex is the largest of the alkaline complexes in Kenya. It has been the subject of several geological and geochemical studies. However, the surface geology puts few constraints on the subsurface shape of the intrusion which we here attempt to determine by simultaneous inversion of gravity and aeromagnetic data.

The major feature of the gravity map is an elliptical high >800g.u. in amplitude centred near Jombo Hill. When the filtered magnetic map is reduced to the pole and pseudo-gravity transformed, a strikingly similar anomaly is revealed, suggesting a common source. Using an iterative least-squares technique, joint inversion of the gravity and unfiltered magnetic data for a three-dimensional model established that both data sets can be adequately modelled by a thick slab tapering upwards from c. 29 km depth and striking approximately E-W. Only the upper c. 18 km is magnetized which we interpret as an indication of the depth to the Curie isotherm. The body is predicted to be of ultramafic composition capped by the exposed syenites and ijolites.

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