FAULT GEOMETRY AND EXTENSION MECHANISMS IN THE CENTRAL KENYA RIFT, EAST-AFRICA - A 3D REMOTE-SENSING APPROACH

In the central zone of the Kenyan Rift, and in particular the region o f half-graben at Baringo-Bogoria, a morphostructural study was carried out using aerial and satellite data of differing scales as well as th ree-dimensional imagery. This zone, situated at the intersection of th e N 150 degrees Aswa lineament and the submeridian eastern branch of t he East African Rift, is a particularly good area in which to study th e role of these two great tectonic trends in the rift evolution since the Cenozoic in detail. The geological context of the region is charac terized by abundant volcanism which has spread out over millions of ye ars. The different periods of volcanic activity are excellent markers of the rift deformation, and they allow us to draw up a chronology of events. The interpretation of the Landsat and SPOT satellite imagery l eads to the definition, on a different scale, of a structural framewor k. The detailed structural studies are put together from methods of 3D teledetection. The 3D images illustrate several types of structures - ''en lanieres'' or grid faults, tilted blocks, rhomb-shaped horsts, i nterference directions - all developed on a large scale. One of the fi rst attempts to statistically process fault geometry and distribution was carried out using numerical data from the Digital Elevation Model (DEM) obtained from the Bogoria area. The interpretation of the geolog ical section established from the DEM and existing geological maps as well as a study of the fault throws, allows us to put forward a stress field compatible with an approximately E-W extension and a chronology of tectonic events. In this kinematic model, the role of Aswa-type tr ansverse faults seems to be totally passive, with only lateral-dextral movements along oblique discontinuities during rift extension, contra ry to the active role proposed by other authors.