THE NORTH TANGANYIKA HYDROTHERMAL FIELDS, EAST-AFRICAN RIFT SYSTEM - THEIR TECTONIC CONTROL AND RELATIONSHIP TO VOLCANISM AND RIFT SEGMENTATION

The two branches of the East African Rift system include numerous hydr othermal fields, which are closely related to the present fault motion and to volcanic and seismic activity. In this study structural data f rom Pemba and Cape Banza hydrothermal fields (western branch, North Ta nganyika, Zaire) are discussed in terms of neotectonic phenomenona. Di fferent types of records, such as fieldwork (onshore and underwater) a nd LANDSAT and SPOT imagery, are used to explain structural controls o n active and fossil hydrothermal systems and their significance. The P emba site is located at the intersection of 000-020-degrees-trending n ormal faults belonging to the Uvira Borde Fault System and a 120-130-d egrees-trending transtensional fault zone and is an area of high seism icity, with events of relatively large magnitude (M(s) < 6.5). The Cap e Banza site occurs at the northern end of the Ubwari Penisula horst. It is bounded by two fault systems trending 015-degrees and is charact erized seismically by events of small magnitude (M(s) < 4). The hydrot hermal area itself is tectonically controlled by structures striking 1 70-180-degrees and 080-degrees. The analysis of both hydrothermal area s demonstrates the rejuvenation of older Proterozoic structures during Recent rift faulting and the location of the hydrothermal activity at the junctions of submeridian and transverse faults. The fault motion is compatible with a regional direction of extension of 090-110-degree s. The Cape Banza and Pemba hydrothermal fields may testify to magma c hambers existing below the junctions of the faults. They appear to for m at structural nodes and may represent a future volcanic province. To gether with the four surface volcanic provinces existing along the wes tern branch, they possibly indicate an incipient rift segmentation rel ated to 'valley-valley' or 'transverse fault-valley' junctions, contra sting with the spacing of the volcanoes measured in the eastern branch . These spacings appear to express the different elastic thicknesses b etween the eastern and western branches of the East African Rift syste m, perhaps related to a difference in stage of evolution of the two br anches.