چكيده فارسي :
Thin lava flows of Gombe Group basalts in the Omo- Turkana depression of southwestern Ethiopia and northern Kenya cover large area. This widely spread lava flows cover the rift floor of the Omo Valley. The aim of this study is to integrate paleomagnetic results with earlier geochronological data to know timing of volcanism and to infer the time for which the present architecture of the basin attained. We collected 80 oriented core drilled samples from eight sites. Rock magnetic and paleomagnetic direction studies were conducted. Pilot specimens imposed to alternating field (AF) demagnetization, thermal (TH) demagnetization and acquisition experiments. Most samples comprise two components of Natural Remnant Magnetization (NRM) directions. The first component of magnetization was easily erased at a range of 5 to 25mT AF demagnetization and 120°C to 250°C TH demagnetization. Stepwise increasing application of magnetic field to selected specimens revealed a saturation magnetization at about 300°C. Magnetization curve results from the acquisition experiment together with TH demagnetization and AF demagnetization of the same specimens indicates titanomagnetite is the dominant magnetic carrier. About 50% of magnetization removed between Temperature ranges of 2500C and 4300C suggesting pseudo single domains as a primary carrier of magnetic remanence. From a total of eight sites, six sites show reversed polarity and two sites show normal polarity. The normal and reversed polarities are 1800 apart, thus they are antipodal to one another. The overall mean direction for 6 sites of reversed polarity is (DS=186.1, IS=- 1.9,KS=38.8, α95=10.9) whereas the two sites with normal polarity yield (DS=348.4, IS=4.6, K=378.9, α95=12.9). By using the available upper age control of Moiti tuff (3.98 Ma) and Naibar tuff (4.02 Ma) which have never been noted intruded by the Gombe Group basalts; basalts from Usno, Turmi and Nklabong correlated with the late Gilbert Chron of Cande and Kent (1995) specifically just above the Cochiti normal sub-Chron (4.18Ma). This suggests wide are covered thin lava flow in southwestern Ethiopia and northern Kenya occurred in a very short period of geologic time between 4.18Ma and 4.02Ma. This short time range probably reveal one of the major volcanic eruption phase in the region. Geomagnetic signatur similarity of this basalt with the overlied fluviolacustrine deposite of the Shungura Formation (Kidane et al., 2015) indicate , the present architecture of the Omo-Turkana depression attained soon after the emplacement of this basalt.
