Recent geological fieldwork, radiometric age dating of volcanic rocks, gravity and seismic reflection surveys have considerably refined our understanding of the tectonic evolution of the northern Kenya Rift. These data reveal that deep, half-graben basins up to 7 km thick were initiated west of Lake Turkana probably during Late Oligocene-Early Miocene times. The basins, bounded by easterly dipping faults, trend along the western side of the rift from Lake Turkana to the Elgayo Escarpment-Tugen Hills area. Some rift basins were episodically active from the Oligocene to the Pliocene while others were only active for a few million years. Some sag basins may have developed during periods of rifting quiescence. In the Turkana area the location of faulting gradually shifted eastwards with time. Volcanism both preceded and accompanied rifting. Within the Turkana area, volcanism moved south and east with time, beginning about 33 Ma (probably preceding half graben formation by a few million years) and continued to the present day. Extension along faults is greater in the Turkana area (between 25-40 km) and decreases southwards to probably 5-10 km or less in the southern Kenya rift. This pattern of extension values agrees with deep seismic refraction work (KRISP), which indicates thin crust in the Turkana area (about 20 km) that thickens to about 35 km south of Lake Baringo. The extension values, crustal thinning, age of volcanism and the timing of faulting to the south all point towards a southerly propagation of the rift.

Exploration in the Rukwa rift, using gravity and seismic reflection surveys, fieldwork, and drilling has defined the structure and stratigraphy of the basin in greater detail than any other part of the Western rift. The stratigraphy comprises Precambrian basement, Karroo sandstones, shales and coals, upper Miocene red beds, and Miocene‐Recent lacustrine and fluvial sediments. During Miocene‐Recent rifting the greatest sediment input apparently came from axial fluvial systems flowing from the northwest and southeast. The southwestern area experienced alternating shallow lacustrine and fluviodeltaic conditions during the Miocene‐Recent. Cenozoic age rift structures have a dominant NW‐SE and a subordinate N‐S trend. The NW‐SE trend tends to follow a Precambrian basement and later Karroo structural trend. NE‐SW seismic lines indicate up to 10 km extension of the Tertiary section in a direction oblique to the probable E‐W regional extension direction. In the southeastern portion of the basin both Karroo and Tertiary‐Quaternary sediments expand into the Lupa fault zone, reaching thicknesses of up to 3 and 7 km, respectively. Tertiary‐Quaternary sediment thicknesses decrease northwestward, accompanied by a decrease in the amount of extension and a broadening of the basin as extension is transferred to the Lake Tanganyika rift.

Vector measurements of the ion transport velocity in the F region above Arecibo (18øN, 50 ø dip angle) are reported for two daytime and five nighttime periods. From these vector measurements both the electric fields and neutral winds iri the F region can be found. Daytime results indii•ate eastward electric fields during the morning, changing to westward near 1400. The most Outstanding feature of the nighttime results is a large (•3 mv/m) westward electric field at 0400. The overall diurnal electric field pattern suggests that an 8-hour tidal mode dominates the wind fields in the dynamo region. Daytime neutral wind measurements indicate the importance of ion drag in determining the neutral winds; the poleward winds are observed to be strongest at •1000, though, according to satellite drag data and evidence presented here, the N-S neutral pressure gradient maximizes in the afternoon. At night the neutral winds are first seen to blow strongly equatorward but then abate and often blow poleward by 0200. Application of the neutral wind and electric field measurements to F region morphology indicates that (1) during sunset strong downward ion transport is the result of poleward neutral winds; (2) the generally occurring (at Arecibo) sudden drop in the height of the F layer near midnight is the result of a reversing wind from equatorward to poleward; and (3) the rise of the F layer often seen at •0400 at Arecibo is the result of I• x B drifts. Finally, there is a strong tendency for the ion motion to be horizontal.Possible reasons for this behavior are discussed. Observatory (18øN) since November 1969. The ion velocity component along the magnetic field is related to the •'rne•idional neutral winds, whereas the components across the magnetic field are due to E g B drifts and thus are direct measurements of the ambient electric fields. Measurements of meridional neutral wirids at the incoherent scatter station of St. Santin have been reported by Vasseur [1969], while ambient E-W electric fields at Jicamarca have been reported by Woodman [1970]. A comprehensive review of ionospheric drift measurements made using incoherent scatter is given by Evans [1972b]. The Arecibo radar has the capability of measuring both winds and electric fields. Thus wind-field interactions can be studied directly. The relative contributions of the winds and the electric fields to various F region morphological phenomena can also be monitored. Since Arecibo is approximately midway in latitude between St. Santin or Millstone Hill and Jicamarca, the Arecibo radar can provide valuable information on the latitudinal variation of the meridional winds and the E-W electric fields. F region ion velocity measurements have been made at the Arecibo

No abstract

Neutral wind observations over the 100-to 200-km height region at Arecibo are presented for a summer, an equinox, and a winter period. Observations confirm that the wind below 110 km is predominately diurnal, with a vertical wavelength of about 20 km, and can be identified with the Sx,x mode. This tide reaches amplitudes of 100 m/s near 100 km but exhibits large day to day and seasonal variation. The vertical energy flux associated with the Sx,x mode is of the order of 0.25 erg/cm•-/s at 100 km. A semidiurnal oscillation dominates the wind field from about 115 km to at least 170 km during the summer and equinox observations. A nighttime intermediate layer of enhanced electron density consistently descends through the Fx valley in the postsunset hours along the convergent null in Vz associated with the semidiurnal winds. The S•.,•. mode dominates the semidiurnal oscillation above 125 km during these periods, reaching a maximum amplitude of 90 m/s at 153 km. Higher-order modes contribute to the semidiurnal oscillation principally below 125 km during spring and summer but appear to dominate at all heights in the winter observations, when the &.,•. mode appears to be largely absent. The vertical energy flux associated with the S•.,•. mode is at least an order of magnitude less than that associated with the Sx,x mode.