We report 26 new heat flow and 13 radiogenic heat production measurements from Zimbabwe, Zambia and Tanzania, together with details and some revisions of 18 previous heat flow measurements by other investigators from Kenya and Tanzania. These measurements come from Archean cratons, Proterozoic mobile belts, and Mesozoic and Cenozoic rifts. Heat flow data from eight new sites in the Archean Zimbabwe Craton are consistent with previous measurements in the Archean Kaapvaal‐Zimbabwe Craton and Limpopo Belt (Kalahari Craton) and do not change the mean heat flow of 47±2 mW m−2 (standard error of the mean) in the Kalahari Craton based on 53 previous measurements. Eight new sites in the Archean Tanzania Craton give a mean heat flow of 34±4 mW m−2. The mean heat flow from nine sites in the Proterozoic Mozambique Belt to the east of the Tanzania Craton in Kenya and Tanzania is 47±4 mW m−2. Twelve measurements in the Mesozoic rifted continental margin in east Africa give a mean heat flow of 68±4 mW m−2; four measurements in the Mesozoic Luangwa and Zambezi Rifts range from 44 to 110 mW m−2 with a mean of 76±14 mW m−2. In comparing heat flow in east and southern Africa, we observe a common heat flow pattern of increasing heat flow away from the centers of the Archean cratons. This pattern suggests a fundamental difference in lithospheric thermal structure between the Archean cratons and the Proterozoic and early Paleozoic mobile belts which surround them. Superimposed on this common pattern are two regional variations in heat flow. Heat flow in the Tanzania Craton is lower by about 13 mW m−2 than in the Kalahari Craton, and in the Mozambique Belt in east Africa heat flow is somewhat lower than in the southern African mobile belts at similar distances from the Archean cratonic margin. The two regional variations can be explained in several ways, none of which can as yet be elevated to a preferred status: (1) by variations in crustal heat production, (2) by thin‐skinned thrusting of the Mozambique Belt over the Tanzania Cratonic margin, (3) by lateral heat transfer from beneath the rift flanks into the rifts, or (4) by lower mantle heat flow beneath all of eastern Africa prior to the Cenozoic development of the East African rift system.

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S U M M A R YThe Bouguer gravity anomaly map of the Zimbabwe and Kaapvaal cratons, the Limpopo Belt and the Mozambique basin shows a regional gravity high in the southeast. Superimposed on this gravity high is a line of elongate gravity highs that is coincident with the Lebombo-Nuanetsi-Sabi volcanics. Gwavava et al. ( 1992) argue that the regional anomaly over this region is the effect of Mesozoic crustal thinning and igneous activity during the breakup of Gondwana. Lithospheric extension resulted in crustal thinning by at least 4 km beneath the central Limpopo Belt.2-D Fourier transforms of Bouguer gravity and topography data have been used to obtain the admittance and coherence throughout the region. Assuming a regionally compensated isostatic model comprising two layers with statistically independent surface and subsurface loading, the predicted coherence was compared to the observed coherence to obtain the best estimate of the effective elastic thickness of the lithospheric plate. This coherence analysis reveals that the Archaean cratons have a minimum effective elastic thickness of about 56 km, whereas that of the area beneath the Mozambique basin is only about 22 km, a value similar to that obtained in other East African rift basins. Hence, the stable cratonic areas are more rigid than the area beneath the Mozambique basin, which was subjected to Mesozoic crustal extension and thinning.An isostatic anomaly map has been computed by filtering the Bouguer gravity in the wavenumber domain with the square root of the predicted coherence function of the whole area generated from the best-fitting two-layer model with an effective elastic thickness of 52 km. The isostatic anomaly map is characterized by (1) a zone of positive isostatic anomalies over the Lebombo-Nuanetsi-Sabi area, which are partly due to crustal thinning and partly to basic plutons emplaced in the upper crust, both resulting from an extensional episode associated with Gondwana breakup, (2) positive isostatic anomalies over the Southern Marginal Zone of the Limpopo Belt, which we interpret as being due to dense granulites within the upper crust, (3) negative isostatic anomalies over the Northern Marginal Zone of the Limpopo Belt, which may have resulted from recent rapid erosion, and (4) negative isostatic anomalies over the Mozambique basin, which are due to the gravity effect of low-density Cretaceous and Tertiary sediments.

A B S T R A C TWe report multisite observations devoted to the main-sequence d Scuti star XX Pyx, conducted as the 17th run of the Delta Scuti Network. Over 125 nights a total of 550 h of usable time-series photometric B-and V-filter data were acquired involving both photoelectric and CCD measurements at eight observatories spread around the world, which represents the most extensive single time-series for any pulsating star other than the Sun obtained so far.We describe our observations and reduction methods, and present the frequency analysis of our new data. First, we detect six new pulsation and five new combination frequencies in the star's light curves. We also discover evidence for amplitude and/or frequency variations of some of the modes during the observations. These can occur on time-scales as short as 20 d and show quite diverse behaviour. To take them into account in the frequency analysis, a so-called non-linear frequency analysis method was developed, allowing us to quantify the temporal variability of the modes and to compensate for it. Following that we continue the frequency search and we also incorporate published multisite observations. In this way, we reveal three more pulsation and two more combination frequencies. In the end, we report a total of 30 significant frequencies ± 22 of which correspond to independent pulsation modes. This is the largest number of independent modes ever detected in the light curves of a d Scuti star.q 2000 RAS

Abstract. Precise geoid models are essential for the conversion of GPS-derived heights to heights above sea level. Such a model is under development for the continent of Africa, as part of the African Geoid Project. A uniform 5' grid of gravity anomalies has been derived from terrestrial gravity data and has been combined with a 5' grid for the marine areas derived from satellite altimetry. The combined data set has been used with the EGM96 geopotential model in a remove-restore process to compute the geoid using two-dimensional convolution. The final result is a 5' grid of geoidal heights covering the land mass of Africa. There are significant gaps in the available terrestrial gravity data -these gaps, coupled with the effects of errors in the DEM used for calculating the Gi term and in interpolating gravity anomalies, mean that the accuracy of this geoid model will be variable and generally less than desirable.Nevertheless, comparison with GPS/levelling data covering a small part of South Africa shows an RMS agreement of better than 10cm. Over a larger region (all of Egypt) the agreement is less satisfactory.