Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.392m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 392
Mathematics
Logic
Bangweulu Basin, Impact Craters, Ring Structure, Zambia
Scientific paper
The Bangweulu Basin (BB) (ca. 29 degrees-31 degrees E, 10 degrees-12 degrees S) is a roughly circular depression, ~150 km in diameter, on the Bangweulu Block of Zambia. The basin, about 1148 m ASL, is occupied by Lakes Bangweulu (~85 km long) and Kampolombo (~20 km long) and the Bangweulu Swamps [1,2]. The basement consists partly of granitoids (~1.8 Ga) together with ~1.1-Ga Katangan cover rocks. To the north, cover rocks of the Mporokoso Group (~1.8-1.3 Ga) form the arcuate Luongo Fold Belt [3], partly defining the perimeter of the outermost ring (R = 125 km) of the Bangweulu structure. Drainage into the BB is centripetal, with one outlet in the south, draining into a tributary of the Luapula River, which then curves in a broad arc toward the north, along the Zambia-Zaire border, before entering Lake Mweru. Rivers entering the BB include the Luansenshi, which rises in the north and flows in an arc to the southeast and south before joining the Chambeshi River, which flows southwest, west, and northwest before entering Lake Bangweulu. There is an arcuate watershed in the west (at R = 100 km), to the west of which rivers drain to the southwest and west into the Luapula River. Several elongate curved sliver-like islands, including Mbawala (~30 x 4 km) and Chisi, are present in Lake Bangweulu. The curvature of the islands follows the arcuate northwest boundary of the lake in a concentric manner. Unlike all the other major lakes in Zambia and surrounding areas (Mweru, Tanganyika, Rukwa, Malawi, and Kariba), which occupy seismically active rift structures [4,5], the Bangweulu Basin is generally aseismic, and is unrelated to rifting. There is a positive aeromagnetic intensity anomaly over the central Bangweulu depression, and there is also a magnetic anomaly density high over the central part of the BB, surrounded by a concentric low [6]. A roughly circular anomaly, outlined by the -140 mgal contour, of the regional Bouguer gravity field is centerd on Lake Bangweulu, surrounded by an arcuate high south of the Luongo Fold Belt [7,8]. There are few heat flow measurements in Zambia [9], but there is no indication that the Bangweulu Basin has abnormally high heat flow, which is present in the Luangwaand Upper Zambezi rifts, as evidenced by numerous hot springs and historical geysers [10]. Satellite imagery of Central Africa clearly shows a roughly circular outline of the Bangweulu Basin, including the lakes and swamps, surrounded by a concentric ring of uplifts. The concentric islands in Lake Bangweulu are reminiscent of the multiple concentric rings around impact basins in other planetary bodies, e.g., Valhalla and Asgard structures on the jovian moon of Callisto. Lunar craters Eratosthenes, Aristarchus, and others also have similar terraced morphologies with concentric rings. Based on the above geomorphological and geophysical features, it is postulated that the Bangweulu Basin represents the eroded remnant of a large multiring impact structure that postdates the Katangan Supergroup. Any possible connection between the Bangweulu structure and the Lukanga swamp (a postulated astrobleme in central Zambia [11]) is unknown at this stage. Ground search for macro- and microscopic shock features in the Bangweulu Basin is planned for 1994. References: [1] Debenham F. (1947) Geog. Rev., 37, 351-368. [2] Thieme J. G. and Johnson R. L. (1976) The 1:1,000,000 Scale Geological Map of the Republic of Zambia, Geol. Surv. Zambia. [3] Andersen L. S. and Unrug R. (1984) Precambrian Res., 25, 187-212. [4] Bram K. (1972). Bull. Seis. Soc. Am., 62, 1211-1216. [5] Fairhead J. D. and Henderson N. B. (1977) Tectonophysics, 41, 19-26. [6] Saviaro K. (1979) Bull. Geol. Surv. Botswana, 22, 159-181. [7] Mazac O. (1974) Tech. Rept. Geol. Surv. Zambia, 76, 40 pp. [8] Cowan I. M. and Pollack H. N. (1977) Nature, 266, 615-617. [9] Chapman D. S. and Pollack H. N. (1975) Nature, 256, 28-30. [10] Legg C. A. (1974) Econ. Rept. Geol. Surv. Zambia, 50, 60 pp. [11] Vrana S. (1985) Meteoritics, 20, 125-139.
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