Rift Kinematics During the Incipient Stages of Continental Extension: Evidence from the Nascent Okavango Rift Basin, Northwest Botswana

Rift kinematics during the incipient stages of continental extension: Evidence from the nascent Okavango rift basin, northwest Botswana

M.P.Modisi* E.A. Atekwana* Department of Geology, University of Botswana, Private Bag 0022, Gaborone, Botswana A.B. Kampunzu T.H. Ngwisanyi Geological Survey of Botswana, Private Bag 14, Lobatse, Botswana

ABSTRACT branch started before 5 Ma (Kampunzu et al., High-resolution aeromagnetic data from the nascent Okavango rift in northwest Botswana 1998), whereas most rift basins of the southwest- provide an unprecedented view of rift kinematics during the incipient stages of continental ern branch are Quaternary. Geomorphological extension. Crosscutting relationships between west-northwest–trending 180 Ma Karoo dikes investigations reveal a major late Pliocene phase and reactivated northeast-trending Proterozoic basement faults are used to document the kine- of uplift of southern Africa (Partridge and Maud, matics of Cenozoic faulting during the initial stages of rifting. Depth estimates to the top of the 1987) that predates the development of the dikes using three-dimensional Euler deconvolution solutions have produced the following inter- Okavango rift. This rift basin hosts the largest in- pretations. (1) The Okavango rift is a half graben with a downthrow of ~200–300 m. (2) The land alluvial fan on Earth, supporting the largest width of the Okavango rift (100 ± 20 km) is similar to that of more mature continental rifts such (~18 000 km2) wetland in southern Africa as the Tanganyika and Baikal rifts. This suggests that the width of continental rifts is acquired (McCarthy et al., 1991). There is agreement that early in their evolution and reflects neither the age and maturity of the rift basin, nor the this fan owes its origin to neotectonic activity re- amount of extension. It is suggested that the cumulative downthrow (sediment infill included) lated to the East African Rift system (Fairhead and subsidence may be a better indicator of the relative maturity of rift basins. (3) Preexisting and Girdler, 1969; Scholz et al., 1976; McCarthy basement faults exert a major control during rifting, and reactivation processes do not occur et al., 1993; Modisi, 2000). synchronously along the entire length of preexisting faults. (4) The Okavango rift is defined by Except for a few isolated outcrops, the rocks normal faults; there is no evidence of major strike-slip faults, thus excluding a pull-apart tec- are largely buried underneath an extensive desert tonic model for this nascent continental rift stage. (5) The preexisting Sekaka shear zone termi- sand cover. Pre-Okavango geologic units include nates the Okavango rift to the south, suggesting that such shear zones represent major barriers the following, from youngest to oldest. (1) Marls, during longitudinal propagation of rifts. This probably explains why such shear zones com- clays, gravels, eolian sands, calcrete, and silcrete monly evolve into accommodation or transfer zones during further evolution of continental rifts. define the 230-m-thick Cenozoic Kalahari beds. (2) Carboniferous to Jurassic Karoo supracrustal Keywords: aeromagnetics, Botswana, kinematics, Okavango, rift. sequences include sedimentary and volcanic rocks. A prominent feature is the west-northwest– INTRODUCTION GEOLOGIC FRAMEWORK trending 180 Ma Karoo dike swarm. (3) Neo- Continental rifts located far from plate margins The East African Rift system includes two proterozoic siliciclastic and carbonate sequences are markers of early stages of continental breakup. branches: (1) an older (>15 Ma) and more compose the Ghanzi-Chobe Supergroup. (4) Understanding the kinematics of rifting during evolved eastern branch that extends from Afar to Mesoproterozoic metavolcanic rocks and related the incipient stages of continental extension has Kenya; and (2) a younger (<15 Ma) less evolved granitoids and gneisses are underlain by Paleo- been a subject of much interest among geoscien- western branch that extends from Albert Lake to proterozoic basement (gneiss, granulite, gran- tists (Rosendahl, 1987). Kinematic models pro- the south of Dombe in Mozambique (Kampunzu itoids). Proterozoic structures (fold axes, thrusts) posed to explain the earliest stages of rift creation et al., 1998) (Fig. 1). In both the eastern and west- trend northeast-southwest; transverse shear zones are usually inferred on the basis of observations ern branches, individual rift basins (50–100 km trend east-west and west-northwest. from more evolved continental rifts such as the long and 20–100 km wide) are linked by transfer East African Rift system, Baikal rift, and Rhine faults or accommodation zones (Rosendahl, DATA ACQUISITION AND PROCESSING graben (e.g., Tiercelin et al., 1988; Strecker et al., 1987; Ebinger, 1989). A network of separate The data set includes high-resolution aero- 1990; Delvaux et al., 1997). In this paper we Quaternary rift basins extends from west of Lake magnetic data (Fig. 2) acquired in 1996 under the present data from the Okavango rift in northwest Tanganyika in the northeast to the Okavango in direction of the Botswana Geological Survey. Botswana, an example of a continental rift in its the southwest. These unconnected rifts have aver- The mean flight elevation was 80 m along north- early stage of evolution. Crosscutting rela- age lengths of 100 km and widths of 40–80 km south lines 250 m apart with east-west tie lines tionships between 180 Ma Karoo dikes and later and define a broad northeast-southwest–trending 1.25 km apart. The International Geomagnetic Cenozoic fault displacement revealed by high- extensional crustal province hereafter named the Reference Field has been removed from the ob- resolution aeromagnetic data afford a unique southwestern branch of the East African Rift sys- served data. The data were gridded with an aver- opportunity to delineate the main faults bounding tem (Fig. 1). This southwestern branch displays age grid cell size of 100 m (Briggs, 1974). The the rift basin and to document the kinematic style geophysical attributes of the East African Rift depth to the top of the dikes was determined and the role of inherited structures during the system, i.e., a regional gravity low (amplitude using Euler’s homogeneity equation (Thompson, earliest stage of continental rifting. <100 mGal) (Girdler, 1975), anomalously high 1982), with depth errors of ±3 m. heat flow (60–90 mWm–2) (Ballard et al., 1987; Sebagenzi et al., 1993), and the seismic activity RESULTS *E-mail: Modisi—[email protected]. Pres- ent address: Atekwana—Department of Geosciences, of a northwest-southeast tensional stress field The most prominent features on the aeromag- Western Michigan University, Kalamazoo, Michigan regime (Fairhead and Henderson, 1977). The netic maps are the west-northwest–trending late 49008, USA. opening of individual rifts within the western Karoo dikes (Fig. 2). This ~70-km-wide dike

Data Repository item 200097 contains additional material related to this article.

Geology; October 2000; v. 28; no. 10; p. 939–942; 6 figures. 939 main areas (Fig. 2) have been selected for detailed analysis of structural features.

Maun Zone Four major northeast-trending faults are evi- dent within this zone: the Kunyere, Thamalakane, Phuti, and Nare faults (Fig. 3). These faults displace the dikes and are also represented by fault scarps on the digital terrain map (Fig. A1). Pre- existing northeast-trending basement fabric con- trols the trend of these faults. Depth estimates to the top of the dikes (Fig. B; see footnote 1) show Figure 1. Map of East African Rift a deepening of the pre-Kalahari surface toward system showing three branches the west (Fig. 4, A–A′). To the east of the Kunyere described in this paper and study area (modified from Kampunzu fault, average depths of 250 m are calculated, et al., 1998). Inset: Map of Africa whereas to the west, depths of 500–550 m are ob- showing position of Okavango tained. From this, we infer dip-slip movements; basin in Botswana. blocks are downthrown to the west consistent with seismic reflection interpretations (Laletsang, 1995). The greatest amount of vertical displacement (~300 m) occurs along the Kunyere fault (Fig. 4, A–A′). The downthrow for the Thamala- kane fault is variable along strike, decreasing from 150 m in the north to 50 m in the south. The amount of vertical displacement is <50 m across the Phuti and Nare faults (Fig. B; see footnote 1). The Kunyere fault is therefore the main west- dipping bounding fault to the east of the rift system, consistent with its higher seismic activity (Reeves, 1972). The Kunyere and Thamalakane faults displace the dike swarm along its entire width, whereas the Phuti and Nare faults only swarm extends from Zimbabwe in the east, In this study we focus mainly on the kinematics of displace the northern portion of the swarm (Fig. 3). through the study area in Botswana, into northeast the younger, postdike faults. This younger tec- The southwestern continuation of these latter Namibia to the northwest (Reeves, 1972). These tonic activity marks the evolution of the Cenozoic faults intersects the southern portion of the dike dikes are superimposed on northeasterly trending Okavango rift. The depth to the top of the dikes is swarm without any displacements. These rela- folds and faults of the Neoproterozoic Ghanzi- used to calculate the vertical displacement across tionships suggest that these are predike faults that Chobe belt and are cut by younger faults (Fig. 2). the faults and to infer their dip directions. Three have been partly reactivated in their northern sec- tion during Cenozoic rifting.

Lake Ngami Zone In addition to the Kunyere and Thamalakane faults, several other faults (e.g., Tsau and Lecha faults) can be identified (Fig. 5) within this zone. About 250 m of displacement is estimated across the westernmost bounding Tsau fault (Fig. C; see footnote 1). The width of the downthrown block bounded by the Tsau fault and the Thamalakane fault in the east is 60 km, defining a sedimentary basin hereafter named the Ngami depocenter (Fig.4,C–C′). The average depth to magnetic sources in this depocenter is 450–550 m, com- pared to 200–300 m outside the basin. This is consistent with available borehole data depths. The Tsau and Lecha faults are inferred to be southeast-

1GSA Data Repository item 200097, Figure A, Digital terrain map of the Okavango rift, and Figures B, C, and D, Euler deconvolution solutions of the Maun, Lake Ngami, and Gumare zones, respectively, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, Figure 2. First vertical derivative aeromagnetic map of Okavango rift. Locations of cross sections [email protected], or at www.geosociety.org/ shown in Figure 4 are indicated. Color scale shown in Figure 3. pubs/ft2000.htm.

940 GEOLOGY, October 2000 Figure 3. First vertical derivative aeromagnetic map of Maun zone. Faults: K—Kunyere; N— Nare; P—Phuti;Th—Thamalakane.

dipping normal faults. All the faults mapped within this zone extend to the southwest and abort against the westerly trending dextral Sekaka shear zone (Fig. 5). There is no evidence of neotectonic activity south of the Sekaka shear zone.

Gumare Zone The Gumare zone includes the northeast- trending Gumare fault. This fault truncates the dikes in the south, but terminates in the middle of the dike swarm to the north with no displacement (Fig. 6), indicating reactivation only in its southern part. The Okavango alluvial fan owes its existence to this fault scarp. Depth estimates (Fig. D; see footnote 1) show <50 m of dip-slip movement along the Gumare fault, suggesting minimal reactivation during the Cenozoic Okavango rifting. The Tsau fault is vertically displaced 250 m, and is therefore the main border fault to the west of the Ngami depocenter.

DISCUSSION AND CONCLUSION Aeromagnetic data, digital elevation, and seismic and geological information have been used to infer the framework of the neotectonic evolution of the Okavango rift. Major active dip-slip faults offsetting late Karoo dikes include the Kunyere, Thamalakane, Nare, Phuti, Lecha, and Tsau faults. The most important downthrow (~250 m) occurs along the Kunyere and Tsau Figure 4. Interpretative structural cross sections faults. The first four define west-dipping syn- of Okavango rift. Profile thetic normal faults, the seismically active Kunyere lines are shown in Figure 2 being the main border fault. To the west, the and fault names in Fig- Lecha and Tsau faults are eastward-dipping normal ure 3, except Ts—Tsau; G—Gumare; L—Lecha. faults (Fig. 4). The Thamalakane and Tsau faults define the outer boundaries of the 60-km-wide Lake Ngami depocenter. The width of the Okavango rift between the Gumare fault to the west and the Nare fault to the east is 120 km, whereas the relief of the basin is <100 m. The thickness of the Cenozoic sediments is <600 m. The width of the Okavango rift is similar to the size of the Tanganyika and Baikal rifts, for which the relief is >1000 m and the thickness of sediment infill is >4 km (e.g., Ebinger, 1989; Logatchev, 1993; Lezzar et al., 1996). Available data suggest that the width of continental rifts is probably acquired very early in their evolution and does not necessarily reflect the age of the rift basins. In contrast, the amount of downthrow and thickness of sediment infill appear to be better indi- cators of the maturity of continental rift basins.

Figure 5. First vertical derivative aeromagnetic map of Lake Ngami zone. Color scale shown in Figure 3. Fault names as in Figures 3 and 4. SSZ—Sekaka shear zone.

GEOLOGY, October 2000 941 Lezzar, K.E., Tiercelin, J.J., de Batist, M., Cohen, A.S., Bandora, T., van Rensbergen, P., Le Turdu, C., Mifundu, W., and Klerkx, J., 1996, New seismic stratigraphy and late Tertiary history of the North Tanganyika Basin, East African Rift system, de- duced from multichannel and high-resolution reflection seismic data and piston core evidence: Basin Research, v. 8, p. 1–28. Logatchev, N.A., 1993, History and geodynamics of the Baikal rift (east Siberia): A review: Bulletin du Centre de la Recherche Exploration Produc- tion Elf Aquitaine, v. 17, p. 353–370. McCarthy, T.S., Stanistreet, I.G., and Cairncross, B., 1991, The sedimentary dynamics of active fluvial channels on the Okavango fan, Botswana: Sedi- mentology, v. 38, p. 471–487. McCarthy, T.S., Green, R.W., and Franey, N.J., 1993, The influence of neo-tectonics on water dispersal in the northeastern regions of the Okavango swamps, Botswana: Journal of African Earth Sci- ences, v. 17, p. 23–32. 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Salis- East Africa: The Malawi rift: Geology, v. 20, and the other assumes that transverse faults origi- bury reviewed an earlier version of the manuscript. p. 1015–1018. nate later, during the evolution of the rift, in order Rosendahl, B.R., 1987, Architecture of the continental REFERENCES CITED rifts with special reference to East Africa: Annual to transfer motion between offset normal faults Ballard, S., Pollack, H.N., and Skinner, N.J., 1987, Ter- Reviews of Earth and Planetary Science, v. 15, (e.g., Rosendhal, 1987). Our data show that the restrial heat flow in Botswana and Namibia: Jour- p. 445–503. major active faults bounding the Okavango rift are nal of Geophysical Research, v. 92, p. 6291–6300. Scholz, C.H., Koczynski, T.A., and Hutchins, D.G., normal faults that exhibit no major evidence of Briggs, I.C., 1974, Machine contouring using mini- 1976, Evidence for incipient rifting in southern mum curvature: Geophysics, v. 39, p. 39–48. 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