Geometry and faults tectonic activity of the Okavango Rift Zone, Botswana: Evidence from magnetotelluric and electrical resistivity tomography imaging

Journal of African Earth Sciences 65 (2012) 61–71

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Geometry and faults tectonic activity of the Okavango Rift Zone, Botswana: Evidence from magnetotelluric and electrical resistivity tomography imaging Kelsey Mosley Bufford a,b, Estella A. Atekwana a,⇑, Mohamed G. Abdelsalam c, Elijah Shemang d, Eliot A. Atekwana a, Kevin Mickus e, Moikwathai Moidaki f, Motsoptse P. Modisi d, Loago Molwalefhe d a

Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK 74078, United States ConocoPhillips Company, 600 North Dairy Ashford, Houston, TX 77079, United States Department of Geological Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, United States d Department of Geology, University of Botswana, Private Bag UB00704, Gaborone, Botswana e Department of Geography, Geology and Planning, Missouri State University, Springfield, MO 65897, United States f Department of Physics, University of Botswana, Private Bag UB00704, Gaborone, Botswana b c

a r t i c l e

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Article history: Received 3 October 2011 Received in revised form 8 January 2012 Accepted 18 January 2012 Available online 28 January 2012 Keywords: East African Rift System Okavango Rift Zone Magnetotellurics Electrical resistivity Earthquakes Half-graben

a b s t r a c t We used Magnetotelluric (MT) and Electrical Resistivity Tomography (ERT) to investigate the geometry and nature of faults activity of the Okavango Rift Zone (ORZ) in Botswana, an incipient rift at the southern tip of the Southwestern Branch of the East African Rift System. The ORZ forms a subtle topographic depression filled with Quaternary lacustrine and fluvio-deltaic sediments and is bounded by NE-trending normal faults that are more prominent in the southeastern portion of the rift basin. An MT model from a regional (140 km) NW–SE trending MT transect shows that much of the rift basin is underlain by a broad asymmetrical low resistivity anomaly that slopes gently (1°) from NW to SE reaching a depth of 300 m. This anomaly suggests that faults in the southeastern part of the rift form a NW-dipping border fault zone and that the lacustrine and fluvio-deltaic sediments contain brackish to saline water filling the broad half-graben structure. Furthermore, MT and ERT models from detailed (4–13 km long) MT transects and resistivity profiles show that one border fault (Thamalakane) and two within-basin faults (Lecha and Tsau) in the southeastern part of the ORZ are characterized by a localized high conductivity anomaly while another border fault (Kunyere) lacks such an anomaly. These localized anomalies are attributed to channelized fresh surface water and saline groundwater percolating through these faults forming ‘‘fault zone conductors’’ and suggest actively displacing faults. The lack of a ‘‘fault zone conductor’’ in the Kunyere fault is interpreted as indicating diminishing displacement on this fault, and that strain was transferred to the Thamalakane fault further to the east. The fluids provide lubricant for the ORZ faults, hence preventing infrequent large magnitude earthquakes, but favoring frequent microseismicity. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The Okavango Rift Zone (ORZ) is an incipient continental rift basin found at the terminal of the Southwestern Branch of the East African Rift System (EARS) (Fig. 1A; Fairhead and Girdler, 1969; Reeves, 1972). It provides a unique opportunity to investigate fault characteristics and activities related to early rifting processes. As such, the ORZ has been the focus of several surface and shallow sub-surface geologic and geophysical investigations (Modisi, ⇑ Corresponding author at: Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK 74078, United States. Tel.: +1 405 744 6361; fax: +1 405 744 7841. E-mail address: [email protected] (E.A. Atekwana). 1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2012.01.004

2000; Modisi et al., 2000; Kinabo et al., 2007, 2008; Laletsang et al., 2007; Shemang and Molwalefhe, 2009) aimed at determining the geometry of the rift zone and the nature and kinematics of associated faults. Other studies have focused on understanding the interplay between neotectonics and surficial processes during the early stages of continental rifting to determine how environmental changes relate to climate and tectonics are recorded in lacustrine and fluvio-deltaic sediments filling the rift basin (Gamrod, 2009; Teter, 2009). A previous geo-electrical study (Shemang and Molwalefhe, 2009) has shown that some of the fault zones associated with the ORZ consist of synthetic and antithetic sets. However, this study imaged the ORZ structures to depths only reaching between 80 and 100 m below the surface. Hence, information on the deeper

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Fig. 1. Digital Elevation Models (DEMs) extracted from the Global 30 Arc Second Elevation Data (GTOPO30, 1 km spatial resolution) showing the extent of the East African Rift System (A) and elements of the Southwestern Branch of the rift system (B). K = Kenya Rift Basin. T = Turkana Rift Basin. TC = Tanzania Craton. A = Albert Rift Basin. TA = Tanganyika Rift Basin. R = Rukwa Rift Basin. M = Malawi Rift Basin. DGC = Damara–Ghanzi–Chobe Orogenic Belt. The Eastern Branch of the East African Rift System constitutes the Kenya and Turkana Rift Basins whereas the Western Branch constitutes the Albert and Tanganyika Rift Basins. The DEM was developed by the US Geological Survey EROS Data Center and was obtained from the website ‘‘www1.gsi.go.jp/geowww/globalmap-gsi/gtopo30/papers/geschd3.html’’.

structure of the basin and sub-surface characteristics of the ORZ faults is limited. Investigations by Kinabo et al. (2007, 2008) have provided important insights into the growth and propagation of faults during the initial stages of rift formation. Kinabo et al. (2008) compared the surface throw on individual faults (measured from the height of the fault topographic escarpments expressed in Shuttle Radar Tomography Mission (SRTM) Digital Elevation Models (DEMs) to the fault throws at depth (calculated from the depth to the top of magnetic sources which represents the depth to the Precambrian crystalline rocks) from aeromagnetic maps and classified the faults in the ORZ as: (1) old and active faults, (2) young and active faults, (3) faults with no recent activity, and (4) faults with waning activity. Additionally, some faults show considerable throw at depth but lack surface expression in terms of topographic escarpment. This suggest a lack of recent activity along these faults and that the deep faults were reactivated but are now concealed by rapid sedimentation associated with the Okavango Delta, or that the faults were reactivated but lacked sufficient upward propagation to rupture the surface, hence became blind normal faults (Kinabo et al., 2008). Determining the neotectonic activity of the different faults in the ORZ is important for deciphering which of these faults are actively contributing to strain accommodation by lengthening and widening of the rift. In this study, we use Magnetotelluric (MT) and Electrical Resistivity Tomography (ERT) imaging to investigate the geometry and nature of faults activity of the ORZ in Botswana. Our specific objectives are to: (1) determine the geometry of the ORZ, (2) determine the shallow sub-surface fault geometry, (3) image the extent of faults at depth, and (4) elucidate the tectonic activity of faults.

2. Geologic and tectonic setting The ORZ is part of the Southwestern Branch of the EARS (Fig. 1A; Fairhead and Girdler et al., 1969; Reeves, 1972; Girdler, 1975; Chapman and Pollack, 1977; Ballard et al., 1987; Sebagenzi et al., 1993; Modisi, 2000; Modisi et al., 2000; Sebagenzi and Kaputo, 2002). This branch consists of a network of 100 km long and 40– 80 km wide Quaternary rift basins distributed along an approximately 250 km wide corridor extending for about 1700 km west of the Tanganyika and Malawi rifts (Fig. 1B; Modisi et al., 2000; Tiercelin et al., 1988). The most notable rifts constituting the Southwestern Branch are the NE-trending Mweru rift to the southwest of the Tanganyika rift and the Luangwa rift to the southeast. The ORZ in northwestern Botswana represents the southwesternmost extent of the Southwestern Branch (Modisi et al., 2000; Kinabo et al., 2007, 2008). The ORZ (Fig. 2) occurs in an inter-cratonic zone between the Congo craton in the northwest and the Kalahari Craton (consisting of the Zimbabwe and Kaapvaal Cratons) to the east–southeast (Fig. 1B). The rift is underlain by the Precambrian Damara and Ghanzi-Chobe organic belt (Kinabo et al., 2007, 2008). This orogenic belt is dominated by NE-trending regional fabric associated with the development of NE-trending folds and faults that spectacularly appear in regional vertical derivative aeromagnetic maps (Modisi et al., 2000; Kinabo et al., 2007, 2008; Shemang and Molwalefhe, 2009). The Precambrian crystalline basement is intruded by the W and NW-trending Karoo dike swarm, thought to have been emplaced at 179 Ma (Le Gall et al., 2002; Tshoso, 2003). The surface geology around the ORZ primarily consists of unconsolidated Quaternary sediments that are restricted to the

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Fig. 2. Geological map of the Okavango Rift Zone. The swamp distribution is obtained from the website http://www.mazalien.nl/weblog/archives/2005/okavango-deltabotswana Location and extent of normal faults from Kinabo et al. (2008). Surface geology is modified from the International Geological Map of Africa 1:5,000,000 scale, Third Edition 1985-1990; a co-publication of CGMW/UNESCO with financial support from PRGM.

ORZ basin, the Quaternary Kalahari alluvium and Holocene lacustrine deposits within paleo-lakes (e.g., Cooke, 1984; Thomas and Shaw, 1991; Ringrose et al., 2005; Fig. 2). The Quaternary unconsolidated sediments of the ORZ are underlain by lacustrine and fluvio-deltaic sediments of varying thickness. Bedrock, which is predominantly Precambrian crystalline rocks of the Damara and Ghanzi-Chobe orogenic belt are exposed to the northwest and southeast of the southwestern end of the ORZ (Fig. 2). The ORZ forms part of a NE-elongated depression known as the Makgadikgadi–Okavango–Zambezi basin with a mean elevation of 850 m (Fig. 3A; Cooke, 1984). The basin is dominated by the SEflowing drainage system of the Okavango Delta, and the Kwando and the Zambezi Rivers (Fig. 2). The ORZ consists of three depocenters represented by Ngami sub-basin in the southwest and the Mababe and Linyanti-Chobe sub-basins to the northeast (Fig. 3a). The Ngami sub-basin represents a graben bounded by the NE-trending Kunyere fault in the southeast and the Tsau fault to the northwest (Fig. 2). Similarly, the Mababe sub-basin is bounded in the northwest and southeast by the Tsau and the Mababe faults, respectively (Fig. 2). Sediments fill of both the Ngami and the Mababe sub-basins may be as thick as 400 and 800 m, respectively (Kinabo et al., 2007). The ORZ is dissected by NE-trending faults that are more dominant on the southeastern side of the basin (Fig. 2). Compared to other rift basins of the EARS where fault escarpments are welldeveloped reaching 100 s of meters, fault escarpment heights in the ORZ range between 6 m along the Ngami sub-basin to 12– 18 m around the Mababe sub-basin to heights as high as 44 m around the Linyanti-Chobe sub-basin (Kinabo et al., 2008). The strike of the main faults bounding the ORZ is 030–050° in the northeast and 060–070° in the south (Kinabo et al., 2007, 2008). Below the surface, the orientation of the ORZ is influenced by the

pre-existing regional fabric of the Precambrian Damara and Ghanzi-Chobe orogenic belt (Modisi et al., 2000; Kinabo et al., 2007, 2008). Growth and propagation of the ORZ faults are thought to be through strain transfer from older, less active faults at the rift floor to younger, more active faults at the margins. The along-strike propagation of these faults appears to have occurred through both hard-linkage and soft-linkage (Kinabo et al., 2008). As a result, strain accommodation along these NE-trending faults was accompanied by the development of subsidiary N and NW-trending faults and fractures (Modisi, 2000). The spatial distribution of the major rift-bounding faults and the subsidiary faults and fractures suggests the presence of a minor right-lateral strike-slip component in addition to the predominant dip-slip component on the ORZ faults. The orientation of and interplay between the different set of faults and fractures is thought to be the result of the accommodation of regional E–W extension of the EARS (Modisi, 2000; Modisi et al., 2000). The age of initiation of the ORZ is unknown. However, paleoenvironmental reconstruction from the sediments of the Ngami sub-basin suggests that SE-flowing rivers related to the today’s Okavango and Kwando Rivers network flowed to the southeast beyond the Thamalakane and Kunyere faults into the Makgadikgadi pans (Fig. 2) until 120 Ka (e.g. Huntsman-Mapila et al., 2006). Between 120 Ka and 40 Ka, uplift along the Zimbabwe–Kalahari axis (Moore, 1999; Moore and Larkin, 2001) and displacement along the NE-trending faults of the ORZ resulted in the impoundment of the proto-Okavango, Kwando, and the upper Zambezi Rivers and the development of the proto-Linyanti-Chobe, Ngami, and Mababe sub-basins (Cooke, 1984; Thomas and Shaw, 1991; Moore and Larkin, 2001; Ringrose et al., 2005). Initiation of rifting in the ORZ may be as young as 40 Ka which is in good agreement with

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Fig. 3. (A) Digital Elevation Model (DEM) of the Okavango Rift Zone extracted from the Shuttle Radar Topography Mission Data (SRTM, 90 m spatial resolution). Shown in this figure is the location of the regional Magnetotelluric (MT) profile used to image the geometry of the Okavango Rift Zone (labeled Profile 1 MT). (B) SRTM-DEM of the Kunyere fault showing the locations of the detailed Magnetotelluric (MT) and Electrical Resistivity Tomography (ERT) profiles that transects the fault (labeled Profile 1 MT and Profile 1 ERT). (C) SRTM-DEM of the Thamalakane fault showing the locations of the detailed Magnetotelluric (MT) and Electrical Resistivity Tomography (ERT) profiles that transects the fault (labeled Profile 2 MT and Profile 2 ERT).

paleo-environmental studies from the Mababe sub-basin which suggest that neotectonic activity of the ORZ may have been initiated between 40 Ka and 27 Ka (Gamrod, 2009). Geochemical and isotopic evidence from sediments from the Mababe sub-basin suggest a major change in the sedimentation and hydrologic regime during this time which is attributed to possible movement along the Linyanti fault (Gamrod, 2009). Prior to this tectonic activity, the entire flow of the then SE-flowing Kwando River was discharging into the Mababe sub-basin (Shaw, 1985) and the displacement along the Linyanti fault and possible Chobe fault diverted the flow of the Kwando River away from the Mababe sub-basin to the Zambezi River (Fig. 2). Concurrent movements along the Gumare, Thamlakane and Kunyere faults impounded the Okavango River and initiated the formation of the present day Okavango Delta. 3. Geophysical methods 3.1. Magnetotelluric (MT) method Two MT profiles were acquired across the ORZ and related faults. The first profile (labeled Profile 1 MT) shown on the SRTM

DEM in Fig. 3A extends for 140 km across the southwestern part of the ORZ. The second profile (labeled Profile 2 MT) which is 15 km long is located 80 km northeast of the first profile. The MT data were collected using Geometric’s Stratagem EH4 system which consists of metal electrodes connected by copper wire (dipole length was 50 m) and induction magnetic coils to measure the magnitude of the Earth’s horizontal electric and magnetic field components, respectively. Measurements were collected within three frequency ranges: 10–1000 Hz, 500 Hz–3 KHz, and 750 Hz– 9.6 KHz. The first two frequency ranges collected natural electromagnetic (EM) signals, while the third frequency range collected EM signals produced by a dual loop antenna placed at least 300 m away from the receiver. Station spacing ranged between 50 and 5000 m, depending on the proximity to faults. The stations were closely spaced when crossing known faults. The data were processed using GEOSYSTEM’s WinGLink program. First, the data were rotated to obtain the optimal tensor impedance estimates as recommended by Whaler and Hautot (2006). Whaler and Hautot (2006) suggested that subsurface structures become evident when the EM field components are measured in, or rotated into, coordinates defined by the electrical

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strike direction of the structure and the one that is perpendicular to it. The rotated EM impedances are named the transverse electric (TE) and transverse magnetic (TM) modes. To ensure that the structures are imaged correctly, Whaler and Hautot (2006) determined the best two-dimensional (2D) approximation of a threedimensional (3D) structure by rotating the impedance tensor such that its diagonal elements were a minimum, on a site-by-site and period-by-period basis. Tournerie and Chouteau (2002) and Park and Wernicke (2003) performed similar steps by first identifying a regional electrical strike and then rotating impedance tensors and magnetic transfer functions to become parallel to the regional strike. The TE and TM mode data of both profiles were rotated to a common strike direction of N60°E (parallel to the general strike of the ORZ faults) and these data were used in the inversion routine. The next step was to invert the TE and TM mode data using a 2D inversion routine such as that developed by Rodi and Mackie (2001). Using this approach, for each inversion, spurious data were removed and the remaining data were smoothed. Both TE and TM mode apparent resistivity and phase were utilized in the inversion process. The data were inverted using a root mean square (RMS) error of 3% for the best fitting models and static shifts were determined during the inversion process. 3.2. Electrical Resistivity Tomography (ERT) method Two dipole–dipole array profiles were acquired across several of the faults bounding the southeastern margin of the ORZ using AGI’s SuperSting R8 Resistivity/IP System with an electrode spacing of 10 m (Fig. 3A–C). In the first profile (labeled Profile 1 ERT

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in Fig. 3B) which traversed the southwestern shore of Lake Ngami, we used 56 electrodes for each spread to produce a 9 km long profile. Each spread was 550 m in length, with spread overlaps every half spread (275 m). In the second profile (labeled Profile 2 ERT in Fig. 3C) located 80 km northeast of Lake Ngami, we used 42 electrodes for each spread to produce a 2.0 km long profile. The spread was 410 m in length with spread overlaps every half spread (205 m). The measured apparent resistivity values were inverted to obtain a model of true subsurface resistivity using the 2D software RES2DINV (Loke and Barker, 1996). The data were robustly inverted using a L2-Norm until and a RMS error of <10% was reached. The final model was imported into Geosoft’s Oasis Montaj software to create a resistivity image for each profile.

4. Results 4.1. Magnetotelluric (MT) surveys In order to evaluate the general geometry of the ORZ, the TM and TE apparent resistivity and phases along MT profile 1 (Fig. 3A) were inverted to produce a MT model for the rift structure (Fig. 4A). The maximum depth (1 km outside the ORZ and 200– 400 m within the rift zone) that was imaged was limited by the maximum frequency collected (less than 1 Hz) and the low resistivity of the lacustrine and fluvio-deltaic sediments (less than 5 ohm m) that fill the ORZ. The MT model shows a pattern in which low resistivity values (lower than 50 ohm m) are bounded in the

Fig. 4. (A) Regional Magnetotelluric (MT) model of the Okavango Rift Zone along Profile 1 MT (see Fig. 3A for location) showing the lacustrine and fluvio-deltaic sediments of the Okavango Rift Zone as being defined by asymmetrical low resistivity anomaly. (B) Topographic profile extracted from the Shuttle Radar Topography Mission (SRTM, 90 m spatial resolution) along Profile 1 MT with 200 vertical exaggeration. (C) Conceptual geological cross-section with 50 vertical exaggeration along Profile 1 MT representing the Okavango Rift Zone as a half-graben.

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northwest and southeast by high resistivity values (higher than 50 ohm m). The region which is characterized by the low resistivity values dominates much of the profile and stretches between locations 30 and 140 km. The regions further to the northwest and southeast, outside the ORZ are characterized by high resistivity values of up to 200 ohm m and these are associated with surface or near surface exposures of the Precambrian crystalline rocks. Additionally, the MT model shows that the low resistivity anomaly is highly asymmetrical. Following the resistivity values of 50 ohm m, this anomaly slopes gently (1°) from its northwestern end, close to the Gumare fault, towards the southeast. The regional slope of this anomaly is accompanied by a topographic expression showing the floor of the rift slopes gently (less than 1°) from the northwest to the southeast (Fig. 4B). On the contrary, the southeastern boundary of this low resistivity anomaly is almost vertical extending from the surface projection of the Kunyere fault down to a depth of 600 m. We interpret the geometry depicted by the distribution of the resistivity values in the MT model as due to increase from northwest to southeast in the thickness of the lacustrine and fluvio-deltaic sediments filling the ORZ (Fig. 4C). These sediments are infiltrated with both surface water and groundwater coming from the Okavango Delta. It is difficult to quantify the exact thickness of these sediments in different parts of the ORZ. Nevertheless, assuming that the resistivity of the lacustrine and fluvio-deltaic sediments is 5 ohm m or less (resistivity values suggested for brackish to saline water of the Okavango Delta (Kgotlhang, 2008), we estimate that the thickness of these sediments increases from almost zero close to the Gumare fault in the northwestern

margin of the ORZ to a maximum thickness of 600 m close to the Kunyere fault to the southeast. Such thickness variation is likely controlled by the evolution of the ORZ as a broad half-graben in which the basin subsidence is controlled by displacement on the Kunyere fault which represents the southeastern boarder fault of the ORZ (Fig. 4C). It is not possible to determine the behavior of this fault at depths beyond the 1 km depth imaged by the MT data. However, it is likely that the fault’s dip shallows out at greater depth possibly at mid-crustal levels or deeper levels. A more detailed examination indicates that the low resistivity anomaly on the MT model constitutes three distinct zones: (1) a region between locations 70 and 90 km in which the base of the anomaly extends down to a depth of 600 m, (2) a region with the lowest resistivity values stretching between locations 95 and 115 km and extending down to a depth of 300 m, and (3) a relatively small region extending between locations 135 and 140 km where the low resistivity anomaly extends down to a depth of 500 m. These three regions most likely reflect the partitioning of the ORZ basin by the Tsau and Lecha faults (Fig. 2). We note here that both the Tsau and the Lecha faults, which occur within the floor of the ORZ, do not have surface expression in terms of topographic escarpments (Fig. 4B). These faults are clearly delineated in aeromagnetic images (Modisi et al., 2000; Kinabo et al., 2007, 2008). To further investigate the geometry of the Kunyere fault near the Ngami sub-basin, closely-spaced MT sounding data covering a 4 km long profile at the southeastern end of the long MT profile (labeled Profile 1 MT in Fig. 3B) were inverted. The MT model (Fig. 5A) shows the Kunyere fault as a narrow zone lying between

Fig. 5. (A) Detailed Magnetotelluric (MT) model of the Kunyere fault along Profile 1 MT (see Fig. 3B for location) showing the structure without low resistivity anomaly. (B) Detailed Magnetotelluric (MT) model of the Thamalakane fault along Profile 2 MT (see Fig. 3C for location) showing the structure as being defined by a low resistivity anomaly.

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locations 2.4 and 2.6 km separating a region of high resistivity values (between 100 and 200 ohm m) in the southeast (Precambrian crystalline rocks) from a region with low resistivity values (between 30 and 2 ohm m) to the northwest (lacustrine and fluviodeltaic sediments). We note that the Kunyere fault itself does not display any low resistivity anomaly. To better understand the along-strike variation in the geometry of the southeastern border faults of the ORZ, the MT data collected along the profile that crosses the Thamalakane fault which is located 80 km northeast of Profile 1 MT (labeled Profile 2 MT in Fig. 3C) were inverted. The MT model (Fig. 5B) shows the Thamalakane fault as a zone lying between locations 11 and 11.5 km, separating a region of high resistivity values (between 100 and 200 ohm m) in the southeast (Precambrian crystalline rocks) from a region with low resistivity values (between 30 and 2 ohm m) to the northwest (lacustrine and fluvio-deltaic sediments). The Thamalakane fault differs from the Kunyere fault in that, the Thamalakane fault is characterized by a sub-vertical zone of low (3 ohm m) resistivity values, especially between the depths of 675 and 475 m in the MT model. 4.2. Electrical Resistivity Tomography (ERT) surveys The ERT method is implemented to further evaluate the nature of the ORZ faults. Fig. 6A shows the ERT results of Profile 1 ERT (Fig. 3B) which traverses the southwestern shore of the Ngami sub-basin and crosses the Kunyere fault characterized by a 5–

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10 m high escarpment (Fig. 6B). Only the first (starting from the southeast) 4200 m of the profile is displayed as the remaining data covering the northwestern extension of the profile imaged this part of the ORZ to a depth of only 50 m due to low electrical resistivity values of the lacustrine and fluvio-deltaic sediments. This ERT profile (Fig. 6A) can broadly be divided into three sections based on their electrical resistivity values: (1) The southeastern section of the profile is characterized by high resistivity values because the Precambrian crystalline rocks are either exposed to the surface or are present buried under thin cover of the Quaternary Kalahari alluvium. Within this section three geoelectric units are observed. An upper layer extending from the surface to a depth of 10 m and consists of resistivity values ranging between 660 and 1300 ohm m. This resistive layer extends for 1500 m northwest-ward from Bothatogo Village before transitioning into a region of low resistivity as the Kunyere fault is approached. This unit is underlain by a discontinuous, 10 m thick layer of less resistive material where resistivity ranges between 40 and 300 ohm m. In turn, this layer is underlain by a more resistive geoelectric unit that extends down to 50 m and has resistivity values ranging between 400 and 5000+ ohm m. In this layer, the resistivity values increase with depth. We interpret the uppermost layer as a thin (10 m) dry Quaternary Kalahari alluvium covering the Precambrian crystalline rocks represented by the lower resistivity layer (Fig. 6C). In the lower layer, the increase of resistivity with depth might be due to varying degree of weathering within the Precambrian crystalline rocks where more weathered material

Fig. 6. Detailed Electrical Resistivity Tomography (ERT) model of the Kunyere fault along Profile 1 ERT (see Fig. 3B for location). The model shows the fault separating a region of high resistivity in the southeast from a region of low resistivity in the northwest. (B) Topographic profile extracted the Shuttle Radar Topography Mission (SRTM, 90 m spatial resolution) along Profile 1 ERT with 200 vertical exaggeration. (C) Conceptual geological cross-section of the Kunyere fault along Profile 1 ERT. LS = Lacustrine and fluvio-deltaic sediments. PCR = Precambrian crystalline rocks. QKA = Quaternary Kalahari Alluvium. MZ = Moist zone.

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is expected to occur at shallow depth. The middle layer which has lower resistivity compared to both the upper and the lower layer might be due to presence of pockets of groundwater at the interface between the Quaternary Kalahari alluvium and the Precambrian crystalline rocks (Fig. 6C). In addition to the presence of the three layers described above, the central part of the southeastern section shows a narrow (100 m) region of low resistivity (40–80 ohm m) that extends between distances 3400 and 3500 m. This region extends throughout the entire depth of the ERT model. The source of this resistivity anomaly might be due to the presence of groundwater circulating through fractured Precambrian crystalline rocks (Fig. 6C). This interpretation is further supported by remote sensing observations where several lineaments possibly faults are mapped southeast of the Kunyere fault in the SRTM DEM in Fig. 3B. (2) The central part of the ERT profile is dominated by a complex geoelectric structure in which a low resistivity unit is sandwiched between high resistivity units extending between locations 2600 m and 3100 m (Fig. 6A). To the northwest of this, a low resistivity region dominates but is interrupted by a high resistivity body (extending between locations 2550 m and 2600 m; Fig. 6A). This complex geoelectric structure which extends between locations 2550 and 3100 m corresponds to the Kunyere fault and this pattern is likely manifesting the interplay between the NW-dip of the fault and surface/groundwater circulation. (3) The northwestern part of the ERT profile (Fig. 6A) is dominated by a relatively homogeneous region of low resistivity where the resistivity values range between 0 and 30 ohm m. We interpret

this region as representing the Quaternary unconsolidated sediments and the underlying lacustrine and fluvio-deltaic sediments which fill the graben bounded between the Kunyere and the Lecha faults (Fig. 2). Fig. 7A shows the ERT results of Profile 2 ERT (Fig. 3C) which is located 80 km northeast of the Ngami sub-basin and crosses the Thamalakane fault. This fault is defined by a 10 m high topographic escarpment (Fig. 7B) and its trace is marked by the Nhabe River (Fig. 3C). No obvious laterally-extending geoelectrical units are observed in this model. However, the model shows a low resistivity section at the central part of the profile where the resistivity values range between 10 and 50 ohm m. This section extends between locations 800 and 1000 m and it corresponds to the Thamalakane fault. To the southeast of the central section, the ERT profile shows a region that is dominated by high resistivity values that ranges between 500 and 5000 ohm m (Fig. 7A). This region is also spotted by irregular low resistivity bodies where resistivity values range between 0 and 50 ohm m. We interpret this part of the ERT model as representing the Precambrian crystalline rocks covered under thin (5 m) Quaternary Kalahari alluvium (Fig. 7C) and with various degrees of fracturing and groundwater content. To the northwest of the ERT section which represents the Thamalakane fault, a cursory three-layer pattern emerges in which a discontinuous high (5000–5500 ohm m) resistivity layer separates a relatively high (100–500 ohm m) resistivity layer at the top from a low ( 10 to 30 ohm m) resistivity layer at the bottom (Fig. 7A). The high resistivity values in this part of the ERT model

Fig. 7. Detailed Electrical Resistivity Tomography (ERT) model of the Thamalakane fault along Profile 2 ERT (see Fig. 3C for location). The model shows the fault as defined by a low resistivity anomaly. (B) Topographic profile extracted the Shuttle Radar Topography Mission (SRTM, 90 m spatial resolution) along Profile 2 ERT with 200 vertical exaggeration. (C) Conceptual geological cross-section of the Thamalakane fault along Profile 2 ERT. LS = Lacustrine and fluvio-deltaic sediments. PCR = Precambrian crystalline rocks. QKA = Quaternary Kalahari Alluvium. C = Calcrete. TF = Thamalakane fault.

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(which is expected to be of low resistivity values representing the lacustrine and fluvio-deltaic sediments) is due to the fact that these sediments in this part of the ORZ are relatively thin compared to those observed in Profile ERT profile 1 (compare Figs. 6C and 7C). The relatively thin sediments section is due to relatively small throw on the Thamalakane fault compared to the Kunyere fault. Kinabo et al. (2008) estimated from aeromagnetic data that the throw of the Thamalakane fault is 80 m compared to the 232–334 m on the Kunyere fault. In addition to the thin nature of the lacustrine and fluvio-deltaic sediments, the source of high resistivity is likely due to presence of a calcrete layer overlying these sediments (Fig. 7C). We observed clacrete on the surface along our traverses. The calcrete is part of the silcrete–calcrete intergrade duricrust which occur extensively in the Kalahari Group sediments (Nash and Shaw, 1998; Ringrose et al., 2002; Nash and McLaren, 2003). The silcrete–calcrete intergrade duricrust can range from a few cm to more than 9 m thick (e.g., Nash et al., 2004) and are composed of carbonate and silica-cemented duricrusts; as such, they are characterized by high resistivities.

5. Discussion 5.1. Okavango Rift Zone (ORZ) geometry The MT models in Fig. 4A suggest that the ORZ is a broad half-graben with sediment thickness increasing from the Gumare fault in the northwest towards the southeast with the Kunyere fault forming the southeastern border fault of the rift structure (Fig. 4C). This is in good agreement with a number of geomorphological, geological and geophysical observations: (1) the floor of the ORZ shows a consistent SE-directed tilt (Fig. 4B) throughout the extent of the rift basin, (2) the southeastern part of the rift is dominated by numerous normal faults that are absent in the northwestern margin of the basin (Fig. 2), (3) the thickness of the lacustrine and fluvio-deltaic sediments within depositional depo-centers such as the Ngami and the Mababe sub-basins in the southeastern margin of the ORZ could be as high as 400 and 800 m, respectively (Kinabo et al., 2007), (4) normal displacement along the Kunyere fault is waning down and the strain is being transferred to the Thamalakane fault to the east and southeast of the Kunyere fault, suggesting that extension of the ORZ is accommodated through widening of the rift basin. This is also in agreement with the presence of normal faults within the Precambrian crystalline basement to the southeast of the Kunyere fault (Fig. 3B), and (5) Micro-seismicity in the ORZ is concentrated in the southeastern part of the rift between the Kunyere and the Thamalakane faults (Hutchins et al., 1976). Kinabo et al. (2007) suggested from forward modeling of regional gravity data, that the ORZ is a ‘‘developing’’ half-graben and that the current shape of the basin is in the form of synformal sag depression. However, the detailed geoelectric structure of the basin imaged by the MT method suggests instead that the geometry of the ORZ can best be described as a well-developed half-graben, similar in geometry to other rift basins of the EARS (Chorowicz, 2005). The presence of a more developed penetrative NE-trending Precambrian regional structure of the Damara and Ghanzi-Chobe organic belt in the southeastern side of the rift compared to its northwestern side might have facilitated the development of the ORZ as a SE-dipping half-graben structure. Sediment supply by SE-flowing drainage systems such as the Okavango, the Kwando, and the upper Zambezi Rivers filled the ORZ half-graben in a roll-over geometry as has been numerically-modeled (e.g. Schlische, 1991) and suggested for other half grabens of the EARS (e.g. Chorowicz, 2005). Detailed geophysical imaging within the rift basin in the southeastern portion shows three regions that reflect the partitioning of

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the ORZ basin by the Tsau and Lecha faults (Fig. 4A) and result from variation in the amount of brackish and saline water contained in the lacustrine and fluvio-deltaic sediments and channelized fluids within the faults. For example, the low resistivity region in the southeastern part of the MT model closely coincides with the graben bounded by the Kunyere and the Lecha faults. Nevertheless, the depth of the low resistivity anomaly in the central region (between 95 and 115 km) cannot be explained entirely by the thickness of the lacustrine and fluvio-deltaic sediments. This anomaly might be due to the presence of lacustrine and fluvio-deltaic sediments saturated with saline water channelized from the Okavango Delta by the Tsau fault providing an additional source of low resistivity. 5.2. Faults activity and seismicity in the Okavango Rift Zone (ORZ) Electrical methods, in particular MT, have been widely used to characterize the nature and activity of faults. Ritter et al. (2005) documented that the conductivity structure of active faults is distinctively different from inactive faults. Active faults are characterized by a distinct ‘‘fault zone conductor’’, while inactive zones tend to lack this zone. For example, segments in the San Andreas Fault that are characterized by high conductivity are associated with the currently seismically-active part of the fault. In contrast, locked segments lack this strong conductive signature (Mackie et al., 1997; Unsworth et al., 1997, 1999, 2000; Bedrosian et al., 2004; Unsworth and Bedrosian, 2004; Ritter et al., 2005). Additionally, these studies suggest that fluids are important in fault activity and earthquake generation. Our electrical models suggest that major faults in the ORZ display different conductivity structures which may be related to the faults activity and associated seismicity. 5.2.1. Fault activity Our work examined two southeastern border faults of the ORZ (Kunyere and Thamalakane faults) and two within-basin faults (Lecha and Tsau faults) – (Fig. 2). Both the Kunyere and the Thamalakane faults are defined by topographic escarpments in the range of 5 and 10 m, respectively (Figs. 6B and 7B). However, the two faults differ in their resistivity structure. Both the MT and the ERT models suggest that the Thamalakane fault has a ‘‘fault zone conductor’’ that is more prominent than the Kunyere fault. The MT model shows the Thamalakane fault as defined by a broad zone (500 m) of low resistivity that is especially apparent at depths between 475 and 675 m (Fig. 5B). This is different from the MT model of the Kunyere fault where the fault is imaged as a narrow (200 m) zone separating a region of high resistivity in the southeast from a region of low resistivity to the northwest (Fig. 5A). Additionally, the ERT model of the Thamalakane fault shows this structure as defined by a broad zone (300 m) of low resistivity surrounded on both sides by regions of high resistivity (Fig. 7A). No such pattern is observed in the ERT model of the Kunyere fault (Fig. 6A). Rather, the Kunyere fault appears to separate a region of high resistivity in the southeast from a region of low resistivity to the northeast. We attribute the low resistivity along the Thamalakane fault to infiltration of surface water from the Nhabe River that follows the trace of the fault (Fig. 3C), as well as channeling of brackish and saline water from the Okavango Delta. We suggest that the presence of a well-developed ‘‘fault zone conductor’’ within the Thamalakane fault indicates that this fault is more active than the Kunyere fault which does not have a well-defined ‘‘fault zone conductor’’. This is in good agreement with Kinabo et al. (2008) who concluded that the tectonic activity is waning along the southwestern segment of the Kunyere fault as strain is being transferred to the younger more active Thamalakane fault. Kinabo et al. (2008) based their conclusion on two observations: (1) the presence of a breccia zone located 2 km to the north-

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west of the topographic escarpment of the Kunyere fault indicating the original location of the fault. This allowed Kinabo et al. (2008) to suggest that the Kunyere fault’s topographic escarpment is retreating due to erosion because the fault became inactive; and (2) Kinabo et al. (2008) noted that the Thamalakane fault constitutes numerous segments arranged as over-lapping right-stepping en-echelon segments indicative of an active fault that is relatively young compared to the Kunyere fault. Unlike the Kunyere and the Thamalakane faults, the Tsau and Lecha faults lack surface expression in the form of topographic escarpments (Fig. 4B). However, these faults are well expressed on the vertical derivative aeromagnetic map (Modisi et al., 2000; Kinabo et al., 2007, 2008). These faults lack topographic expression most likely because they are concealed by rapid sedimentation and these faults deform soft sediments, hence it is unlikely to preserve topographic escarpments. The MT model (Fig. 4A) shows the Lecha and the Tsau faults as associated with zones of low resistivity values; hence the presence of ‘‘fault zone conductors’’ suggesting they are tectonically active. We argue that the Tsau and the Lecha faults are still active and that the lack of topographic escarpment suggests that sedimentation rates exceed the tectonic subsidence caused by displacement on these faults.

cipitation of salts and silica has resulted in the development of saline islands between the many distributaries of the delta (McCarthy and Ellery, 1998). Some studies suggested that evapo-concentration may lead to the transfer of saline water into the deeper aquifer through density fingering (Gieske, 1997) and this has been confirmed by both electrical resistivity (e.g., Bauer et al., 2006) and airborne electromagnetic surveys (Kgotlhang, 2008). Campbell et al. (2006) noted that fresh water within the Okavango Delta has resistivity values of 9–30 ohm m, brackish water has values of 3–8 ohm m, and saline fluids are characterized by <3 ohm m resistivity values. Recently, Kgotlhang (2008) documented that saline water and wet clayey units have resistivity values of <5 ohm m, while brackish water has resistivity values ranging between 5 and 15 ohm m. The MT models (Figs. 4 and 5) show some ORZ faults and other sections of the models with resistivity value of <5 ohm m, suggesting that these regions contain brackish to saline water. Hence, these low resistivity anomalies can be attributed to the effect of both surface water and groundwater from the Okavango Delta being channeled by the ORZ faults to circulate through the lacustrine and fluvio-deltaic sediments filling the basin (McCarthy et al., 1993).

5.2.2. Seismicity Earthquakes in the ORZ are predominantly weak (micro-seismicity), except for large earthquake events that were recorded during the May 1952–May 1953 period, with magnitudes ranging between 5.0 and 6.7 on the Richter scale (Hutchins et al., 1976; Milzow et al., 2009). The region with the highest micro-seismic activity occurs between the Kunyere and Thamalakane faults (Fig. 2; Hutchins et al., 1976). MT studies of the San Andreas Fault suggest that seismic behavior may be controlled by a connected network of fluid-filled cracks within fault zones (Unsworth et al., 1997, 1999, 2000; Mackie et al., 1997; Bedrosian et al., 2004; Unsworth and Bedrosian, 2004; Ritter et al., 2005). Active creeping segments within which micro-seismicity occurs are characterized by wide (750 m) highly-fractured zones with low resistivity values extending down to a depth of 2– 5 km (Caine et al., 1996; Bedrosian et al., 2004; Ritter et al., 2005). These zones of low resistivity (fault zone conductors) are due to the presence of saline fluid-filled voids and fractures within a damaged zone (Anderson et al., 1983; Caine et al., 1996). In contrast, inactive locked segments of the San Andreas Fault Zone that lack micro-seismicity are characterized by narrow ‘‘fault zone conductors’’ (Unsworth et al., 1999). However, these inactive locked segments are usually the sites of infrequent but large (M = 7.8) and damaging earthquakes (Ellsworth, 1990; Ritter et al., 2005). Brodsky and Kanamori (2001) suggests that lubrication by a viscous fluid within a fault zone can reduce the frictional stress during large earthquakes (M > 4) by as much as 30%. Hence, it is likely that active faults in the ORZ such as the Thamalakane fault (which show low resistivity values indicative of the presence of fluids within the fault zone) will accommodate extensional strain though steady creep mechanisms to produce frequent micro-seismicity rather than infrequent large magnitude earthquakes. More earthquake studies are needed to assess in detail the seismicity of the ORZ and this is the focus of an ongoing research project.

6. Conclusions

5.3. Source of low resistivity The MT and ERT models in this study are acquired within the distal end of the Okavango Delta at the southeastern margin of the ORZ where NE-trending faults extend into the delta. Studies have shown that the Okavango Delta is a terminal evaporative system (McCarthy and Ellery, 1998) and more than 95% of the surface water is lost by evapo-transpiration (Dincer et al., 1978). The pre-

We conducted MT and ERT imaging to provide greater insights into the geometry and the nature of faults activity of the ORZ. Our results suggest that the ORZ is a 120 km wide SE-dipping rollover half-graben which is filled by lacustrine and fluvio-deltaic sediments delivered to the basin through SE-flowing rivers including the proto-Okavango, Kwanda and the Zambezi Rivers. The sediments might be as thick as 600 m close to the southeastern boarder faults of the ORZ. The sediment fill of the ORZ halfgraben was controlled by – normal displacement on the Kunyere fault which represents the southeastern border fault – of the basin. However, the MT and ERT structure of the Kunyere fault (lack of ‘‘fault zone conductor’’) suggests that this fault is becoming tectonically-inactive and the strain is transferred further east to the more active Thamalakane fault as indicated by its MT and ERT structure in the form of the presence of a broad ‘‘fault zone conductor’’. The MT and ERT structure of within-basin faults such as the Lecha and the Tsau faults indicate that these faults are still tectonically-active and the lack of topographic escarpments defining these faults is attributed to high rates of sedimentation that exceed tectonic subsidence. The source of the low resistivity observed in the lacustrine and fluvio-deltaic sediments and faults (except the Kunyere fault) might be due to the presence of combined infiltrated surface fresh water as well as brackish and saline groundwater of the Okavango Delta. The presence of fluids within the ORZ fault zones serves as lubricant, providing conditions for the frequent occurrence of micro-seismic activity, which prevents the occurrence of infrequent large magnitude earthquakes.

Acknowledgments Partial funding for this study was provided by the National Science Foundation (NSF-OISE-0738250) under the International Research Experience for Students (IRES) initiative. The University of Botswana provided logistical support for fieldwork. We thank the government of Botswana (Ministry of Education) for providing us with research permits. Participants of IRES project helped with fieldwork and T Halihan helped with the processing of the ERT data. We thank D. Delvaux and J.-J. Tiercelin for detailed and constructive reviews.

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