Review of the Environmental Impact Assessment and Seismic Hazard Reports for the Mphanda Nkuwa proje

Review of the Environmental Impact Assessment and Seismic Hazard reports for the Mphanda Nkuwa project, Tete Province, Mozambique Chris J.H Hartnady Umvoto Africa (Pty) Ltd, PO Box 61, Muizenberg, 7950 South Africa Email: chris@umvoto.com

Executive Summary The present-day break-up of the former African plate into Nubian and Somalian plates is expressed over a broad zone of widespread natural seismicity, partly concentrated within a few discrete belts.

The extensive seismicity and the graben-and-block zonation of the region around the middle and lower Zambezi River drainage system, reflects a tectonic fragmentation of the Nubian plate into smaller microplates, the motions of which are too slow to be easily resolvable in the present state of geodetic monitoring. The possible slow kinematic development of a separate ‘Tete-Chipata microplate’, bounded by the Malawi and Shire Rift structure (Rovuma plate boundary) on the east, reactivated fracture systems following the Sanangoe Shear Zone (SSZ) on the south, and the Luangwa graben on the northwest, provides the seismotectonic setting for discussion of seismic safety aspects of the Mphanda Nkuwa hydroelectric scheme.

The Mphanda Nkuwa dam site is located close to the Chitima-Tchareca Zone (CTZ) of faulting, which follows an older zone of crustal weakness established by the SSZ episode that in turn developed from a zone of oceanic subduction and continental collision between ‘West Gondana’ and ‘South Gondwana’ fragments during supercontinent amalgamation. Reconnaissance morphotectonic assessment of the region around Cahora Bassa and Mphanda Nkuwa indicates that the CTZ and the ‘Estima Fault’ form an en-echelon fault array along the southern boundary of the Tete-Chipata block, probably continuously connected at depth to the main SSZ dislocation within the upper-crustal seismogenic layer. The eastern (‘F1’) segment of the Estima Fault is a recent surface-breaking fault scarp with elements of both normal faulting (southerly downthrow of a few metres) and right-lateral strike-slip faulting (ridge offsets of transacted drainage divides). The combined Estima-TCZ array therefore constitutes a major hazard to the seismic safety of both the Cahora Bassa and proposed Mphanda Nkuwa dams, should future rupture propagate across different segments of the active en-echelon array.

Considering this broader seismotectonic setting, the current determination of a ‘Maximum Credible Earthquake’ (MCE) of moment magnitude (Mw) 6.4 is not realistic. The eastern part of the CTZ, the Chitima fault with a length of ~90 km, is potentially capable of generating a Mw~7.5 earthquake at a distance of only a few kilometres from the Mphanda Nkuwa site. This finding has implications for the planned Design Phase of the Mphanda Nkuwa dam, a future episode of reservoir-triggered seismicity (RTS) related to Mphanda Nkuwa reservoir impoundment, and geophysical invetigations required to ensure the continued safety of the Cahora Bassa dam structure.

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Introduction

The writer was commissioned by Justica Ambiental (JA) to undertake a review of the sixvolume Environmental Impact Assessment (EIA) Report for the Mphanda Nkuwa hydroelectric dam project and to attend a public presentation of the same on the morning of Friday 5 August 2011. The focus of this review is the seismotectonic setting of the project and the possible impact of future reservoir-triggered seismicity (RTS), consequent to the construction of the Mphanda Nkuwa dam and the filling of the new reservoir downstream from the existing Cahora Bassa dam and reservoir.

Following the downloading and partial translation of key parts of the EIA report, namely relevant sections of the Executive Summary [1]1, the volume describing the environmental setting of the project [2], the description of impacts, mitigation measures and EIA findings [3] and the proposed Environmental Management Plan (EMP) [4], the writer travelled to Maputo on Wednesday 3 August. The afternoon of 3 August and the following day was spent in the JA office, reviewing relevant parts of the EIA reports [1-4], assembling other materials and recent scientific literature related to the geology and tectonics of the middle and lower Zambezi River region, and undertaking a form of ‘virtual’ geological fieldwork around the Mphanda Nkuwa site through the medium of Google Earth. During this phase of the review, a desk-top reconnaissance mapping was achieved of some potentially active, tectonic features relevant to the RTS risk. At the end of this phase, some key diagrams were assembled in a Powerpoint presentation and a brief outline of some interim results was outlined to JA members, A. Lemos and D. Ribeiro.

On Friday 5 August, the public presentation of the Mphanda Nkuwa EIA was attended at the Hotel VIP in Maputo in the company of JA members. During this presentation a short outline of the main results of the seismic hazard study was given by Dr Martin Wieland from Poyry Energy SA in Zurich, Switzerland, the specialist consultants to Hidroeléctrica de Mphanda Nkuwa S.A (HMNK) of Maputo, Mozambique. Arising from conversation with Dr Wieland immediately prior to this public meeting, and following some brief interpolations by the writer related to points made in his contribution during the question/discussion time, it was agreed that the writer would have the opportunity for further, more detailed discussions with him that afternoon in the offices of the environmental consultants, Impacto. This meeting commenced shortly after 3 pm and lasted for about three hours.

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Numbered references enclosed in square brackets

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Also in attendance were Prof. Lopo Vasconcelos and Dr Daud Jamal from the Department of Geology at the Universidade Eduardo Mondlane (UEM) in Maputo and staff of HMNK.

During the afternoon meeting Dr Wieland first presented and freely discussed his own Powerpoint presentation with all participants. Then the writer had the opportunity to present some aspects of the presentation that he had prepared for JA on the previous afternoon. Some provisional results of the Google Earth mapping were illustrated with reference to the actual .KMZ file created by the writer over the previous two days.

The writer left Maputo on Saturday 6 August, at which time the seismotectonic report [5] and the seismic hazard study [6] had just been provided in PDF format. The four salient EIA reports [1-4] and the two seismological annexures [5-6] form the main basis of this review.

Seismotectonic Setting of the Zambezi River region

(Sections following under final revision)

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Conclusions

This review reaches the following conclusions: 1. The Mphanda Nkuwa dam site is located close to (within 1 km of) a major fault system, the Chitima-Tchareca Zone (CTZ), which forms a tectonic link between Karoo halfgraben structures of opposite tectonic polarity.

2. The tectonic development of the CTZ as a brittle fracture system during Palaeozoic and Mesozoic (Karoo and Gondwana break-up) times followed the line of an older (Late Pan-African) structure, the Sanangoe Shear Zone (SSZ), a belt of intense ductile deformation that links the Zambezi-Adamastor geosuture in the west to the Lurio Thrust Belt in the east. Both these structures are currently interpreted as major elements within a zone of oceanic subduction and continental collision between ‘West Gondana’ and ‘South Gondwana’ fragments during assembly of the supercontinent.

3. The present-day break-up of the former African plate into major Nubian and Somalian plates is expressed, over a wide zone, by the edge-driven rotation of intervening smaller plates, for which quantitative kinematic descriptions have been derived for two, namely, the Victoria and Rovuma plates.

4. The eastern margin of the Nubian plate in southern Africa is characterized by widespread natural seismicity, partly concentrated within a few discrete belts, such as the Luangwa-Kariba graben, the Mweru graben and Upemba graben, but also diffusely spread through the intervening areas. The extensive seismicity and the graben-andblock zonation of this region, which has had a major influence on the evolution of the modern Zambezi River drainage system, reflects a tectonic fragmentation of the Nubian plate into smaller microplates, the motions of which are too slow to be resolvable with the current spatial distribution and short temporal history of African GPS geodetic networks.

5. The pattern of seismicity and neotectonic faulting around the Middle and Lower segments of the Zambezi River suggests the possible slow kinematic development of a separate ‘Tete-Chipata microplate’, bounded by the Malawi and Shire Rift structure (Rovuma plate boundary) on the east, reactivated fracture systems following the SSZ on the south, and the Luangwa graben on the northwest. The Luangwa faulting and seismicity partly follows the Mwembeshi Shear Zone (or Dislocation), a Late PanAfrican tectonic feature similar to the SSZ.

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6. A quick reconnaissance morphotectonic assessment of the region around Cahora Bassa and Mphanda Nkuwa, using digital elevation data from the Shuttle Radar Topography Mission (SRTM), indicates that both the CTZ and the structure identified in previous seismic hazard studies as the Estima Fault form an en-echelon fault array along the southern boundary of the Tete-Chipata block, constituting a nearly complete fracture system link between the southern end of the Luangwa graben and the northern end of the Shire Rift.

7. The eastern portion of the Estima Fault is also separated as left-stepping en-echelon segment, now identified as the ‘F1 fault’ in the current seismic hazard analysis, and shows morphotectonic evidence of a recent surface-breaking fault scarp with elements of both normal faulting (southerly downthrow of a few metres) and right-lateral strikeslip faulting (ridge offsets of transacted drainage divides).

8. The combined Estima-TCZ array is probably continuously connected at depth to the main SSZ dislocation within the upper-crustal seismogenic layer. It therefore constitutes a major hazard to the seismic safety of both the Cahora Bassa and proposed Mphanda Nkuwa dams, as it cannot be assumed that future rupture nucleation within the deeper part of the seismogenic zone will be limited from propagating across different segments of the active en-echelon array in a major or great seismic event.

9. The determination of a ‘Maximum Credible Earthquake’ (MCE) of moment magnitude (Mw) 6.4 is not realistic, considering the broader seismotectonic setting of the southern boundary of the Tete-Chipata block. Future rupture along the eastern part of the CTZ, the Chitima fault as herein defined with a length of ~90 km, is potentially capable of generating a Mw~7.5 earthquake at a distance of only a few kilometres from the Mphanda Nkuwa site.

10. In the absence of any local knowledge of the detailed state of crustal stress around the Cahora Bassa and Mphanda Nkuwa reservoir sites, it would be imprudent to assume that elements of the Estima-CTZ fault array are not currently close to the end of the stress-build-up phase of the earthquake cycle and that a future episode of reservoirtriggered seismicity (RTS) could not conceivably escalate into a major (>M7) event along some critically-stressed part of this system.

11. Measures to ensure the future safety of the Cahora Bassa dam structure as well as the revision of the scope of the Design Phase of the Mphanda Nkuwa dam, should extend beyond the establishment of a local seismographic network, to include the establishment of modern geodetic benchmarks and continuous GPS monitoring, the

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measurement of local stress systems, and the determination of the flexural rigidity of the lithosphere against vertical loading.

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References

1.

Coba-Impacto-ERM consortium. Estudo de Impacto Ambiental: Vol. 1 - Sumário Executivo (Executive Summary). HMNK Hidroeléctrica de Mphanda Nkuwa S.A., Julho 2011, 80 pp.

2.

Coba-Impacto-ERM consortium. Estudo de Impacto Ambiental: Vol. 3 - O Meio Receptor (The Recipient Environment). HMNK Hidroeléctrica de Mphanda Nkuwa S.A., Julho 2011, 572 pp.

3.

Coba-Impacto-ERM consortium. Estudo de Impacto Ambiental: Vol. 4 - Impactos, Medidas de Mitigação e Conclusões (Impacts, Mitigation Measures and Findings).

HMNK Hidroeléctrica de Mphanda Nkuwa S.A., Julho 2011, 267 pp.

4.

Coba-Impacto-ERM consortium. Estudo de Impacto Ambiental: Vol. 5 - Plano de Gestão Ambiental (Environmental Management Plan). HMNK Hidroeléctrica de Mphanda Nkuwa S.A., Julho 2011, 91 pp.

5.

Poyry Energy AG. Mphanda Nkuwa Hydroelectric Power Plant Project, Mozambique: Seismo-Tectonic Assessment of Project Area. Prepared by T. Dietler & M. Wieland (28 March 2011) for HMNK Hidroelectrica de Mphanda Nkuwa S.A., 33 pp.

6.

Poyry Energy AG. Mphanda Nkuwa Hydroelectric Power Plant Project, Mozambique: Seismic hazard analysis of dam site. Prepared by M. Wieland (12 July 2011) for HMNK Hidroelectrica de Mphanda Nkuwa S.A., 127 pp.

7.

Lahmeyer International, Electricité de France, Knight Piésold Joint Venture (JV LIEDF-KP). Seismic Hazard Assessment Report (Amended version of March 2002).

Annex to Doc. 024 A & B: Draft Final Report on Geological Studies and Seismic Studies, June 2001, cited and reviewed in ref. 5 above.

8.

Itapura, Engenharia, Geologigia e Meio Ambiente Ltda, Avaliação dos Dados e das Informações Sobre Sismicidade Regional e Local para o Empreendimento Hidrelétrico Mphanda Nkuwa , Rio Zambezi, República de Moçambique,. Report to HMNK, São Paulo, Julho 2007, cited and reviewed in ref. 5 above.

9.

International Seismological Centre (ISC), On-line Bulletin, http://www.isc.ac.uk, Internatl. Seis. Cent., Thatcham, United Kingdom. Accessed online at http://www.isc.ac.uk/search/bulletin/rectang.html on 31 July 2011.

10.

Hartnady CJH. Earthquake hazard in Africa: perspectives on the Nubia-Somalia boundary. S Afr J Sci 2002; 98:425-428.

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11.

Hartnady CJH. Seismotectonics of Southern Mozambique. 21st Colloquium on African Geology (CAG21), Maputo (3-5 July 2006), Mozambique, Abstracts Volume, p. 408-410.

12.

Hartnady CJH, Mlisa A,Hartmann S. SRTM morphotectonics of the Urrongas Protorift Swell. 21st Colloquium on African Geology (CAG21), Maputo (3-5 July 2006), Mozambique, Abstracts Volume, p. 411-413.

13.

Calais E, Ebinger C, Hartnady C, Nocquet J-M. Kinematics of the East African Rift from GPS and earthquake slip vector data. In: Yirgu, G., Ebinger, C.J. and Maguire, P.K.H. (eds), The Afar Volcanic Province within the East African Rift System: Geological Society of London Special Publication 2006; 259:9-22.

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Stamps DS, Calais E, Saria E, Hartnady C, Nocquet J-M, Ebinger CJ, Fernandes RM. A kinematic model for the East African Rift. Geophys Res Lett 2008;35:L05304 doi:10.1029/2007GL032781.

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Delvaux D, Barth A. African stress pattern from formal inversion of focal mechanism data. Tectonophysics 2010;482:105-128 doi:10.1016/j.tecto.2009.05.009

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Heidbach O, Tingay M, Barth A, Reinecker J, Kurfeß D, Müller B. Global spatial wave-length analysis of the tectonic intraplate stress pattern. Tectonophysics 2010;482:xxx-yyy.

17.

Johnson SP, De Waele B, Liyungu KA. U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology of granitoid rocks in eastern Zambia: Terrane subdivision of the Mesoproterozoic Southern Irumide Belt.

Tectonics 2006;25:TC6004 doi:10.1029/2006TC001977

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Westerhof ABP, Lehtonen MI, Makitie H, Manninen T, Pekkala Y, Gustafsson B, Tahon A. The Tete-Chipata Belt: A new multiple terrane element from Western Mozambique and Southern Zambia. In: GTK Consortium Geological Surveys in Mozambique 2002–2007, edited by Yrjö Pekkala, Tapio Lehto & Hannu Mäkitie, Geological Survey of Finland, Special Paper 2008;48:145–166.

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Westerhof ABP Tahon A, Koistinen T, Lehto T, Åkerman C. Igneous and tectonic setting of the allochthonous Tete Gabbro-Anorthosite Suite, Mozambique. In: GTK Consortium Geological Surveys in Mozambique 2002–2007, edited by Yrjö Pekkala, Tapio Lehto & Hannu Mäkitie, Geological Survey of Finland, Special Paper 2008;48:191-210.

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Hartnady CJH, Mlisa A, Hay ER. Late Pleistocene protorifts in Southern Africa: SRTM30 and Landsat-7 morphotectonic analysis. 20th Colloquium on African Geology (CAG20) Orleans (2-4 June 2004), France, Abstracts Volume, p. 190.

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Figures

Figure 1. Location of Mphanda Nkuwa study area with magnitude-scaled earthquake epicentres from the online bulletin of the ISC (red circles). Wider area of epicentre search (larger rectangle) covers latitudes 5-22 S, longitudes 21-36 E. Smaller rectangle covers area enlarged in Fig. 2, including the Cahora Bassa reservoir (light blue polygon). Seismographic stations (white triangles) active at the time (September 1963) of the Kariba reservoir-triggered seismicity are shown: BUL – Bulawayo; BHA – Broken Hill (Kabwe); CLK – Chileka; CNG – Changalane; SBD – Sa da Bandeira; LUA – Luanda; TAN – Antananarivo; LWI – Lwiro; HER Hermanus. Epicentre cluster in SE corner of study area is associated with the Mw7.0 earthquake of 2006 Feb 22, the largest recorded seismic event in Mozambique, and its aftershocks.

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Figure 2. Detail of inner study area covering latitudes 11-17 S, longitudes 24-36 E. Symbols as in Fig. 1. The roughly N-S-trending seismic belt associated with the Malawi Rift, extending southwards through the Shire River valley, is evident on the right hand side of the area.

The epicentre cluster in the lower left centre is

associated with the Kariba reservoir (partly obscured) on the middle Zambezi river.

The smaller cluster to the WNW of Kariba is related to the reservoir on the Kafue River at Itezhi-Tezhi. Diffuse belts of micro-earthquakes (M<3) may link the Kariba and Itezhi-Tezhi clusters to neotectonic, surface-breaking faults in the SW corner, across the Zambia-Namibia border. Other diffuse earthquake belts occur around larger epicentres in the Luangwa valley of Zambia (north of Cahora Bassa reservoir), and in the NW part of the study area, extending NNE-SSW from the DRC-Zambia border. Inner white rectangle is included within area enlarged in Fig. 3.

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Figure 3. Part of lower Zambezi River downstream from the Cahora Bassa reservoir (light blue polygon), with indicated area (white rectangle) covered by latitudes 15-16 S, longitudes 32-35 E, selected for more detailed morphotectonic analysis (see Figs.

12-14 and 16-18). ISC earthquake epicentres on right-hand side are associated with the Shire Rift, with larger rift-boundary faults shown (red lines), including the NNESSW Zomba Fault zone (NE of the CLK seismographic station) and the NW-SE Thyolo Fault zone, south of CLK. An array of faults/fractures extends between the Thyolo Fault and the E-W-trending southern boundary fault of the Karoo-age Middle Zambezi rift graben, crossing the Mozambique-Zimbabwe border. Earthquake epicentres around the Cahora Bassa are mostly associated with reservoir-triggered seismicity after 1974.

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Figure 4. Model of Nubian-Somalian (NU-SO) plate motion from Calais et al., 2006 [ref. 13]. The East African Rift System (within dashed block) extends from the Afar triple junction with the Arabian (AR) plate to southern Mozambique via the great African lakes of Albert, Kivu, Tanganyika and Malawi, as marked by the tails of the model SO-NU velocity vectors (blue arrows). Estimated model Euler poles of SONU rotation (blue and green stars) occur near 43 S 28 E.

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Figure 5. Revised kinematic model for the NU-SO plate boundary zone from Stamps et al., 2008 [ref. 14], including the intervening Victoria (VI), Rovuma (RO) and Lwandle (LW) plates. The Mphanda Nkuwa study area (blue rectangle) is located along the NU-RO plate boundary, while the revised SO-NU rotation pole (red star) is located close to the NU-LW boundary south of the submarine Mozambique Ridge, to the northeast of the previously estimated (C06) position [13]. Red bar symbols represent principal horizontal extension direction across rifts, defined by earthquake slip vectors. Red arrows represent velocities (scale in lower left corner) relative to stable NU at continuously recording GPS stations on the VI, RO and SO plates.

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Figures 6A & 6B. A. Earthquake focal mechanisms (coloured beachball symbols) across the NU-SO boundary zone from Delvaux & Barth, 2010 [ref. 15]. Different colours are grouped into numbered rectangles or ‘boxes’, of which the most relevant to the wider study area are boxes 17 (Malawi Rift, including the Shire zone), 18 (Central Mozambique), 20 (Luangwa Graben), 21 (Mweru Graben) and 22 (Upemba Graben). Box 18 contains the Urema Graben, extending southwards from the Shire Rift zone, and boxes 20, 21 and 22 are defined by generally NE-SW alignments of strong (M>6) ‘íntraplate’ earthquakes within the part of the Nubian plate marginal to rift segments 13 (North Tanganyika), 14 (South Tanganyika), 15 (Rukwa Rift) and 16 (Mbeya Triple Junction). B. Interpretation of focal mechanism patterns in terms of crustal stress regimes of normal faulting (NF - red) strike-slip faulting (SS -green) and thrust faulting (TF – blue), with the axes (black lines) of the stress symbol aligned along the local direction of principal horizontal compressive stress (SHmax).

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Figure 6C. Synthesis of horizontal stress (SHmax) orientations combining the boxaveraged results (circles) from the focal mechanism study of Delvaux & Barth, 2010 [ref. 15] and other results (symbols as per legend in upper left corner â&#x20AC;&#x201D; single focal mechanism solutions excluded) from the World Stress Map release 2008 of Heidbach et al., 2010 [ref. 16].

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Figure 7. (a) Simplified tectonic map of Africa : 1, Phanerozoic belts; 2, NeoproterozoicCambrian belts; 3, Mesoproterozoic belts; 4, Paleoproterozoic belts; 5, Archaean Cratons. BB, Bangweulu Block; CFB, Cape Fold Belt; D, Damara Belt; EAO, East African Orogen; I, Irumide Belt; K, Kibaran Belt; LUF, Lufilian Belt; L, Limpopo Belt; MSZ, Mwembeshi Shear Zone; NA, Namaqua Belt; NL, Natal Belt; SIB, Southern Irumide Belt; TC, Tanzania Craton; U, Ubendian Belt-Usagaran Belt; ZC, Zimbabwe Craton. (b) Explanation and distribution of political borders shown in main geological map: DRC, Democratic Republic of Congo; MAL, Malawi; MOZ, Mozambique; TAN, Tanzania; ZIM, Zimbabwe. (c) Simplified geological map showing location of the SIB province, and Zambian terrane subdivisions: C-R, Chewore-Rufunsa Terrane; L-N, Luangwa-Nyimba Terrane; P-S, Petauke-Sinda Terrane; Chp, Chipata Terrane.

From Johnson et al., 2006 [ref. 17, Fig. 1, p. 2 of 29].

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Figure 8. Simplified geological map of the Tete-Chipata Belt (TCB) and the ZambeziLufilian segment of the Damara-Lufilian-Zambezi Belt, from Westerhof et al., 2008a [ref. 18, Fig. 2, p. 149]. Key: MSS = Mugesse Shear Zone, MD = Mwembeshi Dislocation, SSZ = Sanangoe Shear Zone, C.I. = Chewore Inliers, TS = Tete Suite, NMS = Namama Megashear, LTB = Lurio Thrust Belt. Dotted section in the southwestern part of the TCB corresponds to Neoproterozoic (post-Rodinia, early Pan-African) metasediments (mainly pelites and carbonates) with minor metavolcanics, tectonically overlying Mesoproterozoic Mpande Gneiss.. The inferred connection between the SSZ, defining the southern TCB boundary, and the LTB, a major geosuture within the Mozambique Belt, is indicated in this interpretation.

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Figure 9. Provisionally distinguished terranes in the Tete-Chipata Belt (TCB) of northern Tete Province, Mozambique, from Westerhof et al., 2008a [ref. 18, Fig. 9, p. 160].

Here the Sanangoe Shear Zone (SSZ, double-dash line) is interpreted as the main boundary between ‘West Gondwana’ and ‘South Gondwana’ tectonic provinces. The Mphanda Nkuwa dam site is located just north of the SSZ, where it is intersected by a NW-SE lineament (dot-dash line), controlling the location and course of the Lower Zambezi River. It is also situated just to the north of a tectonic window (Rio Chacomama dome) of South Gondwana granitoid gneiss (light pink) overlain at an overthurst contact (barbed line) by the mafic Tete Suite (light grey).

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Figure 10. Simplified geological map of the Tete Suite and surroundings, from Westerhof et al., 2008b [ref. 19, Fig. 1, p 193]. Immediately south of the Sanangoe Shear Zone (SSZ), two tectonic windows of Proterozoic basement (dark pink) in the Tete Suite (grey) are named the Rio Chacocoma dome (west) and Rio Mazoe dome (east).

The Mphanda Nkuwa dam site is located on the lower Zambezi River, just north of the Rio Chacomama dome. Note the more southerly eastward extension of the SSZ, within the Tete Suite, compared to Fig. 9. Note also the â&#x20AC;&#x2DC;mylonite textureâ&#x20AC;&#x2122; symbol on both the South Gondwana Proterozoic basement and the West Gondwana Mussata granitoids, marginal to the Tete Suite.

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Figure 11. Fault/fracture map from the Itapura report [ref. 8], from a reproduction in the Poyry seismotectonic report [ref. 5, Fig. 6, p. 13]. Noteworthy features are: 1) the identification of the Estima Fault (Falha de Estima) and its arcuation from an E-W strike in the west to a SW-NE strike in the west; 2) the apparent role of microearthquake (ML= 2.9) epicentres in its selection as a seismotectonically significant structure; 3) the identification of the Sanangoe Shear Zone (Zona de Cisalhamento Sanangoe) with a generally E-W-striking but slightly arcuate zone of complex fracturing, extending from north of the Mphanda Nkuwa site towards the Shire Rift zone.

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Figure 12. Shuttle Radar Topography Mission (SRTM) topography of area between the Cahora Bassa reservoir (beige tone, upper left border) and the Shire River valley (dark green, centre right border), covering part of the Lower Zambezi River and the Mphanda Nkuwa dam site (yellow circle). â&#x20AC;&#x2DC;Earth tonesâ&#x20AC;&#x2122; colour range (lower left bar scale) represents a linear range of elevation between 275 m (dark green) and 425 m (white), draped over an artificially sun-shaded model (sun azimuth 0 ; sun elevation angle, 25 ; terrain vertical exaggeration, 10x). The selection of the 275-425 m elevation range and the particular sun-shade highlights the geomorphological expression of the fault and fracture system (red lines) extending from Cahora Bassa, past the town of Chitima (formerly Estima, marked C on the image) towards the village of Tchareca (T). The F1 Fault [ref. 5], south of Chitima, is annotated.

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Figure 13. Slope map derived from SRTM elevation model shown in Fig. 12, draped on the same sun-shaded foundation. The rainbow colour range (scale bar at left side) represents a narrow range of very low slope inclination, between 0 (blue-violet) and 5 (red-pink), emphasizing the flat plains within the topography, generally underlain

by Karoo and younger sediments. Part of Cahora Bassa reservoir (upper left corner) and part of the Lower Zambezi River around Tete show as horizontal areas of ~0 slope (dark blue). Areas of slope steeper than 5 (pink tone) with obvious shadows are generally higher topographic zones underlain by pre-Karoo crystalline basement. The Mphanda Nkuwa dam site (yellow circle) is located at the SE end of the gorge section of the Zambezi, upstream from a bend that is structurally controlled by the fault and fracture system (black lines) linking an area of Karoo downthrown to the south against basement around Chitima (C) and a Karoo outcrop downthrown to the north against basement near Tchareca (T).

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\ Figure 14. Regional aspect map of area shown in Fig. 13, where the colour tones represent the facing directions of slopes (indicated by compass points in the legend bar at left). Horizontal areas of no definite slope direction (e.g., water surfaces of Cahora Bassa and the Lower Zambezi River) show as dark red areas. Other red areas are remnant flat land surfaces on some the high topographic prominences (e.g., ~900 m plateaus or tablemounts in the fault-bounded highlands north of Chitima (C), corresponding to the Messandaluz sub-cycle of the African Cycle [ref. 2, Fig. 2.3.4 & p. 41-42] or a ~400 m terrace elevation WNW of Tchareca (T), corresponding to the Median Plateaus of the Zumbo Cycle [op .cit.]).

Morphotectonic lineaments defined by sudden changes of aspect (e.g. the ChitimaTchareca zone) are evident in this kind of diagram. Just SSE of the Mphanda Nkuwa site (black circle), the bevelled western ridges of the Rio Chacomama dome (RCh) at ~400-500 m summit elevations probably also belong to the Zumbo Cycle, predating the most recent incision of the Zambezi River.

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Figure 15. Google Earth map of fault and fracture systems between Cahora Bassa and the western Malawi around the town of Chileka (not to be confused with airport and seismographic station of same name near Blantyre). In the east of this map, the main boundary fault (MBF, thick yellow line) of the NW-SE-trending Lower Zambezi Karoo Basin is on the northeastern side of the basin [‘Muaradzi-Mecondezi subbasin’; ref. 2, Fig. 2.3.7], crossing the Malawi-Mozambique border (light yellow line), and downthrows Karoo strata to the southwest. In the west of this map, the main boundary fault (MBF, red line) of the E-W-trending Middle Zambezi Karoo Basin is on the southern side of the basin, crossing the Zimbabwe-Mozambique border (yellow line), and downthrows Karoo strata to the north. The ‘Estima Fault’ (following ref. 8 and previous Fig. 11) and the F1 fault [ref. 5] are antithetic structures to the MBF, with significant southward downthrow of Karoo strata. The Chitima fault zone (green line) is also antithetic to the Middle Zambezi MBF and has the largest downthrow of Karoo strata against Precambrian crystalline basement. From a point near Cahora Bassa it extends westward over a distance of approximately 90 km, crossing the Zambezi River just south of the Mphanda Nkuwa site (white star symbol). Near Mphanda Nkuwa an E-W-striking systems of faults and fractures (red lines) apparently splays from the Chitima structure and extends towards the faulted boundary of Lower Zambezi Basin Karoo strata near Tchareca. The white rectangle indicates an area selected for more detailed morphotoectnic study (Figs 16-18 below).

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Figure 16. Sun-shaded SRTM digital terrain model of area between latitudes 15.5-16 S, 32.25-33 E, with earth-toned elevation overlay in a narrow 5 m band between 470 m and 475 m, selected to emphasize the evident offset of post-Karoo sediments and land surface across the eastern end of the Estima Faullt and the F1 fault. From earlier Google Earth inspection and the morphotectonic study, the F1 fault appears as a distinct scarp of a few metres elevation difference (downthrow to south), crosscutting valley-and-ridge topography at a high angle (centre-left of image).

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Figure 17. Sun-shaded SRTM digital terrain model with sun azimuth at N90 E to emphasize N-S-trending topographic features. The recent displacement across the surface-breaking scarp of the F1 fault appears to involve also a component of rightlateral displacement that offsets ridge alignments between the foot-wall block (red lines) and the hanging-wall block (blue lines)

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Figure 18. Slope map of area (colour tones as in Fig. 13 above) shown in previous two figures (16 & 17), indicating subtle changes in slope microtopography across the Estima and F1 fault scarps, partly governed by lithological contrast between foot-wall (Karoo) and hanging-wall (post-Karoo; cf. Fig. 10 above) in the case of the Estima fault, but between the same post-Karoo lithology in the case of the western F1 structure.

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