Ruth Fierens

L Droz

Gwenael Jouet

M Rabineau

F Raisson

N Babonneau

Cécile Robin

S J J Jorry

Sedimentary evolution

Keywords: Zambezi turbidite system, Mozambique Channel, multichannel seismic profiles, turbidite, contourite, tectonic, Late Cenozoic

published or not. The documents may come

Sedimentary evolution and effects of structural controls on the development of the

Zambezi mixed turbidite-contourite system (Mozambique Channel, Southwest Indian Ocean) since the Oligocene R. Fierens 1* , L. Droz 1 , G. Jouet 2 , M. Rabineau 1 , F. Raisson 3 , N. Babonneau 4 , C. Robin 5 , S.J. Jorry 2 1: CNRS, Université Bretagne Occidentale, Ifremer, Geo-Ocean, UMR6538, IUEM, 29280, Plouzané, France 2: Ifremer, Université Bretagne Occidentale, CNRS, Geo-Ocean, 29280, Plouzané, France 3: Clastic Sedimentology Group, TotalEnergies, 64000 Pau, France 4: Université Bretagne Occidentale, CNRS, Ifremer, Geo-Ocean, UMR6538, IUEM, 29280, Plouzané, France 5: CNRS, Géosciences Rennes, UMR6118, Université de Rennes, Rennes, 35042, France *Corresponding author: ruth.fierens@univ-brest.fr ABSTRACT High-resolution multichannel seismic reflection data that spans significant parts of the Mozambique margin offshore the Zambezi River permits the study of the Oligocene to present architectural evolution of the Zambezi turbidite system. In this time frame, five major depositional units are recognized that evidence a widespread spatial and temporal occurrence of both turbiditic and contouritic sedimentation. They indicate that the sedimentary regime within the turbidite system changed from dominantly aggradational during the Oligocene to mainly erosional during Miocene to an interplay of erosional and depositional processes during the Plio-Quaternary. Different episodes of incision, linked with the Serpa Pinto, Angoche and Zambezi valleys, are recognized in the upstream portion of J o u r n a l P r e -p r o o f

INTRODUCTION

Tectonic settings and sediment transfer are known to play an important role in the development and characteristics of deep-marine turbidite systems [START_REF] Stow | Sedimentary, Tectonic, and Sea-Level Controls[END_REF]Mutti andNormark, 1987, 1991). Besides possible regulation of upslope sediment flux and sediment delivery configuration (e.g. [START_REF] Stow | Sedimentary, Tectonic, and Sea-Level Controls[END_REF][START_REF] Reading | The classification of deep-sea depositional systems by sediment caliber and feeder system[END_REF][START_REF] Reading | Turbidite systems in deep-water basin margins classified by grain size and feeder system[END_REF], tectonics may deform basin floor topography and hence exert strong control on the morphodynamic evolution of a sedimentary system [START_REF] Alexander | Observations on experimental, nonchannelized, highconcentration turbidity currents and variations in deposits around obstacles[END_REF][START_REF] Bursik | The Effects of Topography on Sedimentation from Particle-Laden Turbulent Density Currents[END_REF][START_REF] Haughton | Evolving turbidite systems on a deforming basin floor, Tabernas, SE Spain[END_REF][START_REF] Morris | Changes in Flow Direction at a Point Caused by Obstacles During Passage of a Density Current[END_REF][START_REF] Mayall | The response of turbidite slope channels to growth-induced seabed topography[END_REF][START_REF] Howlett | Turbidites, topography and tectonics: Evolution of submarine channel-lobe systems in the saltinfluenced Kwanza Basin, offshore Angola[END_REF]. Pre-existing features forming positive relief and/or tectonic movements (by faults, folds, salt or mud diapirs, salt walls, etc.) can disrupt the continuity of sediment flows and cause unusual geometries and sediment distribution patterns as they can lead to:

(1) deflection, or the shifting of sediment routing due to an active structure (e.g., Tabernas-Sorbas Basin: [START_REF] Hodgson | Impact of syndepositional faulting on gravity current behaviour and deep-water stratigraphy: Tabernas-Sorbas Basin, SE Spain[END_REF]Niger Delta: Morgan, 2004; Angolan margin: J o u r n a l P r e -p r o o f [START_REF] Gee | Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways[END_REF]; (2) diversion, when the gravity-driven processes change course because of a pre-existing structure (e.g., Makran margin: [START_REF] Kukowski | Morphotectonics and mechanics of the central Makran accretionary wedge off Pakistan[END_REF]southern Barbados prism: Huyghe et al., 2004;Brazilian slope: Smith, 2004; offshore Tanzania: [START_REF] Maselli | Impact of the East African Rift System on the routing of the deep-water drainage network offshore Tanzania, western Indian Ocean[END_REF]; (3) confinement, which indicates the restriction of turbidity currents by adjacent structures (e.g., Lower Congo Basin: Oluboyo et al., 2014;Levant Basin: Clark and Cartwright, 2009); or (4) blocking, where seafloor relief prevents sedimentation downstream of a structure (e.g., Gulf of Mexico: [START_REF] Rowan | Salt-Sediment Interaction, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico[END_REF][START_REF] Beaubouef | High Resolution Seismic/Sequence Stratigraphic Framework for the Evolution of Pleistocene Intra Slope Basins, Western Gulf of Mexico: J o u r n a l P r e -p r o o f Depositional Models and Reservoir Analogs[END_REF]Annot System: Sinclair and Tomasso, 2002;Fangliao Fan: Hsiung et al., 2018) of sediment pathways. Moreover, the crossing of a tectonic structurewhich occurs if the erosional downcutting of sediment transport systems keeps pace with the rate of structural growth-can lead to a local or more extended topographic constriction of turbidity currents impacting flow behavior and sedimentation patterns (e.g., [START_REF] Gee | Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways[END_REF][START_REF] Morgan | Structural Controls on the Positioning of Submarine Channels on the Lower Slopes of the Niger Delta[END_REF][START_REF] Saller | Linked lowstand delta to basinfloor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems[END_REF][START_REF] Mayall | The response of turbidite slope channels to growth-induced seabed topography[END_REF]. Although the principal anticipated response of deepwater gravity flows to structural elements are known, it is still a challenge to elucidate the depositional architectural evolution of extensive submarine turbidite systems to complex seafloor deformations.

The Mozambique Channel, hosting the Zambezi turbidite system, that has arisen from the break-up of Gondwana and the development of the East African rift system (e.g., [START_REF] Mougenot | Seaward extension of the East African Rift[END_REF][START_REF] Salman | Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique[END_REF][START_REF] Calais | Kinematics of the East African Rift from GPS and earthquake slip vector data[END_REF][START_REF] Leinweber | The Jurassic history of the Africa-Antarctica corridor-new constraints from magnetic data on the conjugate continental margins[END_REF][START_REF] Salman | Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique[END_REF][START_REF] Franke | The offshore East African Rift System: J o u r n a l P r e -p r o o f Structural framework at the toe of a juvenile rift[END_REF][START_REF] Courgeon | The offshore east African rift system: new insights from the Sakalaves seamounts (Davie Ridge, SW Indian Ocean)[END_REF][START_REF] Deville | Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia-Somalia southern plate boundary[END_REF][START_REF] Thompson | New starting point for the Indian Ocean: Second phase of breakup for Gondwana[END_REF]) is known to be a tectonically dynamic region and hence, provides an interesting research area to explore the role of large-scale structural features and processes on the growth and evolution of a deep-water turbidite depositional system. Based on seismic reflection profiles, a number of previous studies on the Zambezi system have already hinted at a significant impact of morpho-tectonic features (i.e., Davie Ridge and Iles Eparses and seamounts) that caused the stable position of the Zambezi Valley and its resulting unusual deep entrenchment [START_REF] Lort | New seismic data in the Mozambique Channel[END_REF][START_REF] Droz | Mozambique upper fan: origin of depositional units[END_REF][START_REF] Castelino | Mesozoic and Early Cenozoic sediment influx and morphology of the Mozambique Basin[END_REF].

Unfortunately, the findings of these studies were hampered due to the low-resolution data J o u r n a l P r e -p r o o f quality and/or insufficient seismic coverage. Other authors (e.g., [START_REF] Breitzke | Seafloor morphology in the Mozambique Channel: evidence for long-term persistent bottom-current flow and deep-reaching eddy activity[END_REF][START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF][START_REF] Miramontes | The influence of bottom currents on the Zambezi Valley morphology (Mozambique Channel, SW Indian Ocean): In situ current J o u r n a l P r e -p r o o f observations and hydrodynamic modelling[END_REF] have suggested the important influence of oceanic bottom currents on the Zambezi Valley morphology. However, it seems unlikely to attribute the total deep entrenchment of the Zambezi Valley (~700 m; [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] merely to erosional activity of deep-water bottom currents (max. 500 m of incision; [START_REF] García | Contourite erosive features caused by the Mediterranean Outflow Water in the Gulf of Cadiz: Quaternary tectonic and oceanographic implications[END_REF][START_REF] Van Rooij | The Le Danois Contourite Depositional System: Interactions between the Mediterranean Outflow Water and the upper Cantabrian slope (North Iberian margin)[END_REF][START_REF] Bozzano | Dropstones in the Mar del Plata Canyon Area (SW Atlantic): Evidence for Provenance, Transport, Distribution, and Oceanographic Implications[END_REF][START_REF] Miramontes | Contourite and mixed turbidite-contourite systems in the Mozambique Channel (SW Indian Ocean): Link between geometry, sediment characteristics and modelled bottom currents[END_REF] and so it has not yet been possible to ascertain the precise cause of the deeply incised morphology of the Zambezi Valley. This study based on a new and extensive high-resolution seismic reflection dataset from the Zambezi depositional system seeks to improve our understanding of the regional sedimentary evolution. The main seismic facies and incisions are examined in order to explore and characterize the timing and extent of erosion and deposition in the Zambezi turbidite system from Oligocene to present-day. This specifically allows us to decipher which mechanisms are responsible for the exceptional deep entrenchment of the Zambezi Valley.

REGIONAL SETTING OF THE MOZAMBIQUE CHANNEL AND ZAMBEZI TURBIDITE

SYSTEM

The Mozambique Channel is located in the western Indian Ocean between Mozambique (East Africa) and Madagascar (centered at 40°E and between 15°S and 42°S) (Fig. 1). Channel showing main sedimentary and structural features (modified from [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF]. TV = Tsiribihina Valley. White star: core U1477 from IODP leg 361 [START_REF] Hall | International Ocean Discovery J o u r n a l P r e -p r o o f Program; Expedition 361[END_REF].

J o u r n a l P r e -p r o o f

Compiled from published literature: (1) [START_REF] Schulz | Variability of Holocene to Late Pleistocene Zambezi riverine sedimentation at the upper continental slope off Mozambique, 15°-21°S[END_REF] (2) [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] (3) Kolla et al., 1980a;(4) [START_REF] Beiersdorf | Placer Deposits of limenite and Zircon on the Zambezi Shelf[END_REF](5) GEBCO, 2014;(6) [START_REF] Wiles | The Zambezi Channel: A new perspective on submarine channel evolution at low latitudes[END_REF](7) Kolla et al., 1980b;(8) [START_REF] Breitzke | Seafloor morphology in the Mozambique Channel: evidence for long-term persistent bottom-current flow and deep-reaching eddy activity[END_REF](9) [START_REF] Raisson | Deep oceanic currents and sea floor interactions offshore SE Africa[END_REF](10) [START_REF] Mahanjane | A geotectonic history of the northern Mozambique Basin including the Beira High -A contribution for the understanding of its development[END_REF](11) Mahanjane, 2014;(12) Schematized from [START_REF] Courgeon | Impact of tectonic and volcanism on the Neogene evolution of isolated carbonate platforms (SW Indian Ocean)[END_REF][START_REF] Courgeon | The offshore east African rift system: new insights from the Sakalaves seamounts (Davie Ridge, SW Indian Ocean)[END_REF][START_REF] Deville | Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia-Somalia southern plate boundary[END_REF][START_REF] Wiles | Surface expression of microplate boundary kinematics: An isolated abyssal hill in the Mozambique Channel[END_REF](13) [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF], partly based on Baby, 2017;(14) [START_REF] Stamps | Role of mantle flow in Nubia-Somalia plate divergence[END_REF].

Structural evolution

The formation of the Mozambique Channel was initiated in the Early Jurassic-Early Cretaceous during the breakup of the Gondwana [START_REF] Salman | Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique[END_REF][START_REF] Leinweber | The Jurassic history of the Africa-Antarctica corridor-new constraints from magnetic data on the conjugate continental margins[END_REF][START_REF] Thompson | New starting point for the Indian Ocean: Second phase of breakup for Gondwana[END_REF]. Multiple rifting phases allowed the East Gondwana (Madagascar, Antarctica, India and Australia) to separate from the African block at ca. 165 Ma and slide southward along the transform Davie Fracture Zone (Fig. 1B) until around 120 Ma (Segoufin and Patriat, 1981;[START_REF] Coffin | Reconstruction of Madagascar and Africa: evidence from the Davie fracture zone and western Somali basin[END_REF][START_REF] Cochran | Somali Basin, Chain Ridge, and origin of the Northern Somali Basin gravity and geoid low[END_REF][START_REF] Gaina | Tectonic interactions between India and Arabia since the Jurassic reconstructed from marine geophysics, ophiolite geology, and seismic tomography[END_REF]. During the Late Cretaceous (95 Ma) Madagascar separated from the Indian block [START_REF] Salman | Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique[END_REF] and acquired its current position at 88 Ma (Late Cretaceous) [START_REF] Reeves | The position of Madagascar within Gondwana and its movements during Gondwana dispersal[END_REF][START_REF] Thomas | The opening of the Indian Ocean: what is the impact on the East African, Madagascar and Antartictic margins, and what are the origins if the aseismic ridges?[END_REF]. The East African continental margin underwent a period of stabilization during the Paleocene and the Eocene, until rifting resumed when the East African Rift System (EARS) developed [START_REF] Salman | Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique[END_REF][START_REF] Calais | Kinematics of the East African Rift from GPS and earthquake slip vector data[END_REF][START_REF] Salman | Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique[END_REF][START_REF] Franke | The offshore East African Rift System: J o u r n a l P r e -p r o o f Structural framework at the toe of a juvenile rift[END_REF]. The eastern branch of EARS initiated in the Oligocene [START_REF] Dawson | Neogene tectonics and volcanicity in the North Tanzania sector of the Gregory Rift Valley: contrasts with the Kenya sector[END_REF][START_REF] Gall | Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times[END_REF] , 2015). This offshore rift segment is characterized by Neogene extension superimposed on earlier strike-slip structures of the Davie Fracture Zone [START_REF] Rabinowitz | The Separation of Madagascar and Africa[END_REF][START_REF] Reeves | The position of Madagascar within Gondwana and its movements during Gondwana dispersal[END_REF][START_REF] Franke | The offshore East African Rift System: J o u r n a l P r e -p r o o f Structural framework at the toe of a juvenile rift[END_REF]. A southwest prolongation of this offshore branch is argued by [START_REF] Courgeon | The offshore east African rift system: new insights from the Sakalaves seamounts (Davie Ridge, SW Indian Ocean)[END_REF], [START_REF] Deville | Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia-Somalia southern plate boundary[END_REF] and [START_REF] Wiles | Surface expression of microplate boundary kinematics: An isolated abyssal hill in the Mozambique Channel[END_REF] on the basis of a NNE-SSW densely distributed fault pattern, extending further south than the Sakalaves to the Mozambique Ridge on the East African Margin. These faults deformed the oceanic lithosphere of the Mozambique Channel in the same direction as the Agulhas-Falkland transform fault zone from at least Miocene times and are spatially associated to the Quathlamba active Seismic Axis (QSA) (Fig. 1B) [START_REF] Stamps | Role of mantle flow in Nubia-Somalia plate divergence[END_REF].

Present-day morphology and main morpho-tectonic features

The Mozambican shelf off the Zambezi River mouth gets connected to the deepest portions of the Mozambique Basin by the long and curvilinear Zambezi Valley that constitutes the main morphologic feature of the Zambezi Fan [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] (Fig. 1). At present, no direct connection is known between the Zambezi River outlet and the Zambezi Valley. This submarine valley has a NW-SE orientation in its upper portion, transverse to the Mozambique Margin. It deflects towards the south when it approaches the Davie Ridge and passes through the Quathlamba Seismic Axis [START_REF] Stamps | Role of mantle flow in Nubia-Somalia plate divergence[END_REF] that is still active today [START_REF] Courgeon | The offshore east African rift system: new insights from the Sakalaves seamounts (Davie Ridge, SW Indian Ocean)[END_REF][START_REF] Deville | Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia-Somalia southern plate boundary[END_REF][START_REF] Wiles | Surface expression of microplate boundary kinematics: An isolated abyssal hill in the Mozambique Channel[END_REF]. The valley runs thereon between the Eparses carbonate platforms [START_REF] Courgeon | Growth and demise of Cenozoic isolated carbonate platforms: New insights from the Mozambique Channel seamounts (SW Indian Ocean)[END_REF] to the west and the Madagascar margin to the east [START_REF] J O U R N A L P R E -P R O O F Delaunay | Les mouvements verticaux de Madagascar (90-0 Ma): une analyse couplée des formes du relief et de l'enregistrement sédimentaire des marges ouest malgaches[END_REF]. At around 22°S, the Zambezi Valley coalesces with the Tsiribihina Valley (TV in Fig. 1B) that originates from the Western Madagascar margin. Approximately at the latitude of the southern tip of Madagascar the valley leads to a rather flat area constituting the Mozambique deep Basin. Between 22°S and 26°S, the valley crosses an elevated area that has undergone structural doming during the Late Miocene [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF].

J o u r n a l P r e -p r o o f

The Davie Ridge is a 1200 km-long prominent N-S trending bathymetric high crossing the Mozambique Channel from the northeastern Mozambique to the southwestern Madagascar margin. It served as a morphological barrier for sediments originating from Madagascar, trapping the sediments in the Morondava Basin until the Miocene (23 Ma, Delaunay, 2018).

Subsequently, this basin was filled in and allowed the Madagascar river inputs to overspill and contribute to the Zambezi Fan via the Tsiribihina Valley.

About 80 km off the Mozambican coast, the NE-SW Beira High (Fig. 1) forms a prominent basement high parallel to the Mozambique margin [START_REF] Mahanjane | A geotectonic history of the northern Mozambique Basin including the Beira High -A contribution for the understanding of its development[END_REF][START_REF] Mueller | The crustal structure of Beira High, central Mozambique-Combined investigation of wide-angle seismic and potential field data[END_REF]. This now buried structure is 300 km long and 100 km wide and served as a morphological barrier for sediments originating from the Mozambique margin until the Middle Miocene.

The central Iles Eparses are steep sloped carbonate platforms developed on top of volcanic edifices relating to an Oligocene to Early Miocene volcanic episode [START_REF] Courgeon | Growth and demise of Cenozoic isolated carbonate platforms: New insights from the Mozambique Channel seamounts (SW Indian Ocean)[END_REF][START_REF] Jorry | Modern sediments and Pleistocene reefs from isolated carbonate platforms (Iles Eparses, SW Indian Ocean): A preliminary study[END_REF]. The Iles Eparses and the Beira High limit a small intraslope basin that has been named the "Intermediate Basin" [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF]. This basin constitutes the more recent depositional system [START_REF] Fierens | Quaternary sediment dispersal in the Zambezi turbidite system (SW Indian Ocean)[END_REF].

Sediment sources

The Mozambique margin records a total sedimentary thickness of ca.6 stwt (~12 km) [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF][START_REF] Ponte | The Zambezi delta (Mozambique channel, East Africa): High resolution dating combining bio-orbital and seismic stratigraphies to determine climate (palaeoprecipitation) and tectonic controls on a passive margin[END_REF] and sediment deposition started at Early Cretaceous.

The main source of sediment filling the Mozambique Basin is the Zambezi River, which is with a catchment area of 1.3 x 10 6 km 2 the fourth largest river on the African continent (Fig. 2) [START_REF] Thomas | The opening of the Indian Ocean: what is the impact on the East African, Madagascar and Antartictic margins, and what are the origins if the aseismic ridges?[END_REF][START_REF] Walford | Solid sediment load history of the Zambezi Delta[END_REF][START_REF] Milliman | River discharge to the coastal ocean: a global synthesis[END_REF].

J o u r n a l P r e -p r o o f The Zambezi River originates in central southern Africa and drains on its eastern portion major parts of the East African Rift System as well as the NE portion of the Kalahari Basin of the South African Plateau (Fig. 2A). It has had a polyphase evolution and has fed the Mozambican margin with fluctuating composition mixture of sediment (Garzanti et al., in press). The Zambezi River has a sediment load of 48 x 10 6 t/yr [START_REF] Nugent | Salt tectonic controls on deep-water turbidite depositional systems: Miocene, southwestern Lower Congo Basin, offshore Angola[END_REF][START_REF] Milliman | Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers[END_REF] and is 2,575 km long with a third of its length at an altitude higher than 850 m.

From the African continent, two other important sources of sedimentary inputs have been observed: the Lurio (during Oligocene times) and Angoche (during Oligocene-Neogene period) watersheds (Fig. 2A). Western Madagascar rivers also contribute, but for a minor J o u r n a l P r e -p r o o f amount, to the deposition in the Zambezi turbidite system (Fig. 2B). Madagascar sediment inputs are originating mainly from the Tsiribihina River (525 km long, catchment area of 49,800 km 2 ) which is connected to the Zambezi Valley by the submarine Tsiribihina Valley.

The Mangoky River which drains the largest watershed in Madagascar (~55,750 km 2 ) is probably also connected to this Tsiribihina Valley. The complex of Onilahy and Fiherenana rivers directly feed the distal turbidite system in the deep Mozambique Basin.

The Zambezi turbidite system

The vast amounts of sediments discharged onto the Mozambican continental shelf by the Zambezi River is attested by the very high sedimentation rate registered for the last climate cycle by IODP drilling U1477 (about 1 m/kyr as a mean sediment rate for the last 120 kyr; Leg 361, [START_REF] Hall | International Ocean Discovery J o u r n a l P r e -p r o o f Program; Expedition 361[END_REF] and during the Late Glacial Maximum (LGM, 26.5-20 ka BP) as attested by the MOZ4-CS17 core (reaching about 2-3 m/kyr, [START_REF] Zindorf | The evolution of early diagenetic processes at the Mozambique margin during the last glacial-interglacial transition[END_REF] both cores retrieved on the upper slope in extension of the Zambezi Delta (Fig. 1B). [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] showed that the fluvial input of the Zambezi River is deposited into two main depocenters (Fig. 3) that together compose the Zambezi turbidite system: the Zambezi Fan, which is fed by the 1500 km long entrenched Zambezi Valley that transfers sediments to the deep basin in a vast zone of distal deposition ("Depositional Area", Fig. 3) and a Ponded Fan located in the Intermediate Basin, limited by the Mozambican slope and the Iles Eparses.

J o u r n a l P r e -p r o o f The Zambezi Fan has been studied previously by multiple studies (e.g., Simpson, 1974;Kolla et al., 1980aKolla et al., , 1980b;;[START_REF] Droz | Mozambique upper fan: origin of depositional units[END_REF][START_REF] Breitzke | Seafloor morphology in the Mozambique Channel: evidence for long-term persistent bottom-current flow and deep-reaching eddy activity[END_REF]Wiles et al., 2017a;[START_REF] Miramontes | The influence of bottom currents on the Zambezi Valley morphology (Mozambique Channel, SW Indian Ocean): In situ current J o u r n a l P r e -p r o o f observations and hydrodynamic modelling[END_REF][START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF][START_REF] Fierens | Quaternary sediment dispersal in the Zambezi turbidite system (SW Indian Ocean)[END_REF]. From these we know that the Zambezi Valley and the distal Depositional Area are characterized by coarse-grained deposits (Simpson, 1974;Kolla et al., 1980b;[START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF]. The presence of sediment waves [START_REF] Breitzke | Seafloor morphology in the Mozambique Channel: evidence for long-term persistent bottom-current flow and deep-reaching eddy activity[END_REF][START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] and the flank erosion of the Zambezi Valley [START_REF] Miramontes | The influence of bottom currents on the Zambezi Valley morphology (Mozambique Channel, SW Indian Ocean): In situ current J o u r n a l P r e -p r o o f observations and hydrodynamic modelling[END_REF] reveal important sediment reworking by strong bottom currents. In contrast, the Ponded Fan has been more poorly studied [START_REF] Wiles | Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel[END_REF][START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF][START_REF] Fierens | Quaternary sediment dispersal in the Zambezi turbidite system (SW Indian Ocean)[END_REF]. Based on sub-bottom profiler data, [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] suggested that the Ponded Fan mainly consists of fine-grained turbidites with thin sheet-like, coarse-grained interbeds.

The current Zambezi Fan was established from Oligocene times when the Zambezi Delta progradation was initiated [START_REF] Droz | Mozambique upper fan: origin of depositional units[END_REF][START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF]. Since then, large quantities of sediment have been drained from the Zambezi watershed causing a substantial progradation of the Mozambique shelf [START_REF] Walford | Solid sediment load history of the Zambezi Delta[END_REF][START_REF] Ponte | The Zambezi delta (Mozambique channel, East Africa): High resolution dating combining bio-orbital and seismic stratigraphies to determine climate (palaeoprecipitation) and tectonic controls on a passive margin[END_REF] (Fig. 4).

J o u r n a l P r e -p r o o f

During Pleistocene times, the continental margin progradation was most important and mass transport activity was common [START_REF] Ponte | The Zambezi delta (Mozambique channel, East Africa): High resolution dating combining bio-orbital and seismic stratigraphies to determine climate (palaeoprecipitation) and tectonic controls on a passive margin[END_REF]. The architecture of the Zambezi Fan is poorly known, except for the feeding axes of the fan. [START_REF] Droz | Mozambique upper fan: origin of depositional units[END_REF] have shown that the initiation of the Zambezi Fan, during Oligocene times was related to the Serpa Pinto Valley, a N-S feeding axis close to the Davie Ridge (Figs. [START_REF] Imbo | The Gebra Slide: a submarine slide on the Trinity Peninsula Margin, Antarctica[END_REF]3). This valley received sedimentary inputs from the Lurio catchement [START_REF] Roquette | La marge transformante nord-Mozambicaine : bilan érosionsédimentation[END_REF][START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF] (Fig. 2A). The shift to the present-day NW-SE Zambezi Valley occurred in the Mid-Miocene possibly in response to tectonic activity induced by the development of the East African Rift System that affected the northern Mozambique margin. [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF] indicates that during Oligocene-Neogene times, sediments were supplied by the Angoche (Fig. 2A) and Zambezi watershed by several channeling systems that join the Serpa Pinto Valley (Oligocene) and later the Zambezi Valley (from Middle Miocene). Since the Middle Miocene, sediments from Madagascar contributed to the feeding of the Zambezi system (see section 2.2), funneled in the Tsiribihina Valley (Delaunay, 2018).

MATERIAL AND METHOD

This work is based mainly on high-resolution seismic data that were acquired during three cruises carried out as part of the PAMELA (PAssive Margin Exploration LAboratories) project J o u r n a l P r e -p r o o f [START_REF] Bourillet | PAMELA "PASSIVE MARGINS EXPLORATION LABORATORIES[END_REF]: PTOLEMEE [START_REF] Jorry | PTOLEMEE cruise[END_REF], PAMELA-MOZ2 [START_REF] Robin | PAMELA-MOZ02 cruise[END_REF] and PAMELA-MOZ4 [START_REF] Jouet | PAMELA-MOZ04 cruise[END_REF] (Fig. 5). The PAMELA data are unevenly distributed across the study area with a majority located around the Zambezi Valley (Fig. 5A).

The seismic lines are more dispersed on the Mozambican continental slope (N to NE of Iles Eparses) and in the distal Mozambique Basin.

On the Mozambican slope, the dataset was complemented with seismic data made available by Total. Additional seismic data from the MD163-MoBaMaSiS expedition [START_REF] Reichert | MD 163 / MOBAMASIS cruise[END_REF] were also used.

PAMELA seismic data acquisitions

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This study used approximately 23,000 km of multichannel seismic data (Fig. 5) that were acquired aboard of the L'Atalante and Pourquoi Pas? research vessels. The key parameters of the seismic acquisitions are given in Table 1. The multichannel seismic data image the sediment cover up to ca. 2-3 seconds two-way time (stwt), depending on the nature of deposits and the bathymetric conditions. Table 1: Acquisition parameters of the high-resolution multichannel seismic systems used during the PTOLEMEE, PAMELA-MOZ2 and PAMELA-MOZ4 cruises.

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PAMELA data processing

The seismic data were processed using Ifremer in-house softwares Qc-SISPEED © (PTOLEMEE, MOZ2) and SolidQC © (MOZ4). The processing sequence includes quality controls of navigation and seismic data, binning, SEGD to SEGY format conversion, passband filtering (15-150 Hz), correction of source delay and stacking and migration at constant velocity (1500 m/s).

Seismic data interpretation

Seismic interpretation was performed following the seismic stratigraphy principles [START_REF] Mitchum | Seismic stratigraphy and global changes of sea level: Part 6. Stratigraphic interpretation of seismic reflection patterns in depositional sequences: Section 2. Application of seismic reflection configuration to stratigraphic interpretation[END_REF]. This approach involves the recognition and correlation of seismic units (definition according to [START_REF] Mitchum | Seismic stratigraphy and global changes of sea level: Part 6. Stratigraphic interpretation of seismic reflection patterns in depositional sequences: Section 2. Application of seismic reflection configuration to stratigraphic interpretation[END_REF] complemented by an analysis of the seismic facies. The limits of these units are recognized by reflection terminations (onlaps, downlaps, erosional truncations, and toplaps). The geometry of reflections within an individual seismic unit is described using seismic facies analysis. Seismic facies are defined by looking at the internal and external configurations of the seismic units. Internal reflection characters are described by the continuity, amplitude, frequency and geometry of the reflections. The upper and lower boundaries of the seismic facies define its 2-or 3-dimensional external shape (e.g.

J o u r n a l P r e -p r o o f

wedge, lens or sheet geometry). The stratigraphic framework established by [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF] on the Mozambique margin (Fig. 4) served as a basis for our study.

The processed data (navigation and SEGY seismic files) were imported into the IHS Kingdom Suite ® software that was used to display and analyze the data. Isopach maps were created with grid cell sizes of 200x200 m and produced in second two-way travel-time (stwt).

A time-depth conversion is provided using an approximate velocity of 2000 m/s [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF]. With regard to the large size of the study area and the generally low density of the seismic data, the interpolation of seismic interpretations has been locally difficult. We have chosen to downgrade the isopach maps by showing a maximum of five classes of thickness (0.015-0.25, 0.25-0.5, 0.5-0.75, 0.75-1.0 and 1.0-1.634 stwt). The lowest value (0.015 stwt) corresponds to the vertical resolution of the seismic data with the lowest resolution (PTOLEMEE and MOZ2 data, see Table 1). These isopach maps were displayed by using ArcGIS® v10.3.1 (World Mercator map projection).

RESULTS AND INTERPRETATION

Seismic facies and process-based interpretation

The high-resolution multichannel (24-and 48-channels) seismic profiles of the Mozambique Channel reveal three main types of seismic facies that are those typically encountered on continental margins and in turbidite systems (e.g., [START_REF] Winker | High-resolution seismic stratigraphy of a late Pleistocene submarine fan ponded by salt-withdrawal mini-basins on the Gulf of Mexico continental slope[END_REF][START_REF] Piper | Outcrop-scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California[END_REF][START_REF] Babonneau | Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan[END_REF][START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3-D seismic data, Niger Delta continental slope[END_REF]: stratified (S), chaotic (C) and transparent (T)

facies. Main characteristics of the facies are summarized in Table 2 and examples are provided in Fig. 6.

J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f the distal end of the Zambezi Valley where coarse-grained sediments dominate (Kolla et al., 1980a, b). In addition, local flat top lens-shaped seismic units are identified on the Madagascar slope where it is thought to image coarse-grained turbidites. The chaotic facies are also observed as contorted reflections that are tilted towards the valley axis (Cb), which are interpreted as slumped deposits from the valley flanks in agreement with the interpretation of other authors (e.g., [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF][START_REF] Janocko | The diversity of deep-water sinuous channel belts and slope valley-fill complexes[END_REF]. At the foot of the seamounts of the Iles Eparses, the Cc chaotic facies is interpreted as carbonated turbidites with hemipelagic muds [START_REF] Counts | Sedimentation adjacent to atolls and volcano-cored carbonate platforms in the Mozambique Channel (SW Indian Ocean)[END_REF].

The transparent facies (T) are observed as homogeneously transparent inside the Zambezi Valley (Ta) or as mostly transparent with faint and contorted reflections at various sites of the Mozambique margin and basin (Tb). These facies are usually interpreted as mass-transport deposits (MTDs) (e.g., [START_REF] Imbo | The Gebra Slide: a submarine slide on the Trinity Peninsula Margin, Antarctica[END_REF][START_REF] Garziglia | Mass-transport deposits on the Rosetta province (NW Nile deep-sea turbidite system, Egyptian margin): Characteristics, distribution, and potential causal processes[END_REF][START_REF] Loncke | Multi-scale slope instabilities along the Nile deep-sea fan, Egyptian margin: a general overview[END_REF][START_REF] Dennielou | Major modification of sediment routing by a large Mass Transport Deposit in the Gulf of Lions (Western Mediterranean)[END_REF][START_REF] Badhani | Morphology of retrogressive failures in the Eastern Rhone interfluve during the last glacial maximum (Gulf of Lions, Western Mediterranean)[END_REF].

Architecture and stratigraphy of the Zambezi turbidite system

The Mozambique Basin feeding networks

The PAMELA data with a water depth up to 2500 m and data from Total between 2500 m and 1000 m water depth show that the upper portion of the Zambezi Valley (i.e. most northwestern part) has been fed by two main converging tributary networks (Fig. 7A), the youngest Northern and the oldest Southern Zambezi network [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF].

Tributaries of these networks do not show clear connections to the upper slope and disappear halfway up the slope. The absence of connection to the uppermost slope is not due to a lack of data, but rather because of the absence of lasting incisions upstream of the slope. The Intermediate Basin, in contrast, was fed by a loose network of parallel valleys distributed homogeneously along the Mozambique slope (black lines in Fig. 7A).

J o u r n a l P r e -p r o o f Downstream, the Zambezi Valley receives three main tributaries on its left hand-side: the Angoche, Serpa Pinto and Tsiribihina valleys (Fig. 7A). The valley that originates northwards from the Angoche Basin is referred to as the Angoche Valley in this paper. The Serpa Pinto

Valley [START_REF] Droz | Mozambique upper fan: origin of depositional units[END_REF] originates from the Lurio watershed and possible other northeastern African drainage basins [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF] and runs adjacent to the Davie Ridge.

These two paleo-tributaries have a North-South orientation and provided sediments from the Northern Mozambique rivers to the Zambezi Fan (Fig. 7B). The Tsiribihina Valley originates from the Western Madagascar margin (Tsiribihina and Mangoky drainage basins) and is currently providing sediments to the lower portion of the Zambezi Fan.

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Seismic units

Based on the seismic facies analysis and interpretation (see Section 4.1) we identified four main facies types: fined-grained overbank turbidites (lateral levees), coarse-grained turbidites (mainly channel fills or lobe complexes), fine-grained contourites (drifts) and masstransport deposits (MTDs, restricted in valleys or more widespread in the distal depositional area) (Fig. 8). These facies are organized into five main seismic units (U1 to U5) that are regionally correlated (Fig. 8A,9). U1 to U4 are stacked up in the Zambezi Fan, while U5 corresponds to the Ponded Fan in the Intermediate Basin. Figure 10 shows the thickness maps (in sec TWT) of the 5 units that could be correlated throughout the study area.

Additional units (Ua to Ug on Figs. 8A,9) are observed locally in the Zambezi Fan, but the low density of seismic data prevents their correlation and they are not considered in this paper.

Towards the distal depositional area (Fig. 8B), we could not individualize U1 to U4, therefore, a single 1000 m-thick seismic unit called Depositional Area (DA) has been considered in this area.

J o u r n a l P r e -p r o o f A general spatial overview (from north to south) of the architecture of the Zambezi Fan with the distribution of the seismic units is provided in figure 9. The distribution and isochore maps of the five main seismic units are provided in figure 10.

Unit U1: Channel-Levee Complex

Unit U1 (Figs. 8A,9) is wedge-shaped and composed of the association of high-amplitude channel infilling facies (S3a) interpreted as coarse-grained turbidites and lateral low to medium amplitude, high-frequency S1b facies interpreted as fine-grained turbidites (Fig. 6 and Table 2). The convergent configuration distally from the valley is typical of overbank deposits (levees).

Unit U1 originates in the northern most part of the study area, in relation with the N-S oriented Serpa Pinto and Angoche valleys (Fig. 10A). The depositional history of the Serpa Pinto Valley appears especially complex with multiple episodes of cut-and-fill in link with overbank deposition and intercalated with small mass-transport deposits inside the channels (Fig. 11). Consequently, U1 is interpreted as a channel-levee complex comprising several channel-levee systems. Available data are too limited to allow an extensive interpretation of the activity of this valley, and therefore it has been considered as a thick single event in this study. In addition, in this northern area, overbank deposition from both the Serpa Pinto Valley to the east and the Angoche Valley to the west indicates that both valleys have been active

synchronously and served as feeding axes to the so called Serpa Pinto channel-levee Complex (Fig. 9A,10A).

U1, including the Angoche contribution, shows its maximal lateral extent (approximately 260 km) and thickness (0.5 stwt, i.e. ~500 m) in the upstream northern part (Fig. 10A). Close to 9A).

The basal channels of the Serpa Pinto and Angoche Valley show limited erosion in the Eocene-Oligocene strata, and appear as a flat and wide erosional unconformity (Fig. 11).

Downstream the confluence with the Zambezi Valley (Fig. 8A, 9B to 9D), the Serpa Pinto

Valley is more deeply incised, and disappears downstream because of over-incision of the Zambezi Valley (see Section 4.2.4). Where the channels are still observed they are filled in with typical channel infilling facies, i.e. discontinuous, high-amplitude and low-frequency reflections (S3b in Fig. 6). The distribution of unit U1 (Fig. 10A) shows an approximate N-S direction. The channels appear relatively close to the present-day position of the Zambezi Valley except in the downstream extension of Unit U1 where they diverge toward the west (Fig. 10A). U1 is identified and correlated downstream to a latitude just a bit further south than the Tsiribihina confluence (Fig. 10A), however, the downstream extension of this unit remains uncertain.

J o u r n a l P r e -p r o o f terminal lobes, respectively. These seismic characteristics suggest that unit U2 is composed of channel-mouth deposits, distinctive of distal depositional environments.

The distribution of unit U2 (Fig. 10B) shows a pear shape with an average NE-SW orientation, i.e. oblique to the current Zambezi Valley (Fig. 10). This unit is up to 0.4 stwt thick (~400 m, Fig 10) and is generally widespread (up to ~330 km of lateral extent in its downstream portion, on Fig. 10B), more developed at the right hand-side of the Zambezi Valley. At its northeastern limit, unit U2 is mainly recognized on the left hand-side of the Zambezi Valley (Fig. 8A). Based on available data, it was not possible to extend this unit more to the north, so that we may only hypothesize on its origin (see Section 5.1). The southern limit of U2 extends more distally compared to unit U1. U2 is incised by the paleoincisions of both the Zambezi and Tsiribihina Valley (Figs. 8A,9E), which might indicate that unit U2 pre-existed the development of these valleys or that the incision of these valleys has endured after the deposition of U2.

Unit U3: Contourite drift on the western side of the Zambezi Valley

Unit U3 (Figs. 8A; 9) is composed of continuous low-amplitude, mostly high-frequency reflections (facies S1a). This unit is up to 0.7 stwt (~ 700 m) thick and shows a slightly domed wedged shape with bidirectional thinning. Close to the Zambezi Valley, the lateral thinning is partly due to erosion on the Zambezi Valley flanks. Reflections are parallel under the thickest part of the wedge and are affected by sediment bedforms that are observed in the whole vertical extent of the unit (Figs. 9, 13A). These bedforms, observed on bathymetric and subbottom profiler data [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF] are interpreted as bottom current controlled sediment waves. Based on the shape, the internal configuration and the similarities with other contourite drifts identified in the Mozambique Channel (Fig. 13B) or in literature [START_REF] Hernández-Molina | Giant mounded drifts in the Argentine Continental Margin: Origins, and global implications for the history of thermohaline circulation[END_REF][START_REF] Miramontes | Submarine landslides in the Northern Tyrrhenian Sea and relationship with the turbiditic and contouritic deposits: morphology, stratigraphy, geotechnics and modelling[END_REF], U3 is interpreted as a contourite drift.

Westward at the distal and basal limit of the Intermediate Basin, approaching the Iles Eparses, unit U3 appears to be locally structured by high reliefs wherein reflections are lost and the facies becomes mostly transparent (Fig. 8A). These reliefs are associated to a dense J o u r n a l P r e -p r o o f network of faults [START_REF] Deville | Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia-Somalia southern plate boundary[END_REF], mainly observed in the underlying units (Ud and Ue)

and evoke structural domes (associated to volcanism?).

Closer to the Iles Eparses seamounts, high-amplitude reflections are intercalated in the dominantly low-amplitude facies. They could represent detrital carbonate sediments originating from the islands, synchronous with the contourite deposition [START_REF] Counts | Sedimentation adjacent to atolls and volcano-cored carbonate platforms in the Mozambique Channel (SW Indian Ocean)[END_REF].

Unit U3 is solely observed along the right (looking downstream) border of the Zambezi Valley (Fig. 10C) and is locally less developed towards the SE of Iles Eparses (Fig. 13C). The width of the contourite drift reaches up to ~300 km (Fig. 10C) and the unit is recognized along at least 680 km from upstream to downstream. It is important to note that the northwestern and southern limits of unit U3 remain uncertain due to the lack of data.

J o u r n a l P r e -p r o o f ), similar to well-known levees of channel-levee systems (e.g. Congo, Amazon, Indus, Mississippi). We therefore interpret this facies as fine-grained overbank turbidites (probably alternating with very fine hemipelagic deposits). The stacking of these turbidites resulted in a ca. 0.7 stwt (~700 m) thick seismic unit (up to 0.9 stwt, ~900 m, locally) (Fig. 10D). Smaller units of inferred coarse-grained turbiditic (Uf, see Fig. 9 and section hereafter) and MTD (Ug, see Fig. 9 and section hereafter) origin are intercalated locally within U4. In addition, towards the east, lateral change in facies and external configuration to wavy parallel reflections (Figs.

8A, 14B) suggest a possible contemporaneous contouritic sedimentation on the Madagascar slope and along the Davie Ridge (Fig. 13B). This transition is arbitrary since there is probably a complete gradation between turbiditic and contouritic deposits.

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Unit U5: Ponded turbidites confined in the Intermediate Basin

Unit U5 is wedge-shaped (Fig. 15) and consists in two main depocenters up to 1.6 stwt (~1600 m) thick (Fig. 10E) along the Mozambique base of slope. It shows numerous small incisions inside it evoking erosional channels and is composed of seismic sub-units that migrate alternately to the NE and SW (Fig. 16D) indicating shifts of depocenters over time.

The main core of unit U5, between the Mozambique slope and the Iles Eparses, is composed of an alternation of high-amplitude facies (probably coarse-grained turbidites) and more transparent or even chaotic levels (respectively small but recurrent MTDs and erosional channels) that suggest a cyclic sedimentation (Fig. 16). The base of U5 shows large irregular undulations (erosional structures?) (Figs. 15,16A) where contourites (possibly related to the

J o u r n a l P r e -p r o o f

Neogene drift proposed by [START_REF] Raisson | Deep oceanic currents and sea floor interactions offshore SE Africa[END_REF] are suspected to be affected by structural doming (Fig. 8).

In its distal most part, between the Iles Eparses and the Zambezi Valley (Fig. 8A), unit U5 appears as an infilling unit composed of continuous, strong amplitude and low frequency reflections with onlapping terminations (facies S2, Figs. 6, 8A) on the supposed contourite drift (Fig. 17).

In the center part of the Intermediate Basin near the Iles Eparses, a transparent cover rests unconformably on the turbiditic deposits (Figs. 16). On sub-bottom profiles, this cover reveals continuous, high frequency reflections with increasing amplitude towards the top (Fig. 16C).

These characteristics at the top of the cover suggest an increase in coarse-grained material in the youngest strata. This cover has a variable thickness (ca. 0-100 m) and a limited distribution area. Owing to these characteristics, we suggest that this cover consists of an alternation of contourites or fine-grained turbidites with hemipelagic sediments. Any firm attribution of this superficial cover would need ground truthing by coring.

J o u r n a l P r e -p r o o f deposited late (Late Quaternary?). According to this stratigraphic framework, the Miocene interval and especially the Late Miocene are very thin (only some hundreds of meters in Fig. 8A).

In the distal depositional area (DA), the timing of deposition is different from upstream:

turbidite facies are absent during the Oligocene and only began to deposit during the Miocene. It must also be noticed also that contrarily to upstream, the Miocene deposits (especially the Late Miocene) are much thicker.

The Zambezi Valley incisions

While the Oligocene, the Early Miocene and the Plio-Quaternary periods were dominantly characterized by thick aggrading deposits, the Late Miocene period is distinguished by a limited turbiditic deposition (Fig. 8A) and a deep entrenchment of the valley.

The Zambezi Valley is currently deeply incised with relief exceeding 700 m in the middle portion of the valley [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF]. It evolved during the Miocene through several phases of incision and infilling (Figs. 18,19), some of them observed continuously all along the Zambezi Valley. Three main incisions, namely Angoche2 (A2), Zambezi1 (Z1) and Zambezi2 (Z2) from the oldest to the youngest (Figs. 9, 18, 19) are identified. Additionally, a fourth incision (Zambezi3, Z3) corresponds to very recent erosion, well expressed on bathymetric data of the valley floor [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF]. Besides some local shifts that may be observed (e.g., profile 7, Fig. 18 and profile 13, Fig. 19), A2, Z1 and Z2 occur mostly vertically under the current Zambezi Valley, indicating that this valley did not undergo major migration since its formation. Multiple additional minor erosional unconformities inside the valley and in the depositional area (purple unconformities in Fig. 19) are observed but cannot be followed from upstream to downstream.

J o u r n a l P r e -p r o o f

A2: Beginning of over-deepening of the Zambezi Valley

Incision A2 is the deepest incision observed on available seismic data. It is recognized only in the upstream portion of the Zambezi Valley, not far upstream from the Angoche Valley-Zambezi Valley confluence (Fig. 18 profile 1). A2 is therefore thought to relate to the Angoche Valley. This incision disappears rapidly downstream because of its erosion by the following incision Z1 (Fig. 18).

As proposed above, the Angoche Valley may have contributed to the feeding of the fan simultaneously with the Serpa Pinto Valley during an initial stage of activity (Angoche1, Fig. 9A). A2 is the trace of the latest incision by the Angoche Valley, when the Serpa Pinto Valley was no longer active. It implies that the Angoche Valley probably deeply incised during its second stage of activity, resulting in the total disappearance of the previous A1 erosional course (Fig. 18, profile 1, 2).

Z1: Main incision resulting in the first occurrence of the upstream part of the Zambezi

Valley

Incision Z1 is identified upstream from the Angoche Valley confluence. Consequently, it is considered to indicate the first occurrence of the valley that funneled the inputs of the Zambezi River. It marks the definitive installation of the valley at its present-day position on the central Mozambique margin.

Z1 eroded down to the upper strata of the Eocene sequence (Fig. 18). It is identified all along the Zambezi Valley (Figs. 18,19). The depth below the current valley floor evolves irregularly from up to ~0.4 stwt (profiles 1 and 8, Fig. 18) to down to ~0.2 stwt (profile 6, Fig. 18), and the depth below sea level varies from 4.4 stwt (profile 1, Fig. 18) to 5.6 stwt on profile 15 (Fig. 19) where it is tentatively identified.

J o u r n a l P r e -p r o o f

Its infill is very thick (more than 0.4 stwt, ~400 m, on profiles 4 to 6, Fig. 18) and is generally made of high-amplitude stratified facies indicating coarse-grained material, with facies that vary on profiles from chaotic to transparent to stratified (Fig. 18,19).

Z2 and Z3: later incisions of the Zambezi Valley

Incision Z2 is the last main incision of the Zambezi Valley. Its infill is characterized by a 0.2 stwt (~200 m) thick transparent body, identified continuously from the upper reaches of the valley to about 100 km southwards of the Tsiribihina confluence. This transparent mass is interpreted as a mass transport deposit (MTD). Above the MTD, a thin layer (at the seismic scale) of sediments is observed, mostly in the upstream portion of the valley.

Erosion Z3 is observed all along the Zambezi Valley and represents the youngest erosional events. Compared to previous incision events, this erosive period appears rather negligible.

It resulted in local over-deepening of the valley floor up to 0.063 stwt (~63 m) (profiles 1-2, Fig. 18) and gave the valley its current morphology (see [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF].

DISCUSSION

Origin of depositional units

U1: Serpa Pinto channel-levee complex with possible first stage of Angoche Valley

As mentioned previously (Section 4.2.3) unit U1 deposits are related to the Serpa Pinto

Valley. The Serpa Pinto Valley received inputs from the northern part of Mozambique (Lurio and probably other northeastern African drainage basins, [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF], indicating a 1100 km long transfer of sediments downstream to the most distal area where the channel-levee complex is identified with certainty (Figs. 9E,10A). Moreover, in the northernmost part of the study area (Fig. 9A), overbank deposition is observed from both the Serpa Pinto and the Angoche Valley. This indicates that there was possibly a contemporaneous first stage of Angoche Valley (Angoche1, Fig. 9) that contributed to the channel-levee complex.

J o u r n a l P r e -p r o o f

The U1 channel-levee complex, which was deposited during the Oligocene, is coeval to the building of the Zambezi giant 1.8 stwt-thick contourite drift on the southern bank of the upper Zambezi Valley (Fig 1B ,20A) [START_REF] Raisson | Deep oceanic currents and sea floor interactions offshore SE Africa[END_REF][START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF][START_REF] Thiéblemont | Seismic stratigraphic framework and depositional history for Cretaceous and Cenozoic contourite depositional systems of the Mozambique Channel, SW Indian Ocean[END_REF].

The juxtaposition of 2 major sedimentation mechanisms raises the question of their possible interaction. It is probable that the dramatic increase in terrigenous sedimentation brought into the basin by valleys [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF], associated with the major modification of global oceanic circulation [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF][START_REF] Thiéblemont | Seismic stratigraphic framework and depositional history for Cretaceous and Cenozoic contourite depositional systems of the Mozambique Channel, SW Indian Ocean[END_REF] controls the construction of the giant drift.

It is noticeable that the giant drift builds approximately where the northeastward flowing deep waters were interpreted to turn back to the South [START_REF] Thiéblemont | Seismic stratigraphic framework and depositional history for Cretaceous and Cenozoic contourite depositional systems of the Mozambique Channel, SW Indian Ocean[END_REF].

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U3-U4: Mixed contouritic-turbiditic sedimentation

Units U3 and U4 have been described separately because we do not have any stratigraphic information to give a relative chronology. However, owing to the top Miocene horizon of [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF], they were both deposited during the Plio-Quaternary, and we think that they were probably contemporaneous.

Unit U3, identified as a contourite drift at a water depth of ca. 2800 to 3500 m, may have been generated by bottom currents associated to the northward flowing Mozambique Undercurrent, which contain the North Atlantic Deep Water (NADW) at this particular depth range [START_REF] Van Aken | North Atlantic deep water in the south-western Indian Ocean[END_REF]. This interpretation is consistent with the observation of Valley. In addition, contourites appear to have been constantly deposited, either synchronously or in alternation with levee deposition, especially in areas approaching high reliefs, such as the Davie Ridge (Fig. 13B).

U5: Ponded turbidites from the Mozambique slope

The mainly coarse-grained turbidites were fed to the Intermediate Basin through a network of parallel valleys originating from the Mozambique slope off the Zambezi River mouth (Fig. 7A). Several channels have been identified, however the density of channels on the slope is rather small (ca. 50 km between each main channel). These valleys disappear halfway up the slope in agreement with the absence of canyons on the highest part of the slope [START_REF] Jouet | PAMELA-MOZ04 cruise[END_REF], demonstrating that there is no current connection between the shelf and the basin (as it was also observed for the Zambezi Valley). [START_REF] Wiles | Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel[END_REF] showed that channels in the southwestern portion of the basin (see Fig. 1B) probably transfer some of the Zambezi inputs towards the Bourcart-Hall Depression southeast of the Iles Eparses.

J o u r n a l P r e -p r o o f

Based on the stratigraphy of [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF], the Intermediate Basin began to fill in during the Upper Miocene, while deposition had mainly ceased in the Zambezi Fan that was mainly subject to erosion inside the valley (see Section 5.2), except in the distal depositional area. This is the only period of time during which data highlight a clear absence of synchronicity between both turbidite accumulation sites of the Mozambique margin. Later on, during the Plio-Quaternary, the Intermediate Basin was filled in and sediment deposition happened in the Zambezi Fan.

The thickness map (Fig. 10E) is consistent with that of [START_REF] Thiéblemont | Seismic stratigraphic framework and depositional history for Cretaceous and Cenozoic contourite depositional systems of the Mozambique Channel, SW Indian Ocean[END_REF] (Fig. 20D).

Upstream-downstream evolution of the incisions and effects of the Miocene doming

The depth profiles of incisions A2 to Z2 (Fig. 21B) show that A2 and Z1 incisions suffered a post-depositional deformation. This deformation is well expressed by the depth profiles of Top Eocene and Top Oligocene horizons observed between km 100 and km 900. It was described by [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF] as an episode of doming during the Miocene (Fig. 21C).

J o u r n a l P r e -p r o o f The continuous deformation resulted in a constant adjustment of the Z1 incision depth profile by entrenchment in order to compensate the uplift movement and to establish a new equilibrium state. The deep entrenchment of Z1 incision resulted in the erosion and disappearance of A2 (and probably also A1). The Z1 depth profile is not at an equilibrium state, it is assumed that it was abandoned prematurely before it had time to reestablish a full equilibrium profile.

The structural deformation on the Zambezi Valley during the Miocene had significant influence on the capacity to transfer sediments downstream. The continuous erosion in link with the elevation of the seafloor during the Miocene resulted in an over-incision of the Z1 incision, the production of important volumes of reworked sediments and the absence of levee deposits related to over-deepening of the valley floor.

The lacking, eroded Oligocene strata are estimated to a minimum of 0.4 stwt thickness (~400 m) and occur at least along 700 km (from profile PTO-SR-094 to profile MOZ2-SR-13, Fig. 21B). If the shape of the Z1 incision is approximated by a half rectangular prism of ca. 700 km long, 400 m high and 4 km wide (mean wideness of Zambezi1 incision, see Fig. 18), a rough minimum volume of 560 km 3 of eroded sediments is estimated. This reworked J o u r n a l P r e -p r o o f sedimentary volume is probably deposited downstream. At the time the incision occurred, it was unrelated to input by the Zambezi and other feeding rivers and therefore should be taken into consideration for the erosion-sedimentation balance calculations in source to sink studies. Conversely, the upstream portion of the Z1 incision (Fig. 21B, from km 50 to 150)

shows a concave upward-shape suggesting that it could have served as a trap for the Zambezi River inputs. This would have decreased the sedimentary volume able to reach the deeper portions of the Zambezi Fan. However, it can be assumed that the volume of trapped sediments inside Z1 is negligible with regards to the eroded Oligocene strata volumes (from km 150 to km 850, Fig. 21B).

Distinct modifications in the Zambezi turbidite system development

The Zambezi turbidite system experienced drastic modifications of sediment deposition during its evolution. Since Oligocene, sediment feeding axes migrated westward for the upstream portion of the Zambezi Valley (from SP-A1 to A2 to Z1-Z3) and a general southward migration of deposits happened. This was partly accompanied by successive variations in sedimentological regimes alternating between mainly aggradational and erosional phases (Fig. 22). The Oligocene was dominated by mainly fine-grained deposits and aggradational processes with the deposition of the Serpa Pinto channel-levee complex (U1). Limited erosional processes were restricted to the basal surface of the channel and it was possibly combined with a first stage of Angoche Valley activity (Fig. 9A). The deposition evolved during Early Miocene into coarse-grained sediments that are interpreted as distal turbiditic channel-mouth deposits of the Serpa Pinto system (U2). Their installation on previous more proximal channel-levees could reflect the retrogradation of the system. The origin of the change in depositional regime and possible retrogradation is unknown, but is probably related to the offshore tectonic deformation in the eastern branch of the East African Rift [START_REF] Courgeon | The offshore east African rift system: new insights from the Sakalaves seamounts (Davie Ridge, SW Indian Ocean)[END_REF], which would provoke changes in sediment flux supplied from North Mozambique.

The thickness map of Miocene sediments (Fig. 20B) shows that deposition started in the Intermediate Basin, while the Zambezi Fan is dominated by erosional processes during Middle to Late Miocene. This with two successive incisions (A2 and Z1) that indicate a shift of the position of the source, from the Angoche to the Zambezi drainage basin. In the Late Miocene, the incisions were affected by a progressive structural doming in the central part of the Mozambique Channel [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF]. Oligocene strata were eroded and the Angoche2 incision was probably totally erased by the Z1 erosional phase in response to the long-lasting elevation of the seafloor during Late Miocene. This incision period must have produced an important volume of material (estimated to 560 km 3 , see section 5.2) that is probably deposited downstream in the Depositional Area. It was however not possible to individualize the seismic units associated to this episode of strong erosion.

Later on in the Late Miocene a new aggradational stage happened were the Z1 incision was infilled with onlapping sediments probably in order to establish a new equilibrium profile and deposition started in the Intermediate Basin.

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The Plio-Pleistocene is characterized by a combination of aggradational and erosional events that resulted in a diversification of deposits. The Zambezi Fan has been shown to be dominantly erosional during very recent times [START_REF] Fierens | Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)[END_REF][START_REF] Miramontes | The influence of bottom currents on the Zambezi Valley morphology (Mozambique Channel, SW Indian Ocean): In situ current J o u r n a l P r e -p r o o f observations and hydrodynamic modelling[END_REF]. The erosional regime is attested by multiple generations of incisions in the thalweg (corresponding to the Z3 incision), erosion of the valley flanks possibly by bottom currents, and absence (or rarity) of fine-grained deposits (pirating of the turbulent suspension cloud by contouritic currents?). Contrarily, in the Ponded Fan, the depositional regime is mainly aggradational.

CONCLUSION

Academic high-resolution seismic data complemented with industrial data allowed deciphering the Oligocene to present architectural evolution of the Zambezi turbidite system including the Zambezi channelized Fan and the Intermediate Ponded Fan. Seismic interpretation allowed distinguishing five major depositional units and four principal incisional J o u r n a l P r e -p r o o f episodes. The respective depositional timing of these units is established based on the regional stratigraphy from [START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF].

The main results can be summarized as follows:

 The Zambezi turbidite system is shown to be composed of both turbidites and contourites that were deposited mostly synchronously along the Mozambique and Madagascar margins. The history of the turbidite system shows an alternation of aggradational and erosional processes.

Figure 1 :

 1 Figure 1: (A) Location of the Mozambique Channel in the southwest Indian Ocean, with

  and is prolonged offshore along the North Mozambique coastline and northern part of the Mozambique Channel (Mougenot et al., 1986; Franke et J o u r n a l P r e -p r o o f al.

Figure 2 :

 2 Figure 2: The Zambezi River and Madagascar Rivers catchments. (A) Position on the African

Figure 3 :

 3 Figure 3: Major elements composing the Zambezi turbidite system (modified from Fierens et

Figure 4 :

 4 Figure 4: Stratigraphic architecture of the Mozambique margin (Ponte et al., 2019) (see location on Fig. 5).

Figure 5 :

 5 Figure 5: Data used in this study. PAMELA high-resolution multichannel seismic data set. Magenta lines: PTOLEMEE cruise (Jorry, 2014); orange lines: MOZ2 cruise (Robin and

Figure 6 :

 6 Figure 6: Examples of the seismic facies encountered in the Zambezi turbidite system

Figure 7 :

 7 Figure 7: Marine feeding axes of the Zambezi turbidite system and continental sediment

Figure 8 :

 8 Figure 8: Seismic sections (with their location) illustrating the seismic facies, the regional

J

  o u r n a l P r e -p r o o f the Davie Ridge a significant contribution of contouritic sedimentation led to the deposition of thick contouritic drifts (S1a in Figs 6 and

Figure 11 :

 11 Figure 11: The Serpa Pinto Valley depositional succession. (A): Close up of seismic profile

Figure 12 :

 12 Figure 12: (A): Close-up of unit 2 showing the possible stacking of small coarse-grained

Figure 13 :

 13 Figure 13: (A): Detail of unit U3 that is characterized by S1a facies affected by sediment

14A

Figure 14 :

 14 Figure 14: Close up of unit U4 that characterizes the left (looking downstream) border of the

Figure 15 :

 15 Figure 15: The architecture of the Ponded Fan. (A) NW-SE seismic section with stratigraphy established by Ponte (2018) and (B) corresponding line

Figure 17 :

 17 Figure 17: The distal end of unit U5 (Ponded Fan). (A): Sub-bottom profile illustrating the

Figures 18 :

 18 Figures 18: Upstream-downstream evolution of the incisions in the Zambezi Valley upstream

Figure 19 :

 19 Figure 19: Upstream-downstream evolution of the incisions in the Zambezi Valley

Figure 20 :

 20 Figure 20: Composite thickness maps including the Zambezi turbidite system (this study)

  [START_REF] Miramontes | The influence of bottom currents on the Zambezi Valley morphology (Mozambique Channel, SW Indian Ocean): In situ current J o u r n a l P r e -p r o o f observations and hydrodynamic modelling[END_REF] that the present-day NADW partly flows inside the Zambezi Valley J o u r n a l P r e -p r o o f and contribute to the flank erosion and over-widening of the valley. This large contourite drift which developed on the southern bank of the upper Zambezi Valley, just where it bends from a NW-SE to N-S trend (pink dotted polygon in Fig.20C), is shifted to the SE with regard to the giant Oligocene drift(Fig 20A) and extends more to the SW. The Plio-Quaternary contouritic drift is associated to a sea floor relief (green polygon in Fig20C) that was formerly called Neogene drift by[START_REF] Raisson | Deep oceanic currents and sea floor interactions offshore SE Africa[END_REF]. Here again we can suppose that the drift benefited from the terrigenous supply of turbidity current overflows (by possible sediment pirating) whose origin is not clear (as it can come from turbidity currents from the Zambezi Valley and/or distal sediment upwelling to the north).On the left border of the Zambezi Valley and both sides of the Tsiribihina Valley fine-grained levee turbidites dominate (Fig.8A, 9B-C). The U4 deposit thus indicates that there is a synchronicity of different terrigenous input sources coming from the Mozambique and Madagascar margins. Moreover, the lenticular units Uf and Ug (present in the upper part of unit U4) attest to recurrent inputs from Madagascar that are not funneled into the Tsiribihina

Figure 21 :

 21 Figure 21: Effects of the Miocene doming in the central Mozambique Channel on the

Figure 22 :

 22 Figure 22: West-east schematic illustration of the timing of seismic unit deposition and main

  Fine-grained turbidites are observed at the left overbank of the Zambezi Fan, while mainly coarse-grained turbidites are present in the Intermediate Basin and Depositional Area. Contourites occur on the right bank of the Zambezi Valley as well as in the Angoche basin. Additionally, this period is characterized by synchronicity of different terrigenous input sources (Zambezi and Madagascar margin). The Zambezi Valley (incision Z2; Figs. 18, 19) is in a rather equilibrium state. It was lately infilled by a MTD, observed 800 km along the Zambezi Valley more downstream than the Tsiribihina confluence (Fig. 18, 19 profiles 1-11). Considering the maximum thickness of this MTD (0.2 stwt, ~200 m), the mean wideness of the Z2 incision (4 km), and a rectangular prism as an approximation of the shape of the MTD, a rough estimation of the volume of transported sediments can be calculated to 640 km 3 . Together with the MTDs in the distal Depositional Area, the occurrence of these mass transport deposits attests to the recurrence of instabilities in the Plio-Pleistocene period, also shown on the Mozambique slope by Ponte (2018).



  Stratigraphic correlations place the Serpa Pinto (and Angoche1) deposition (unit U1) during the Oligocene, the distal turbiditic deposition (unit U2) during Early Miocene, the Angoche2 and Zambezi1 incisions during Middle -Late Miocene and the Zambezi2 incision at the beginning of the Pliocene. Units U3 and U4 were deposited during the Plio-Quaternary.  Succession of valleys that fed the sedimentary units evidences an anticlockwise shift of feeding axes from the Serpa Pinto Valley to the Angoche Valley and finally towards the Zambezi Valley. This goes together with a shift of the drainage basins that fed the fan, from the northern most watersheds of the Mozambique (Lurio and possible other northern rivers) to the Angoche watershed (more south), then to the Zambezi watershed (central Mozambique).  The main incision event occurred during the Late Miocene, when the valley suffered the effects of a structural doming in the central part of the Mozambique Channel. The continuous elevation of the seafloor forced the profound entrenchment of the Zambezi1 incision (~400 m of erosion), which supposedly is cause of the observed absence of lateral levees proximal to the Zambezi Valley. Products of this strong erosional regime (estimated to 560 km 3 of reworked sediments) are supposed to be transported to the distal Depositional Area. Most turbiditic deposits in the studied area are either fine-grained levee turbidites (units U1 and U4) or coarse-grained channel-mouth turbidites (U2, U5 and Depositional Area).J o u r n a l P r e -p r o o f Mass-movement processes, recurrent on the Mozambique slope[START_REF] Ponte | La marge africaine du canal du Mozambique (le système turbiditique du Zambèze) : une approche « Source to Sink[END_REF] since the Pliocene have been observed mainly in the Zambezi Valley, in the distal depositional area, and on the Madagascar margin.  Contouritic sedimentation is at least continuous since the Oligocene. The drift sediments are supposed to be supplied by turbiditic processes (current overflow) and both (i.e. turbiditic and contouritic) depositional processes occur most often synchronous (i.e. U3 during U4 levee deposition, possibly during northern Serpa Pinto deposition and close to the Davie Ridge). Besides the two thickest Oligocene and Miocene drifts upstream on the margin, a prominent Plio-Quaternary N-S contourite drift (U3) is identified on the right flank of the Zambezi Valley where bottom current controlled bedforms are observed. This study has important implications for the current understanding of deep-marine turbidite systems. It demonstrates the sensitivity of large depositional systems to changing basin floor topography created by progressive structural deformation in terms of the architectural elements and the associated capacity to transfer sediments downstream.

Table 2 :

 2 Description and interpretation of the seismic facies encountered in the Zambezi

	Facies class	Facies characters	Sub-class Configuration	Location	Interpretation
					Parallel, or
				S1a	bi-directionally convergent	Right hand-side of the Zambezi Valley	Contourite drift
	STRATIFIED	S1 S2	High Co, Low to medium A, High F High Co, High A, High F	S1b S2	with undulations Uni-directionally convergent away from a valley or channel axis Parallel and onlaping (infilling)	Left hand-side of the Zambezi Valley and levees of distal channel-levees Distal Intermediate Basin	Fine-grained overspill turbidites alternating with very fine hemipelagic deposits Alternation of hemipelagic deposits and sheet-like turbiditic
							sediments
		S3	High Co, High A, Low F	S3a S3b	Parallel (infilling valleys/depressions) Parallel, wide extension	Zambezi Valley fill Right hand-side of the Zambezi Valley	Coarse-grained turbidites Coarse-grained turbidites
	CHAOTIC	C	Low Co, High A, Low F	Ca Cb	Basal unconformity, flat topped lens shape Contorted and divergent from valley walls	Madagascar margin and distal Depositional Area of the Zambezi Valley Zambezi Valley fill	Coarse-grained distal turbidites Slumped coarse-grained deposits
				Cc	Contorted	Eparses hills and sea-mounts	Carbonate deposits
	TRANSPARENT	T	Very low A and F	Ta Tb	Entirely transparent, isopach Some contorted reflections	Inside the Zambezi Valley Basin Mozambique slope, Zambezi Valley, distal Depositional Area and Intermediate	Mass-transport deposit Mass-transport deposit

turbidite system. Facies characters are described in term of the continuity (Co), amplitude (A) and frequency (F) of the reflections.

J o u r n a l P r e -p r o o f

ACKNOWLEDGEMENTS

The PhD thesis of Ruth Fierens was co-funded by TOTAL and IFREMER as part of the PAMELA (Passive Margin Exploration Laboratories) scientific project. The PAMELA project is a scientific project led by Ifremer and TOTAL in collaboration with Université de Bretagne Occidentale, Université Rennes 1, Université Pierre and Marie Curie, CNRS and IFP-EN.

We thank the Captains and crew members of the 2014 and 2015 cruises PTOLEMEE and PAMELA-MOZ2 onboard the R/V L'Atalante and PAMELA-MOZ4 onboard of R/V Pourquoi pas? as well as the technicians from Genavir that ensured acquisitions of the geophysical data.

The onboard scientific teams of PAMELA surveys are greatly thanked for their contribution to data acquisitions. Seismic data were analyzed using Kingdom Suite software, kindly made available to UBO by IHS in the framework of their Educational Grant Program. The authors are also grateful to Jacob A. Covault and David Hodgson for their critical reading and constructive J o u r n a l P r e -p r o o f discussions at a preliminary stage of this work. We greatly acknowledge the reviews from the Editor in Chief Istvan Csato and from an anonymous reviewer who greatly helped improving the paper.

Supplementary material A

Supplementary material A: Uninterpreted profiles of figure 9.  Five major depositional units are identified and demonstrate both turbiditic and contouritic deposits that occur most often synchronously.

 The Zambezi Fan is characterized by various episodes of incision that evidence multiple shifts of feeding axes since Oligocene.

 Progressive structural doming during Late Miocene caused a deep entrenchment of the Zambezi Valley.

J o u r n a l P r e -p r o o f

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: