Mozambique Basin Research Articles

Oceanic and atmospheric influences on the variability of phytoplankton bloom in the Southwestern Indian Ocean

The phytoplankton bloom developed in the southwestern Indian Ocean during austral summer is unique in its occurrence. Interannual and intraannual variability of this large phytoplankton bloom were studied using satellite derived, model and reanalysis data together with hydrographic observations. The study shows that the bloom is not confined to Madagascar basin alone, but also developed in the Mozambique Basin as well as in the southern Mozambique Channel. The strongest bloom event in the Madagascar and Mozambique basin since twelve years (1997–2008) occurred during January 2008. Intraannual variability of the phytoplankton bloom is linked to the upwelling along the south coast of Madagascar, precipitation along the east coast of Madagascar, light limitation and local mesoscale circulation features. Distribution pattern of the mesoscale eddies is found to play an important role in inducing the interannual variability of the bloom. The study reveals the dominance of the mesoscale eddies during the bloom events and the distinct association of chlorophyll maxima with either anti -cyclonic or cyclonic eddies. Flow pattern of the East Madagascar Current also shows interannual variability, which constrains the distribution of the bloom in different basins. ENSO is likely to have a direct and remote impact in inducing the interannual variability of the bloom.

Advanced insights into magmatism and volcanism of the Mozambique Ridge and Mozambique Basin in the view of new potential field data

SUMMARY A new plate tectonic model is presented which describes the emplacement of the Mozambique Ridge off southeast Africa as the result of long lasting volcanic activity (140–122 Ma) during the initial opening between Africa and Antarctica. Thus, an oceanic origin for the Mozambique Ridge is proposed. This model is based on a new and systematic high resolution magnetic anomaly data set acquired across the Mozambique Ridge and throughout the Mozambique Basin. Data from the Mozambique Basin allow the identification of Mesozoic magnetic anomalies from M0r to M26 (124.61–155.3 Ma) with previously unmatched accuracy. Small-scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin. Additionally, a bend in the major fracture zones ‘F’ and ‘E’ between M17r and M18n (142.84–144.04 Ma) and a recognized sinusoidal change in spreading direction with an amplitude of about 15° indicate that the basin experienced several small scale changes in spreading direction through time. A maximum change in spreading direction to almost 0° at around M11n (135.69 Ma) is followed by a short lived increase in spreading half rate from 23.5 km Ma−1 to about 27.5 km Ma−1 in the time frame from M10r to M9n (134.30–132.83 Ma). We propose that this is related to the initial opening of the South Atlantic Ocean represented by the onset of seafloor spreading between the Falkland Plateau and Africa in the conjugate Georgia and Natal basins. Across the Mozambique Ridge, high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times. A simple 2-D gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 Ma. Together with new and verified rotation parameters from the Mozambique Basin and its conjugate, the Riiser-Larsen Sea, Antarctica, a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time.

Modes of the southern extension of the East Madagascar Current

Data sets from satellite observations and a nested high‐resolution model are used to study a source region of the Agulhas Current. Altimeter‐derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long‐term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross‐correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1‐month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21°S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55°E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC.

Late Jurassic and Cretaceous palynostratigraphy of the onshore Rovuma Basin, Northern Mozambique

Late Jurassic and Cretaceous palynomorph assemblages were recovered from the Macomia, Pemba, and Mifume formations from the onshore Rovuma Basin, northern Mozambique. These assemblages provide new evidence for an Aptian‐Albian age for the Macomia Formation and the upper member of Pemba Formation, and confirm that these two stratigraphic units are coeval and laterally continuous. The lower member of the Pemba Formation contains Kimmeridgian‐Tithonian palynomorphs, thus documenting for the first time the existence of Upper Jurassic strata north of Nacala in the onshore Rovuma Basin. The rich and diverse dinoflagellate cyst assemblage recovered from the Mifume Formation is of late Campanian age. This indicates an early Late Cretaceous hiatus between the sandstones of Pemba Formation, and the marls of the overlying Mifume Formation. The hiatus corresponds to a break in sedimentation prior to a rapid, global marine transgression, recognised in the Rovuma and Mozambique basins, during the middle Campanian, associated with the onset of Gondwana fragmentation. Earlier, Late Jurassic faulting produced local half‐grabens filled with continental debris, for example the N'Gapa Formation of northern Mozambique.

A model of plate kinematics in Gondwana breakup

SUMMARY An accurate model of relative plate motions in Gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones (FZ) from the Riiser-Larsen Sea and Mozambique Basin. Used predictively, the model precisely locates kinematic markers in the West Somali Basin, which allows the conclusion that the spreading centres in the West Somali and Mozambique basins and the Riiser-Larsen Sea formed parts of the boundary between the same two plates. The locations of FZ and less well-defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the West Somali Basin and northern Natal Valley to the West Enderby Basin and Lazarev Sea during chron M10n. Small independently moving plates thus played no role in the breakup of this core part of Gondwana. In an inversion procedure, the data from these areas yield more precise finite rotations that describe the history of the two plates’ separation. Breakup is most simply interpreted to have occurred in coincidence with Karoo volcanism, and a reconstruction based on the rotations shows the Lebombo and Mateke-Sabi monoclines and the Mozambique and Astrid ridges as two sets of conjugate volcanic margins. Madagascar’s pre-drift position can be used as a constraint to reassess the positions of India and Sri Lanka in the supercontinent.

LATE JURASSIC AND CRETACEOUS PALYNOSTRATIGRAPHY OF THE ONSHORE ROVUMA BASIN, NORTHERN MOZAMBIQUE

Abstract Late Jurassic and Cretaceous palynomorph assemblages were recovered from the Macomia, Pemba, and Mifume formations from the onshore Rovuma Basin, northern Mozambique. These assemblages provide new evidence for an Aptian–Albian age for the Macomia Formation and the upper member of Pemba Formation, and confirm that these two stratigraphic units are coeval and laterally continuous. The lower member of the Pemba Formation contains Kimmeridgian–Tithonian palynomorphs, thus documenting for the first time the existence of Upper Jurassic strata north of Nacala in the onshore Rovuma Basin. The rich and diverse dinoflagellate cyst assemblage recovered from the Mifume Formation is of late Campanian age. This indicates an early Late Cretaceous hiatus between the sandstones of Pemba Formation, and the marls of the overlying Mifume Formation. The hiatus corresponds to a break in sedimentation prior to a rapid, global marine transgression, recognised in the Rovuma and Mozambique basins, during the middle Campan...

The effect of environmental controls on the metal content in ferromanganese crusts and nodules from the Mozambique Ridge and in the Mozambique Basin, southwestern Indian Ocean

Major and minor element compositions of a suite of ferromanganese nodules and crusts dredged from the Mozambique Ridge on the RV Sonne cruise in 2005 are combined with previously published data from both the Mozambique Ridge and adjacent Mozambique Basin, in order to examine the relationship between regional variations in chemical composition and environmental controls on ferromanganese mineralisation. Three principal environmental controls on ferromanganese nodule and crust composition have previously been recognised: primary productivity in the surface waters, the oxygen minimum layer, and the calcium carbonate compensation depth. Of these three, the oxygen minimum layer is identified as having the greatest controlling influence on the composition of the ferromanganese deposits of the Mozambique Ridge and Basin, with primary productivity and the calcium carbonate compensation depth being secondary in importance. As a result, three different mineralisation zones can be defined in the study area: deepwater basin deposits, ridge flank deposits formed below the base of the oxygen minimum layer, and ridge crest deposits formed within the oxygen minimum layer. Nodules and crusts from these different zones display distinct variations in chemistry and mineralisation style.

Tectonic types of deepwater basins in the Indian Ocean

Among 16 deepwater basins located in the central Indian Ocean and along its western, eastern, and southern margins, the central, perioceanic, and perispreading tectonic types are recognized. The Central, Cocos, Wharton, and Crozet basins belong to the first type. The second type comprises the Somalia, Mascarene, Madagascar, Mozambique, and Agulhas basins localized along the western margin of the ocean; the Argo, Gascoyne, Cuvier, and Perth basins that are situated along its eastern periphery; and the African-Antarctic Basin in the southern periphery. The South Australian and Australian-Antarctic basins pertain to the third type. Spatially and tectonically, the pericontinental basins are conjugated with continental blocks in the ocean (rises, plateaus, microcontinents). Together, they make up specific tectonic systems that extend parallel to the continents. The formation of such systems is controlled by horizontal movement of continental blocks and tectonic subsidence of the oceanic bottom.

The Mesozoic breakup of the Weddell Sea

A new set of rotations is presented that describes a refined model for the early opening of the Weddell Sea between South America and Antarctica and the Mesozoic breakup of Gondwana. Published high‐resolution aeromagnetic data from the eastern Weddell Sea and additional track data farther west in the Weddell Sea were used to constrain the new model for the opening of the Weddell Sea. Rotation parameters derived for the South America–Antarctica spreading regime were combined with constraints on the South America–Africa and Africa‐Antarctica spreading systems to calculate a refined model for the Mesozoic breakup of Gondwana. Thereafter, at the time when the north‐south oriented separation between Africa and Antarctica is initiated by rifting in the Somali and Mozambique basins (∼167 Ma), stretching and extension takes place in a basin comprising continental crust of the Filchner‐Ronne Shelf, the Falkland Island block and the Maurice Ewing Bank. The first true ocean floor in the Weddell Sea is formed at about 147 Ma, after rifting between the Antarctic Peninsula and southernmost South America occurred. This is about 15–20 Myr later than previously estimated. Separation between South America and Antarctica takes place at slow spreading rates (14–12 mm/yr half rate) from 147 to 122 Ma and after 122 Ma (M2) at ultraslow spreading rates (∼8 mm/yr half rate) with little change in the NNW spreading direction throughout this time. A revised age range is proposed for the formation of the Explora Wedge (150–138 Ma), which is more than 30 Myr later than previously published (∼183 Ma).

Motion of Nubia relative to Antarctica since 11 Ma: Implications for Nubia-Somalia, Pacific–North America, and India-Eurasia motion

Recent estimates of the rotation between Nubia and Somalia have resulted in disparate poles of rotation for the motion since 3.16 Ma (southwest of South Africa) compared with that since 11.03 Ma (near the east tip of Brazil). Here we use magnetic anomaly profiles unavailable in prior Nubia-Antarctica motion studies to significantly revise the estimate of the rotation between Nubia and Antarctica since 11.03 Ma. We use this newly estimated rotation to construct revised estimates of Nubia-Somalia, Pacific–North America, and India-Eurasia motion. The new Nubia-Somalia rotation indicates substantial displacement of Somalia relative to Nubia over the past 11.03 m.y.: 129 ± 62 km extensional, 90 ± 42 km right-lateral transtensional, and 52 ± 21 km right-lateral transtensional near the northern extremity of the East African Rift, the northern Mozambique Basin, and the Andrew Bain Fracture Zone complex, respectively. The substantial rotation between Nubia and Somalia implies that prior plate motion estimates based on a circuit through Africa are biased by 60–85 km at 11.03 Ma and perhaps by much more for earlier reconstructions. Our results imply that India-Eurasia motion since 11.03 Ma has been ∼12% (∼5 mm yr−1) slower than, and ∼20° clockwise of, estimates that neglect Nubia-Somalia motion. Our results further imply that Pacific–North America displacement since 11.03 Ma has been 5°–10° clockwise of prior estimates and require 58–75 km less extensional displacement across the Basin and Range since 11.03 Ma than inferred before.

Eddies and dipoles around South Madagascar: formation, pathways and large-scale impact

During a hydrographic cruise in March 2001, we encountered a dipole-like vortex structure directly southwest of Madagascar. The cruise formed part of the Dutch-South African Agulhas Current Sources Experiment (ACSEX). Direct current observations with a lowered ADCP showed that the multipole involved a deep-reaching central jet, with over 20 cm/s speeds still at 2000 m depth, with two contra-rotating eddies on either side. Both eddies had a size of about 250 km. A subsequent analysis of the combined TOPEX-POSEIDON/ERS altimeter dataset revealed the regular formation of dipoles around south Madagascar, where the East Madagascar Current (EMC) separates from the shelf. Between April 1995 and June 2000, 16 such features could be identified, six of which were formed between August 1996 and May 1997 and six between May 1999 and June 2000. These periods of enhanced dipole formation coincided with the negative phases of the Indian Ocean Dipole and El Niño cycles, suggesting a connection between these climate modes and the interannual variability in the periods of dipole train formation. The temperature–salinity characteristics of the eddies from the ACSEX dipole were identical to those measured in the EMC during the same cruise. The cyclonic eddy appeared to have drawn its waters from the inshore side of the EMC, suggesting it was formed as a lee-eddy on the cyclonic flank of the separating jet. The data indicate that the eddies carry their water along. At the observed scales, one dipole per year would feed 8 Sv (1 Sv=10 6 m 3/s) of EMC-derived water into the Agulhas retroflection region. In the Mozambique Basin, the eddy pairs behaved quite irregularly, with many of them splitting and interacting with other eddies in the region. The anticyclones then propagated mostly westward, while the cyclone tracks diverged more between west and southwest and were influenced by the interaction with the Mozambique Plateau. A very regular train of dipoles started in December 1999 and continued through 2000. They were spawned roughly at 2-month intervals, stayed remarkably coherent and triggered an unusually early retroflection of the Agulhas Current in late 2000. Thus, interannual variability originating in the equatorial climate modes of the Indian Ocean seems to result in variability of the Agulhas retroflection and associated ring shedding.

A near-synoptic survey of the Southwest Indian Ocean

Abstract This study focused on the southwest region of the Indian Ocean, where the poleward-directed Agulhas Current is born, and where dense waters filter through fractures in the Southwest Indian Ridge to form an equatorward-directed deep boundary current east of Madagascar. Both represent major circulation features of the Indian Ocean: the Agulhas in its role as a western-boundary current closes the wind-driven subtropical gyre; the deep western-boundary current renews the bottom waters of the Madagascar, Mascarene, and Somali basins to the north. A regional, quasi-synoptic survey of the Southwest Indian Ocean carried out as part of the World Ocean Circulation Experiment Hydrographic Program from May through July of 1995 occupied a cruise track that comprised a closed “box” in the Southwest Indian Ocean. Full-depth water properties and direct-velocity measurements were used to diagnose the circulation patterns as a function of depth and to estimate the transports of the major currents. The synoptic circulation was quantified by the construction of a referenced geostrophic velocity field. Water-mass distributions, direct-velocity measurements, and mass conservation within bounded regions guided the placement of the level of no motion. Errors in the reference scheme and from synoptic-scale circulation features such as eddies and internal waves that are aliased by the hydrographic sampling led to uncertainties in the transport estimates. The upper-ocean Agulhas transport (neutral density, γ n −3 , depths≲2000 m) was estimated to be 76×10 6 m 3 s −1 . Contributions to the Agulhas consisted of 29×10 6 m 3 s −1 from the westward limb of the subtropical gyre south of 25°S, 20×10 6 m 3 s −1 from the poleward flow east of Madagascar, which subsequently turns to move west at about 25°S, and an additional poleward flow of 18×10 6 m 3 s −1 through the Mozambique Channel. Bathymetry strongly controls the deep and bottom circulation: the African Coast, the Mozambique Plateau, and the Madagascar Ridge support deep western-boundary currents along their eastern margins; however, the Natal Valley and the Mozambique Basin are blocked to meridional flow beneath 2800 and 3000 m, respectively. East of the Madagascar Ridge, net northward deep transport is small. In the Madagascar and Mascarene Basins, a deep cyclonic circulation is present: high-latitude deep waters move northward along the western boundary, while North Indian Deep Water flows southward in the basin interior. The deep western-boundary current along the east coast of Madagascar carries about 3×10 6 m 3 s −1 of bottom water ( γ n >28.11 kg m −3 ) northward.

Pore microgeometry analysis in low-resistivity sandstone reservoirs : Cerepi, A. et al. Journal of Petroleum Science and Engineering, 2002, 35, (3–4), 205–232

The water saturation of a hydrocarbon bearing formation is usually calculated via the conductivity of the formation water, as defined by Archie, 1942. However, in some formations such conventionally calculated water saturation contradicts values calculated from the Nuclear Magnetic Resonance (NMR) tool and from the Drill Stem Test (DST). Here we discuss such a case of low resistivity pay zones in the J- and K-Reservoirs, of the Lower Grudja Formation of Maastrichtian age, onshore Mozambique basin, in which conventional log analysis contradicts water saturation calculated from the NMR and by the Drill Stem Test.For both, the J-Reservoir and K-Reservoir, the water saturation (Sw) was calculated using the Archie equation and the Simandoux equation, that accounts for clay bound water but not for silt, and the results were compared to NMR-calculated water saturation. A new algorithm that improves the accuracy of water saturation calculation was introduced. This new algorithm takes into account the clay- and silt-bound water within the overall reservoir resistivity. It is demonstrated that the clay- and silt-bound water must be included in water saturation calculations, where the formations are rich in silt and clay. The Simandoux equation and the new algorithm yield significant improvement in water saturation results for the J- and K-Reservoirs, when compared to water saturation calculated using the Archie's equation.

The tracer signature of Antarctic Bottom Water and its spread in the Southwest Indian Ocean: Part I—CFC-derived translation rate and topographic control around the Southwest Indian Ridge and the Conrad Rise

In this paper we use a temperature and salinity based mixing model to assess the dilution of Antarctic Bottom Water (AABW) as it moves away from the Weddell Sea and into the Southwest Indian Ocean. By combining these results with CFC tracer measurements we have been able to make direct estimates of the large-scale translation rates of AABW in this region. We confirm that there is a major northward flow of AABW via a gap in the Southwest Indian Ridge at 30°E, and thence across the Agulhas Basin into the Mozambique Basin, with a translation rate from the Greenwich Meridian of 0.8–1.0 cm s −1 and a volume transport between the two basins of 1.5×10 6 m 3 s −1. A second, smaller flow cuts the Del Cano Rise through the Prince Edward Fracture Zone but is indistinguishable from the general bottom waters once on the northern side of the rise. The third flow moves eastward along the southern flank of the Del Cano Rise to pass north of the Conrad Rise. This has bottom velocities of 0.7 cm s −1 and a volume transport of 1.6×10 6 m 3 s −1. This water is probably the source of the AABW-rich Circumpolar Deep Water that flows through the gap to the west of Crozet Island, and which is traceable again at stations on the northern flanks of the ridge. Flow between the Conrad Rise and the Del Cano Rise is complicated by the influence of a fourth flow, the AABW that passes south of the former and thence into the Crozet Basin via the Crozet-Kerguelen Gap. We suggest that a portion of this flow loops into the channel between the Del Cano Rise and the Conrad Rise, modifying the bottom waters at the easternmost stations within this channel. We will go on in Part 2 of this paper to use these results to estimate the dissolution rates of silica in the SWINDEX area.

An Early Cretaceous extinct spreading center in the northern Natal valley

We have identified an extinct E–W spreading center in the northern Natal valley on the basis of magnetic anomalies which was active from chron M11 (∼133 Ma) to ∼125.3 Ma, just before chron M2 (∼124 Ma) in the Early Cretaceous. Seafloor spreading in the northern Natal valley accounts for approximately 170 km of north–south motion between the Mozambique Ridge and Africa. This extension resolves the predicted overlap of the continental (central and southern) Mozambique Ridge and Antarctica in the chron M2 to M11 reconstructions from Mesozoic finite rotation parameters for Africa and Antarctica. In addition, the magnetic data reveal that the Mozambique Ridge was an independent microplate from at least 133 to 125 Ma. The northern Natal valley extinct spreading center connects to the spreading center separating the Mozambique Basin and the Riiser-Larsen Sea to the east. It follows that the northern Mozambique Ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center. To the west the extinct spreading center connects to the spreading center separating the southern Natal valley and Georgia Basin via a transform fault. Prior to chron M11, there is still a problem with the overlap of Mozambique Ridge if it is assumed to be fixed with respect to either the African or Antarctic plates. Some of the overlap can be accounted for by Jurassic deformation of the Mozambique Ridge, Mozambique Basin, and Dronning Maud land. It appears though that the Mozambique Ridge was an independent microplate from the breakup of Gondwana, ∼160 Ma, until it became part of the African plate, ∼125 Ma.

Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge

Circulation pathways and transports of the Antarctic Circumpolar Current (ACC) and Agulhas Return Current (ARC) around the complex bathymetry of the Southwest Indian Ridge and Del Caño Rise have been established from two hydrographic surveys and two‐year long moorings. After crossing the Southwest Indian Ridge in a 110 Sv (1 Sv = 106 m3s−1) flow concentrated at 48°S, the ACC fragments before being reconcentrated into several branches by the bathymetry. In particular 30–40 Sv turns north around the eastern flank of the Del Caño Rise, turns west as part of anticyclonic flow round the Del Caño Rise finally returning eastward as part of the Crozet Front. Eddy variability from both satellite data and moorings is low over the bathymetry of the Del Caño Rise, so the strong currents found between the Del Caño Rise and the Crozet Plateau are believed to be permanent features. This largely barotropic, time‐independent transport is the major pathway for the ACC to enter the Crozet Basin. The anticyclonic flow also transfers water from the Mozambique Basin across the Southwest Indian Ridge to the south side of the Del Caño Rise. Given the evidence for fragmentation of the ACC where it crosses the Southwest Indian Ridge and the concentration of several jets into the Crozet Front from the ACC, Subtropical Front and Agulhas Return Current, it is misleading to relate standard frontal definitions such as the Polar Front and Subantarctic Front to the major transport pathways.

Deep inflow into the Mozambique Basin

More than 200 conductivity‐temperature‐depth (CTD) stations were worked around the Southwest Indian Ridge and Del Caño Rise as part of the World Ocean Circulation Experiment. A selection of these data provides information about the inflow of bottom water into the Mozambique Basin. The basin is closed below 3000 m, yet the inflow is significantly large, of order 1 Sv (1 Sv = 106 m3 s−1). Estimates of the basin‐scale upwelling at 4000 m suggest that the vertical velocity is also large, 10 × 10−5 cm s−1 or more, an order of magnitude greater than global ocean estimates. Examination of the characteristics of the bottom water in the Mozambique and Agulhas Basins and the Prince Edward Fracture Zone shows that bottom water enters the Mozambique Basin from the Agulhas Basin and also directly from the Enderby Basin. Most of the transport enters the Mozambique Basin via the Agulhas Basin, where two regions of northward flow below 4000 m are found. The major flow, on the eastern flank of the Mozambique Ridge, is through and above the deep, extending (5900 m) trench that connects the Agulhas and Mozambique Basins. The second, weaker flow enters the Transkei Basin along the deep eastern flank of the Agulhas Plateau, then turning east into the Mozambique Basin. The only source of bottom water to the Agulhas Basin is the Enderby Basin, but a more direct route between the Enderby and Mozambique Basins exists via the Prince Edward fracture, which extends deeper than 4000 m throughout its length and links the two basins directly across the Southwest Indian Ridge. Full depth CTD stations trace the changing characteristics of the deep and bottom water in the fracture, and moored current meter data show the strength and persistence of the throughflow. Strong mixing with the overlying deep water elevates the salt content of the bottom water by comparison with the other water in the Mozambique Basin. Thus two distinct bottom waters of the Mozambique Basin originate in the same place (the Enderby Basin), and their different characteristics are solely a function of the routes they have taken and the processes encountered along the different pathways.

Mechanisms of isostatic compensation of the Zimbabwe and Kaapvaal cratons, the Limpopo Belt and the Mozambique basin

SUMMARY The Bouguer gravity anomaly map of the Zimbabwe and Kaapvaal cratons, the Limpopo Belt and the Mozambique basin shows a regional gravity high in the southeast. Superimposed on this gravity high is a line of elongate gravity highs that is coincident with the Lebombo-Nuanetsi-Sabi volcanics. Gwavava et al. ( 1992) argue that the regional anomaly over this region is the effect of Mesozoic crustal thinning and igneous activity during the breakup of Gondwana. Lithospheric extension resulted in crustal thinning by at least 4 km beneath the central Limpopo Belt. 2-D Fourier transforms of Bouguer gravity and topography data have been used to obtain the admittance and coherence throughout the region. Assuming a regionally compensated isostatic model comprising two layers with statistically independent surface and subsurface loading, the predicted coherence was compared to the observed coherence to obtain the best estimate of the effective elastic thickness of the lithospheric plate. This coherence analysis reveals that the Archaean cratons have a minimum effective elastic thickness of about 56 km, whereas that of the area beneath the Mozambique basin is only about 22 km, a value similar to that obtained in other East African rift basins. Hence, the stable cratonic areas are more rigid than the area beneath the Mozambique basin, which was subjected to Mesozoic crustal extension and thinning. An isostatic anomaly map has been computed by filtering the Bouguer gravity in the wavenumber domain with the square root of the predicted coherence function of the whole area generated from the best-fitting two-layer model with an effective elastic thickness of 52 km. The isostatic anomaly map is characterized by (1) a zone of positive isostatic anomalies over the Lebombo-Nuanetsi-Sabi area, which are partly due to crustal thinning and partly to basic plutons emplaced in the upper crust, both resulting from an extensional episode associated with Gondwana breakup, (2) positive isostatic anomalies over the Southern Marginal Zone of the Limpopo Belt, which we interpret as being due to dense granulites within the upper crust, (3) negative isostatic anomalies over the Northern Marginal Zone of the Limpopo Belt, which may have resulted from recent rapid erosion, and (4) negative isostatic anomalies over the Mozambique basin, which are due to the gravity effect of low-density Cretaceous and Tertiary sediments.

Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique

Two major sedimentary basins have been identified on the continental margin of Mozambique—Mozambique Basin and Ruvuma Basin. The formation of the basins is related to the break-up of Gondwana and opening of the western part of the Indian Ocean. The basins are relatively young, having developed discordantly to the structural plan of Gondwana sedimentary basins. The history of the formation of the East African continental margin sedimentary basins within Mozambique has been studied on the basis of the present-day concept of Gondwana break-up and Madagascar's drift with respect to Africa. Two stages in the history of the East African basins can be recognized: late-Gondwana and post-Gondwana. The late-Gondwana stage (303-157 Ma) is typified by sedimentary and volcanogenic rocks of the Karoo Group. The post-Gondwana stage (157-0 Ma) corresponds to the period of active Gondwana break-up and the formation of Indian Ocean marginal basins. The Mozambique Basin occupies both central and southern parts of the coastal plain of Mozambique, extending onto the continental shelf and slope. The sedimentary fill is composed of Upper Jurassic, Cretaceous and Cenozoic rocks which discordantly overly the Karoo basalts. The Upper Jurassic occurs as continental red-beds, mostly distributed within buried grabens. Cretaceous rocks occur as terrigenous sediments of continental and marine genesis. Cenozoic deposits are of predominantly marine and deltaic origin. The Ruvuma Basin, situated in the north of Mozambique, is part of an extended East African marginal basin which includes parts of the coastal plains and continental margins of Tanzania and Kenya. The basement of the basin is composed of crystalline and metamorphic rocks of pre-Cambrian age. The sedimentary fill is represented by continental terrigenous Karoo sediments, marine and lagoonal Jurassic deposits, and marine and deltaic rocks of Cretaceous and Cenozoic age. The main difference between the sedimentary fill of the Mozambique and Ruvuma basins is related to the changing sedimentary environment in the period of Gondwana break-up, Jurassic and Early Cretaceous, when the drift of Madagascar and sea-floor spreading took place and the marine basin gradually extended southwards between the East African margin and Madagascar. From the Late Cretaceous on, the sedimentary environment within both basins was similar, the Mozambique and Ruvuma basins forming a single system on the East African continental margin. The main stages of the basins' development are illustrated by a series of palaeogeographic reconstruction maps.

A hydrographic section across the subtropical South Indian Ocean

Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O 2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 10 6 m 3 s −1. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel. Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 10 6 m 3 s −1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 10 6 m 3 s −1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 10 6 mm 3 s −1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 10 6 m 3 s −1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10 −5 cm s −1. This estimate, perhaps uncertain by 20–30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.