Geochemical Correlation Between Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province, South Africa and the Ahlmannryggen Group of the Grunehogna Province, Antarctica

The whole-rock geochemistry of metamorphosed greywackes, arenites and arkoses within the Mesoproterozoic Namaqua-Natal-Maudheim Province is interpreted with the aim of establishing geochemical correlations and defining common sediment source terrains. Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province were sampled from their type area in the Mfongosi Valley. Metagreywackes from the northern limits of the Mfongosi Valley, directly adjacent to the Kaapvaal Craton, show ocean island arc signatures while metagreywackes from the southern limits of the Mfongosi Valley, near the contact with the Madidima Thrust of the Natal nappe zone, show mainly active continental margin signatures. Interleaved, geochemically distinct low-Ca+Na, high-K metamorphosed arkoses to lithic arkoses indicate a minor passive margin sediment component. Geochemical classification of low-grade Ahlmannryggen Group greywackes, arenites and arkoses of the Grunehogna Province, Antarctica, indicates both active and passive continental margin sediment sources. An oceanic island arc signature is not evident in Ahlmannryggen Group data. The active continental margin signature in both Natal Sector and Grunehogna Province metasediments potentially provides for a common link between these terranes. Discriminant Function Analysis, using three pre-defined provenance sub-sets within the Mfongosi Group and two pre-defined provenance sub-sets within the Ahlmannryggen Group, indicate that metasediments with active continental margin signatures from both groups are geochemically identical, implying that the active continental margin of the Grunehogna Province shed immature sediments westwards (African azimuths) into the developing, narrow or restricted Mesoproterozoic ‘Mfongosi Basin.’ This was accompanied by minor sediment influx from a stable continental platform, potentially the Kaapvaal Craton. Oblique and diachronous collision, initiated in the southwestern portions of the combined Natal Sector/Grunehogna Province system produced a laterally variable Mfongosi Group, which formed in the ‘Mfongosi Basin’. Coarse-grained sediments dominated in its eastern portions while basalts with thin sapropelite units dominated in its western portions.     

Distinct brief major events in the Karoo large igneous province clarified by new Ar/Ar ages on the Lesotho basalts

Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new ⁴⁰Ar/³⁹Ar age data, Geology 33, 745–748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the ⁴⁰Ar/³⁹Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2σ). We derived an apparent maximum duration for this event of  0.8 Ma by neglecting correlated errors embedded in the age uncertainties.

A critical review of previous ages obtained on the Lesotho sequence [Duncan R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127–18138] shows that groundmass analyses are unreliable for high-resolution geochronology, due to alteration and ³⁹Ar recoil effects. Discrepancy between our ages and a previous plagioclase age at  184 Ma obtained by the later workers is tentatively attributed to the heterogeneity of the monitor used and/or cryptic excess ⁴⁰Ar. The current age database suggests that at least three temporally and spatially distinct brief major events (the Lesotho and southern Botswana lava piles and the Okavango dyke swarm) are so far recognized in the Karoo province. Identification of brief and volumetrically important Karoo magmatic events allows detecting the migration of the Karoo magmatism and potentially the stress regime that affected the southern African lithosphere at this time. A filtered compilation of 60 ages obtained with homogeneous intercalibrated standards suggests a shorter duration for the main pulses of the magmatism between 3 and 4.5 Ma, compared to a whole province duration of  10 Ma, between  182 and  172 Ma.     

A new pathway for Deep water exchange between the Natal Valley and Mozambique Basin?

Although global thermohaline circulation pathways are fairly well known, the same cannot be said for local circulation pathways. Within the southwest Indian Ocean specifically there is little consensus regarding the finer point of thermohaline circulation. We present recently collected multibeam bathymetry and PARASOUND data from the northern Natal Valley and Mozambique Ridge, southwest Indian Ocean. These data show the Ariel Graben, a prominent feature in this region, creates a deep saddle across the Mozambique Ridge at ca. 28°S connecting the northern Natal Valley with the Mozambique Basin. Results show a west to east change in bathymetric and echo character across the northern flank of the Ariel Graben. Whereby eroded plastered sediment drifts in the west give way to aggrading plastered sediment drift in the midgraben, terminating in a field of seafloor undulations in the east. In contrast, the southern flank of the Ariel Graben exhibits an overall rugged character with sediments ponding in bathymetric depressions in between rugged sub/outcrop. It is postulated that this change in sea-floor character is the manifestation of deep water flow through the Ariel Graben. Current flow stripping, due to increased curvature of the graben axis, results in preferential deposition of suspended load in an area of limited accommodation space consequently developing an over-steepened plastered drift. These deposited sediments overcome the necessary shear stresses, resulting in soft sediment deformation in the form of down-slope growth faulting (creep) and generation of undulating sea-floor morphology. Contrary to previous views, our works suggests that water flows from west to east across the Mozambique Ridge via the Ariel Graben.     

An archeomagnetic analysis of burnt grain bin floors from ca. 1200 to 1250 AD Iron-Age South Africa

Recognition of the rapid decay of Earth’s magnetic field over the last 150 years, chronicled in magnetic observatory and satellite data, highlights the need for a higher resolution record of geomagnetic field behavior over the past millennium. Such a record would help us better understand the nature of the recent dramatic changes. A limitation of the existing database is undersampling of the Southern Hemisphere. Here we investigate the potential of obtaining archeomagnetic data from Iron-Age burnt grain bins from southern Africa. These structures preserve oriented material that can record both paleodirections and paleointensity information. Directional data collected from three sites (ca. 1200–1250 AD) fall 9–22° to the East of predictions. Thellier–Coe and Shaw paleointensity results differ from model values by ～15%. The consistency of results between the three sites suggests that further investigations of these materials with different ages could markedly improve the current spatial distribution of the archeomagnetic database.     

Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel

Sediment delivery to the abyssal regions of the oceans is an integral process in the source to sink cycle of material derived from adjacent continents and islands. The Zambezi River, the largest in southern Africa, delivers vast amounts of material to the inner continental shelf of central Mozambique. The aim of this contribution is to better constrain sediment transport pathways to the abyssal plains using the latest, regional, high-resolution multibeam bathymetry data available, taking into account the effects of bottom water circulation, antecedent basin morphology and sea-level change. Results show that sediment transport and delivery to the abyssal plains is partitioned into three distinct domains; southern, central and northern. Sediment partitioning is primarily controlled by changes in continental shelf and shelf-break morphology under the influence of a clockwise rotating shelf circulation system. However, changes in sea-level have an overarching control on sediment delivery to particular domains. During highstand conditions, such as today, limited sediment delivery to the submarine Zambezi Valley and Channel is proposed, with increased sediment delivery to the deepwater basin being envisaged during regression and lowstand conditions. However, there is a pronounced along-strike variation in sediment transport during the sea-level cycle due to changes in the width, depth and orientation of the shelf. This combination of features outlines a sequence stratigraphic concept not generally considered in the strike-aligned shelf-slope-abyssal continuum.     

New age constraints for a short pulse in Ross orogen deformation triggered by East–West Gondwana suturing

Constraints on the timing of deformation within the Antarctic Ross orogenic belt are important for understanding early Paleozoic tectonic activity accompanying the assembly of Gondwana. One of the best areas for constraining the tectonic evolution of the Ross orogenic belt is the Holyoake Range in the central Transantarctic Mountains where previous work shows an abrupt change in the stratigraphic succession of the Cambrian Byrd Group possibly related to the inception of tectonism within the orogen at  515 Ma. To further constrain deformational timing, we conducted ⁴⁰Ar–³⁹Ar analyses on magmatic phases of the Holyoake Gabbro, which cross-cuts folded Lower Cambrian Shackleton Limestone of the Byrd Group. Our analyses yield average ages of 506.7 ± 3.6 Ma (2 sigma; MSWD = 0.9) and 504.1 ± 2.4 Ma (2 sigma; MSWD = 1.3) for hornblende from the early and late magmatic phases, respectively. Deformation of the Shackleton Limestone therefore occurred prior to 506.7 ± 3.6 Ma, which is  12 m.y. after the siliciclastic drowning of the carbonate platform inferred to record the start of Ross tectonism in the central Transantarctic Mountains. On a regional scale, the data are consistent with a short pulse of deformation, which was probably related to global plate motion changes following final suturing of East and West Gondwana.     

Seafloor morphology in the Mozambique Channel: evidence for long-term persistent bottom-current flow and deep-reaching eddy activity

The Mozambique Channel plays a key role in the exchange of surface water masses between the Indian and Atlantic Oceans and forms a topographic barrier for meridional deep and bottom water circulation due to its northward shoaling water depths. New high-resolution bathymetry and sub-bottom profiler data show that due to these topographic constraints a peculiar seafloor morphology has evolved, which exhibits a large variety of current-controlled bedforms. The most spectacular bedforms are giant erosional scours in the southwest, where northward spreading Antarctic Bottom Water is topographically blocked to the north and deflected to the east forming furrows, channels and steep sediment waves along its flow path. Farther north, in the water depth range of North Atlantic Deep Water, the seafloor is strongly shaped by deep-reaching eddies. Steep, upslope migrating sediment waves in the west have formed beneath the southward flow of anticyclonic Mozambique Channel eddies (MCEs). Arcuate bedforms in the middle evolved through an interaction of the northward flow of MCEs with crevasse splays from a breach in the western Zambezi Channel levee. Hummocky bedforms in the east result from an interplay of East Madagascar Current eddies with overspill deposits of the crevasse and Zambezi Channel. All bedforms are draped with sediments indicating that the present-day current velocities are not strong enough to erode sediments. Hence, it can be concluded that the seafloor morphology developed during earlier times, when bottom-current velocities were stronger. Assuming a sedimentation rate of 20 m/Ma and a drape of at least 50 m thickness the bedforms may have developed during the Pliocene Epoch or earlier.