The structure of the crust, the uppermost mantle, and the mantle transition zone beneath Madagascar

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. October 2017.Since the arc assembly and continental collision of the East African Orogen some 640 million years ago, Madagascar has gone through several geodynamic and tectonic episodes that have formed and subsequently modified its lithosphere. This thesis aims to investigate the structure of the crust, the uppermost mantle, and the mantle transition zone beneath Madagascar to gain insights into the relationship between present-day lithosphere structure and tectonic evolution, and to evaluate candidate models for the origin of the Cenozoic intraplate volcanism. To address these issues, local, regional, and teleseismic events recorded by several temporary seismic networks; the MAdagascar-COmoros-MOzambique (MACOMO), the SEismological signatures in the Lithosphere/Asthenosphere system of SOuthern MAdagascar (SELASOMA), and the Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel (RHUM-RUM) were used to complement the seismic events recorded by the permanent seismic stations in Madagascar. The different methods used and the primary results of this study are explained in each section of this thesis. In the first part of this thesis, crustal and uppermost mantle structure beneath Madagascar was studied by analyzing receiver functions using an H-κ stacking technique and a joint inversion with Rayleigh-wave phase-velocity measurements. Results reflect the eastward and northward progressive development of the western sedimentary basins of Madagascar. The thickness of the Malagasy crust ranges between 18 km and 46 km. The thinnest crust (18-36 km thick) is located beneath the western basins and it is due to the Mesozoic rifting of Madagascar from eastern Africa. The slight thinning of the crust (31-36 km thick) along the east coast may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including thickness, Poisson’s ratio, average shear-wave velocity, thickness of mafic lower crust, show little evidence of secular variation. Slow shear-wave velocity of the uppermost mantle (4.2-4.3 km/s) are observed beneath the northern tip, central part and southwestern region of the island, which encompass major Cenozoic volcanic provinces in Madagascar. The second part of the thesis describes a seismic tomography study that determines the lateral variation of Pn-wave velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show an average uppermost mantle Pn-velocity of 8.1 km/s. However, zones of relatively low-Pn-velocity (~7.9 km/s) are found beneath the Cenozoic volcanic provinces in the northern, central, and southwestern region of the island. These low-Pn-velocity zones are attributed to thermal anomalies that are associated with upwelling of hot mantle materials that gave rise to the Cenozoic volcanism. The direction of Pn anisotropy shows a dominant NW-SE direction of fast-polarization in the northern region and around the Ranostara shear zone, in the south-central Madagascar. The anisotropy in the uppermost mantle beneath these regions aligns with the existing geological framework, e.g. volcanic complex and shear zones, and can be attributed to a fossil anisotropy. The Pn anisotropy in the southwestern region, around the Morondava basin, is E-W to NE-SW-oriented. It can be attributed either to the mantle flow from plate motion, the African superplume, or the Mesozoic rifting from Africa. Results from this study do not show any substantial evidence of the formation of a diffuse boundary of the Lwandle plate, cutting through the central region of Madagascar. Station static delays reflect the significant variation in the Moho depth beneath the island. In the third part of the thesis, the thickness of the mantle transition zone beneath Madagascar, which is sensitive to the surrounding temperature variation, has been estimated by stacking receiver functions. Single-station and common-conversionpoint stacking procedures show no detectable thinning of the mantle transition zone and thus no evidence for a thermal anomaly in the mantle under Madagascar that extends as deep as the mantle transition zone. Therefore, this study supports an upper mantle origin for the Cenozoic volcanism. However, the resolution of the study is not sufficient to rule out the presence of a narrow thermal anomaly as might arise from a plume tail. Overall, the findings in this research are broadly consistent with the crustal and upper mantle structure of Madagascar determined by previous studies, but provides significantly greater detail with regard to the crustal and uppermost mantle structure as more seismic stations were used.LG201

An investigation of the 27 July 2018 bolide and meteorite fall over Benenitra, southwestern Madagascar

Several dozen stones of an ordinary chondrite meteorite fell in and around the town of Benenitra in southwestern Madagascar during the early evening of 27 July 2018, minutes after a widely observed meteor fireball (bolide) transit and detonation. The event was confirmed by low-frequency infrasound recordings received at ~17h15 UTC (Coordinated Universal Time; 19h15 local time) at the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) infrasound station I33MG near Antananarivo, 542 km north-northeast of Benenitra. An energy release equivalent to 2.038 kt of TNT was calculated from the infrasound signals. Seismograph readings at the SKRH station 77 km north-northwest of Benenitra recorded a twostage signal consistent with the arrivals of an initial air-coupled ground wave at 16h48:08 UTC and a stronger pulse at 16h49:22 UTC linked directly to the atmospheric pressure wave. The infrasound and seismic signal arrival times suggest that the bolide entry and detonation occurred at approximately 18h46 local time (16h46 UTC), entry was from the northwest, and the detonation hypocentre was located within ~20 km of Benenitra. Despite meteorite debris being found among buildings within Benenitra, there was no damage to structures or injuries reported. Eyewitness accounts and photographic records indicate that approximately 75 mostly intact stones were collected; however, the remoteness of the area, the rugged nature of the terrain and sales of fragments to meteorite collectors have limited scientific analysis of the fall and the extent of the strewn field. The total mass of recovered stones is estimated at between 20 kg and 30 kg, with one fragment of 11.2 kg and several of ~1 kg. Petrographic and mineral chemical analyses indicate that the stones belong to the L6 class of ordinary chondrites. Cosmogenic radionuclide analysis confirms that the fall is linked to the bolide event. The name Benenitra has been officially accepted by the Meteoritical Bulletin Database.Significance: Eyewitness reports, CTBTO infrasound records, seismograph records and cosmogenic radionuclide analysis confirm a meteorite fall over Benenitra on 27 July 2018. Petrographic and geochemical analyses confirm that the meteorite is an L6 ordinary chondrite. Recovery of meteorite falls is rare; this is Madagascar’s second known meteorite fall and the first that can be linked to a bolide. Regional and global science monitoring networks can be interrogated to improve the understanding of bolide events. Interaction of scientists with local communities is important to dispel misunderstandings around scientific phenomena, and can improve collection of data

Coherent Seismic Anisotropy Pattern Across Southern Africa Revealed by Shear Wave Splitting Measurements

We report new PKS, SKS, and SKKS splitting measurements for 88 seismic stations in Namibia, Botswana, South Africa, and Mozambique. When combined with measurements from previous studies, the ensemble of measurements shows a fairly uniform NNE to NE (∼41° on average) fast-polarization direction (ϕ) and delay time (δt) (∼0.7 s on average) across the entire southern African subcontinent. It is difficult to attribute the NNE-NE ϕ direction to just one source of anisotropy either within the lithospheric or sublithospheric mantle. We instead propose the observed anisotropy pattern could result from a combination of several sources that together give rise to a pervasive NNE-NE ϕ direction; (a) fossil anisotropy in the lithospheric mantle resulting from the Neoproterozoic collision of the Congo and Kalahari cratons to form the Damara Belt, (b) movement of the African plate over the asthenosphere, and (c) flow in the upper mantle induced by the African Superplume. In addition, a contribution from anisotropy in the lowermost mantle in the vicinity of the African large low shear velocity province cannot be ruled out

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

International audienc

Coherent Seismic Anisotropy Pattern Across Southern Africa Revealed by Shear Wave Splitting Measurements

Abstract We report new PKS, SKS, and SKKS splitting measurements for 88 seismic stations in Namibia, Botswana, South Africa, and Mozambique. When combined with measurements from previous studies, the ensemble of measurements shows a fairly uniform NNE to NE (∼41° on average) fast‐polarization direction (ϕ) and delay time (δt) (∼0.7 s on average) across the entire southern African subcontinent. It is difficult to attribute the NNE‐NE ϕ direction to just one source of anisotropy either within the lithospheric or sublithospheric mantle. We instead propose the observed anisotropy pattern could result from a combination of several sources that together give rise to a pervasive NNE‐NE ϕ direction; (a) fossil anisotropy in the lithospheric mantle resulting from the Neoproterozoic collision of the Congo and Kalahari cratons to form the Damara Belt, (b) movement of the African plate over the asthenosphere, and (c) flow in the upper mantle induced by the African Superplume. In addition, a contribution from anisotropy in the lowermost mantle in the vicinity of the African large low shear velocity province cannot be ruled out

An investigation of the 27 July 2018 bolide and meteorite fall over Benenitra, southwestern Madagascar

Several dozen stones of an ordinary chondrite meteorite fell in and around the town of Benenitra in southwestern Madagascar during the early evening of 27 July 2018, minutes after a widely observed meteor fireball (bolide) transit and detonation. The event was confirmed by low-frequency infrasound recordings received at ~17h15 UTC (Coordinated Universal Time; 19h15 local time) at the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) infrasound station I33MG near Antananarivo, 542 km north-northeast of Benenitra. An energy release equivalent to 2.038 kt of TNT was calculated from the infrasound signals. Seismograph readings at the SKRH station 77 km north-northwest of Benenitra recorded a twostage signal consistent with the arrivals of an initial air-coupled ground wave at 16h48:08 UTC and a stronger pulse at 16h49:22 UTC linked directly to the atmospheric pressure wave. The infrasound and seismic signal arrival times suggest that the bolide entry and detonation occurred at approximately 18h46 local time (16h46 UTC), entry was from the northwest, and the detonation hypocentre was located within ~20 km of Benenitra. Despite meteorite debris being found among buildings within Benenitra, there was no damage to structures or injuries reported. Eyewitness accounts and photographic records indicate that approximately 75 mostly intact stones were collected; however, the remoteness of the area, the rugged nature of the terrain and sales of fragments to meteorite collectors have limited scientific analysis of the fall and the extent of the strewn field. The total mass of recovered stones is estimated at between 20 kg and 30 kg, with one fragment of 11.2 kg and several of ~1 kg. Petrographic and mineral chemical analyses indicate that the stones belong to the L6 class of ordinary chondrites. Cosmogenic radionuclide analysis confirms that the fall is linked to the bolide event. The name Benenitra has been officially accepted by the Meteoritical Bulletin Database.Significance: Eyewitness reports, CTBTO infrasound records, seismograph records and cosmogenic radionuclide analysis confirm a meteorite fall over Benenitra on 27 July 2018. Petrographic and geochemical analyses confirm that the meteorite is an L6 ordinary chondrite. Recovery of meteorite falls is rare; this is Madagascar’s second known meteorite fall and the first that can be linked to a bolide. Regional and global science monitoring networks can be interrogated to improve the understanding of bolide events. Interaction of scientists with local communities is important to dispel misunderstandings around scientific phenomena, and can improve collection of data

Shear velocity structure of the crust and upper mantle of Madagascar derived from surface wave tomography

International audienceThe crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until <1 Ma, but the sources of which remains uncertain. Combined analysis of three complementary surface wave methods (ambient noise, Rayleigh wave cross-correlations, and two-plane-wave) illuminate the upper mantle down to depths of 150 km. The phase-velocity measurements from the three methods for periods of 8–182 s are combined at each node and interpolated to generate the first 3-D shear-velocity model for sub-Madagascar velocity structure. Shallow (upper 10 km) low-shear-velocity regions correlate well with sedimentary basins along the west coast. Upper mantle low-shear-velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism. These anomalies appear distinct at depths <100 km, suggesting that any connection between the zones lies at depths greater than the resolution of surface-wave tomography. An additional low-shear velocity anomaly is also identified at depths 50–150 km beneath the southwest region of intraplate volcanism. We interpret these three low-velocity regions as upwelling asthenosphere beneath the island, producing high-elevation topography and relatively low-volume magmatism