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. / LG2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/24986 |
| Date | January 2017 |
| Creators | Andriampenomanana Ny Ony, Elamahalala Fenitra Sy Tanjona |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (xx, 297 pages), application/pdf |
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