Open Session - SWISS GEOSCIENCE MEETINGs

1.1
Evolution of the Morondava basin shoulder in western Madagascar - an
apatite fission-track study
Giese Jörg*, Seward Diane** & Schreurs Guido*
*Institute of Geological Sciences, University of Bern, Baltzerstrasse 1 & 3, CH-3012 Bern, Switzerland
(giese@geo.unibe.ch)
**Geological Insitute, ETH Zürich, Leonhardstrasse 19, CH-8092 Zürich, Switzerland
The Morondava basin is the largest and oldest of three Phanerozoic sedimentary basins in Madagascar and is bound to the
east by the crystalline basement of Proterozoic age and stretches into the Mozambique Channel to the west (Fig. 1a). The
basin is related to the break-up of Gondwana and the separation of Madagascar from eastern Africa. Extensional structures
very often reactivated pre-existing ductile structures within the basement (e.g. Montenat et al. 1993; Piqué et al. 1999). The
structural style changed from development of local pull apart basins during the deposition of the oldest sedimentary units
of Late Carboniferous to Late Permian times, the Sakoa Group, to transtensional and later pure tensional strain during the
deposition of the mainly siliciclastic sediments of the Sakamena and Isalo Groups in Middle Permian to Early Jurassic times
(Schandelmeier et al. 2004). After a gap in sedimentation, final separation between Madagascar and Eastern Africa was achieved
during Middle Jurassic times (Geiger et al. 2004) and Madagascar was displaced relatively southward along the Davie
Ridge and parallel oriented fault systems (Coffin and Rabinowicz 1992).
Apatite fission track (AFT) analysis and thermal modelling of a series of 5 basement and 4 sediment samples (depositional
age varies between Late Permian and Late Triassic) along a transect across the eastern border of the Morondava basin (Fig.
1b) was carried out in order to refine the exhumation history, topographic evolution and low temperature thermal history
of the Morondava basin shoulder. Here, the boundary between the basement and the sediment is characterized by an escarpment
with an altitude difference of about 1000 m within ≈10 km of horizontal east-west distance.
Four consecutive basement and all sedimentary samples show progressively younger AFT ages towards the basin. Furthermore,
the apparent AFT ages of the sediments are all younger than the corresponding stratigraphic ages, indicating partial annealing
of the fission tracks. This partial annealing is most likely caused by an extensive sedimentary overburden, whose
thickness increases towards the basin. Modelled time – temperature paths also indicate reburial of the area after sedimentation
to temperatures between ≈60 and 80°C for the eastern samples and up to 120°C for the westernmost sample.
REFERENCES
Besairie, H. 1969/70: Cartes géologique de Madagascar (1:500.000), Service Géologique de Madagascar, Antananarivo.
Coffin, M.F. & Rabinowicz, P.D. 1992: The Mesozoic East African and Madagascan conjugate continental margins;
stratigraphy and tectonics. In: Watkins, J.S., Feng, Z. & Mc Millen, K.J. (Eds.), Geology and Geophysics of Continental
Margins. AAPG, 207-240.
Geiger, M., Clark, D.N. & Mette, W. 2004: Reappraisal of the timing of the break-up of Gondwana based on sedimentological
and seismic evidence from the Morondava Basin Madagascar. J. Afr. Earth Sci. 38, 363-381.
Montenat, C., Ramahavory, L. & Croisile, M. 1993: La séquence tectono-sédimentaire de la marge ouest-malgache au Jurassic
(bassin de Morondava, Madagascar). C. R. Acad. Sci., Ser. II 317(6), 811-818.
Pique, A., Laville, E., Bignot, G., Rabarimanana, M. & Thouin, C. 1999: L´ouverture et le dévelopment du bassin de
Morondava (Madagascar) du Carbonifère supérieur au Jurassic moyen. Données stratigraphiques, sédimentaires,
paléontologique et structurales. J. Afr. Earth Sci. 28(4), 931-948.
Schandelmeier, H., Bremer, F. & Holl, H.G. 2004: Kinematic evolution of the Morondava rift basin of SW Madagascar – from
wrench tectonics to normal extension. J. Afr. Earth Sci. 38, 321-330.
Seward, D., Grujic, D. & Schreurs, G. 2004: An insight into the breakup of Gondwana: identifying events through lowtemperature
thermochronology from the basement rocks of Madagascar. Tectonics 23, 1-20.
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Symposium 1: Structural Geology, Tectonics and Geodynamics