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