OFFSHORE AND ONSHORE EVIDENCE FOR A POLYPHASED UPLIFT HISTORY AND UPPER MIOCENE DOMING OF MADAGASCAR
Résumé
Madagascar is an Archean to Neoproterozoic continental crust surrounded by transform,
oblique and divergent margins: the oblique Morondava Basin to the west, pounded by the
Davie Fracture Zone, and to the north, the divergent Mahajanga (Majunga) Basin connected to
the Somali Oceanic Basin. This 1600 km long island is a high axial plateau with elevations ranging
from 1200 to 1800m. The top of the plateau corresponds to weathered planation surfaces
(etchplains), bounded by more or less high scarps.
We here present geological arguments for the age and the timing of the Madagascar Plateau.
This analysis is based on a double, coupled analysis of the onshore geomorphology (stepped
planation surfaces) and the offshore margin stratigraphy (seismic stratigraphy and wells).
The geomorphological analysis is based on a characterization, a mapping and dating of stepped
planation surfaces (mantled to stripped etchplains, pediments to pediplains). The dating
is based on their geometrical relationships with dated magmatic rocks. The difference of elevation
between two planation surfaces (corresponding to local base level) providing a proxy
of the uplift. The sequence stratigraphic and seismic analysis is based on a biostratigraphic
reevaluation of 4 industrial wells (foraminifers and nannofossils on cuttings), several hundreds
of kilometers of industrial and oceanographic research seismic surveys. Uplift periods are characterized
by (1) seaward tilting of the margins overlain by planar reflectors, (2) forced regression
wedges, (3) upstream erosions of older sediments recorded by fauna/flora reworking and
(4) major paleogeographic changes within the Morondava basin.
(1) During Middle Cretaceous (99 to 85 Ma), Madagascar undergoes a major volcanic event
with the onset of the Madagascar Trap, contemporaneous to Madagascar-India separation.
The major mid Cretaceous erosion surface is described onshore as the result of a pre to
syn-trap Uplift.
(2) During Paleocene to Lower Miocene times (66 to 20 Ma), Madagascar is a quite flat low
elevation domain with remnants of an oldest pre-Madagascar Trap (90 Ma) surface. This low relief is highly weathered with growth of numerous lateritic profiles and surrounded by
large carbonate platforms with no siliciclastic sand influx.
(3) The Late Miocene is the paroxysm of uplift with (1) a tilting of the margin (Morondova), (2)
an increase of the siliciclastic sand flux since middle Miocene and (3) a major stepping of
dated planation surfaces.
(4) The end result of this uplift is a convex up shape pattern for the end Cretaceous surface
weathered during Eocene times, creating the present-day dome morphology (with a central
plateau) of Madagascar.
(5) The amplitude of this uplift can be estimated based on the present-day elevation of Late
Eocene lagoonal sediments located 100 km north-east of Toliara and now at an elevation of
900m. If the absolute sea level was around 50 m (Miller et al., 2005) above present-day sea
level during Late Eocene times, this means a surface uplift of around 850 m.
(6) The mechanism of this uplift has to explain a very long wavelength deformation (x1000 km)
necessary due to mantle dynamics. The relationships with the other East African domes
(Ethiopia, East Africa, South Africa) are discussed.
This study was funded by TOTAL and IFREMER in the frame of the research project PAMELA (Passive
Margin Exploration Laboratories).