FIG. 7 BATHYMETRY, SAKALAVE MOUNTS
ridge and their imprint is often recognizable as lenses-forms on the seismic
above the major unconformity. Paleocene is absent on the ridge and was not
drilled by Glomar Challenger Leg 25,
thus the major unconformity has to be
considered of Cretaceous to Paleocene
Variations of the Cretaceous sedimentary environments are well documented at
the median parts of the ridge (SU7704,
SU7720, 84DR06, 84DR09). The oldest
known pelagic sediments of the Davie
Ridge are Late Cretaceous oozes. These
sediments locally cover the eroded crystalline basement and in two cases were
dated of Middle Coniacian (SU7720)
and Upper Campanian to Maastrichtian (SU7704). They are particularly soft,
supporting the hypothesis that subsidence associated with overlying sediments was limited at the median parts
of the ridge.
The volcanics are present in both
northern and southern parts of the
ridge below Eocene pelagic sediments
and above eroded crystalline basement, consistent with the presence of
magnetic signatures. Basalts are alkaline aphyric or phenocrystic rocks
emplaced under hypabyssal, and in
some cases, subaerial conditions. The
absence of mylonitic texture in the
basalts may suggest absence of stress
and low-temperature deformation during emplacement and
absence of transform fault movements. The absence of pillow features and of ultrabasic rocks discounts transition between oceanic and continental crust, indicating that the volcanic activity of the ridge appeared on a continental setting
after cessation of the southwards movement of Madagascar.
The geochemistry of the volcanics shows mixing between
a differentiated basaltic melt situated in shallow depths and a
sialic component, suggesting the influence of subcontinental
lithosphere. 8 These volcanics are chemically and petrogeneti-cally comparable with the Early Cretaceous volcanism along
the southeastern margin of Africa and penecontemporaneous
with the Late Cretaceous volcanic activity of western Madagascar which preceded or accompanied local distensions.
They coincide with the initiation of the transcurrent motion
between India and Madagascar at 83 Ma (Anomaly 34) and
with the subsequent northwards movement of India.
From fracture zone to ridge
Local distensions along the DFZ during Cretaceous induced
either the first Cretaceous pelagic sedimentation or hypabys-
sal to subaerial volcanic activity with intense hydrothermal-
ism. A major Cretaceous unconformity developed on both
flanks of the ridge, yielding two important features:
• The spatial symmetry of the volcanic activity, present
at the northern and southern parts of the ridge.
• The Late Cretaceous and Eocene sedimentation, prominent at the median and southern parts of the ridge.
These features suggest that during Late Cretaceous and
Eocene the local distensions accompanying lithospheric fissuring were transgressive southwards while the northern
part of the ridge remained close to sea level for a long period.
Eocene outcrops are, in fact, better exposed at the southern
parts of the ridge.
The products of this erosion settled on the flanks of the