Quantification des flux sédimentaires et de la subsidence du bassin ...

tel-00790852, version 1 - 21 Feb 2013
(Figure 8). The lines show an increasing slope with increasing age of clinoforms. Topset of
clinoforms (Figures 2A and 8) can reach 15m/km (1.5 %, 0,86°) foresets can reach 50-
62m/km (5-2.6.2 % or 2.86-3.5 °) and bottomsets 3m/km. All lines showed a common
rotation point located at 13 km landward of present day coast. As we assume that topsets
represent the same paleoenvironment and the same paleobathymetry at time of deposition
which is proven for at least the last 434 000 years (Rabineau et al., 2005, Bassetti et al.,
2008), they correspond to a caracteristic slope at time of deposition. We considered this slope
to be the same as present day slope of the shelf i. e. 1,5m/km. ( 0,086 °). Observed
increasing slopes are due to post-depositional tilting of the margin related to subsidence.
However detailed analysis of slope adjustment performed on the outer shelf showed that the
landward part of clinoforms did not fit well with real observed geometries (figure 8, see zoom
8B). Further investigation on seismic profiles showed that around 32 km from the coast we
observed a small but systematic deformation of reflectors (Figure 7 and 8). All reflectors
from early Pliocene to p14 show the same deformation, whereas reflector q10 is much less
deformed and D30 not deformed. We therefore conclude that there is a tectonic deformation
with 50 m uplift in the landward part of the section. This deformation can be followed on
several lines and mapped in the western area (see discussion and Figure 9). The age of
reflectors extrapolated between p1 and D30 (see method section) led us conclude that the
uplift occurs around q10, i.e. 1 Ma.
If we restore the landward part of the section before this deformation we then find a good
adjustment of topsets, that can be propagated down to p1 clinoform with the same previously
defined rotation point.
Present-day approximation-line of the sea-floor lies at 150m at Km 70. The tilt between
approximation of p1 and the approximation of present day sea-floor brings line-p1 at 1430 m
at Km70 Figure 10), i.e. 1280 m of subsidence at 70 km from present-day coast. The angle of
tilt (between early Pliocene to present day) is therefore =0.88° (=1,54%) (see definition on
Figure 02). The first visible prism p1 on the shelf (appearing at Km6) lies directly on the
messinian surface (dated at 5,33 Ma). If we date this first prism as 5,3 Ma we found a total
subsidence rates of 49 m/Ma at 19 km from the coast to 241 m/Ma at 70 km from the coast. In
other words, the tilt of the shelf is of 0,16 °/Ma = 0,28 %/Ma.
The two methods to estimate subsidence rate are in good agreement, this second method is
more precise than our calulation of accommodation because it describes the 2D evolution of
the subsidence.
This value 241m/Ma is exactly the same order as the subsidence calculated for the last
434,000 years (250 m/Ma) (Rabineau et al., 2006). This common value calculated at two very