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

tel-00790852, version 1 - 21 Feb 2013
surface however appear to be about planar and parallel to each other in this area. Wells on the
shelf showed that those Upper Miocene deposits are very shallow (Cravatte et al., 1974
describes a brackish to inner shelf environment at both Mistral and Tramontane ; In Autan1
well (which is located much further on the upper slope) paleoenvironments are described as
inner to middle shelf. We therefore think that hose Miocene deposits have been deposited as
horizontal strata. The amount of Pioquaternary subsidence should therefore have tilted them,
which is not what we observe. We therefore think that Miocene strat have been deformed
before the Pliocene-Quaternary which is probably related to an isostatic re-adjustement
related to the Messinian erosional and salinity crisis. This suggestion has been proposed by
many authors (Ryan, 1976 ; Norman & Chase, 1986; Mauffret et al., 2001, Gorini et al., 2005
; Ryan, 2011). The drop of sea-level, the intense erosion on the shelf (over 1.5 twtt amount of
sediments that have been eroded in our area Bache, 2008, Bache et al. 2009) must have
generated an isostatic rebound. Our results on Pliocene-Quaternary subsidence therefore also
give, a direct estimate of this isostatic rebound. Miocene strata have been uplifted during the
MSC and then subsided again during the Pliocene-Quaternary. As they are back to horizontal
at present day, the amount of Plioquateranry subsidence equals the amount of uplift during the
crisis (i.e. 1280 m at Km 70). We can therefore approximate the rebound during the crisis
to be about 1.3 Km in this area. This value is of the same order than that evaluated by
Mauffret et al., 2001 who found 1.7 km of uplift in the offshore part of the Albères Massif
(i.e. about 20 km south of our estimate near the Pyrenees) which is not fully compensated by
Pliocene-Quaternary subsidence as the landward dipping geometry of the strata on the outer
shelf demonstrate. Considering the duration of the crisis to be 700 000 ka at the most
(CIESM, 2008) we found a rate of more than 1830 m/Ma which shows how rapid vertical
movement can be.
The value we found here is far greater than the values obtained with isostatic modelling by
Govers et al., (2009) in the Gulf of Lion. Govers described both the effect of the dessication
creating an uplift of the margin and the effect of evaporites deposition (creating a load) (see
his figure 9). But this uplift is limited to 300 to 600 m (from 100 Km offshore to present-day
shoreline), furthermore the uplift is partly compensated by subsidence effect of evaporites
deposition. However the model does not take into account the huge amount of erosion on the
shelf (more than 1 km thick, Bache et al., 2009) nor the huge amount of terrigeneous and
evaporitic deposits (over 3 km thick Bache et al., 2009).
• Meaning of the hinge point between subsidence values at 200 Km relation to deep structure
of the margin.