Quantification de la subsidence et du réajustement isostatique messinien à partir de paléomarqueurs sédimentaires : exemples en Méditerranée Occidentale
Abstract
Passive margins are characterised by an important tectonic and thermal
subsidence, which favours a good preservation of sedimentary sequences.
This sedimentation in turn enhances the subsidence because of
loading effects. We present here a direct method based on sedimentary
markers seen on seismic data, to evaluate total subsidence rates from the
coast to the outer shelf and to the deep basin in the Gulf of Lion, from
the beginning of massive salt deposition up to present day (the last circa
6 Ma) with minimal theoretical assumptions.
On the shelf, the Pliocene-Quaternary subsidence shows a seaward tilt
reaching a rate of 240 m/Ma ( 15 m/Ma) at the shelf break (70
km from the present day coastline) (i.e. a total angle of rotation of
0.88°(0.16°/Ma).We also quantifed for the first time the Messinian salinity
crisis isostatic rebound of the outer shelf. This value is reaches up to
1.3 km of uplift during the crisis around the Herault-Sète canyon heads
(around 1.8 km/Ma). On the slope, we also find a seaward tilting subsidence
from Km 90 to Km 180 with a measured angle of 1.41°. From 180
Km to the deepest part of the basin, the total subsidence is then almost
vertical and reaches 960 m/Ma ( 40 m/Ma) during the last 5.7 Ma (
0.25 Ma) in the deepest part of the basin.
The subsidence is organised in three compartments that seem related to
the very deep structure of the margin during the opening of the Liguroprovencal
basin. These very high total subsidence rates enable high sedimentation
rates along the margin with sediments provided by the Rhône
river flowing from the Alps, which in turn enable the detailed record of
climate evolution during Pliocene-Quaternary that make of the Gulf of
Lion a a unique archive.
These results have been extended in 3D during Estelle Leroux PhD but
also onland during Stéphane Molliex postdoc and towards the Valencia
basin were similar measurements are under process within Romain
Pellen PHD (see their respective presentations).