QUANTICATION OF VERTICAL MOVEMENT OF LOW ELEVATION TOPOGRAPHY FROM A NEW COMPILATION OF SEA-LEVEL CURVES AND SCATTERED MARINE DEPOSITS (ARMORICAN MASSIF, WESTERN FRANCE)
Résumé
A wide range of methods were developed to quantify vertical motions at plate boundaries and
to track their spatial distribution, their wavelength and the associated deformation mechanism.
However, they cannot be used for intraplate domains due to too low amplitudes of vertical
movements or too old motions (e.g. thermochronology, cosmogenic isotope studies). Here
we propose a new method for quantifying low amplitude vertical movements experienced by
intraplate areas by combining dating of marine sediments and correcting their elevation from
their depositional bathymetry and global sea level height from a global sea level curve. For
accurate quantifications, we re-assess the reliability of published global sea level curves and
constructed a composite curve combining their most reliable intervals : i) from ca. 100 to 35
Ma («greenhouse» period), curve which reflects ocean basin volume changes and ii) from ca.
40 to 0 Ma («icehouse» period) curves which reflect ocean water volume changes, which iii) _t
for these respective periods with a curve which account for both driving factors. We investigate
the poorly constrained Paleogene to Neogene vertical motions and deformation history
of the Armorican Massif, a western European Variscan basement (France) characterized by a
low relief, low elevation topography and twice buried then exhumed from Jurassic to Paleocene
times. Numerous well dated Cenozoic marine sedimentary remnants are scattered upon
this massif and are thus key archives to apply our new method for quantifying low amplitude
vertical movements. Vertical movements of low amplitude ranging from 54.5 m of subsidence
to 85.0 m of uplift are identified. Their spatial distribution argues for deformations of medium
wavelength (x 100 km) which can be related to the deformation history of the northwestern
European lithosphere with : i) a phase of no deformation from 38 to 34 Ma, recognized at the
northwestern European scale, ii) a 30 to 3.6 Ma low-subsiding phase, possibly due to the location
of the massif upon a syncline of a buckled lithosphere and iii) a 2.6 to 0 Ma uplift either
related to the intensification of the Africa-Apulia convergence or enhanced climate effects on
this long-term uplift.