QUANTIFICATION OF VERTICAL MOVEMENT OF LOW ELEVATION TOPOGRAPHY COMBINING A NEW COMPILATION OF GLOBAL SEALEVEL CURVES AND SCATTERED MARINE DEPOSITS (ARMORICAN MASSIF, WESTERN FRANCE)
Résumé
A wide range of methods are available to quantify Earth’s surface vertical movements but most of these
methods cannot track low amplitude (< 1 km, e.g. thermochronology) or old (> 5 Ma, e.g. cosmogenic isotope
studies) vertical movements where deformation is damped. The difference between the present-day elevation
of ancient sea-level markers (deduced from well dated marine deposits corrected from their bathymetry of
deposition) and a global sea-level (GSL) curve are sometimes used to estimate these vertical movements. Here,
we formalized this method by re-assessing the reliability of published GSL curves to build a composite curve
that combines the most reliable ones at each stage, based on the potential bias and uncertainties inherent to
each curve. We suggest i) that curves which reflect ocean basin volume changes are suitable for the ca. 100 to
35 Ma “greenhouse” period ii) whereas curves that reflects ocean water volume changes are better suited for
the ca. 35 to 0 Ma “icehouse” interval and iii) that, for these respective periods, the fit is best when using
curves that accounts for both volume changes. We used this composite GSL curve to investigate the poorly
constrained Paleogene to Neogene vertical motions of the Armorican Massif (western France). It is
characterized by a low elevation topography, a Variscan basement with numerous well dated Cenozoic marine
deposits scattered upon it. Using our method, we identify low amplitude vertical movements ranging from 66
m of subsidence to 89 m of uplift over that time period. Their spatial distribution argues for a preferred scale
of deformation at medium wavelengths (i.e., order 100 km), which we relate to the deformation history of
northwestern European lithosphere in three distinct episodes: i) a phase of no deformation between 38 and 34
Ma, that has been previously recognized at the scale of northwestern Europe, ii) a phase of low subsidence
between 30 and 3.6 Ma, possibly related to buckling of the lithosphere and iii) a phase of more pronounced
uplift between 2.6 Ma and present, which we relate to the acceleration of the Africa-Apulia convergence or to
enhanced erosion in the rapidly cooling climate of the Pleistocene.