CENOZOIC DEFORMATION TIMING AND REGIME OF THE ARMORICAN MASSIF (WESTERN FRANCE): NEW INSIGHTS FROM A REPROCESSED HIGH RESOLUTION SEISMIC PROFILE (RENNES BASIN)
Résumé
The Armorican Massif is one of numerous Variscan basement blocks of Western Europe characterized
by low to moderate elevation plateaus (e.g. Massif Central, Rhenish Massif) which have experienced several
episodes of burial and exhumation in the Mesozoic and/or Cenozoic. This massif was twice buried then
exhumed between Jurassic and Paleocene times in response to relative movements between Iberia and Eurasia.
However, as other western European shields, its Paleogene to Neogene deformation history is still poorly
constrained despite the presence of numerous Cenozoic (Ypresian to Piacenzian; 56-2.6 Ma) lacustrine to
shallow marine deposits scattered upon it. Some of these deposits, mainly the Bartonian to Rupelian (41-28
Ma) ones, are preserved in small narrow grabens bounded by N140E faults. The Rennes Basin, a 2 to 4 km
width for 400 m depth basin, is the largest and well-documented one with: i) a well-dated borehole which cross
the whole Bartonian to Piacenzian series and reach the underlying basement (CDB1 Borehole, CINERGY
Project, 2010); ii) a 5-km long high resolution seismic reflection profile which cut across the basin (HR1
profile, GeoFrance 3D program, 2000 – reprocessed in 2012).
Here, we investigate this reprocessed seismic profile to bring new constraints about the Cenozoic
deformation history of the Armoricain Massif. We converted the CDB1 borehole into seismic velocity and
some shallow seismic reflectors in depth in order to date these reflectors then correlate seismic facies together
with sedimentary facies of CDB1 and 58 neighboring shallow boreholes (50 m max.) from the French borehole
database (BSS, via Infoterre).
We show that Bartonian to lowermost Rupelian (41-33 Ma) series, which are affected by slight
synsedimentary deformation, are gently bended and collapsed between two hidden highly dipping reverse
faults. These faults are subsequently sealed by Lower Rupelian tabular and sparsely faulted deposits.
Outcropping Late Langhian to Serravallian (15-11.6 Ma) crags (“faluns”) rest on and overflow these deposits
above an angular unconformity (Upper Oligocene – Lower Miocene gap). They are affected by superficial
normal faults gently dipping toward the pre-existing weathered basement. The sedimentary succession ends
with Pliocene fluvial to estuarine deposits which overlain a second unconformity.
Together with field observations, our results argue for Priabonian (?) to Early Rupelian transtensional
to transpressional deformations associated with strike-slip movements along the N140E faults related to a NS
tectonic stress. These deformations can be integrated within the Western European tectonic framework. They
are likely to be related to the growth of numerous small sedimentary basins during Oligocene to middle
Miocene times (ca. 35 – 10 Ma) along the western side of the British Isles (e.g. Cornwall) in a largely strikeslip
regime, which led to local basin inversions. They also coeval with Oligocene (Eocene?) to Miocene shortwavelength
deformation observed in surrounding basins and on the northwestern European platform that
includes strike-slip to compressive folding (South Armorican Margin), major basin inversion (Western
Approaches Basin) and NNE-SSW striking left-lateral transtensional wrenching of the European Cenozoic
Rift System.