THE PERCHED PALEOSURFACES OF THE PYRENEES -CANTABRIAN BELT
Abstract
Perched paleosurfaces of the Central and Eastern Pyrenees have been first described by
geomorphologists for a long time (Birot, 1937; Lagasquie, 1982; Calvet, 1996) and more recently
by geologists (Bosch et al., 2015; Monod et al., 2015; Bosch, in progress). These studies
are of peculiar interest since they provide robust constraints for the timing and mechanisms of
relief creation in a collision belt.
Pyrenean paleosurfaces have been recognized over large areas in the Axial Zone between
the Aure valley to the West and the Cerdagne and Capcir grabens to the East. The actual elevation
of theses surfaces varies between 1000 and 2500 m above sea level. In Cerdagne, the graben
filling has been dated of Vallesian age (Middle to Late Miocene). Based on this constraint,
the entire Pyrenean perched paleosurfaces have been considered of Miocene age. Since the
palynofacies of the Vallesian sediments is interpreted as a near-sea level deposit, the present
elevation of the paleosurfaces should have been acquired after the Vallesian assuming that
all paleosurfaces have nearly a Miocene age. In the vicinity of Cerdagne and Capcir grabens,
the central Pyrenees paleosurface appears connected to the uppermost terraced paleosurface,
suggesting an age prior to the Vallesian times.
A common feature of the Pyrenean paleosurfaces is the preservation of thick lateritic
weathering profiles. In the Eastern Pyrenees, the paleosurfaces display a terraced pattern with
three or more levels, the highest being likely the oldest. In the Central Pyrenees (from the Aston
Massif to the East to the Neouvielle Massif to the West), The single paleosurface is apparently
offset along numerous faults. Field work will be conducted in order to collect structural data
allowing a detailed tectonic evolution to be reconstructed.
In the Louron valley (close to the Aure valley), the central Pyrenean paleosurface is apparently
downthrown along N160 trending faults facing the valley that define a graben-like feature
with an overall NS trend. Field work will be conducted to check the structural relationships
of such possible features with the reverse faults and thrusts linked to the Pyrenean compression.
Preliminary investigations show that fault planes here bear shallow dipping slickenslides
indicating a probable transtensional mechanism. A second set of normal faults trending N100
to N130, parallel to the belt, seems to offset the central Pyrenean paleosurface. These faults are
observed in the Massif of Aston (SW of Ax les Thermes) where a detailed tectonic analysis will
be conducted.
Paleosurfaces will be followed from the Aure valley (central Pyrenees) to the East in order
to decipher their origin and evolution: are they terraced surfaces or a single surface offset
along recent faults having a general N160 and N100-130 orientation? Special attention will
be payed to the occurrence of the preserved lateritic weathering profiles that allows reconstructing
the lateral correlation of the paleosurface remnants. Finally, we plan to map these
paleosurface remnants from the central Pyrenees to the Cantabric belt with a special attention
to possible faults that might offset some portions. An attempt to date the weathering profiles
using syn-weathering minerals (adulaires) will be conducted in the meantime. These data will
collectively help better understanding the significance of paleosurfaces in collision belts.
This work is part of the project “Source to Sink”, we thank Total and BRGM