Crust-mantle interactions and crustal deformations, some geological observations
Résumé
Crustal deformations at plate boundaries or intracontinental are governed by the relative movements of plates, and
most published models consider the lithosphere as the main stress guide in extensional or compressional contexts.
The possible contribution of the underlying asthenospheric flow to crustal deformation through viscous coupling
is often neglected. Since the early days of plate tectonics, and even earlier, two schools of thought have been
developed in parallel whether mantle convection is considered or not. This reflects nowadays in the difficulty of
reconciling lithospheric-scale models and global-scale convection models to explain tectonic features observed
at the surface. Still, recent studies reemphasized the role of mantle convection in shaping mountain belts or rifts
and the consequences of different styles of convection on the geometry and kinematics of mountain belts. We
present here a number of geological observations in convergent or divergent contexts that may suggest a strong
coupling between asthenospheric flow and crustal deformation. Several of these examples, especially in extensional
contexts, show a deformation distributed over wide zones, accommodated by shallow-dipping shear zones and
with a constant asymmetry over large distances. This is the case of the Mediterranean back-arc basins, such as
the Aegean Sea, the northern Tyrrhenian Sea or the Alboran domain, where extension is taken up by shallowdipping
extensional shear zones and normal faults with a constant asymmetry. A similar image is also observed
across the Gulf of Lion passive margin that also belongs to the Mediterranean back-arc basins. Such is also the
case of some of the Atlantic passive margins where shallow-dipping normal faults and extensional shear zones
control the extraction of the lower crust and the mantle with a constant asymmetry across the entire margin.
Finally, the distribution and geometry of normal faults across the Afar region also show a constant asymmetry. We
discuss these contexts and search for the main controlling parameters for this asymmetric distributed deformation.
These parameters include an original heterogeneity of the crust and lithosphere (tectonic heritage) and a possible
contribution of the underlying asthenospheric flow. We discuss the relations between the observed asymmetry and
the direction and sense of the mantle flow underneath. Finally, we extend this question to larger-scale processes
such as obduction and continental collision.