Geodynamics since the Jurassic and mantle convection.
Résumé
The dynamics of mantle convection and its relation to plate motion and
plate boundary deformation has been so far mostly studied through numerical
modeling but as the rheology of the mantle is not precisely
known, testing the models is difficult. If one could relate deep convection
with the geological evolution recorded through time, one may have
access to time constants of convection cycles, slab penetration and slab
detachment, tearing. We look for such possible connexions in the tectonic
history of the Tethys Ocean. The tectonic history of the Neo-
Tethys ocean and of the mountain belts formed during its progressive
closure are characterized by (1) the fragmentation of southern continental
masses and fast northward drift of smaller continents that end up
subducting and colliding with the southern margin of the northern continents,
(2) one major compressional event in the Late Cretaceous leading
to generalized obduction on the northern margins of Africa and Apulia
contemporaneous with generalized compression in the northern part of
Africa and part of Eurasia and (3) opening and closure of backarc basins
formed within the southern margin of Eurasia, especially in the Mediterranean
region. Based on reconstructions and a synthesis of various
indicators of mantle convection, such as plate velocities, oceanic lithosphere
production or super plumes activity, we discuss a model describing
the relations between lithospheric-scale geodynamics in the Tethys
region and mantle convection cycles, slab penetration and detachments.
We propose that the fragmentation of Africa (Apulia, Arabia) resulted
from the drag of the underlying mantle flowing northward and that the
Late Cretaceous compressional event and large-scale obduction resulted
from the penetration of the Tethyan slab in the lower mantle. The noncoaxial
component induced by basal drag induced the underthrusting of
the old and cold continental mantle below the younger oceanic mantle,
leading to obduction.