The many impacts of building mountain belts on plate tectonics and mantle flow
Résumé
During the Cenozoic, the number of orogens on Earth increased. This observation readily indicates that in the same
time, compression in the lithosphere became gradually more and more important. Such an increase of stresses in
the lithosphere can impact on plate tectonics and mantle dynamics.
We show that mountain belts at plate boundaries increasingly obstruct plate tectonics, slowing down and reorienting
their motions. In turn, this changes the dynamic and kinematic surface conditions of the underlying flowing
mantle. Ultimately, this modifies the pattern of mantle flow. This forcing could explain many first order features
of Cenozoic plate tectonics and mantle flow. Among these, one can cite the compression of passive margins, the
important variations in the rates of spreading at oceanic ridges, or the initiation of subduction, the onset of obduction,
for the lithosphere. In the mantle, such change in boundary condition redesigns the pattern of mantle flow
and, consequently, the oceanic lithosphere cooling.
In order to test this hypothesis we first present thermo-mechanical numerical models of mantle convection above
which a lithosphere rests. Our results show that when collision occurs, the mantle flow is highly modified, which
leads to (i) increasing shear stresses below the lithosphere and (ii) to a modification of the convection style. In
turn, the transition between a “free” convection (mobile lid) and an “upset” convection (stagnant –or sluggish- lid)
highly impacts the dynamics of the lithosphere at the surface of the Earth. Thereby, on the basis of these models
and a variety of real examples, we show that on the other side of a collision zone, passive margins become squeezed
and can undergo compression, which may ultimately evolve into subduction or obduction.We also show that much
further, due to the blocking of the lithosphere, spreading rates decrease at the ridge, a fact that may explain a variety
of features such as the low magmatism of ultraslow spreading ridges or the departure of slow spreading ridges from
the half-space cooling model.