When mountain belts disrupt mantle flow: from natural evidences to numerical modelling
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.
Here, we show that such mountain belts, at plate boundaries, increasingly obstruct plate tectonics, slowing
down and reorienting their motions. In turn, it changes the dynamic and kinematic surface conditions of the
underlying flowing mantle, which ultimately modifies the pattern of mantle flow. Such forcing could explain many
first order features of Cenozoic plate tectonics and mantle flow. Among others, at lithospheric scale, one can
cite the compression of passive margins, the important variations in the rates of spreading at oceanic ridges, the
initiation of subductions, or the onset of obductions. In the mantle, such changes in boundary conditions redesign
the flow pattern and, consequently, disturb the oceanic lithosphere cooling.
In order to test this hypothesis we first present thermo-mechanical numerical models of mantle convection
above which a lithosphere is resting on top. Our results show that when collision occurs, the mantle flow is
strongly modified, which leads to (i) increasing shear stresses below the lithosphere and (ii) a modification of the
convection style. In turn, the transition between a “free” convection (mobile lid) and a “disturbed” convection
(stagnant - or sluggish - lid) highly impacts the dynamics of the lithosphere at the surface. Thereby, on the basis
of these models and a variety of real examples, we show that on the other side of a lithosphere presenting a
collision zone, passive margins become squeezed and can undergo compression, which may ultimately evolve
into subduction initiation or obduction. We also show that much further, due to the blocking of the lithosphere,
spreading rates decrease at the ridge, which 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.