Impact of convection in the mantle transition zone on long-term lithospheric extension and subsequent convergence and subduction initiation

Lorenzo Giuseppe Candioti (1) , Stefan Markus Schmalholz (1) , Thibault Duretz (2)

1 ISTE - Institut des sciences de la terre [Lausanne]
2 GR - Géosciences Rennes

Modelling the long-term geodynamic history (>100 Myrs) of orogens such as the Alps, including the pre-orogenic extension and cooling stages, remains challenging. For example, significant heat loss due to thermal diffusion over large time scales leads to unrealistic temperature and viscosity fields in the models. Modelling mantle convection in the transition zone below the thermal lithosphere-asthenosphere boundary (LAB) provides a mechanism to decrease the heat loss and stabilize the LAB in depth. Thermal effects of this convection can be modelled by either (1) directly modelling mantle convection, or by (2) artificially scaling the thermal parameters of the sub-lithospheric mantle without modelling convection. We perform 2D high resolution (2 x 1 km grid spacing) thermo-mechanical numerical simulations of more than 120 Myrs of lithospheric deformation. The models include the mantle transition zone down to a depth of 660 km to model the Western Alpine cycle including three subsequent deformation phases: (1) formation of a ca. 400 km wide basin of exhumed mantle bounded by two hyper-extended passive margins during a 30 Myrs extension period. (2) Thermal relaxation of the margin system for 70 Myrs with no far-field tectonic activity. (3) Convergence of the passive margin system leading to subduction initiation, basin closure and orogenic wedge formation. We perform two types of simulations: (1) We resolve and model sub-lithospheric convection during lithospheric extension, cooling and convergence. (2) We scale the thermal conductivity of the sub-lithospheric mantle without modelling and resolving the convection. Results show more intense convection during extension and cooling for models with realistic conductivity compared to models with scaled conductivity, but a similar evolution of the passive margin structure. Resolving sub-lithospheric mantle convection leads to more pronounced heterogeneity in, for example, density, viscosity or temperature fields. During convergence, the enhanced heterogeneity facilitates the initiation of a single asymmetric subduction. Parameterising sub-lithospheric mantle convection by scaling thermal conductivities leads to a more homogeneous sub-lithospheric mantle and results in initiation of two, opposite dipping asymmetric subduction zones. Authors

Domaines: Tectonique

Lorenzo Giuseppe Candioti, Stefan Markus Schmalholz, Thibault Duretz. Impact of convection in the mantle transition zone on long-term lithospheric extension and subsequent convergence and subduction initiation. American Geophysical Union Fall Meeting 2019, Dec 2019, San Francisco, United States. , pp.T21G-0411, 2019. ⟨insu-02396578⟩