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Impact of convection in the mantle transition zone on long-term lithospheric deformation during the Alpine cycle

Abstract : The geodynamic history of the Western Alpine orogeny comprises periods of distinct phases of lithospheric deformation. Ultra-slow to slow spreading during ca. 60 Myrs formed the ca. 300-400 km wide Piemonte- Liguria basin. This basin was bounded by magma-poor, hyper-extended continental margins of the European and Adriatic plate and consisted mainly of exhumed and partially serpentinized sub-continental mantle. Subsequently, post-extension cooling took place for ca. 70 Myrs with insignificant tectonic activity. Then, convergence of the basin-margin system started at ca. 90 Ma causing the closure of the basin during subduction and the formation of an orogenic wedge during continent-continent collision. Modelling the long-term geodynamic history (>100 Myrs) of orogens such as the Alps, including the preorogenic 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 convection can be modelled by either (1) directly modelling small-scale mantle convection, or by (2) artificially scaling the thermal parameters of the sub-lithospheric mantle without modelling convection. We perform 2D high resolution thermo-mechanical numerical simulations of more than 200 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. 350 km wide basin of exhumed mantle bounded by two hyper-extended passive margins during a 60 Myrs rifting 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 sublithospheric mantle without modelling and resolving the convection. We discuss and quantify the general differences of the model results for the extension, cooling and subduction phase. We further quantify differences in effective viscosities in the models and their impact on the extension and subduction dynamics.
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Conference papers
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https://hal-insu.archives-ouvertes.fr/insu-02392444
Contributor : Isabelle Dubigeon <>
Submitted on : Wednesday, December 4, 2019 - 8:50:45 AM
Last modification on : Friday, December 6, 2019 - 1:18:24 AM

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Lorenzo Candioti, Stefan Schmalholz, Thibault Duretz. Impact of convection in the mantle transition zone on long-term lithospheric deformation during the Alpine cycle. Emile Argand Conference on Alpine Geological Studies 2019, Sep 2019, Sion, Switzerland. pp.14. ⟨insu-02392444⟩

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