Mathematical modelling of collision orogens

Résumé 0

The formation of orogenic belts, like the Western Alps and the Himalayas, is an impressive manifestation of Plate Tectonics. Many physical processes, such as subduction initiation, leading to the formation of collisional orogens are embedded in long-term (>160 Myr) geodynamic cycles often involving subsequent phases of extension, cooling without plate deformation and convergence. Observations from orogenic belts indicate deep burial and exhumation of continental and oceanic crustal rocks during mountain building. In fact, high-pressure (>1.3 GPa) and ultrahigh-pressure (>2.7 GPa), (U)HP, mineral assemblages were first discovered in continental rocks from the Western Alps in 1984 and have since been observed in mountain ranges worldwide. Quantifying the physical mechanisms driving the formation and exhumation of (U)HP units provides crucial insights into plate tectonic processes forming orogenic belts. Many geodynamic aspects related to mountain building remain elusive, including: (i) structural and thermal inheritance impacting on subduction-related processes, (ii) the magnitude and relative importance of forces driving the formation of orogenic belts and (iii) the mechanisms explaining the formation and exhumation of (U)HP units. This thesis addresses these aspects and aims at shedding new light on the formation of orogenic belts embedded in long-term geodynamic cycles of coupled lithosphere--upper mantle deformation. To this end, two-dimensional high-resolution petrological-thermomechanical numerical models are presented and the following deformation phases are modelled: (i) formation of hyperextended magma-poor margins and opening of a marine basin floored by exhumed mantle, (ii) cooling without plate deformation establishing upper mantle convection, (iii) model-internally consistent subduction initiation and closure of the marine basin and (iv) the formation and exhumation of coherent (U)HP units during mountain building. Deformation velocities are constrained by plate motion reconstructions from the European and Adriatic plate. The impact of upper mantle convection on the subduction phase, the dominating forces driving the dynamics within growing collisional orogens, and mechanisms for (U)HP rock exhumation are investigated. It is demonstrated that: (i) hyperextension, cooling without plate deformation, subduction initiation and the evolution of a subduction zone coupled to upper mantle convection can be predicted by a single model based on the fundamental laws of physics. The location of subduction initiation and the polarity of the evolving subduction forms spontaneously, i.e. without a priori prescription. The vigour of upper mantle convection controls whether single-slab or divergent double-slab subduction evolves. (ii) Shear force-dominated orogen dynamics form thrust wedges and major crustal volumes of the subducting plate escape subduction early. In contrast, buoyancy force-dominated orogen dynamics involve deep burial and either exhumation of minor crustal volumes close to the surface, or relamination of major crustal volumes below the upper plate. (iii) Buckling-induced local upper plate extension triggering buoyant uplift of coherent (U)HP units is a feasible synconvergent exhumation mechanism. Predicted peak metamorphic conditions, exhumation velocities and the deformation history of the exhumed coherent (U)HP units are consistent with petrological estimates, structural observations and geophysical data from natural (U)HP terranes such as the Western Alps and the Himalayas. The petrological-thermomechanical models presented here predict several first-order processes related to the burial and exhumation cycle of the crust within mountain building. Therefore, the models are transferable, robust and provide new insights into the long-term plate tectonic processes forming orogenic belts.