Dynamics of subduction systems with opposite polarity in adjacent segments: application to the Westernmost Mediterranean
[eng] The objective of this thesis is to study the first-order dynamics of subduction systems characterized by opposite dip polarity in adjacent plate segments. The absence of previous studies analyzing the geodynamic evolution of these systems has defined the research strategy of the present work....
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| Tipo de documento: | tese |
| Estado: | Versão publicada |
| Data de publicação: | 2020 |
| País: | España |
| Recursos: | Universidad de Barcelona |
| Repositório: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/180170 |
| Acesso em linha: | https://hdl.handle.net/2445/180170 http://hdl.handle.net/10803/672453 |
| Access Level: | Acceso aberto |
| Palavra-chave: | Subducció Geodinàmica Models geològics Mediterrània occidental Subduction Geodynamics Geological modeling Western Mediterranean |
| Resumo: | [eng] The objective of this thesis is to study the first-order dynamics of subduction systems characterized by opposite dip polarity in adjacent plate segments. The absence of previous studies analyzing the geodynamic evolution of these systems has defined the research strategy of the present work. Consequently, the thesis consists of three different parts combining analog experiments and numerical models of very simple double subduction systems and its application to the Westernmost Mediterranean where a subduction system with opposite polarity in adjacent plate segments (double polarity subduction) has been proposed to explain the formation and evolution of this region. Part 1. Analog experiments of subduction systems with opposite polarity in adjacent segments Firstly, I have studied the first-order plate dynamics of subduction systems with opposite polarity in adjacent plate segments by means of analog experiments. Laboratory experiments have been carried out in the Laboratory of Experimental Tectonics in Roma Tre University during a two month stay in Rome (Italy) under the advice of Prof. Dr. Francesca Funiciello and Prof. Dr. Claudio Faccenna. A total of eighteen experiments have been designed, including four with a single plate subduction as reference models. The laboratory experiments are composed of one or two separates plates made of silicon putty representing the lithosphere and glucose syrup representing the mantle. The plates are fixed at their back edge to enforce a slab rollback behavior and subduction is started by deflecting manually the leading edge of the plate (i.e., initial slab pull). Different setups have been designed to test the influence of the width of plates and the initial separation between them on the evolution of the system. Results show that the mantle flow induced by both plates is asymmetric relative to the axis of each plate causing a progressive merging of the toroidal cells that prevents a steady state phase of the subduction process and generates a net outward drag perpendicular to the plates. Trench retreat velocities depend on the relative position of the trenches, increasing when trenches approach to each other and decreasing when they separate after their intersection. Part 2. Reproducing analog experiments of subduction systems with numerical modeling Secondly, some of the previous laboratory experiments have been performed by means of numerical modeling to compare and complement previous results and quantify the relevant physical parameters characterizing a double polarity subduction system. Around thirty-five numerical models, in addition to preliminary tests, have been performed although only fifteen are presented in this thesis showing the most outstanding and satisfactory results. The set of the numerical models have been run in the supercomputer MARENOSTRUM 4 (Barcelona Supercomputing Center, Spain) and BRUTUS (Swiss Federal Institute of Technology, Switzerland), with an average computation time of 3 weeks per model. The 3D numerical setup is chosen with similar material parameters, geometry and dimensions as for previous subuction analog models consisting on one or two viscous plates descending into the upper mantle in opposite directions. Plates are fixed at their trailing edge to enforce roll-back behavior during density-driven subduction. A small perturbation is initially imposed to initiate subduction. Firstly, computational domain size, boundary conditions, rheology and thickness of plates are tested to find the numerical model that best represents analog experiments. Secondly, relevant parameters controlling the double polarity subduction process are studied by means of numerical techniques. Results show that the most suitable numerical boundary conditions to reasonably reproduce the analog results are free-slip at the lateral boundaries and no-slip at the bottom of the model. Lateral boundary conditions affect the evolution of the system at short distances allowing for a reduction of the size of the model domain relative to the analog model and to increase the resolution and saving computation time. Complementing previous experiments of double polarity subduction, numerical results show that the induced mantle flow generates a stress coupling between the adjacent plates slowing down the overall subduction process and producing lateral movement of plates and asymmetrical deformation of the slabs and trenches. Part 3. The Alboran and Algerian basins (Westernmost Mediterranean). A case study of double subduction with opposite polarity in adjacent segments. Finally, a 3D numerical model of double subduction with opposite polarity in adjacent plate segments has been performed simulating the tectonic setting of the Westernmost Mediterranean. The evolution of the Alboran-Tethys slab (Betic-Rif slab) is reproduced in this tectonic scenario studying the influence of the adjacent plate segment. Around forty numerical models have been performed varying physical and geometrical parameters, including preliminary numerical tests. In this thesis only the final double polarity subduction model and a single plate subduction model are presented. The numerical models have been run in the supercomputer MARENOSTRUM 4 (Barcelona Supercomputing Center, Spain) with an average computation time of 4 weeks per model. The model setup consists of two oceanic plate segments with a visco-plastic rheology subducting in opposite directions into the viscous upper mantle and starting at 35 Ma. In the present-day Alboran Basin region, the plate segment corresponding to the Alboran-Tethys slab (Betic-Rif slab) dips to the southeast with the trailing edge fixed to the Iberian margin. A continental African plate segment is included at the west side of this plate. In the present Algerian Basin region the plate segment (Tell-Kabylies slab) dips to the northwest and the trailing edge is fixed to the African margin. A small slab perturbation to account for the Africa- Iberia convergence prior to 35 Ma is initially imposed and the subsequent subduction process is driven by Rayleigh-Taylor instability. In addition, a reference model including only the Alboran-Tethys slab has been performed in order to study the influence of an adjacent plate segment subducting in opposite direction. Results show that the progressive curvature of the Alboran-Tethys slab is due to the lack of a transform zone in its connection with the Atlantic oceanic to the west and the strong segmentation of the African margin. This produces larger retreat velocities in the eastern side of the slab, where a transform zone separates the Alboran segment from the Algerian segment, than in the western side. Trench retreat velocities of both plate segments are measured concluding that the opening of the Alboran Basin occurs around 22 Ma. The influence of the adjacent Algerian segment generates an asymmetrical flow pattern around both trenches slowing down the overall subduction process of the Alboran plate segment. |
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