Sustained indentation in 2-D models of continental collision involving whole mantle subduction

Continental collision zones form at convergent plate boundaries after the negatively buoyant oceanic lithosphere subducts entirely into the Earth’s mantle. Consequently, orogenesis commences, and the colliding continents are sutured together. During the collision, plate convergence and motion of the sutured boundary towards the overriding plate are manifest in its deformation, as is the case for the long-term (∼50 Ma) and nearly constant convergence rate at the India–Eurasia collisional zone that hosts the Himalaya. However, despite the long history of modelling subduction-collision systems, it remains unclear what drives this convergence, especially in models where subduction is driven solely by buoyancy forces. This paper presents dynamic self-consistent buoyancy-driven 2-D whole-mantle scale numerical models of subduction-and-collision processes to explore variations in density and rheological stratification of the colliding continent and overriding plate (OP) viscosity (a proxy for OP strength) that facilitate post-collisional convergence and collisional boundary migration. In models with a moderately buoyant indenting continent, the collisional boundary advance is comparatively low (0.1–0.6 cm yr–1), and convergence is driven by the dense continental lithospheric mantle that continues to subduct as it decouples from its deforming crust. Conversely, models with a highly buoyant indenting continent show sustained indentation at 0.5–1.5 cm yr–1 until the slab detaches. Furthermore, models with a weaker OP and lower backarc viscosity show an enhanced propensity for indentation by a positively buoyant continent. These models additionally highlight the role of whole mantle flow induced by the sinking of the detached slab in the lower mantle as it sustains slow convergence at an average rate of 0.36 cm yr–1 for ∼25 Myr after break-off as well as prevents the residual slab from educting. In previous buoyancy-driven partial mantle depth models such eduction does generally occur, given that free-sinking of the detached slab in the mantle is not modelled. Although these findings widen the understanding of the long-term convergence of indenting continents, the lower post-collisional advance rates (0.3–1.5 cm yr–1) compared to India’s approximate 1000–2000 km of northward indentation during the last 50 Myr attest to the need for 3-D models

UWGeodynamics: A teaching and research tool for numerical geodynamic modelling

International audienceThe UWGeodynamics module facilitates development of 2D and 3D thermo-mechanicalgeodynamic models (Subduction, Rift, Passive Margins, Orogenic systems etc.). It isdesigned to be used for research and teaching, and combined the flexibility of the Under-world Application Programming Interface, (Moresi, Dufour, & Mühlhaus, 2002, Moresi,Dufour, & Mühlhaus (2003), Moresi et al. (2007)) with a structured workflow.Designing geodynamic numerical models can be a daunting task which often requiresgood understanding of the numerical code. UWGeodynamics provides a simple interfacewith examples to get you started with development of numerical models. Users can startdesigning their models without any pre-existing knowledge of programming. Expert userscan easily modify the framework and adapt it to more specific needs. The code can be runin parallel on multiple CPUs on personal computers and/or High Performance Computingsystems.Although UWGeodynamics has been primarily designed to address geodynamic problems,it can also be used to teach fluid dynamics and material mechanics.UWGeodynamics uses the flexibility of the Python language and the Jupyter Notebookenvironment, which allows leveraging the wide range of scientific libraries available fromthe Python community. It also facilitates the coupling with existing scientific Pythonmodules such as Badlands (Salles, Ding, & Brocard, 2018).The functionalities include:•Dimensional input values, using user’s choice of physical units.•Automated and transparent scaling of dimensional values.•Sets of predefined geometries that can be combined to define the initial geometryof a model.•Handles Newtonian and non-Newtonian rheologies (Viscous, Visco-plastic andVisco-elasto-plastic).•Database of common rheologies used in geodynamics, which can be personalised /extended by users.•Simple definition of kinematic, stress, and thermal boundary conditions.•Lithostatic pressure calculation•Thermal equilibrium (steady-state) calculation.•Pseudo Isostasy using a range of kinematic or stress boundary conditions.•Partial melt calculation and associated change in viscosity / heat production.•Simple definition of passive tracers and grid of tracers.•Simple Phase changes•2-way coupling with the surface processes model pyBadlands (Salles et al., 2018).UWGeo comes with a series of examples, benchmarks and tutorial setups that can be usedas cookbook recipes. They provide a wide range of teaching materials useful to introducenumerical geodynamic modeling to students.New functionalities are constantly being added to the code and contributions are morethan welcomed. You can access the full documentation online athttps://uwgeodynamics.readthedocs.i