9 research outputs found
On the dynamics of continental rifting: a numerical modelling approach
Get PDFPassive margins around the world commonly feature evidence of syn-rift small scale contractional deformation, such as reverse faulting and low amplitude folding, as well as evidence of basin depth inversions. Often, these features have no corresponding change in plate motion, and seem in conflict with the kinematic understanding that areas undergoing continental rifting should record only extensional deformation and subsidence. It has been proposed that syn-rift basin inversion may form as a result of gravitational body forces developing because of the upwelling asthenospheric dome beneath the rifted region, known as rift push. The aim of this thesis is to investigate the role these gravitational forces can play on the evolution of passive margins during rifting, and to quantify the role the upwelling asthenosphere plays in this process. To achieve this, we use high-resolution thermomechanical numerical modelling in both two and three dimensions, with non-linear, temperature, stress, and strain-dependent rheologies, to map the evolution of the stress regime throughout the lithosphere as the margins develop. Two dimensional numerical experiments show that the upwelling asthenosphere is capable of driving rift opening, and that transient compressional stress of up to 30 MPa develop within localized regions of the passive margin. When coupled with simplified surface processes allowing the formation of sedimentary basins, the experiments show that rift basins can undergo multiple phases of compression with no change to the applied rift kinematics, as they tend to localise compressive stress. We also explore the role of rift-push in the fundamentally three-dimensional context of continental rifting close to an Euler pole. It has been proposed that diachronous upwelling of the asthenosphere could induce compressional structures along the strike of the rift axis. Our experiments show that as the asthenosphere reaches break-up earlier at one end of the rift, a component of rift-push force is orthogonal to the rift axis (as per the previous experiments), and a second component is parallel to the rift axis, as the relatively unthinned areas of lithosphere are juxtaposed against the upwelled asthenospheric dome along the strike of the rift. In combination, these force components produce transcurrent and compressional stress within the developing passive margins, which matches earthquake focal mechanism data from similar realworld examples. Finally, to facilitate the access of thermo-mechanical modelling to a broader community of structural geologists and tectonicists, we have designed a prebuilt toolbox named the Lithospheric Modelling Recipes which includes a customizable 2D and 3D lithospheric model. In doing so, we aim to a) give structural geologists and tectonicists a user-friendly self-consistent framework to test their conceptual ideas, b) expand the pool of expertise and ideas tapping into geodynamic modelling, and c) enable a large number of geoscientists to critically assess geodynamic models, and d) contribute to the reproducibility of thermo-mechanical modelling
The role of surface processes in basin inversion and breakup unconformity
No full textInternational audienceIn the context of continental extension, transient compressional episodes (stress inversion) and phases of uplift (depth inversion) are commonly recorded with no corresponding change in plate motion. Changes in gravitational potential energy during the rifting process have been invoked as a possible source of compressional stresses, but their magnitude, timing, and relationship with depth inversions remain unclear. Using high-resolution two-dimensional numerical experiments of the full rifting process, we track the dynamic interplay between the far-field tectonic forces, loading and unloading of the surface via surface processes, and gravitational body forces. Our results show that rift basins tend to localize compressive stresses; they record transient phases of compressional stresses as high as 30 MPa and experience a profound depth inversion, 2 km in magnitude, when sediment supply ceases, providing an additional driver for the breakup unconformity, a well-documented phase of regional uplift typically associated with continental breakup
The role of asthenospheric flow during rift propagation and breakup
No full textInternational audienceContinental rifting precedes the breakup of continents, leading to the formation of passive margins and oceanic lithosphere. Although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling, or passive rifting caused by far-field extensional stresses, it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere. Three-dimensional numerical experiments of rifting near an Euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis-parallel motion in the asthenosphere dome, but also to promote along-axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole. We show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time-dependent phases of compressional and transcurrent stress regimes, despite an overall divergent plate setting. Our experiments predict overdeepened bathymetry at the tip of the propagating rift, as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting. We also explain the episodes of basin inversion documented in many rifted continental margins
Supplemental Material: The role of surface processes in basin inversion and breakup unconformity
No full textAdditional detail concerning the numerical method, internal and boundary conditions, and numerical parameters, and access to outputs of the entire suite of the numerical experiments. </p
UWGeodynamics: A teaching and research tool for numerical geodynamic modelling
No full textInternational 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
