Guillaume Duclaux

Three-dimensional thermo-mechanical modelling unravels the complex deformation history of oblique rifts, transtensional margins, and gneiss domes

Throughout the Wilson cycle the obliquity between lithospheric plate motion direction and nascent or existing plate boundaries prompts the development of intricate three-dimensional (3-D) tectonic systems. Where oblique divergence dominates, as in the vast majority of continental rift and incipient oceanic domains, deformation is typically transtensional and strongly partitioned. Large stretching in the brittle upper crust is primarily achieved by the accumulation of displacement on fault networks of various complexities, while viscous flow and anastomosed shear zones dominate in the ductile lower crust. The occurrence of contrasting and superimposed structures and fabrics in these regions has fuelled countless debates about driving forces and tectonic interpretations, including timing of deformation.

But, there is a silver lining. Over the past three decades thermo-mechanical numerical modelling has transformed the way we look at deformation in the lithosphere. Thanks to the recent progress of computational methods, numerical models have evolved from 2-D to 3-D, and reached sufficiently high resolution that their predictions can be compared with analogue experiments and natural systems. More than just generating aesthetically pleasing pictures, outputs from numerical models contain a rich source of quantitative information that can be used to measure deformation quantities in 2- and 3-D. Post-processing adds value to the modelling and aims to produce visual information that will resonate to seasoned structural geologists, and assist with comparing experimental and observational data in order to unravel the deformation history of past and present tectonic regions.

Here, I will present a set of high-resolution 3-D thermo-mechanical finite element models of a simplified layered lithosphere, which combine non-linear viscous and strain-weakening frictional-plastic rheologies, undergoing oblique extension. These models explore the relative importance of varying initial inherited structures geometry, and crustal rheologies during oblique rifting and transdome formation. To analyse these models we apply newly developed methods to measure and visualise (1) the spatiotemporal evolution of active faults distribution and local stress orientation at the surface, and (2) the progressive development of finite strain patterns in the lower crust.

The analysis of the relative timing and distribution of structures and fabrics in the continental crust allows us to propose a deformation sequence for the models, and discuss the underlying controls governing deformation style in the obliquely extending continental crust. We found that in transtension and thanks to local stress rotations, extension-orthogonal fault arrays preferentially form at the surface during the early segmentation stage of the lithosphere to accommodate the thinning of the upper crust. The progressive rotation and overprinting of active structures at the surface after segmentation of the rift, along with the narrowing of the active deformation zone, suggests an increasing mechanical coupling between the brittle upper crust and the rising viscous sub-lithospheric mantle, that ultimately ends up with oblique breakup. In decoupled models with a weak viscous crustal rheology, strain partitioning promotes the development of oblique gneiss domes in the lower crust, while anastomosed fault networks form at the surface.

Conclusions drawn from our 3D thermo-mechanical numerical models are consistent with observations from oblique continental rift and transtensional margins in East Africa, and gneiss domes in the French Massif Central. This work confirms the importance of the rheological layering of the lithosphere and the geometry of inherited structures for controlling the characteristic orientation and spatiotemporal distribution of deformation in transtensional systems.