Lithospheric-scale experiments of continental rifting monitored in an X-Ray CT scanner

When simulating lithosphere-scale rifting processes, analogue modellers have their model lithosphere float on top of a dense fluid representing the sub-lithospheric mantle (i.e. the asthenosphere). Such models provide crucial insights into rift evolution, but monitoring model-internal deformation has always been a major challenge. Here we present the results of new rifting experiments performed with a novel lithospheric-scale modelling machine that allows for X-ray CT-scanner, uniquely revealing the models’ internal evolution.

Our models involve a 4-layer lithosphere, with brittle layers for the competent upper crust and upper lithospheric mantel, and viscous layers for the ductile lower crust and lower lithospheric mantle. This model lithosphere is placed in a basin of glucose syrup simulating the asthenosphere and contained by mobile sidewalls. When stretching the model by moving these sidewalls apart (inducing either orthogonal or oblique extension), deformation is accompanied by syrup flow and isostatic compensation. A weakness within the upper mantle serves to localize deformation along the central axis of the model. We use photogrammetry and PIV techniques for detailed analysis of surface deformation, whereas CT imagery and PIV analysis of CT-sections provide unprecedented insights into internal model evolution.

We find that early on in orthogonal extension models, deformation initiates along the weakness in the upper mantle layer. This deformation is then transferred into the upper crust via shear zones in the lower crust, generating a dual graben structure there. In parts of the model, one of the grabens can become dominant and as extension progresses, so that a large shear zone cutting through the whole lithosphere forms (asymmetric, simple-shear rifting). In other parts of the model deformation may be more distributed so that both grabens are well-developed (symmetric, pure shear rifting). Meanwhile, the on-going stretching and thinning of the lithosphere splits the upper mantle layer, and the simulated lower mantle (and especially the asthenosphere) rises towards the model surface, bringing the lower mantle layer in contact with the lower crustal layer (i.e. necking of the lithosphere).

In oblique extension models initial deformation also localizes in the upper mantle layer, but no clear surface structures develops (except for a broad topographic depression along the central model axis). By increasing the extension velocity and thus the coupling between the upper mantle and upper crust, faulting initiated in the upper crust, creating two bands of en echelon grabens. Also in these models, we observe lithospheric necking.

Our (final stage) model results are similar to previous works. Yet the new CT-imagery provides the first-ever direct insights (both qualitative and quantitative) into the internal evolution of lithospheric-scale rift models. Furthermore, this new and versatile modelling machine in combination with our CT-scanning abilities provides a broad range of opportunities for advanced future lithospheric-scale modelling studies.

 

 

Figure: 3D CT image of an oblique extension model. UC: upper crust, LC: Lower crust, ULM: upper lithospheric mantle, LLM: lower lithospheric mantle, As: asthenosphere