During my honours year in 2019 I ran a suite of numerical experiments investigating the structural styles of fold-thrust belts brought on by the mechanical layering of competent and incompetent units within a stratigraphy.

Understanding how the mechanical stratigraphy influences structural styles across various scales could be very useful in predicting displacement along faults and inferring fold styles.

The layering of these models consists of both short wavelength and long wavelength competency oscillations (e.g. a unit of sandstone will oscilate in strength within an overall stratigraphic package – see figure below).

We find that multiple wavelengths of deformation are activated in our models. A large-scale wavelength of deformation forms due to the mechanical contrasts within the overall stratigraphic package, and a more local wavelength of deformation forms as a result of mechanical heterogoniety within single units. This smaller wavelength of deformation gives way to a more complex and heterogenous set of structures.

Investigating the ratio of competent to incompetent units as a control of structural style

Previous studies have proposed that it is the ratio of competent to incompetent (C:I) rocks that has the largest control on deformation style (Ferrill and Morris, 2008). Our experiments show that the C:I ratio may have a strong influence but does not solely control the structural style. For example in the animations below the C:I ratio vary greatly, but the structural styles are very similar.

As opposed to solely the ratio of competent to incompetent rocks controlling the deformation style, it is likely a combination of both the ratio and the location of the competent units relative to the incompetent units that control deformation style. The models below have the same C:I ratio but exhibit very different structural styles.