July 28, 2020

Fault-propagation folds in a sandbox model

Posted by larryohanlon

By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

The sandbox model shown below developed nice fault-propagation folds during its deformation.

These anticlines are recognizable as fault-propagation folds because the fault that offsets the deepest blue layer does not cut upward through the entire section. Displacement along the fault at depth is accommodated by folding of the overlying, un-faulted layers. The image below shows generally where the fault, drawn in red, terminates.

Compressional fault-propagation anticlines often have a vertical or slightly overturned forelimb (the left limb of the fold above). Bed thickness in the upper folded part of the structure remains fairly constant, without obvious significant thickening in the hinge or thinning by “smearing” in either the footwall of the fault or the forelimb in the shallower parts of the structure. The model shows slight thickness variations, but these probably result from locally uneven layer deposition during setup.

The fault-propagation deformation style seen here probably results from the mechanical characteristics of the layer pack, which is dominated by the weak white microbeads. The uppermost light blue layers are quite strong and brittle, and the deepest thick blue layer is stronger than the microbeads but weaker than light blue. The very thin dark blue layers are intended to only provide a reference point to track deformation, but they exert some mechanical influence on how the white layers behave. In a real-world example, the forelimb of a fault-propagation fold would likely contain many small structures resulting from steepening of the bedding to a vertical or overturned orientation. The image below suggests what these small-scale faults might look like in the model, if more very thin layers were present to reveal them.

The forelimb is likely cut by a large number of very low displacement forethrusts. The beds that have rotated to vertical probably experienced slip in the bedding planes to accommodate the steep tilting. Above, I have drawn this bedding plane slip as occurring before the small forethrusts, which offset the slip plane. In reality, it’s all probably happening at the same time to create a zone of great small-scale structural complexity.

The structure at the center of the two images below is a fault-propagation fold that has undergone fault breakthrough due to continued shortening. Here, fault offset cuts all the way through the section.

Below, I have drawn faults onto the structure.

Much has been written about fault-propagation folding in compressional settings, and the diagram below by Suppe (sourced here) shows examples of breakthrough structures.

The breakthrough model shows both high-angle and low-angle breakthroughs. Although the model structure is completely fault-cut, its fault-propagation history is apparent because the deepest dark blue layer is much more displaced from its footwall cutoff than the orange layer upsection. The image below illustrates the differing offsets with yellow lines. Some of the displacement affecting the deep blue layer is accommodated in the breakthrough faults offsetting the orange layer, but these breakthrough displacements can’t account for the full displacement of the thick blue layer. This offset disparity indicates that for a period of the deformation sequence, the deep blue layer was being offset by a thrust fault, while overlying layers were accommodating the shortening by folding above the propagating fault.

I like to offer at least one real-world example of structures produced in the models I present, so here is a cross section from Suppe (1985) of the Meilin Anticline of western Taiwan, interpreted to have a simple and straightforward fault-propagation style (sourced here). Note the scale bar…this is a kilometers-scale (or miles) structure, and the models shown here are intended to represent the behavior of a few kilometers of sedimentary rock in a compressional fold belt.

Compared to the first models in this post, it bears some resemblance, particularly if the top half of the model below were lost to erosion.

This post was originally published in The Geo Models blog. It was reproduced here with permission.