April 22, 2020

Listric normal faults with glass microbeads

Posted by larryohanlon

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

Deformation associated with listric (downward flattening) normal faults produces very interesting patterns. A natural example revealed by sesimic imaging is shown below, sourced from this site:

Rollover anticline features like the one shown above are often associated with deposition of clastic sediment atop a very weak and mobile substrate, like salt or overpressured shale. This deformation style can be replicated in physical models using silicone putty or elevated pore fluid pressure at the base of a layer pack. These models are extremely sophisticated and require significant time and material investment, and their results are quite striking.

Granular media with very extreme mechanical contrasts can be used to create listric normal faults and rollover-type structures in a more expedient way–the model shown below can be made on a desktop in a few minutes (gelling and slicing it still takes a while!). While the weak glass microbeads do not move or deform under loading and stress in the same way as silicone putty, the curving fault geometry needed to create progressive tilting of layers and the recognizable rollover shape can be produced, and deformation rate has no impact on the model’s evolution.

In the image above, the pink and green layers are material deposited during movement of the model. The mechanical contrast between the pre-movement yellow layers, which fault at a high angle, and the underlying weak microbeads produces the downward-flattening breakaway fault and associated rollover. The microbeads are a frictional material, but the friction between the particles is very, very low. With movement of the left side of the model away from the fixed right side and steady addition of sand layers during movement, the microbeads can be “squeezed” out from beneath the growing basin fill and underlying rigid base due to their greater ductility. This leaves a thin seam of microbeads above the model’s base.

Deformation is produced by moving a base plate beneath half of the initial layer pack. The major breakaway fault forms on the moving part of the model (see the GIF further down). In the image above, the base plate is beneath the left side of the model, with the large red arrow indicating movement direction. The edge of the base plate (which is thin, plastic-coated freezer paper) is just left of where the lowermost pink layer touches the top of the yellow layers near the bottom of the image.

Curvature of the main fault is key to creating the significant rotation/tilting of the oldest layers. As the left side of the model is withdrawn, the hanging wall (fixed part of the model here) must change its shape to match the changing shape (downward flattening) of the main fault. The gap shown in the GIF above never forms; the hanging wall is in constant contact with the fault surface/footwall, and deformation is progressive. The shape change requires the top of the hanging wall bock to stretch, which is accommodated by normal faults. These faults cut the pre-movement yellow layers and pink and green layers deposited atop them during movement. Progressive deformation causes early faults to rotate to unusual dip angles.

The effects of stretching of the hanging wall block can be observed on the surface of this type of model during movement. The main breakaway fault is at the far left; downward movement of the hanging wall is focused here and can be seen on the model surface. This forces stretching of the hanging wall further to the right, which creates small rotating blocks to accommodate the stretching.

Below, the yellow arrow points to the syncline labeled in the GIF. The broad anticline labeled in the GIF is down and left of the point of the arrow; continued movement of the model caused a crestal graben (the thin bedded, triangular zone down and left of the yellow arrow) to develop on the crest of the anticline.

The narrow syncline forms between two small rotating blocks. The block to the right of the arrow point rotated counter-clockwise, which the block to its immediate left rotated clockwise. The tilting of these blocks can be observed in the GIF showing the surface of the model. Block rotation resulted from stretching of the layering as it moved from the curved portion of the master fault onto the flatter portion, as shown in the conceptual drawing. This is a very localized example of the deformation that occurs when a layer pack is stretched above an elastic base to produce a “domino block” style. You can see the syncline form in the area between these rotating blocks in the GIF showing the model surface.

This post was originally published on The Geo Models blog.