By Philip S. Prince
Inversion of normal faults under compression is a popular topic in structural geology, but thrust or reverse faults can also be inverted in extension. Like inverting normal faults, favorable fault cases are an important part of this process, especially when rendered in a sandbox model like the one shown below.
This model developed an initial steep thrust fault due to oblique shortening. The thrust, along which the yellow layer moved upward, was reactivated during direct extension to offset the interbedded green-blue and pink-blue horizons. These interbedded horizons post-compression and extension pre-date. The annotated image below shows the main thrust fault below the offset yellow interval and associated extensional fault segments. The yellow fault segment was the first to move during the first extension; the black extension of the main traction plane started to move later in the extension phase. The white arrow indicates a purely extensional segment that expanded from the existing thrust during reactivation … the strata it displaces would yet be deposited during the compressional/thrust phase.
A particularly interesting detail in this model is the apparent reverse fault and mild anticline in the blue-green part above the yellow thrust structure. Although it initially appears related to the traction/compression phase of the model, it is actually a purely extensional structure related to downdip steepening of the initial extensional fault motion. The image below highlights the fault segments moving during the first extension; the abrupt downward steepening as the shallow fault deflects into the thrust plane forces the folding of the blue-green sequence.
The development of this model revolves around the steepness of the initial compression/tension structure, which is steep due to the highly oblique shortening that caused it. Direct shortening would have produced a shallower error that would not be reactivated during lengthening. The GIF below gives an overall sense of the orientation of oblique shortening, followed by direct lengthening.
During the extensional phase, faint signs of the various extensional fault strands and the associated folding can be seen on the surface of the model. In the images below, the location of these features is indicated on the cross-section of the model already shown.
The deformation sequence for this model involved considerable syn-compressional deposition, with greensand deposited along the flanks of the prograding thrust anticline. This setting was used to produce the final compression geometry with the yellow range on the tensile wall, completely surrounded by green. The yellow sand (dolomite sand) is stronger and more brittle than the green sand (quartz sand), so this geometry provides an interesting strength contrast zone within the layer package. Drawing errors and the location of the shortening surface on the cross section would look something like the annotated image below.
This model is in fact the opposite of a normal fault reactivation or basin inversion setting, which would require direct extensional movement followed by oblique shortening to invert the existing normal faults. At least in the sandbox model world, both of these inversion methods require oblique compression to produce thrust or reverse faults with sufficient steepness to match normal faults, whose dips are always steep in these models. A model of basin inversion produced by the “inverse” of the setup shown here is shown below.
One detail of the thrust inversion setup that may affect the somewhat lower dip of the initial normal fault segment is the presence of the weak microbead detachment layer on the thrust ramp. During shortening, the microbead layer is carried up the ramp. The microbeads have very low internal friction and are extremely unstable on the ramp at its high dip. As soon as compression is released and the layer pack can “relax”, collapse within the steeply dipping microbead horizon begins immediately.
Finally, the overall appearance of the model disc is shown here. This setup consumes a pitiful amount of whatever sand color is used for the growth sequence. The model was developed on a flat, flat base plate. The slight curve at the base seen here is the result of careless slicing and drying!
Although this is a highly idealized setup with a specific outcome in mind, structures such as this are developed in environments where stress regimes can change significantly and the potential for rapid settlement and syn-deformation is high. Back-arc settings are a good example, as subduction angle, behavior of the subducting plate, shifts in orientation of plate motion, etc. can greatly alter deformation and depositional conditions in the overriding plate. Such a setting may experience extension and deposition, localizing compressional inversion to produce isolated thrust structures such as the one shown above, followed by a return to the extensional or transtensional stress regime to cause collapse. I like this model because it nicely shows the importance of stress field orientation and the independent variable of material strength on deformation patterns.
Philip Prince is a project geologist with Appalachian Landslide Consultants, PLLC, in Asheville, North Carolina. He also performs geologic mapping in the Virginia Valley and Ridge for the Virginia Department of Mines, Minerals and Energy. More posts related to his field experiences and remote sensing work can be found at princegeology.com.
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