August 6, 2018

Normal or reverse? Tricky interpretations near high-displacement faults

Posted by larryohanlon

By Philip Prince, The Geo Models

I made this little compressional wedge model to test the mechanical contrast between various sands. The results weren’t quite what I was hoping for, but one very interesting geometry occurred just below the “backstop” thrust sheet. At the crest of the anticline just below the backstop thrust, the thin yellow layer is abruptly offset in a “down” direction, towards the base of the model. The offset isn’t large, but it is noticeable, particularly because of its orientation and apparent motion sense. Is it the product of extensional, normal-sense deformation? If not, how does it result from compression?

To go ahead and answer the question, it’s definitely not an extensional feature. A look above and below in the model shows no further evidence for normal-sense movement, and as the model maker I can tell you there wasn’t any! The feature is a small rotated thrust. Specifically, it’s a small backthrust that formed very early in the deformation sequence prior to being rotated almost 90 degrees.

The structure is not smeared and dragged by the overriding thrust sheet because its rotation is easily accommodated by slip on the white microbead layers. The microbead layers become particularly active once the rotate just a bit, as this brings them into an ideal orientation to slip within the stress field of a compressional wedge. Significant slip on fault 1 coupled with less slip on fault 2 is one component of rotation. Additional rotation occurs as fault 2 flattens moving up-section.

Passive slip in the microbeads along the front of the structure is also necessary. Strata overlying the odd structure would have reflected all of these steps, but they have thrust away and lost to erosion. This model experienced considerable erosion during shortening, constantly resetting stress trajectories and bringing new fault planes to life.

The model also shows the concept of “fold decapitation.” The rotated backthrust feature actually formed within a larger anticline at the leading edge of the backstop thrust sheet ( scroll back up a couple of figures). Subsequent detachment in the microbead layers within the anticline allowed its upper portions to be thrust away and lost to erosion. Remnants of the beheaded fold are apparent in the dark green layers.

The rotated backthrust is a small feature in the model, but it would be vary apparent during field mapping were it exhumed by erosion in a fold-thrust belt setting. Rotations in the footwall of high-displacement thrust systems can be very confusing when outcrop is limited. If rotation is sufficient to make sense of motion unclear, it’s possible to end up trying to explain phases of movement that didn’t occur.

The structural style of the model also bears close resemblance to interpretation of a section of the sub-Andes in Salta Province, Argentina, where out-of-sequence movement and multiple detachments lead to complex anticlinal structures (Echavarria et al. 2009). The model section is reflected to match the published interpretation shown here.

This post was originally published here