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Negative inversion of a sandbox model thrust fault

The Field

Inversion of normal faults during compression is a popular topic in structural geology, but thrust or reverse faults can also be inverted by extension. Like inverting normal faults, favorable fault dips are an important part of this process, particularly when it is reproduced in a sandbox model like the one shown here. A detail of the thrust inversion setup that may impact the slightly lower dip of the initial normal fault segment is the presence of the weak microbead decollement layer on the thrust ramp. During shortening, the microbead layer is carried up the ramp. The microbeads have very low internal friction and are exceedingly 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.