A duplex is a both a package of rock and a type structure. Old fault exposed by roadcut near Harrisburg, Pennsylvania. ...

A duplex is a type of thrust fault where there is a roof thrust and a floor thrust or decollement which split around a package of rock run parallel to each other. The package of rock is generally quite thick. Within the package of rock, several imbricate splays rise off the floor thrust and join to the roof thrust, isolating horsts of rock. USGS image In physical geography and geology, a horst is the raised fault block bounded by normal faults. ...

The duplex generally forms from several deformations, but may form within one prolonged deformation. Duplexing is an efficient way of accommodating tectonic shortening.

The floor thrust and roof thrusts generally climb up or down through the geological section. Duplexes can be foreland dipping or hinterland dipping.

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Appalachian folding, thrusting and duplexing Photos of structures and faults