The term is derived from the double storeyed wall-to-wall (to prevent from cold) houses built first in Canada and then built probably everywhere else in the world. DUPLEX is a thrust sheet (mass) which is bounded by a 'fioor thrust' and a 'roof thrust' Often, duplex structures are imbricated. Blind thrusts are those below the erosion surface but their presence is indicated by shortening in the overlying sequence which is achieved by disharmonic detachment folding.

A system of imbricate thrust faults that branch off from a floor thrust below and curve upward to join a roof thrust at a branch line is called a duplex. The fault-bound bodies of rock in the duplex are horses. Duplexes have a variety of forms depending on the amount of displacement of the individual horses.

3. Foreland dipping

Understanding the formation of duplex systems provides a basis for our understanding how thrust faults develop in a complex imbricate thrust fan.

To understand how thrust systems form, we need to know the sequence of development of the faults in duplexes and imbricate fans. The structure of duplexes can give us clues as to the order of thrusting and folding in a given situation. In most, but not all, cases thrust duplexes form as a result of the stepwise advance of the sole fault into the footwall and the progressive incorporation of the resulting horses into the hanging wall. Five possible models for the formation of duplexes have been advanced. The first three involve progressive faulting into the footwall with different amounts of displacement of older horses relative to newly formed horses (hinterland, antiformal, foreland). The fourth involves different ordering of thrust fault formation, with faulting into the hanging wall (out-of-sequence thrust). The fifth involves cross cutting of older faults by younger faults in the hanging wall.

1. Hinterland-dipping duplexes- The general process lies in that the successive thrust faults cut into the footwall forming horses. Younger faults tilt the previous fault and the horse back toward the hinterland. New horses do not move beyond the tip of the next successive thrust fault into the footwall Displacement is progressively transferred from the faults bounding the duplexes to the floor thrust and the roof thrust is never active as a distinct fault.

The sole fault ramps progressively into the footwall toward the foreland. Horses of the footwall are incorporated into the hanging wall.The roof thrust above the duplex structure is never active as a distinct fault. Segments of the upper glide plane are inactive by the time they become part of the roof thrustActivity ceases at different times on the various segments of the roof thrustThe segments above the youngest horse have the longest history of thrusting. 2 Antiformal Stack: Duplex horses move to or just beyond the start of the next horse.

3 Foreland-dipping duplex model: If the displacement is still greater, so that only the rear end of the youngest horse overlaps the front end of the incipient horse, a foreland dipping duplex develops.

4 - Out-of-sequence thrusting: Ramping thrusts into the hanging wall block - examples of this development are not numerous.

5 - Duplex truncation: Duplex

formation by truncation of a pre-existing imbricate thrust fan by a younger overriding roof thrust examples of this are rare.