Thrust/reverse faults are dip-slip faults on which the hanging wall block has moved up relative to the footwall block. They accommodate horizontal shortening of the Earth’s crust. Reverse faults with dips shallower than 45 degrees are called thrust faults.
-A thrust sheet, or nappe is a hanging wall block above a very low angle thrust.
-Autochthon, a large region of rock that has not been moved and is close to its original location.
Shape of Thrust Faults
Most thrust faults exhibit similar deformation features intrinsic to faults. Thrust faults usually emplace older rocks on top of younger rocks, with the exceptions of when the strata dip more steeply in the same direction as the fault or in areas of complex deformation where the stratigraphy is overturned or folded.
At depth, thrust faults are usually listric faults that curve toward shallow or horizonatal dips with increasing depth. Some other features that one might also find at a thrust fault are:
-Klippe, an isolated remnant of the allochthon that has been separated from the main sheet through erosion
-Window or fenster, a hole through the thrust sheet that exposes an isolated area underneath
-When thrust faults have an alternating flat ramp geometry, movements from the faults cause fault-bend folds to form in the hanging wall block
The most common example of large thrust systems are the thrust faults in foreland fold and thrust belts which mark the margins of major orogenic belts. Parts of this system include:
-Foreland, the area in front of the thrust towards which the thrust sheet moves, region behind is the hinterland – systems are generally curved and extend 100’s to 1000’s of kilometers.
-Thrust faults in a system don’t need to have the same vergence, back thrusts can develop with the opposite vergence through tectonic wedging
Kinematic Models of Thrust Fault Systems
Model 1. Duplex structures result from the progressive cutting of the thrust fault into the footwall block (Fig 5.14)
Model 2. Duplex develops by stepwise retreat as the thrust fault frontal ramp steps back into the hanging wall block, leaving horses in the footwall. Upper glide horizon remains the same after each stepwise retreat of the frontal ramp (Fig 5.18 A)
Model 3. Same as 2, but the upper glide horizon steps up in the structure with each stepwise retreat of the frontal ramp (Fig 5.18 B)