Almost all solid substances break under pressure, if the pressure is great enough and pushing (or pulling) in the right directions. The layers of rock within the Earth’s crust are no exception to this rule. Push or pull on them hard enough, and rocks deep below the surface will break like a board kicked by a martial artist or tear apart like a towel caught in a tug of war between two puppies. The surfaces along which these breaks and tears occur are called faults by earth scientists. Most of the time, these faults are great, sloping planes that cut across many miles of the crust.
Faults are grouped depending on which way blocks of rock on the two sides of the fault plane move. The block above an inclined fault is termed the “hanging wall,” the block below the fault plane is called the “footwall.” If the hanging wall block moves downward, the fault is called a “normal fault” because it’s normal to expect something to slide down a slope. When the hanging wall moves upward, which happens when the rocks are compressed, the fault is called a “reverse fault.” Sliding upward on a slope is the reverse of what you’d expect. If the blocks appear to move sideways relative to each other instead of up or down, the fault is called a strike-slip fault. That name is because the movement, or “slip,” is along the length, or “strike,” of the fault.
Faults often cut the crust at angles that are close to vertical. There are also special cases of reverse faults, however, that lie at an angle of less than 45 degrees – sometimes nearly horizontal. These faults are called ‘thrust faults” because the hanging wall block is pushed, or thrust, over the top of the rocks under the fault, often for long distances. Hanging wall blocks of thrust faults are known as “thrust sheets,” because they are very thin compared to their width and length, like a sheet of paper.
Thrust faults are most common where two tectonic plates are colliding. The tremendous compression in the collision zone causes the crust to shorten, and a thrust fault is a very efficient way to achieve this shortening. Thrusting is most effective and easiest to recognize in layered rocks, in particular sedimentary rocks.
Thrust faults often run almost parallel to a soft layer of rock, such as shale, for long distances. If the shale layer ends or an obstruction occurs, the fault will bend upward at a steeper angle through harder rocks until it encounters another relatively soft layer, at which it bends to follow the layer again. Such changes in orientation are called flats (nearly horizontal) and ramps (more steeply dipping). Large thrust faults may pass through several ramp-flat changes along their length. Rocks in the thrust sheet usually lie almost horizontal above a flat, but are more steeply inclined above the ramps. The change from flat to dipping and back to flat is a form of folding
Because thrust faults occur at sites of continental collision, it is common for many faults to occur through time. When this happens, a complex of many stacked, or imbricate, thrust faults may result. Early thrust faults can end up included within the thrust sheets of younger faults, and rocks already bent and folded by the first fault may be folded again by subsequent faults, often several times. In an area of complex thrust faulting, it is not unusual to find rock layers so bent and folded that they are upside down.
Faults can be located in outcrop or by subsurface studies as lines or planes along which rock layers and other features are clearly mismatch. The key for recognition of a reverse fault, including a thrust fault, is comparing the age of the rocks on either side of the fault. The movement along a reverse fault always places older rocks in the thrust sheet on top of younger rocks in the footwall. This violates the law of superposition, a basic principle of geology. In complexly-faulted zones, the same rock layer or set of layers may occur several times in a stack. This “repeated section” is another key to identification of thrust faults.
Thrusting takes place at almost all collisional tectonic boundaries. At the present, thrusting is occurring along the plate boundaries between Africa and Europe (the Alps and Pyrenees), the Arabian plate and Asia (Zagros fault belt of Iran) and between the Indian subcontinent and Asia (the Himalayas). In North America, great thrust-fault complexes built the Appalachian Mountains chain several hundred million years ago and the Canadian Rockies about 65 million years ago.
