A reverse fault is a specific type of fracture in the Earth’s crust where rocks on either side have moved relative to one another. Geological faults are features that represent a significant break in the rock layers. Movement along these fractures, known as the fault plane, is what allows the blocks of crust to be displaced. This displacement is a fundamental process in how the Earth’s surface changes over time.
Defining Fault Geometry
To understand the mechanics of a reverse fault, it is necessary to identify the two major blocks of rock separated by the angled fault plane. These blocks are termed the hanging wall and the footwall, a terminology borrowed from early English mining. The hanging wall is the mass of rock resting directly above the fault plane.
The footwall is the block of rock that lies beneath the fault plane. The angle at which the fault plane dips into the Earth is an important characteristic. The relative movement of the hanging wall and footwall is used to classify the fault type.
The Driving Force: Compressional Stress
The formation of a reverse fault is driven by compressional stress, which occurs when forces push blocks of rock together, causing them to be squeezed. This intense pressure must build up until it exceeds the inherent strength of the rock mass, leading to a sudden fracture.
Compressional stress fundamentally results in the shortening and thickening of the Earth’s crust. In contrast, tensional stress pulls rocks apart, which is the force responsible for a normal fault.
This constant squeezing and shortening deformation is often associated with the building of major mountain ranges. When this stress is suddenly released during the fault’s slip, it is the direct cause of an earthquake.
Mechanism of Reverse Fault Movement
The mechanism of a reverse fault is defined by the upward movement of the hanging wall relative to the footwall. As the crust is subjected to compressional forces, the rock blocks are shoved toward one another, resulting in one block being thrust up over the other. This action is described as a dip-slip movement, meaning the displacement is predominantly vertical along the fault plane.
The result of this upward motion is a net shortening of the crustal area, reducing the total horizontal distance spanned by the rock layers. Reverse faults typically have a fault plane that dips at greater than 45 degrees from the horizontal.
A specific subtype is the thrust fault, defined by a shallower dip angle. Thrust faults are reverse faults where the angle of the fault plane is less than 45 degrees.
Tectonic Environments of Formation
Reverse faults are primarily found in geological settings where tectonic plates are actively moving toward each other. These regions are known as convergent plate boundaries. The collision of large-scale lithospheric plates generates the necessary compressional stress to drive reverse faulting.
One common environment is a continental collision zone, where two continental plates move together. The Himalayas, for example, owe much of their significant uplift and size to a series of reverse and thrust faults formed by the ongoing collision of the Indian and Eurasian plates.
Reverse faults also form in subduction zones, where one tectonic plate is forced beneath another. The intense compression in the overriding plate generates these upward-moving faults.