The Earth’s crust is a dynamic layer fractured into numerous segments called tectonic plates, which are in constant, slow motion. The boundaries where these plates meet are sites of intense geological activity, defined by the relative movement between the adjacent plates. The immense forces created by plate motion are released through fractures in the rock called faults. Identifying the specific type of fault allows geologists to determine the underlying tectonic stress and the type of plate boundary responsible for the deformation.
The Anatomy of a Reverse Fault
A fault is a planar fracture in a rock mass where the rocks on either side have moved relative to one another. Geologists use the terms hanging wall and footwall, which originated from old mining practices, to define motion. The footwall is the rock mass beneath the fault plane, while the hanging wall is the rock mass above it. A reverse fault is defined by vertical movement where the hanging wall block moves upward relative to the footwall block. This upward movement results in the shortening and thickening of the Earth’s crust in that region.
The movement is a direct result of compressional stress, which is a force that pushes rock masses together. This compression forces the hanging wall to override the footwall, effectively pushing one section of crust on top of the other. Reverse faults with a shallow dip angle (typically less than 45 degrees) are often called thrust faults.
Convergent Boundaries: The Source of Compression
The geological setting that generates the necessary compressional stress to form a reverse fault is the convergent plate boundary. Convergent boundaries are regions where two tectonic plates actively move toward each other, generating the powerful squeezing forces required for crustal shortening.
When two plates converge, intense compression results. If an oceanic plate converges with a continental plate, the denser oceanic plate slides beneath the lighter continental plate in a process called subduction. This generates powerful friction and compression in the overlying plate, often resulting in massive reverse faults, sometimes called megathrust faults.
The second scenario involves two continental plates colliding, such as the ongoing formation of the Himalayas. Since continental crust is relatively buoyant and resists subduction, the landmasses crumple and fold against each other. This intense, sustained compression pushes rock layers upward and outward, forming vast mountain ranges and producing numerous reverse and thrust faults as the crust is stacked upon itself. The presence of a reverse fault is geological evidence that the area is, or was, undergoing plate convergence.
The Tectonic Context: Reverse Faults and Other Boundary Types
While the reverse fault is the signature of a convergent boundary, the other two major boundary types are also associated with unique fault styles reflecting their distinct stress environments. Divergent plate boundaries occur where two plates move away from each other, a process that places the crust under tensional stress, or pulling apart. This extensional force results in the formation of normal faults.
In a normal fault, the hanging wall moves downward relative to the footwall, allowing the crust to stretch and thin out. These faults are common features along mid-ocean ridges and continental rift valleys, such as the East African Rift Zone. The downward slip on a normal fault is the exact opposite of the upward movement seen in a reverse fault, which clearly distinguishes the tensional setting from the convergent one.
The third major setting is the transform plate boundary, where plates slide horizontally past one another instead of colliding or separating. This side-by-side motion creates shear stress, resulting in a strike-slip fault. Strike-slip faults, exemplified by the San Andreas Fault in California, show movement that is almost entirely horizontal with little vertical displacement. Because reverse faults uniquely indicate crustal shortening due to compression, they serve as a powerful identifier for past or present convergent tectonics.