A geological fault is a fracture within the Earth’s crust where rock blocks on either side have moved relative to one another, a process known as displacement. Scientists classify these fractures based on the direction of movement and the forces that caused the break. Both reverse and thrust faults form when the crust is subjected to immense squeezing forces, known as compressional stress. These two fault types are expressions of the same underlying mechanism but differ significantly in their geometry and scale of movement.
Shared Identity: The Mechanics of Compressional Faulting
The unifying characteristic between reverse faults and thrust faults is that both result from compressional forces pushing rock masses together. This squeezing causes the crust to shorten horizontally and thicken vertically. Geologists define two key rock blocks relative to the inclined fault plane: the hanging wall (the rock mass situated above the fault plane) and the footwall (the mass beneath it).
The shared movement defining both fault types is the upward motion of the hanging wall relative to the footwall. This upward displacement against gravity is the defining feature of all compressional dip-slip faults. The result is that older rock layers are often pushed up and over younger layers. This characteristic movement distinguishes them from normal faults, which are caused by extensional stress.
Reverse Faults: Steep Angle Displacement
A reverse fault is defined by the steep angle of its fault plane, known as the dip. A fault is classified as a reverse fault if its dip angle measures greater than 45 degrees from the horizontal. This steep geometry means that the displacement along the fault has a substantial vertical component. The movement results in localized crustal shortening and a sharp, observable offset of rock layers.
Because of the steep inclination, movement is directed more upward than sideways. This results in the fault accommodating compression by stacking the blocks vertically in a concentrated area. Reverse faults generally describe structures that are smaller in scale and more localized. The forces acting on the rock are primarily directed perpendicular to the steep fault plane, leading to significant vertical uplift.
Thrust Faults: Low Angle and Regional Significance
The distinction between the two fault types is fundamentally based on the angle of the fault plane. A thrust fault is a specific type of reverse fault where the dip angle is 45 degrees or less, often approaching horizontal at less than 15 degrees. This shallow angle alters the nature of the displacement, shifting the movement from primarily vertical to predominantly horizontal.
This low-angle geometry allows the hanging wall block to be pushed laterally for great distances over the footwall. This massive horizontal transport of rock is known as a thrust sheet or nappe, and it can move tens to hundreds of kilometers. The resultant movement is large-scale crustal shortening and thickening, responsible for the formation of major mountain ranges, such as the Rocky Mountains and the Alps.
The shallow inclination allows compressional forces to effectively slide massive sheets of rock over underlying units. This process of stacking rock layers is known as thin-skinned deformation. Thrust faults represent regional geological deformation, accommodating compression across broad zones at convergent plate boundaries. The existence of older rock resting on top of younger rock over a wide area is a definitive sign of thrust faulting.