A geological fault is a fracture within the Earth’s crust where blocks of rock move relative to each other. This movement can happen suddenly, causing an earthquake, or slowly over a long period, known as creep. Faults are categorized based on the orientation of the fracture plane and the direction of the rock movement. A dip-slip fault is a fracture where the movement of the rock blocks is primarily vertical, occurring parallel to the dip of the fault plane.
Core Components and Direction of Movement
Understanding the mechanism of a dip-slip fault requires defining the two blocks of rock separated by the fracture surface: the hanging wall and the footwall. The hanging wall is the mass of rock situated directly above the inclined fault plane. This term originated from miners who would hang their lanterns on this upper block while working.
The footwall is the block of rock located beneath the fault plane. The fault surface can be at any angle between horizontal and vertical; this angle is the fault’s dip. In a dip-slip scenario, the blocks move along this inclined surface, meaning the displacement is entirely in the up-or-down direction, parallel to the dip.
The actual distance and direction of this displacement is called the slip. For a dip-slip fault, the slip is measured along the fault plane, indicating how far the hanging wall has moved vertically relative to the footwall. This vertical motion distinguishes dip-slip faults from strike-slip faults, where the movement is predominantly horizontal and parallel to the strike of the fault.
The Primary Types of Dip-Slip Faults
Dip-slip faults are divided into two categories based on the relative movement of the hanging wall and the footwall: the normal fault and the reverse fault. These types represent different ways the Earth’s crust responds to force. A normal fault involves the hanging wall block moving downward relative to the footwall block.
This downward motion lengthens the crust horizontally, creating space as the blocks pull apart. Normal faults are found in regions where the crust is being stretched, such as rift valleys. The Basin and Range Province in the Western United States is an example of this structural feature.
A reverse fault is characterized by the hanging wall block moving upward relative to the footwall block. This upward movement causes the blocks to overlap, resulting in the shortening and thickening of the crust. Reverse faults are associated with mountain-building processes where rock layers are compressed.
A specialized form of the reverse fault is the thrust fault, defined by having a shallow dip angle, usually 45 degrees or less. The low angle allows the upper block to be pushed a considerable distance horizontally over the lower block. Reverse and thrust faults are the mechanisms responsible for the formation of large mountain ranges.
Tectonic Stress and Geological Context
The formation of dip-slip faults is a consequence of the forces generated by plate tectonics. The two types of dip-slip faults are caused by distinct stress regimes acting on the crust. Normal faults occur in areas dominated by tensional stress, which is a pulling force that stretches the rock mass apart.
This tensional environment is found at divergent plate boundaries, where tectonic plates are moving away from each other. Mid-ocean ridges and continental rift zones, such as the East African Rift, are settings where normal faulting dominates. The stretching of the crust thins the lithosphere, often creating down-dropped valleys called grabens bounded by normal faults.
Reverse and thrust faults are generated by compressional stress, which is a pushing force that squeezes the rock mass together. This stress regime is characteristic of convergent plate boundaries, where tectonic plates collide. Subduction zones, where one plate is forced beneath another, and continental collision zones, like the Himalayas, are locations for compressional faulting.
The intense compression shortens the crust, forcing rock blocks upward and over one another, leading to uplift in major mountain belts. The classification of a fault indicates the underlying tectonic forces at work in that part of the Earth’s crust.