One of the greatest geologists of the past century was the Scottish geologist Ernest Masson Anderson (1877–1960), who in his (now classic) work The Dynamics of Faulting and Dyke Formation with Application to Britain (Edinburgh, 1942, 1951) systematized our knowledge of the geometry and stress-fields of various faults.
For a three-dimensional rock volume, Anderson visualized three principal axes of stress, all of which are compressional but with different magnitudes: maximum (σ1), intermediate (σ2) and minimum (σ3). He reasonably assumed that shear stress at the ground-air or ground-water interface is zero: no shear occurs in fluids (of course, hurricanes may uproot trees and blow off roofs, but they are too weak to produce faults and earthquakes). Therefore, one of the principal stress axes must be vertical and increase with depth as the rock overburden (lithostatic pressure) increases; the other two stress axes are horizontal.
The direction of fault movement is such that fracture opens along the minimum stress axis and the slip occurs as the rock wedge containing the maximum stress axis moves inward. The angle between the maximum stress axis and the shear plane is called the angle of internal friction, and studies show that this angle is about 30° for most rocks.
Anderson’s stress model is strictly applicable if we assume that the deforming rock is isotropic (homogeneous throughout fault surface) and that structural deformation is coaxial (the stress axes do not rotate). In reality, rock types exhibit different mechanical strengths and inherit pre-existing fractures, and in the larger frame of the Earth’s crust, stresses may rotate. Nevertheless, Anderson’s elegant model provides a basic scheme for studying the geomechanics of faulting.
Classification of Faults
Faulting is a kind of strain (permanent deformation) in rock in response to stress which is usually supplied by the motion of tectonic plates relative to one another. As stress (force applied per unit area) builds up in a block of rock, a point reaches when the stress surpasses the rock strength and the rock then ruptures (yields to the stress).
Based on slip (direction of movement) of fault section and orientation of the stress axes, faults are broadly categorized into three types: normal, reverse, and strike-slip faults.
A normal fault is a dip-slip fault in which the hanging-wall has moved down relative to the footwall. Normal faults are produced by extensional stresses in which the maximum principal stress (rock overburden) is vertical. The faulting takes place at a point at depth when lithostatic pressure exceeds the rock strength and horizontal stress is reduced along an axis. Geometrical considerations dictate that such a fault plane dips at greater than 45°, or more precisely at 60° (that is, 45° plus 30°/2, where 30° is the angle of internal friction). Although the majority of normal faults are indeed high-angle, low-angle normal faults also occur because fault surface is not necessarily isotropic.
A very low-angle normal fault at the base of an extending block is called a detachment fault. In this case, a series of extensional faults, sometimes having a listric (‘spoon-shape’ or ‘concave upward’) shape, join at the detachment. A low-angle normal fault that develops on top of, parallel but in an opposite direction to a thrust sheet is a lag fault. Such an extensional fault forms almost simultaneously with the thrust fault at the base of the thrust sheet, and plays an important role in the tectonic exhumation of deep-seated rocks.
A reverse fault is a dip-slip fault in which the hanging-wall has moved upward, over the footwall. Reverse faults are produced by compressional stresses in which the maximum principal stress is horizontal and the minimum stress is vertical. In this way, the fault section is shortened in the direction of maximum compression and the fault dips at less than 45°, or in theory, strictly at 30° (i.e. 45° minus 30°/2, where 30° is the angle of internal friction). However, in nature steeply or shallowly dipping reverse faults do occur because of variations in the properties of rocks (such as their relative strength) on a fault surface.
A thrust is a low-angle reverse fault. In orogenic belts, such as the Alps, a thrust fault may transport a thick package of folded rocks over many kilometers; such a thrust sheet is called nappe in French and decke in German. Nappes, some of which have moved for over 100 km, have long been a paradoxical phenomenon in structural geology, and geologists have tried to explain them as a result of gravity gliding of rock on an orogenic slope (towards foreland); hydrothermal fluid lubrication along thrust planes; and incremental movement of the thrust over millions of years. Probably all these processes happen. Plate collisional tectonics provides the fundamental stress mechanism for the generation of large thrust sheets.
Seismic images from orogenic belts show that thrust faults are often rooted in a basal detachment or decollement. Moreover, in orogenic belts, thrust faults become younger toward the foreland; this sequence is referred to as foreland-propagating or piggy-back faults.
The terms overthrust and underthrust are sometimes used for low-angle, regional thrust faults with the implication that hanging-wall and footwall respectively was the active element in the thrust movement (although it is difficult to verify this). Upthrust is a high-angle thrust with a great amount of uplift, often involving basement rupture.
Reverse faults and associated folds may deform the basement rocks (thick-skinned deformation), or only sedimentary cover detached from the basement (thin-skinned deformation), or occasionally both the basement and sedimentary cover respectively in the hinterland and foreland of a mountain system.
A strike-slip fault is a nearly vertical dip-slip fault in which fault blocks move horizontally, parallel to the fault strike. In this kind of fault, both the maximum and minimum principal stresses are horizontal while the intermediate stress is vertical. The direction of strike slip may be left-lateral (sinistral) or right-lateral (dextral) with respect to an observer. Large strike-slip faults are also called wrench or transcurrent faults. A mega-shear is a continental-scale zone of deformation produced by strike-slip movement.
Regional strike-slip faults are usually composed of several strands. Sometimes two segments of a strike-slip fault partly overlap but are also separated by a step-over, jog or bend; the latter area is usually deformed by transtensional (releasing bend) or transpressional (straining bend) structures depending on the directions of strike-slip movements and step-overs.
Fault Inversion and Growth Fault
Tectonic inversion is the reactivation of a dip-slip fault resulting in the reversal of the sense of fault throw. Positive inversion is the changing of a normal fault to a reverse fault (reverse-reactivation); negative inversion is the changing of a reverse fault to a normal fault (normal-reactivation). Tectonic inversion occurs when a basin is a subjected to a new but contrasting tectonic regime, and stresses find it easier to build up in pre-existing faults.
Non-tectonic gravity-driven faults are also common, especially in sedimentary basins. These include faults generated by ductile movement of salt and shale, and also those caused by gravity gliding of strata on a slope. The latter include growth normal faults and toe-thrusts on passive continental margins, triggered by deltaic sediment overburden and continental slope.
An important way of analyzing faults in sedimentary basins is to note their relations to strata. For example, a bedding-parallel fault or ‘flat’ often moves along an incompetent (weak) layer, while a ramp develops in more competent (rigid) layers. Syntectonic sedimentation results in thicker growth strata on the down-thrown block which can then be used to identify pre-tectonic and post-tectonic sediments.
Part I - Know Your Faults!