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. 45° minus 30°/2, where 30° is the angle of internal friction). 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. Reverse faults are produced by compressional stresses in which the maximum principal stress is horizontal and the minimum stress is vertical. 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.Ī reverse fault is a dip-slip fault in which the hanging-wall has moved upward, over the footwall. A low-angle normal fault that develops on top of, parallel but in an opposite direction to a thrust sheet is a lag fault. In this case, a series of extensional faults, sometimes having a listric (‘spoon-shape’ or ‘concave upward’) shape, join at the detachment. Although the majority of normal faults are indeed high-angle, low-angle normal faults also occur because fault surface is not necessarily isotropic.Ī very low-angle normal fault at the base of an extending block is called a detachment fault. 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). The faulting takes place at a point at depth when lithostatic pressure exceeds the rock strength and horizontal stress is reduced along an axis. Normal faults are produced by extensional stresses in which the maximum principal stress (rock overburden) is vertical. Source: Rasoul Sorkhabi 2012 A normal fault is a dip-slip fault in which the hanging-wall has moved down relative to the footwall. 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).īased 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. Classification of Faultsįaulting 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. Nevertheless, Anderson’s elegant model provides a basic scheme for studying the geomechanics of faulting. 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. 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.Īnderson’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). 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. 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. 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). 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.įor 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).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |