Fault

Fault, in Mining and Geology, means the fracture of a stratum of rock, or of a series of strata, accompanied by dislocation. Miners sometimes call a fault a "heave" or "trouble," or term it an "upthrow" or a "downthrow fault," according as the bed in which they have been working is carried to a higher or to a lower level, the same fault being known by either name according to the direction from which it is approached. In geology the term upthrow is applied to that side of the fault on which the beds are higher, and downthrow to the other side, although the actual movement of the broken strata may have been upward, downward, or both. The line of fracture, or hade (a b), is sometimes vertical, coinciding with the throw, but generally it is inclined. In this case it usually dips or hades downwards towards the downthrow side of the fault, and faults in which this is the case are termed normal. A fault which hades towards the upthrow is termed reversed. In the case of a normal fault affecting horizontal strata the same bed cannot be pierced twice by a vertical shaft as it may be in the case of a reversed fault. The actual displacement of one part of a bed by a fault with reference to the other part, measured along the hade, is called the heave (c d), and may be resolved, like any other oblique movement, into an apparent vertical displacement and an apparent lateral displacement. The latter of these apparent movements is termed the width (c e) of the fault; the former is termed the throw (e d), and is the more readily ascertained. It may amount to thousands of feet. It is, however, remarkable that the great mass of rock uplifted above the general surface in such cases (g a) has commonly been so planed down by denudation that faults on a gigantic scale may make no difference in the surface levels. On a small scale faults may be sharply-defined lines; but even then the harder parts of the fractured surfaces will generally be polished into what are called slicken sides. More often the broken ends of rock are bent or crushed, the line of fault being sometimes occupied by a cemented breccia known as faultrock, which may be yards in thickness. The general direction of a fault along the surface, indicated by a white line on the maps of the Geological Survey, is termed its strike. Faults, being fractures that have commonly resulted from the giving of rocks under similar strains to those which have folded or inclined them, often coincide either with the dip (q.v.) or strike (q.v.) of the beds - i.e. they are either parallel to, or at right angles to, the'main folds, and are accordingly known as dip-faults or strike-faults. In place of a single fault we sometimes find a succession of parallel faults close to one another, each throwing the beds a little farther apart. These are called step-faults. Two normal faults hading in opposite directions produce what is known as a trough-fault. Faults are of the greatest importance in mining, since they often seriously interrupt the continuity of seams of coal or of mineral veins. They sometimes serve as preexisting fissures to direct the intrusion of igneous dykes, or to become lined with mineral ores, and so converted into veins. Interrupting the underground drainage, they are often marked by a line of springs, either at the surface or deep-seated, as in that at Burton-on-Trent, which intercepts the gypsum-charged waters from Needwood Forest and supplies the deep wells of the Burton breweries. Bringing rocks of entirely different characters together, they are often marked by abrupt change in scenery, as in the two great faults which cross Scotland. One of these runs from Loch Lomond to near Stonehaven, over 120 miles, with a throw of several thousand feet, bringing the crystalline Archeean rocks of the Highlands against the Old Red Sandstone, and tilting the latter on end for a thickness, or breadth of outcrop, of 2 miles. This forms the "Highland line" of Strathmore. The other fault runs from Dunbar to Girvan, with a throw that amounts in some places to 15,000 feet, and produces a contrast of scenery almost as marked as that of the Highland line.