TECTONICS. - It is that part of geology (v.) which deals with: a) the conditions of position (attitude), form and mutual relations of the rocks that constitute the Earth's crust or lithosphere; b) the causes to which these conditions may be attributed (tectogenesis in general; orogenesis, if it concerns the origin of mountains in particular). It is also called structural geology.
Tectonics is divided into: a) tectonics of stratified or sedimentary rocks (v.); b) tectonics of eruptive rocks (v.); c) tectonics of metamorphic rocks (v.). The present arrangement of any rock mass considered in itself and in its relations with others does not generally correspond to original conditions, but is the result of the movements to which it has been subjected through geological time. In other words, the rock masses that we see constituting the reliefs, or in any case the emerged parts of the lithosphere, are generally displaced with respect to the position they occupied when they were formed. This fact is especially evident for masses of stratified rocks, inasmuch as this manner of presenting themselves is proof of their origin through deposition or sedimentation (for the most part at the bottom of marine basins) of inorganic materials very frequently intermixed with organic remains (fossils), e.g., shells.
The displacement of stratified masses can occur according to two distinct and sometimes combined processes, that is, through the effect of folding or crumpling, or through fracturing into blocks, which in tectonic movements are displaced relative to one another, with a prevailing vertical component upward or downward (displacements by faults).
An elementary part of a stratified mass is the stratum, whose thickness (or power) is given by the measurement of the normal common to the two plane surfaces that bound it. The surfaces that separate one stratum from another are called joints.
Rarely is a stratified block displaced while preserving the horizontality that, over a greater or lesser extent, characterizes the individual strata, as a consequence of their origin through sedimentation. A non-horizontal stratum is said to be inclined; the inclination lies between zero and 90 degrees (vertical stratum). In the study — for the most varied purposes — of the tectonics of a non-horizontal stratified series, it is necessary to establish the position in space of the individual strata, whether they have remained flat (displacement by faults) or have undergone a curvature (displacement by folds). This is achieved by determining the strike of the stratum and its inclination together with the direction of its dip.
The strike is given by the angle that a horizontal line drawn on the surface of the stratum forms with the north-south line, that is, with the geographic meridian. For this purpose the geologist's compass is used, which gives directly magnetic north, from which one passes to geographic north by knowing the current magnetic declination of the locality. The inclination corresponds to the angle that the surface (plane) of the stratum forms with the horizontal plane (a simple accessory device of geologist's compasses allows this measurement). It is necessary, however, to indicate the cardinal point toward which the stratum dips.
In fold tectonics, the curvature of a stratum may have its convexity facing upward, and the fold is then said to be anticlinal; in the opposite case, synclinal. If the two parts (limbs, flanks) that constitute the anticline or syncline are symmetrical or not with respect to the axial plane, that is, the plane that passes through the hinge, or the locus of the points of maximum convexity, the fold is called symmetrical or asymmetrical. The axial plane may be vertical or inclined even to horizontal. The result is oblique and overturned folds, even to recumbent, in which one limb is normal and the other overturned. In the first, the strata appear in normal stratigraphic succession; in the second, in inverted succession. Associations of folds derive their characteristics (style) from the type that predominates in them (regular, isoclinal, fan-shaped, zig-zag, etc.).
Special tectonic forms are: dome structures or pontive structures, in which the strata have a periclinal or quaquaversal arrangement (every section is an anticline); basin or negative structures (every section is a syncline).
Recumbent folds, through the elongation of their normal limb and the extreme stretching of their overturned limb, give rise to nappe or overthrust sheet structures (Alpine style).
Faults, as mentioned, result from fractures followed by displacement. This occurs along the plane of separation of two blocks or packages of strata, called the fault plane, which may be vertical or variously inclined. If one of the blocks is displaced relative to the other as if it had slid downward along the inclined plane of the fault, this is called a normal, tension, or gravity fault. If instead a block has risen along the inclined fault plane, this is called a reverse or compression fault. Displacements can also occur tangentially (faults with horizontal displacement).
Faults can be associated in various ways in groups or systems (parallel, radiating, etc.) with a mutual arrangement in step fashion (Germanic style) on one side only or on two sides. In the second case there may be a raised block (tectonic pillar, Horst), followed on both sides by progressively lowered blocks, or a lowered block (trough or Graben) comprised between two series of progressively raised blocks (classic example, the "Rhine Graben").
There exist transitional terms between fold tectonics and fault tectonics (fold-faults), and likewise within the same region the two types may combine or succeed one another in time.