TECTONICS. – geology (v.) which deals with: a) the conditions of position (attitude), form, and mutual relationships of the rocks constituting the earth’s crust or lithosphere; b) the causes to which these conditions may be attributed (tectogenesis in general; orogenesis, when it concerns the origin of mountains in particular). It is also called structural geology.
T. is divided into: a) t. of stratified or sedimentary rocks (v.); b) t. of eruptive rocks (v.); c) t. of metamorphic rocks (v.). The present configuration of any rock mass, considered in itself and in its relations with others, generally does not correspond to original conditions, but is the result of the movements to which it has been subjected throughout geological time. In other words, the rock masses that we see forming elevations, or in any case emerged parts of the lithosphere, are generally displaced with respect to the position they occupied when they were formed. This fact is especially evident in masses of stratified rocks, since this mode of occurrence is evidence of their origin through the deposition or sedimentation (mostly at the bottom of marine basins) of inorganic materials very frequently mixed with organic remains (fossils), e.g., shells.
The displacement of stratified masses may occur through two distinct and sometimes combined processes, namely as a result of folding or corrugation, or through fracturing into blocks which, during tectonic movements, are shifted relative to one another, with a predominantly vertical component upwards or downwards (displacements along faults).
The elementary part of a stratified mass is the stratum, whose thickness (or power) is given by the measurement of the common normal to the two plane surfaces delimiting II. The surfaces separating one stratum from another are called contacts.
Rarely is a stratified block displaced while retaining the horizontality which, 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 (a vertical stratum). In studying – for the most varied purposes – the t. of a non-horizontal stratified series, it is necessary to establish the position in space of the individual strata, whether they have remained planar (displacement along faults) or have undergone curvature (displacement through 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 which a horizontal line drawn on the surface of the stratum forms with the north–south line, that is, with the geographical meridian. The geologist’s compass is used for this purpose; it directly gives magnetic north, from which one passes to geographical north by knowing the present magnetic declination of the locality. The inclination corresponds to the angle which the surface (plane) of the stratum forms with the horizontal plane (a simple accessory device on geologist’s compasses permits this measurement). It is nevertheless necessary to indicate the cardinal point towards which the stratum dips.
In fold t., the curvature of a stratum may have its convexity directed upwards, and the fold is then called an anticline; in the opposite case, a syncline. If the two parts (limbs, flanks) constituting the anticline or syncline are symmetrical or not with respect to the axial plane, that is, the plane passing 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 up to the horizontal. The resulting folds are oblique and overturned, extending to recumbent folds, in which one limb is normal and the other overturned. In the former, the strata occur in normal stratigraphic succession; in the latter, in inverted succession. Fold associations derive their characteristics (style) from the type predominating in them (regular, isoclinal, fan-shaped, zigzag, etc.).
Special tectonic forms are: dome-shaped or pontive structures, in which the strata have a periclinal or quaquaversal arrangement (every section is an anticline); basin-shaped or basin or negative structures (every section is a syncline).
Recumbent folds, through the lengthening 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 takes place along the plane separating two blocks or packets of strata, called the fault plane, which may be vertical or variously inclined. If one of the blocks is displaced with respect to the other as though it had slid downwards along the inclined plane of the fault, the fault is called normal, extensional, or gravitational. If, on the other hand, one block has risen along the inclined fault plane, the fault is called reverse or compressional. Displacements may also take place tangentially (faults with horizontal displacement).
Faults may be variously associated in groups or systems (parallel, radiating, etc.), with a step-like arrangement (Germanic style) on one side or on both sides. In the second case, there may be an uplifted block (tectonic pillar, Horst), flanked on both sides by blocks successively lowered, or a downthrown block (graben or Graben) enclosed between two series of blocks successively uplifted (classic example, the “Rhine Graben”).
There are transitional forms between fold t. and fault t. (fold-faults), and likewise, in one and the same region, the two types may combine or follow one another over time.