TECTONICS
Tectonics is that branch of geology (v.) which deals with: a) the conditions of position (attitude), form, and reciprocal relationships 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, when it concerns the origin of mountains in particular). It is also called structural geology.
Tectonics is subdivided into: a) tectonics of stratified or sedimentary rocks (v.); b) tectonics of igneous rocks (v.); c) tectonics of metamorphic rocks (v.). The present arrangement of any rock mass, considered in itself and in its relationships with others, generally does not correspond to original conditions but is the result of 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 their mode of occurrence is proof of their origin through deposition or sedimentation (mostly at the bottom of marine basins) of inorganic materials frequently mixed with organic remains (fossils), such as shells.
The displacement of stratified masses can occur through two distinct and sometimes combined processes, namely by folding or corrugation, or by fracturing into blocks that, in tectonic movements, are shifted relative to one another with a predominant vertical component upward or downward (displacements by faults).
The elementary part of a stratified mass is the stratum, whose thickness (or power) is given by the measure of the common perpendicular to the two plane surfaces that delimit II. The surfaces that separate one stratum from another are called bedding planes.
Rarely is a stratified block displaced while retaining the horizontality that, over a more or less extensive area, characterizes individual strata depending on their origin through sedimentation. A non-horizontal stratum is said to be inclined; the inclination ranges between zero and 90 degrees (vertical stratum). In the study—undertaken for the most varied purposes—of the tectonics of a non-horizontal stratified series, it is necessary to establish the spatial position of the individual strata, whether they remain planar (displacement by faults) or have undergone curvature (displacement by folds). This is achieved by determining the strike of the stratum, its dip, and the direction of its plunge.
The strike is given by the angle that a horizontal line drawn on the surface of the stratum forms with the north-south line, i.e., with the geographic meridian. For this purpose, the geologist’s compass is used, which directly indicates magnetic north, from which one proceeds to geographic north according to the current magnetic declination of the locality. The dip corresponds to the angle that the surface (plane) of the stratum forms with the horizontal plane (a simple accessory device of the geologist’s compass allows this measurement). It is, however, necessary to indicate the cardinal point toward which the stratum plunges.
In fold tectonics, the curvature of a stratum may have its convexity directed upward, and the fold is then said to be anticlinal; in the opposite case, it is synclinal. If the two limbs (sides, flaps) that constitute 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 points of maximum convexity—the fold is said to be symmetrical or asymmetrical. The axial plane may be vertical or inclined up to horizontal. The result is oblique and overturned folds, even recumbent ones, 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. The associations of folds derive their characteristics (style) from the type that predominates (regular, isoclinal, fan-shaped, zigzag, etc.).
Special tectonic forms include: dome or positive structures, in which the strata have periclinal or quaquaversal arrangement (every section is an anticline); trough or basin or negative structures (every section is a syncline).
Recumbent folds, through elongation of their normal limb and extreme stretching of the overturned limb, give rise to nappe or overthrust 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 slipped downward along the inclined fault plane, the fault is said to be normal or of extension or of gravity. If, instead, a block has moved upward along the inclined fault plane, the fault is called a reverse or compression fault. Displacements may also occur tangentially (faults with horizontal displacement).
Faults may be variously associated in groups or systems (parallel, radiating, etc.) with a reciprocal arrangement in step-like fashion (Germanic style) on one side or both sides. In the latter case, there may be a raised block (tectonic pillar, Horst), followed on both sides by progressively lower blocks, or a depressed block (trough or Graben) enclosed between two series of progressively elevated blocks (classic example: the “Rhine Graben”).
There are transitional forms between fold tectonics and fault tectonics (fold-faults), and likewise in the same region the two types may combine or succeed one another in time.