Introduction. Strict application of the principles of original horizontality and superposition to sedimentary strata which no longer conform to these criteria demand the invocation of some type of dynamic post-depositional geologic event or events responsible for producing the observed stratal deformation. Structural geology is that branch of the science which is concerned with the determination of the three-dimensional architecture of a rock body as well as the identification and proper sequencing of those dynamic geologic events responsible for producing the observed structural features of the rock body and those of its neighbors in a particular region. While tectonicists are concerned with structural features that occur on a large scale such as the dimensions of lithospheric plates, structural geologists are concerned with features that occur within and between individual outcrops of rock to those which may only be revealed through examination (of a thin section) of a rock sample under a microscope.
Geometry of Tilted Strata. The two basic observations made by a structural geologist in the field at a particular rock outcrop of tilted sedimentary strata are strike and dip. The strike of a tilted sedimentary bed is, by definition, the line formed by the intersection of a tilted bed surface with the horizontal. The resulting line formed by the intersection of the bedding plane with the horizontal is by definition horizontal and its orientation with respect to geographic azimuth (in degrees) is measured and recorded. There are always two possible orientations which can be assigned to a line of strike. The (true) dip of a tilted sedimentary bed is the angle of inclination of the bedding surface, measured down-slope from the horizontal, in a direction which is orthogonal (perpendicular) to the strike. Since the dip direction is always perpendicular to the strike, the dip is assigned a simple number which corresponds to the number of degrees from the horizontal at which the strata is tilted. Flat-lying strata possess no dip and therefore would be assigned a value of 0 degrees. Vertically-standing (or upright) strata possess a dip of 90 degrees.Faults. A fault is a planar discontinuity which transcends (penetrates) a rock body and across which there has been relative displacement. That is, the rock body on one side of a fault has been moved (or displaced) with respect to the rock body on the opposite side of a fault. Faults are not to be confused with rock fractures or rock joints which are merely planar discontinuities which have not been accompanied by any relative rock displacements across them. Faults can be either brittle or plastic deformation features, associated with the breaking of large rock masses into smaller, relatively undeformed masses across narrow zones of high strain (deformation). Fault nomenclature and classification is based on the relative motion of the adjoining rock masses, the dip angle of the fault surface, and the types and orientation of stresses that created the faulting in the first place. There are four principal fault types, namely, normal, strike-slip (transcurrent), reverse (high-angle), and thrust (low-angle) faults [Figures 1 and 2]. Faults may also be composite, that is, they are a combination of two (or more) fault motion components. Faults may be short-lived or reveal great antiquity. They are present everywhere in the Earth's crust and upper mantle. They may have a length and width measured in centimeters, or they may extend uninterrupted for thousands of kilometers. Displacements of rock masses across faults is always accompanied by the release of stored energy in the rocks expressed in the form of an earthquake.Folds. The folding of rock strata in response to dynamic geologic events is a commonplace and well-documented phenomenon. Over great lengths ("geologic lengths") of time, rocks under stress can deform plastically, that is, they can flow like toothpaste being extruded from a tube.
Folding is often intimately associated with faulting and they are best observed in mountain belts,
being the products of the orogenic (mountain-building) process. Generally speaking, rocks buried at great depth will tend to respond to stress by bending as opposed to breaking. That is, folding of rocks is more likely to occur at great depth than faulting, and vice-versa. Whether or not
rocks deform by folding as opposed to faulting also depends upon the type of rock being deformed, its texture and composition, its strength and that of adjacent rock layers, and also upon the presence (or absence) of fluids (such as water). Fold type and nomenclature is based
upon their shape, symmetry, bed thickness, and orientation with respect to the horizontal. Vertically upward-arched folds are called anticlines whereas vertically downward-arched folds are called synclines. The plane which symmetrically divides a fold into bilateral parts (limbs) is the axial plane. Folds in which opposing limbs are parallel are referred to as isoclinal folds. Isoclinal folds in which the axial plane is horizontal is a recumbent fold. Fold dimensions vary from microscopic to kilometer lengths [Figures 3-6]. Axial planes are usually oriented perpendicular to the principal compressive stress direction. The vertical piling-up (imbrication) of many folded strata is a feature characteristic of most mountain (orogenic) belts.
Questions.
1. How does a structural geologist's work differ from that of a tectonicist?
2. Define strike and dip and how they are related.
3. Define a fault. How do faults differ from fractures and joints?
4. Define the four basic fault types.
5. Upon which criteria is fault nomenclature based?
6. Which dynamic geologic phenomenon is always associated with faulting?
7. How do folds differ from faults?
8. Which criteria and factors contribute to folding of rocks as opposed to faulting of rocks?
