The previous chapter dealt with planar geological surfaces. A geological surface which is curved is said to be folded. Most folding is the result of crustal deformation whereby rock layering such as bedding has been subjected to a shortening in a direction within the layering. To demonstrate this place both hands on a tablecloth and draw them together; the shortening of the tablecloth results in a number of folds.

Structural geology is obviously one of the more important subjects for geoscientists working in petroleum industry. Folds and faults in deformed rocks make traps for hydrocarbon accumulation. Also, large-scale deformations, the so-called tectonics, control the architecture of petroliferous sedimentary basins. It is the primary job of a structural geologist to interpret geological map and field data, and infer geometry of large scale folds and faults. However, geoscientists with varied specializations and working with different kinds of data may also be called upon to make structural interpretations.

Folds in ductile shear zones are common structures that have a variety of origins. These can be pre-existing folds that become modified or folds developed during the shearing event. Among the syn-shearing folds, a second subdivision is based on the relative age of the folded surface, which can be pre-existing or newly formed during the shearing event. In each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame. The final fold geometry depends on the vorticity within the shear zone, the rheology and the initial orientation of the folded surface relative to the kinematic framework. It follows that folds are complex structures, difficult to use as kinematic indicators. However, in shear zones where undeformed wall rocks with pre-shear structures are accessible and where kinematics can be well established, folds can provide a valuable natural means to understand the initiation and evolution of structures under non-coaxial regimes. We point to the need of discriminating among different plausible categories, based on the nature of the folded surface and on the inherent structural features of each category. <...>

Number 1
The Casanova Complex of the Northern Apennines: a melange formed on a distal passive continental margin
A structural analysis of the Moine Thrust Zone between Loch Eriboll and Foinaven, NW Scotland
The origin of tensile fracture lineaments
Subcritical crack propagation in rocks: theory, experimental results and applications
Calculations of volume change in ductile band structures
The problem of shearing along axial plane foliations

I have been teaching Structural Geology and Tectonics for the past four decades and during this period, I always felt a need for a glossary or definitional dictionary devoted to this branch. I therefory, made an attempt in this direction. The editor for while doing the groundwork of the book made use of Glossary of Geology edited by R. Bates & J. Jackson (Am. Geo!. Inst., 1980); Glossary of Geology in Hindi edited by myself (CSTT Govt. of India 1996); Introduction to the structure of the earth by E. Spencer; Tectonic Geology by myself (2006); Geology: an Introduction by myself (2004).

Structural Geology is the study of deformed rocks. To do so, we define the geometries of rock bodies in three dimensions. Then, we measure or infer the translations, rotations, and strains experienced by rocks both during, and particularly since, their formation based on indicators of what they looked like prior to their deformation. Finally, we try to infer the stresses that produced the deformation based on our knowledge of material properties. Structure is closely related to various fields of engineering mechanics, structural engineering, and material science. <...>

Number 1
Folds and the strain ellipsoid: a general model
Patterns of fold interference: influence of early fold shapes
Brittle deformation in pitted pebble conglomerates
Kinematics of late Mesozoic thrusting, Pilot Mountains, west-central Nevada, U.S.A.
An inverse problem in microtectonics for the determination of stress tensors from fault striation analysis
The effect of the basal-prism mechanism switch on fabric development during plastic deformation of quartzite
Mineral reactions participating in intragranular fracture propagation: implications for stress corrosion cracking
Diapirism and gravity tectonics: report of a Tectonic Studies Group conference held at Leeds University, 25-26 March 1980

Hercynian mylonite belts in the eastern Pyrenees: an example of shear zones associated with late folding
The Caledonian thrust and shear zones of N.W. Scotland

Shear zones in the Precambrian crust of Southern Africa
The role of shear belts in the structural evolution of the South Harris igneous complex
The South Armorican shear zone
Shear zones in the granodioritic massifs of the Central Pyrenees and the behaviour of these massifs during the Alpine orogenesis
The Western Meseta shear zone, a major and permanent feature of the Hercynian belt in Morocco
Shear zones in the Iberian Arc